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Induction accelerators are appealing for heavy-ion driven inertial fusion energy (HIF) because of their high efficiency and their demonstrated capability to accelerate high beam current (≥10 kA in some applications). For the HIF application, accomplishments and challenges are summarized. HIF research and development has demonstrated the production of single ion beams with the required emittance, current, and energy suitable for injection into an induction linear accelerator. Driver scale beams have been transported in quadrupole channels of the order of 10% of the number of quadrupoles of a driver. We review the design and operation of induction accelerators and the relevant aspects of their use as drivers for HIF. We describe intermediate research steps that would provide the basis for a heavy-ion research facility capable of heating matter to fusion relevant temperatures and densities, and also to test and demonstrate an accelerator architecture that scales well to a fusion power plant.

We have measured electron and gas emission from 1 MeV K + impact on surfaces near grazing incidence on the High-Current Experiment (HCX) at LBNL. Electron emission coefficients reach values of 130, whereas gas desorption coefficients are near 10 4 . Mitigation techniques are being studied: a bead-blasted rough surface reduces electron emission by a factor of 10 and gas desorption by a factor of 2. Diagnostics are installed on HCX, between and within quadrupole magnets, to measure the beam halo loss, net charge and expelled ions, from which we infer gas density, electron trapping, and the effects of mitigation techniques. Here we discuss a new diagnostic technique that measures gas pressure and electron ionization rates within quadrupole magnets during the beam transit.

W.I-07 EXPERIMENTAL STUDY OF THE TRANSPORT LIMITS OF INTENSE HEAVY ION BEAMS IN THE HCX 1 L. R. Prost (FNAL), F. M. Bieniosek (LBNL), C.M. Celata (LBNL), A. Faltens (LBNL), P. A. Seidl (LBNL), W. L. Waldron (LBNL) R. Cohen (LLNL), A. Friedman (LLNL), M. Kireeff Covo (LLNL), S.M. Lund (LLNL), A.W. Molvik (LLNL) I. Haber (UM) FNAL: Fermi National Accelerator Laboratory, Batavia, IL, 60510 USA LBNL: Lawrence Berkeley National Laboratory, Berkeley, CA, 94720-8201 LLNL: Lawrence Livermore National Laboratory, Livermore, CA 94550 USA UM: University of Maryland, College Park, MD 20742-3511 Corresponding author: Lionel Prost FERMILAB P.O. Box 500 – MS 306 Batavia, IL, 60510 USA Tel: Fax: E-mail: lprost@fnal.gov Abstract The High Current Experiment (HCX) at Lawrence Berkeley National Laboratory is part of the US program to explore heavy-ion beam transport at a scale representative of the low-energy end of an induction linac driver for fusion energy production. The primary mission of this experiment is to investigate aperture fill factors acceptable for the transport of space-charge-dominated heavy-ion beams at high space-charge intensity (line charge density up to ~ 0.2 µC/m) over long pulse durations (4 µs) in alternating gradient focusing lattices of electrostatic or magnetic quadrupoles. The experiment also contributes to the practical baseline knowledge of intense beam manipulations necessary for the design, construction and operation of a heavy ion driver for inertial fusion. This experiment is testing transport issues resulting from nonlinear space-charge effects and collective modes, beam centroid alignment and beam steering, matching, image charges, halo, electron cloud effects, and longitudinal bunch control. We first present the results for a coasting 1 MeV K + ion beam transported through the first ten electrostatic transport quadrupoles, measured with optical beam-imaging and double-slit phase-space diagnostics. This includes studies at two different radial fill factors (60% and 80%), for which the beam transverse distribution was characterized in detail. Additionally, beam This work performed under the auspices of the U.S Department of Energy by University of California, Lawrence Livermore and Lawrence Berkeley National Laboratories under contracts No. W-7405-Eng-48 and DE-AC03-76SF00098.

The High-Current Experiment (HCX) at Lawrence Berkeley National Laboratory is part of the US program to explore heavy-ion beam transport at a scale representative of the low-energy end of an induction linac driver for fusion energy production. Four pulsed magnetic quadrupoles are being used to study gas and electron effects with a 0.2 A, 1-MeV K + beam. The magnets, originally designed and built for a prototype pulsed magnetic quadrupole array, have an elliptical beam tube (6×10 cm) and iron yoke. The magnet coil and field length are ≈31 cm, and operating gradients are 10–40 T/m. To establish that the field quality of the prototype quadrupoles is satisfactory for the experiments, a 1-cm pickup loop was used to measure the flux B r ( θ ) at the magnet mid-plane and also at the lead and return ends. A longer probe was used to measure the integrated flux of B θ ( θ ) along the magnet. The field quality appears satisfactory for the short transport experiments through these quadrupoles.

When a short-duration, small-amplitude energy perturbation is applied to a heavy-ion beam, positive- and negative-going longitudinal space–charge waves are generated on the beam. Longitudinal diagnostic kickers that provide ∼1% energy perturbation have been implemented for generating space–charge waves on HCX and NTX beams. The kickers consist of specially designed fast pulse generators combined with an existing aperture or ESQ structure to provide the longitudinal perturbation to the beam. The amplitudes of the resulting density waves are ∼10%, measured a few meters downstream of the kicker. The time-of-flight (TOF) of the wave provides an accurate measure of the beam energy. The TOF measurements will be described. Comparison of measured waves with a simple 1-D fluid model of the beam will be presented.

