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An alternate technique for heavy ion final transport, from the driver to the target, is by the use of the self-standing Z-pinched plasma channel. Experiments conducted at the Lawrence Berkeley National Laboratory have produced 40 cm long stable plasma channels with a peak discharge current of 55 kA in a 7 Torr nitrogen gas fill. These channels are produced using a double pulse discharge scheme, namely, a pre-pulse discharge and a main capacitor bank discharge. It is postulated that the channel’s insensitivity to MHDinstabilities within the time scale relevant to beam transport is due to the wall effect the pre-pulse discharge creates. This is accomplished by leaving a gas density depression on the channel’s axis after hydrodynamic expansion. Since the pre-pulse discharge creates the initial conditions for the main bank Z-pinch, it is critical to understand how to control and engineer the pre-pulse. Here we present some of the results of ongoing experiments geared to understand the underlying physics of the LBNL Z-pinch plasma channel. Schlieren and phase contrast measurements show the radial propagation of a shock wave during the pre-pulse discharge and suggest indirectly the evidence of the on axis gas density depression, that is believed to be 5 1 10 of the original gas fill pressure. For the main bank Z-pinch, interferometry show an integrated electron line density of 1.6 � 10 17 cm � 2 for a 15 kV discharge on axis. These measurements coupled with Faraday rotation measurements will indicate ultimately the current density distribution in the channel. This data will be used to benchmark simulation codes. # 2001 Elsevier Science B.V. All rights reserved.

An alternate technique for heavy ion beam transport inside the chamber of an inertial confinement fusion reactor, is by the use of a self standing Z-pinch discharge. The focusing properties of such a channel are determined by the magnetic field distribution inside the plasma column. In order to allow measuremets of the magnetic field in the LBNL channel experiment, a Faraday polarimeter has been developed, consisting of a high power cw CO2 laser ð10:6 mmÞ and two HgCdTe detectors working on the half-shadow principle. The polarimeter was designed for a sensitivity of 0.018 since the expected peak rotation was only about one degree. Preliminary measurements with reduced sensitivity indicate that the current flow is confined within a small cylinder of ca. 1 cm diameter. # 2001 Elsevier Science B.V. All rights reserved.

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.

Adiabatic plasma lens and Z -pinch channel transport represent attractive alternatives for the final transport of Heavy-Ion Fusion (HIF) beams to target. Production of stable, reliable, precisely timed Z -pinches is a first step to establish this method of channel transport for HIF. Stable Z -pinches at currents greater than 30 kA over a distance of 400 mm were routinely produced at 3–10 Torr. The technique uses four distinct steps: (1) laser ionization; (2) laser initiated resistor; (3) prepulse Z -pinch; and (4) high-current Z -pinch. The time interval between laser trigger and main-bank spark-gap trigger is controllable to less than 10 ns, limited by the firing characteristics of the spark gap. Optical channel diameters measured with a gated TV camera and correlated with a current probe are approximately those needed for transport in fusion chamber. No large or small scale instabilities were observed before the first current peak. A symmetric two-sided return path had a stable target interaction, even with inductively unbalanced currents. As expected, a single return path, 90° to a single-pinch channel had unstable alignment on the target, due to higher magnetic pressure on the inside of the 90° bend.

A heavy ion linear induction accelerator is considered to be the leading driver candidate for an Inertial Fusion Energy reactor. To deliver a space-charge-dominated beam at the appropriate energy (several GeV), such an accelerator would be several kilometers in length. Since total length has a strong influence on accelerator cost, we are considering the potential advantages and practical implementation of a recirculating induction accelerator. To address the critical scientific and technical challenges of a recirculating space-charge-dominated heavy ion beam, we have begun to develop the elements of a scaled ``small recirculator``. An operating recirculator must demonstrate full beam control including multi-lap operation, beam insertion/extraction, acceleration and pulse compression. At present, experiments have been conducted using a 2mA, 80keV K{sup +} beam transported through a 45{degree} bend; experiments on a 90{degree} bend with five induction modulators will begin soon. This paper briefly summarizes the recirculator specifications and operational features and reports the latest experimental data as well as the developmental status of beam diagnostics.

We present intense-beam simulations with the WARP code that are being carried out in support of the Heavy-Ion Fusion experimental programs at Lawrence Livermore National Laboratory (LLNL) and Lawrence Berkeley National Laboratory (LBNL). The WARP code is an electrostatic particle-in-cell code with an extensive hierarchy of simulation capabilities. Two experiments are analyzed. First, simulations are presented on an 80 keV, 2 mA K‘ bent transport channel at LLNL that employs an alternating-gradient lattice of magnetic quadrupoles for beam focusing and electric dipoles for beam bending. Issues on dispersion-induced changes in beam quality on the transition from straight- to bent-lattice sections are explored. The second experiment analyzed is a 2 MeV, 800 mA, driver-scale injector and matching section at LBNL that is based on a K‘ source and an alternating-gradient lattice of electrostatic quadrupoles biased to accelerate, focus, and match the beam. Issues on beam quality, space-charge waves, and beam hollowing are explored. Published by Elsevier Science B.V.

As part of an ongoing exploration of final transport schemes based on plasma channels, a point design of a final focus and reactor system is being developed. Six MJ of 3–4 GeV Pb ions are delivered to a target in two opposing current-carrying plasma channels within a modified HYLIFE II reactor filled with 5 Torr of Xe gas. The transition from the superconducting quadrupoles to the small entrance hole in the reactor is provided by a tapered plasma lens. The required channel parameters of 50 kA current and 10 mm size have been demonstrated to be feasible in recent Lawrence Berkeley National Laboratory (LBNL) experiments. The proposed system relaxes requirements on the incoming beam and provides life of the plant protection of all critical components around the reactor. Engineering design work is paralleled with beam and channel simulations from the final quadrupoles to the target.

The US is developing the physics and technology of induction accelerators for heavy-ion beam-driven inertial fusion. The recirculating induction accelerator repeatedly passes beams through the same set of accelerating and focusing elements, thereby reducing both the length and gradient of the accelerator structure. This promises an attractive driver cost, if the technical challenges associated with recirculation can be met. Point designs for recirculator drivers were developed in a multi-year study by LLNL, LBNL, and FM Technologies, and that work is briefly reviewed here. To validate major elements of the recirculator