The High Current Experiment (HCX) is being assembled at Lawrence Berkeley National Laboratory as part of the U.S. program to explore heavy ion beam transport at a scale representative of the low-energy end of an induction linac driver for fusion energy production. The primary mission of this experiment is to investigate aperture fill factors acceptable for the transport of space-charge dominated heavy ion beams at high space-charge intensity (line-charge density ∼ 0.2 μC/m) over long pulse durations (>4 μs). This machine will test transport issues at a driver-relevant scale resulting from nonlinear space-charge effects and collective modes, beam centroid alignment and beam steering, matching, image charges, halo, lost-particle induced electron effects, and longitudinal bunch control. We present the first experimental results carried out with the coasting K+ ion beam transported through the first 10 electrostatic transport quadrupoles and associated diagnostics. Later phases of the experiment will include more electrostatic lattice periods to allow more sensitive tests of emittance growth, and also magnetic quadrupoles to explore similar issues in magnetic channels with a full driver scale beam.

A one-tenth-scale version of a final focus subsystem for a heavy-ion-fusion driver has been built and used for experimental tests of concept. By properly scaling the parameters that relate ion energy and mass, current, emittance, and focusing fields, the transverse dynamics of a representative driver final focus have been replicated in a small laboratory beam. Whereas the driver beam parameters considered are 10 GeV Bi+ at 1.25 kA, the scaled experiment used a 95 μA beam of 160 keV Cs+ ions brought to a ballistic focus through a series of six quadrupole magnets. The measured focal spot size was consistent with calculations in the report of the design on which the experiment is based. In a second experimental program, a 400 μA beam was propagated through the focal system and partially neutralized after the last magnet using electrons released from a hot tungsten filament to test the predictions of the benefits of neutralization. The increase in beam current resulted in a corresponding increase in spot radi...

In the planned heavy-ion Integrated Research Experiments (IRE), final beam kinetic energy will be low compared to a driver, giving perveance significantly larger than the driver, and making final focus to a small spot comparatively much more difficult. Splitting of beams ahead of the final focus system would allow economical transport of large beams through the IRE accelerator, while maintaining focusability. Since splitting decouples the number of beams transported in the accelerator from what is required at final focus, it could also lower the cost of a driver. In this paper we consider the effects on beam quality of transverse beam splitting. The system of most interest consists of one or more consecutive magnetic septa. PIC simulations of this case are discussed, along with engineering considerations.

An overview of experiments is presented, in which the physical dimensions, transverse emittance and generalized perveance are scaled to explore driver-relevant beam dynamics. Among these are beam merging, focusing to a small spot, and bending and recirculating beams. The Virtual National Laboratory for Heavy Ion Fusion (VNL) is also developing two driver-scale beam experiments involving heavy-ion beams with I beam ∼1 A to provide guidance for the design of an Integrated Research Experiment (IRE) for driver system studies within the next 5 years. Multiple-beam sources and injectors are being designed and a one-beam module will be built and tested. Another experimental effort will be the transport of such a beam through ∼100 magnetic quadrupoles. The experiment will determine transport limits at high aperture fill factors, beam halo formation, and the influence on beam properties of secondary electron production and neutral atom desorption from the vacuum pipe wall. Research into driver technology will be briefly presented, including the development of ferromagnetic core materials, induction core pulsers, multiple-beam quadrupole arrays and plasma channel formation experiments for pinched transport in reactor chambers.

We simulate fusion power plant driver efficiency by pulsing small induction cores at 5 Hz (a typical projected power plant repetition rate), with a resistive load in the secondary winding that is scaled to simulate the beam loading for induction acceleration. Starting from a power plant driver design that is based on other constraints, we obtain the core mass and acceleration efficiency for several energy ranges of the driver accelerator and for three magnetic alloys. The resistor in the secondary is chosen to give the same acceleration efficiency, the ratio of beam energy gain to energy input to the core module (core plus acceleration gap), as was computed for the driver. The pulser consists of a capacitor switched by FETs, Field Effect Transistors, which are gated on for the desired pulse duration. The energy to the resistor is evaluated during the portion of the pulse that is adequately flat. We present data over a range of 0.6–5 μs pulse lengths. With 1 μs pulses, the acceleration efficiency at 5 Hz is measured to be 75%, 52%, and 32% for thin-tape-wound cores of nanocrystalline, amorphous, and 3% silicon steel materials respectively, including only core losses. The efficiency increases for shorter pulse durations.

A multi-channel quadrupole array has been proposed to increase beam intensity and reduce space charge effects in a heavy ion fusion driver. A single array unit composed of several quadrupole magnets, each with its own beam line, will be placed within a ferromagnetic accelerating core whose cost is directly affected by the array size. A large number of focusing arrays will be needed along the accelerating path. The use of a superconducting quadrupole magnet array will increase the field and reduce overall cost. The authors report here on the design of a compact 3/spl times/3 superconducting quadrupole magnet array. The overall array diameter and length including the cryostat is 900/spl times/700 mm. Each of the 9 quadrupole magnets has a 79 mm warm bore and an operating gradient of 50 T/m over an effective magnetic length of 320 mm.

The approach to heavy-ion-driven inertial fusion studied most extensively in the US uses induction modulators and cores to accelerate and confine the beam longitudinally. The intrinsic peak-current capabilities of induction machines, together with their flexible pulse formats, provide a suitable match to the high peak-power requirement of a heavy-ion fusion target. However, as in the RF case, where combinations of linacs, synchrotrons, and storage rings offer a number of choices to be examined in designing an optimal system, the induction approach also allows a number of architectures, from which choices must be made. We review the main classes of architecture for induction drivers that have been studied to date. The main choice of accelerator structure is that between the linac and the recirculator, the latter being composed of several rings. Hybrid designs are also possible. Other design questions include which focusing system (electric quadrupole, magnetic quadrupole, or solenoid) to use, whether or not to merge beams, and what number of beams to use – all of which must be answered as a function of ion energy throughout the machine. Also, the optimal charge state and mass must be chosen. These different architectures and beam parameters lead to different emittances and imply different constraints on the final focus. The advantages and uncertainties of these various architectures will be discussed.

Current amplification of heavy-ion beams is an integral feature of the induction linac approach to heavy-ion fusion (HIF). In this paper we report on amplification experiments conducted on a single beam of the Multiple Beam Experiment (MBE-4), a heavy-ion (Cs+) induction linac. Earlier MBE-4 experiments [H. Meuth et al., Nucl. Instrum. Methods Phys. Res. A 278, 153 (1989)] had demonstrated up-to-9× current amplification but had been accompanied by an up-to-2× increase of normalized transverse emittance. Experiments to pinpoint the causes of this emittance growth indicated various factors were responsible, including focusing aberrations and mismatch difficulties between the injector diode and the accelerator transport lattice, a localized quadrupole misalignment problem, and the interaction of transversely large beams with the nonlinear elements of the focusing lattice. Following ameliorative measures, new current amplification experiments, both with and without acceleration, showed that current amplifications of up to 3× and line charge density increases of up to ≈2× could be achieved without increasing the beam’s normalized transverse emittance. Finally, both the transverse beam dynamics, and beam current and energy measurements were accurately modeled by numerical particle-in-cell simulations and longitudinal dynamics codes, respectively.

Transverse beam combining is a cost-saving option employed in many designs for induction linac heavy ion fusion drivers. The resultant transverse emittance increase, due predominantly to anharmonic space charge forces, must be kept minimal so that the beam remains focusable at the target. A prototype combining experiment has been built using the MBE-4 experimental apparatus. Four new sources produce up to 6.7 mA Cs+ beams at 200 keV. The ion sources are angled toward each other so that the beams converge. Focusing upstream of the merge consists of four quadrupoles and a final combined-function element (quadrupole and dipole). All lattice elements are electrostatic. Owing to the small distance between beams at the last element (about 3–4 mm), the electrodes here are a cage of small rods, each at different voltage. The beams emerge into the 30-period transport lattice of MBE-4 where emittance growth due to merging, as well as the subsequent evolution of the distribution function, can be diagnosed. The combiner design, simulation predictions and preliminary results from the experiment are presented.

This note was originally one of the parts of the work on a 50 MeV and 500 MeV Rb{sup +} driver and part of work on delay lines for a 60 GeV U{sup +12} driver. It is slightly expanded here to make it more generally applicable. The emphasis is on beam manipulations such as joining and separating beams at the two ends of a driver and providing various time delays between beams as required by the target.

A 2 MV, 800 mA, K+ injector for heavy-ion fusion studies is under construction. This new injector is a one-beam version of the proposed 4-beam ILSE injector. A new 36-module MARX is being built to achieve a 5 μs flat top. The high-voltage generator is stiff (

The Heavy-Ion Fusion Accelerator Research Program at Lawrence Berkeley Laboratory has proposed building a 10 MeV induction linac systems experiment, ILSE, to investigate accelerator physics and beam manipulations which are needed or desirable for an induction linac driver. This paper describes the experiments proposed for ILSE: transverse beam combining, drift compression, bending of space-charge-dominated beams, final focus, recirculation, and some studies of beam propagation in the environment of the reactor chamber.

This study group was initiated to consider whether there were any ''show-stopper'' issues with accelerators for heavy-ion warm-dense matter (WDM) and heavy-ion inertial fusion energy (HIF), and to prioritize them. Showstopper issues would appear as limits to beam current; that is, the beam would be well-behaved below the current limit, and significantly degraded in current or emittance if the current limit were exceeded at some region of an accelerator. We identified 14 issues: 1-6 could be addressed in the near term, 7-10 are potentially attractive solutions to performance and cost issues but are not yet fully characterized, 11-12 involve multibeam effects that cannot be more than partially studied in near-term facilities, and 13-14 involve new issues that are present in some novel driver concepts. Comparing the issues with the new experimental, simulation, and theoretical tools that we have developed, it is apparent that our new capabilities provide an opportunity to re-examine and significantly increase our understanding of the number one issue--halo growth and mitigation.

The High Current Experiment at Lawrence Berkeley National Laboratory is part of the U.S. program to explore heavy-ion beam transport at a scale representative of the low-energy end of an induction linac driver for fusion energy production. The primary mission of this experiment is to investigate aperture fill factors acceptable for the transport of space-charge-dominated heavy-ion beams at high intensity (line charge density ∼0.2 μC/m) over long pulse durations (4 μs) in alternating gradient focusing lattices of electrostatic or magnetic quadrupoles. This experiment is testing transport issues resulting from nonlinear space-charge effects and collective modes, beam centroid alignment and steering, envelope matching, image charges and focusing field nonlinearities, halo, and electron and gas cloud effects. We present the results for a coasting 1 MeV K^{+} ion beam transported through ten electrostatic quadrupoles. The measurements cover two different fill factor studies (60% and 80% of the clear aperture radius) for which the transverse phase space of the beam was characterized in detail, along with beam energy measurements and the first halo measurements. Electrostatic quadrupole transport at high beam fill factor (≈80%) is achieved with acceptable emittance growth and beam loss, even though the initial beam distribution is not ideal (but the emittance is low) nor in thermal equilibrium. We achieved good envelope control, and rematching may only be needed every ten lattice periods (at 80% fill factor) in a longer lattice of similar design. We also show that understanding and controlling the time dependence of the envelope parameters is critical to achieving high fill factors, notably because of the injector and matching section dynamics.

Heavy‐ion induction linacs for inertial fusion energy and high‐energy density physics have an economic incentive to minimize the clearance between the beam edge and the aperture wall. This increases the risk from electron clouds and gas desorbed from walls. We have measured electron and gas emission from 1 MeV K+ incident on surfaces near grazing incidence on the High‐Current Experiment (HCX) at LBNL. Electron emission coefficients reach values >100, whereas gas desorption coefficients are near 104. Mitigation techniques are being studied: A bead‐blasted rough surface reduces electron emission by a factor of 10 and gas desorption by a factor of 2. We also discuss the results of beam transport (of 0.03–0.18 A K+) through four pulsed room‐temperature magnetic quadrupoles in the HCX at LBNL. Diagnostics are installed on HCX, between and within quadrupole magnets, to measure the beam halo loss, net charge and expelled ions, from which we infer gas density, electron trapping, and the effects of mitigation tech...

The pulsed drift-tube accelerator (DTA) concept was revived by Joe Kwan and John Staples and is being considered for the HEDP/WDM application. It could be used to reach the full energy or as an intermediate accelerator between the diode and a high gradient accelerator such as multi-beam r.f. In the earliest LBNL HIF proposals and conceptual drivers it was used as an extended injector to reach energies where an induction linac with magnetic quadrupoles is the best choice. For HEDP, because of the very short pulse duration, the DTA could provide an acceleration rate of about 1MV/m. This note is divided into two parts: the first, a design based on existing experience; the second, an optimistic extrapolation. The first accelerates 16 parallel K{sup +} beams at a constant line charge density of 0.25{micro} C/m per beam to 10 MeV; the second uses a stripper and charge selector at around 4MeV followed by further acceleration to reach 40 MeV. Both benefit from more compact sources than the present 2MV injector source, although that beam is the basis of the first design and is a viable option. A pulsed drift-tube accelerator was the first major HIF experiment at LBNL. It was designedmore » to produce a 2{micro}s rectangular 1 Ampere C{sub s}{sup +} beam at 2MeV. It ran comfortably at 1.6MeV for several years, then at lower voltages and currents for other experiments, and remnants of that experiment are in use in present experiments, still running 25 years later. The 1A current, completely equivalent to 1.8A K{sup +}, was chosen to be intermediate between the beamlets appropriate for a multi-beam accelerator, and a single beam of, say, 10A, at injection energies. The original driver scenarios using one large beam on each side of the reactor rapidly fell out of favor because of the very high transverse and longitudinal fields from the beam space charge, circa 1MV/cm and 250 kV/cm respectively, near the chamber and because of aberrations in focusing a large diameter beam down to a 1mm radius spot at a distance of 10m. Almost all subsequent work and the present concept have invoked multiple beams. For HEDP the major differences are that the focal distance can be centimeters instead of meters, provided strong-enough lenses exist and they do, thereby allowing much higher transverse and longitudinal emittances than driver concepts, and focusing parallel small beams is easier than one big beam.« less

We report the successful development and commissioning of new diagnostics for electron cloud and gas desorption studies on the High Current Experiment (HCX) at LBNL. These measure the time-dependent gas density in a 0.03 - 0.2 A 1 MeV K{sup +} ion beam inside of a quadrupole magnet, and the three possible sources of electrons: from ionization of gas, from beam tube walls, and from an end wall where the beam dumps. The accumulation of electrons in the beam is determined by measuring the net charge of the beam with capacitive electrodes, and other diagnostics being developed. These diagnostics are complemented by a suppressor electrode that controls the electron source from an end wall, and clearing electrodes that remove (and measure the current of) electrons from drift regions between quadrupole magnets. Measurements of electron emission coefficients and gas desorption coefficients near grazing angle enable us to infer the magnitude of beam loss needed to produce the measured electron emission and the resulting gas desorption. These new tools provide the capability to explore new areas of electron-cloud effects science, and to make quantitative comparisons with simulation.

Modern diagnostic techniques provide detailed information on beam conditions in injector, transport, and final focus experiments in the HIF-VNL. Parameters of interest include beam current, beam energy, transverse and longitudinal distributions, emittance, and space charge neutralization. Imaging techniques, based on kapton films and optical scintillators, complement and, in some cases, may replace conventional techniques based on slit scans. Time-resolved optical diagnostics that provide 4-D transverse information on the experimental beams are in operation on the existing experiments. Current work includes a compact optical diagnostic suitable for insertion in transport lines, improved algorithms for optical data analysis and interpretation, a high-resolution electrostatic energy analyzer, and an electron beam probe. A longitudinal diagnostic kicker generates longitudinal space-charge waves that travel on the beam. Time of flight of the space-charge waves and an electrostatic energy analyzer provide an absolute measure of the beam energy. Special diagnostics to detect secondary electrons and gases desorbed from the wall have been developed.

090101-1 Transverse beam combining is a cost-saving option employed in many designs for heavy ion fusion drivers. However, the resultant transverse phase space dilution must be minimized so as not to sacrifice focusability at the target. A prototype combining experiment has been completed employing four 3-mA Cs beams injected at 160 keV. The focusing elements upstream of the merge consist of four quadrupoles and a final combined-function element (quadrupole and dipole). Following the merge, the resultant single beam is transported in a single alternating gradient channel where the subsequent evolution of the distribution function is diagnosed. The results are in fair agreement with particle-incell simulations. They indicate that for some heavy ion fusion driver designs, the phase space dilution from merging is acceptable.

We present the design of a pulsed quadrupole magnet array for side by side transport of multiple high current particle beams in induction linear accelerators. The design is directly applicable to low field superconducting quadrupole arrays and can be modified for high fields. The magnetic design minimizes current requirements by placing a modified cosine 2/spl theta/ current distribution inside an elliptical iron boundary. The resulting magnet aperture more closely matches the beam's elliptical cross-section, and only the space the beam occupies is magnetized to the required field quality. By eliminating unneeded high field area in the magnet aperture, energy requirements are halved, as compared to an equivalent circular design. The magnet serves double duty as a UHV vacuum chamber/beam tube, and reduced radial thickness results from lower current requirements. Each interior magnet's neighbors serve as the return flux paths and the poles are placed as close as possible to each other to facilitate this. All the above features serve to reduce the beam-to-beam spacing, leading to significant reductions in the size and cost of the induction cores which surround the array, as well as the pulsed power supplies which drive them. The design shown here is for a 21 beam array, yet it is suitable for larger or smaller numbers of beams, as required.

A multibeam injector being built to provide 16-500 mA beams of C/sup +/ at 2 MeV energy is presented. The sources are carbon cathode multiple arcs that provide plasma to a double-grid plasma switch. The plasma switch is a planar electrostatic device that prevents ions from entering the extraction gap before the extraction voltage pulse is fired. Data on the performance of this source in a test stand have been obtained. The 1- mu s current pulse will be injected into the acceleration column approximately 30 mu s after the voltage pulse is applied to the column. This allows voltages to equilibrate and the flat spot in the voltage pulse to be reached. The ions are accelerated in a column that consists of titanium electrodes mounted inside aluminum oxide insulator modules. Most of the 16 beam holes are in thick plates which are machined to provide a set of alternating aperture lenses. These lenses focus the beam and inhibit propagation of back streaming electrons. >

The design of a high-current heavy-ion induction linac driver for inertial confinement fusion is optimized by adjusting the acceleration units along the length of the accelerator to match the beam current, energy, and pulse duration at any location. At the low-energy end of the machine the optimum is a large number of electrostatically focused parallel beamlets, whereas at higher energies the optimum is a smaller number of magnetically focused beams. ILSE parallels this strategy by using 16 electrostatically focused beamlets at the low end followed by four magnetically focused beams after beam combining. Descriptions are given of the electric focusing section, the magnetic focus accelerator, the induction cores and pulsers, and the ILSE alignment systems. >

An update on the development of the 500-mA/beam sixteen-beam injector being built at Lawrence Berkeley Laboratory is presented. An inductively graded Marx bank provides the acceleration potential on the electrostatic column. A carbon arc source provides the pulsed current for the injector. Recent results on extracted beam parameters, column performance, generator performance and system design changes are reported. The carbon ion beam is diagnosed with Faraday cups and with a double-slit emittance measurement system. Controls for the final machine are also discussed. >

In ILSE (Induction Linac Systems Experiment), sixteen intense parallel ion beams are to be transversely combined into four by dispersionless double bends. Emittance growth due to electrostatic energy redistribution and to the geometry is evaluated. Most bending elements are electric, and alternate with AG electrostatic quadrupoles similar to those upstream. The final elements are magnetic, combining focusing and 'unbending'. Electrode shapes and pulsed-current arrays (with very small clearances), as well as mechanical and electric features of the combiner, are described. >

A program is underway to construct a 2 MV, 800 mA, K/sup +/ injector for heavy ion fusion. The electrostatic quadrupole injector configuration consists of a zeolite source, a diode of up to 1 MV, together with several electrostatic quadrupole units to simultaneously focus and accelerate the beam to 2 MV. The key issues of source technology, high voltage breakdown, beam aberrations, and transient effects are discussed. Results from ongoing experiments and simulations are presented. >

High-voltage electrostatic quadrupoles are used for focusing ion beams at low energies in the induction linac approach to heavy ion driven inertial confinement fusion for the production of electrical power. The transportable beam line charge density depends linearly on the operating voltage of the quadrupoles, so an experimental program was conducted to find the voltage breakdown dependence on the overall size of the quadrupoles which would then allow determination of the best geometry and operating voltage. The quadrupole electrodes are usually stainless steel cylinders with hemispherical end caps, mounted on stainless steel end plates. The end plates are precisely positioned with respect to each other and the vacuum chamber with alumina insulators with shielded triple points. It is advantageous for beam transport to employ an array of multiple beams for which a rather large number of interdigitated electrodes forms an array of quadrupoles. The trade-offs between very large numbers of small channels and a smaller number of large channels, and the dependence of the choice on the voltage breakdown dependence is discussed. With present understanding, the optimum is about 100 beamlets focused with quadrupoles which have a beam aperture radius of about 2.3 cm and art operated with about 150 kV between electrodes.

An induction linac test module suitable for investigating the drive requirements and the longitudinal coupling impedance of a high-power ion induction linac has been constructed by the heavy ion fusion (HIF) group at LBL (Lawrence Berkeley Laboratory). The induction linac heavy ion driver for inertial confinement fusion (ICF) as presently envisioned uses multiple parallel beams which are transported in separate focusing channels but accelerated together in the induction modules. The resulting induction modules consequently have large beam apertures-1-2 m in diameter-and correspondingly large outside diameters. The module geometry is related to a low-frequency gap capacity and high-frequency structural resonances, which are affected by the magnetic core loading and the module pulser impedance. A description of the test module and preliminary results are presented. >

An injector is being developed at LBL (Lawrence Berkeley Laboratory) that would serve as the front end of a scaled induction linac accelerator technology experiment in heavy ion fusion. The ion mass being used is in the range 10-18. It is a multi-beam device intended to accelerate up to 2 MeV with 500 mA in each beam. The first half of the accelerating column has been built and experiments with one carbon beam are underway at the 1 MeV level. >

Transverse beam combining is a cost-saving option employed in many designs for induction linac heavy ion fusion drivers. The resultant transverse emittance increase, due predominantly to anharmonic space charge forces, must be kept minimal so that the beam remains focusable at the target. A prototype combining experiment has been built and preliminary results are presented. Four sources each produce up to 4.8 mA Cs/sup +/ beams at 160 keV. Focusing upstream of the merge consists of four quadrupoles and a final combined-function element (quadrupole and dipole). All lattice elements of the prototype are electrostatic. Due to the small distance between beams near the merge (/spl sim/3-4 mm), the electrodes here are a cage of small rods, each at different voltage.

The Heavy Ion Fusion Program at the Lawrence Berkeley Laboratory is conducting experiments in the transport and acceleration of "driverlike" beams. The single beam coming from the four-to-one beam combiner will be transported in a lattice of pulsed magnetic quadrupoles. The present beam transport consists of high field, short aspect ratio magnetic quadrupoles to maximize the transportable current. This design could also be converted to be superconducting for future uses in a driver. The pulsed quadrupole will develop a maximum field of two Tesla and will be housed within the induction accelerator cells at the appropriate lattice period. Hardware implementation of the physics requirements and full parameter testing will be described.

The promise of inertial fusion energy driven by heavy ion beams requires the development of accelerators that produce ion currents (/spl sim/100's Amperes/beam) and ion energies (/spl sim/1 - 10 GeV) that have not been achieved simultaneously in any existing accelerator. The high currents imply high generalized perveances, large tune depressions, and high space charge potentials of the beam center relative to the beam pipe. Many of the scientific issues associated with ion beams of high perveance and large tune depression have been addressed over the last two decades on scaled experiments at Lawrence Berkeley and Lawrence Livermore National Laboratories, the University of Maryland, and elsewhere. The additional requirement of high space charge potential (or equivalently high line charge density) gives rise to effects (particularly the role of electrons in beam transport) which must be understood before proceeding to a large scale accelerator. The first phase of a new series of experiments in the Heavy Ion Fusion Virtual National Laboratory (HIF VNL), the High Current Experiments (HCX), is now beginning at LBNL. The mission of the HCX is to transport beams with driver line charge density so as to investigate the physics of this regime, including constraints on the maximum radial filling factor of the beam through the pipe. This factor is important for determining both cost and reliability of a driver scale accelerator. The HCX will provide data for design of the next steps in the sequence of experiments leading to an inertial fusion energy power plant. The focus of the program after the HCX will be on integration of all of the manipulations required for a driver. In the near term following HCX, an Integrated Beam Experiment (IBX) of the same general scale as the HCX is envisioned. The step which bridges the gap between the IBX and an engineering test facility for fusion has been designated the Integrated Research Experiment (IRE). The IRE (like the IBX) will provide an integrated test of the beam physics necessary for a driver, but in addition will provide target and chamber data. This paper will review the experimental and theoretical progress in heavy ion accelerator driver research from the scaled experiments through the present experiments and will discuss plans for the IRE.

This paper reports progress in the HCX experimental program since the last HIF-VNL Program Advisory Committee Review (February 14-15 2002). On July 25 2002 the experiment was shut down for about four weeks to move the control room. A principal area of effort has been to obtain and evaluate the first experimental results carried out with a matched and well-aligned K + ion beam transported through 10 electrostatic transport quadrupoles. These are the main results and highlights to date: A1. There is no emittance growth within the sensitivity of the diagnostics, and little beam loss. The beam centroid is aligned to within 0.5 mm and 2 mrad of the central axis of the channel, and the envelope mismatch amplitude is

In a typical heavy ion fusion driver design, the induction linac requires a multiple beam injector. We present a conceptual design for an 84-beam injector system with each beam channel carrying 0.5 A of beam current. Each channel starts with a 10-cm diameter surface ionization source followed by a 1.7 MV electrostatic Einzel lens-type preaccelerator and an electrostatic quadrupole (ESQ) matching section. The preaccelerator and matching section are 0.7 m and 5.0 m long respectively. The array has an overall diameter of 3.0 m at the ion source end and 1.0 m diameter at the exit.

The High Current Experiment (HCX) is being built to explore heavy-ion beam transport at a scale appropriate to the low-energy end of a driver for fusion energy production. The primary mission of this experiment is to investigate aperture fill factors acceptable for the transport of space-charge dominated heavy-ion beams at high space-charge intensity (line-charge density /spl sim/0.2 /spl mu/C/m) over long pulse durations (3-10 /spl mu/sec). A single beam transport channel will be used to evaluate scientific and technological issues resulting from the transport of an intense beam subject to applied field nonlinearities, envelope mismatch, misalignment-induced centroid excursions, imperfect vacuum, halo, background gas and electron effects resulting from lost beam ions. Emphasis will be on the influence of these effects on beam control and limiting degradations in beam quality (emittance growth). Electrostatic (Phase I) and magnetic (Phase II) quadrupole focusing lattices have been designed and future phases of the experiment may involve acceleration and/or pulse compression. The Phase I lattice is presently under construction and simulations to better predict machine performance are being carried out. Here we overview: the scientific objectives of the overall project, processes that will be explored, and transport lattices developed.

Vacuum ballistic focusing is the straightforward method to obtain a heavy ion beam spot size necessary to drive an inertial confinement fusion target. The beam is first expanded then focused to obtain the desired convergence angles at the exit of the last element. This is done in an attempt to achieve a focal spot size in which emittance is the limiting factor; however, aberrations and space charge will influence the spot radius. Proper scaling of particle energy, mass, beam current, beam emittance, and magnetic field replicates the dynamics of a full driver beam at the focus in a small laboratory experiment. By scaling the beam current to ~;100 mu A, 160 keV Cs+ has been used to study experimentally a proposed driver design at one-tenth scale. Once a nominal focal spot is achieved, the magnet strengths are deliberately de-tuned to simulate the effect of an off-momentum slice of the beam. Additionally, several methods will be used to inject electrons into beam following the last focusing element in order to study the neutralization of space charge and its effect on the focus. Transverse phase space and beam current density measurements at various stages of the focus will be presented as well spot size measurements from the various trials. This data will be compared to the results of a PIC model of the experiment.

Vacuum ballistic focusing may be a straightforward method to obtain the heavy ion beam spot size necessary to drive an inertial confinement fusion target. Proper scaling of particle energy, mass, beam current, beam emittance, and magnetic field replicates the dynamics of a full driver beam in a small laboratory experiment. By expanding the beam and then focusing to a very small spot, the effects of aberrations and space charge on this method of final focus can be studied. To date, 200 /spl mu/A of 120 keV K/sup +/ has been focused to test the matching and focusing elements. A recently designed high brightness contact ionization source is being tested for upcoming focusing measurements with an 87 /spl mu/A Cs beam that will duplicate the dynamics of a proposed driver design at one-tenth scale. Transverse phase space and beam current density at various stages of the focus will be presented. Follow-on measurements studying electron neutralization of space charge and its effect on the focus will be explored.

Heavy ion fusion (HIF) sources will be low-emittance, high current (0.5-1 A), and suitable for injection into a multiple beam induction linac, where the diameter of each beam aperture at the beginning of the accelerator is 3-6 cm. Practical transport limits involved in matching the beam to the accelerator aperture in a reasonably-sized structure correspond to several mA/cm/sup 2/ of space charge limited flow with an injector extraction voltage of 1-2.5 MV. We have built a J/spl ap/8 mA/cm/sup 2/ Cs/sup +/ contact ionization source for a 160 kV diode; this represents a factor of four increase in the Cs/sup +/ current density over previous sources for HIF. The emitter is sintered iridium, operating at 950

Elise is a heavy-ion induction linear accelerator that will demonstrate beam manipulations required in a driver for inertial fusion energy. With a line charge density similar to that of heavy-ions drivers, Elise will accelerate a beam pulse (duration of 1 μs or more) of K + ions from an initial energy of 2 MeV to a final energy 5 MeV or more. In the present design, the Elise electrostatic quadrupoles (ESQs) will have an aperture of radius 2.33 cm operating at ± 59 kV. The half-lattice periods range from 21 to 31 cm. The entire machine will be approximately 30 m long, half of which is the induction accelerator with the remaining half being the injector (including the Marx generator) and the matching section. Elise will be built in a way that allows future expansion into the full Induction Linear Accelerator Systems Experiments (ILSE) configuration, so it will have an array of four ESQ focusing channels capable of transporting up to a total of 3.2 A of beam current. Elise will also have an active alignment system with an alignment tolerance of less than 0.1 mm. Initially, only one beam channel will be used during nominal Elise operation. At the currently expected funding rate, the construction time will be 4.75 years, with FY95 being an extra year for research and development before construction. Total project cost is estimated to be US$25.9m, including contingency costs.

Induction linacs are among the most powerful accelerators in existence. They have accelerated electron bunches of several kiloamperes, and are being investigated as drivers for heavy ion driven inertial confinement fusion (HIF), which requires peak beam currents of kiloamperes and average beam powers of some tens of megawatts. The requirement for waste transmutation with an 800 MeV proton or deuteron beam with an average current of 50 mA and an average power of 40 MW lies midway between the electron machines and the heavy ion machines in overall difficulty. Much of the technology and understanding of beam physics carries over from the previous machines to the new requirements. The induction linac allows use of a very large beam aperture, which may turn out to be crucial to reducing beam loss and machine activation from the beam halo. The major issues addressed here are transport of high intensity beams, availability of sources, efficiency of acceleration, and the state of the needed technology for the waste treatment application. Because of the transformer-like action of an induction core and the accompanying magnetizing current, induction linacs make the most economic sense and have the highest efficiencies with large beam currents. Based on present understandingmore » of beam transport limits, induction core magnetizing current requirements, and pulse modulators, the efficiencies could be very high. The study of beam transport at high intensities has been the major activity of the HIF community. Beam transport and sources are limiting at low energies but are not significant constraints at the higher energies. As will be shown, the proton beams will be space-charge-dominated, for which the emittance has only a minor effect on the overall beam diameter but does determine the density falloff at the beam edge.« less

Previous heavy-ion fusion driver system studies suggest that transverse beam combining significantly reduces driver cost. In a combiner, several beams are brought together to a common transport channel which accommodates the increased line charge density. Combining intense beams increases the transverse emittance mainly due to the heating of the beam by space charge forces as the non-uniform original beam configuration becomes more uniform. The combiner itself introduces additional aberrations, which are small for the present design. Those aberrations are due to the reduced available space for the focusing electrodes and reduced clearance from the beamlets to the surrounding electrodes, thereby generating field aberrations and larger image forces. These aberrations can also lead to particle loss. We have studied a particular design of the proposed induction linac system experiment (ILSE) combiner which is a first-order achromat that tolerates a rather large fractional head-to-tail momentum tilt of ± 10%. Using a 2D particle-in-cell code we have found that ≈7% of particles are lost in the combiner. The emittance growth after the combiner is large enough so that the emittance growth due to combiner aberrations is unimportant. The scaled projection to a driver shows the growth is small enough to be tolerated. At present, methods of improving combiner design to reduce particle loss and to minimize emittance growth are being studied.

Voltage-holding data for three quadrupole electrode sizes and inter-electrode spacings are reported. The dependence of the breakdown voltage on system size and its influence on the optimum quadrupole size for beam transport in a multiple beam array are discussed. >

Intense beams of heavy ions are well suited for heating matter to regimes of emerging interest. A new facility, NDCX-II, will enable studies of warm dense matter at ∼1 eV and near-solid density, and of heavy-ion inertial fusion target physics relevant to electric power production. For these applications the beam must deposit its energy rapidly, before the target can expand significantly. To form such pulses, ion beams are temporally compressed in neutralizing plasma; current amplification factors of ∼50–100 are routinely obtained on the Neutralized Drift Compression Experiment (NDCX) at the Lawrence Berkeley National Laboratory. In the NDCX-II physics design, an initial non-neutralized compression renders the pulse short enough that existing high-voltage pulsed power can be employed. This compression is first halted and then reversed by the beam’s longitudinal space-charge field. Downstream induction cells provide acceleration and impose the head-to-tail velocity gradient that leads to the final neutraliz...

Longitudinal beam compression and the possibility of concurrent pulse shaping have been the subject of previous studies using analytical methods and 1D

A sixteen beam injector to supply 500mA per beam of C/sup +/ ions, which was initially designed and partially constructed at Los Alamos National Laboratory, is under further development at LBL. We report on development work involving single and multiple carbon arc sources. These sources are useful because they can easily supply the 25 mA/cm/sup 2/ needed for the device. They are pulsed sources which do not cause unacceptable background gas loads in the column. We present emittance and reproducibility data. Acceleration is by a high gradient column using a succession of aperture lenses to focus the beam. The electrodes are arranged with holes for parallel beams inside 28 inch alumina insulators. Data on beam propagation and electron trapping in the column is presented. The acceleration potential is provided by a Marx bank with inductive grading along the structure. The resulting 34 ..mu..sec rise time pulse allows the accelerating potential to equilibrate along the column before the current is injected. Acceleration occurs during a 1 ..mu..sec window at the peak of the pulse, kn which the voltage is flat within 0.1%. 8 refs., 7 figs.

This report describes a 20 channel ``stepper'' magnet assembly designed to cause the incoming Bevatron particle beam to scan across the 1.83 m hydrogen bubble chamber in discrete steps. The purpose of this device is to avoid superimposing tracks. Stepping was triggered by an ``events'' counter placed in the beam. The system was built with low voltage, silicon controlled rectifiers. Magnet pulsing was accomplished in two parts, rapid rise of field was produced by a fast circuit and pulse length was maintained by a slow circuit. Cost was approximately $15 000, exclusive of the surplus components.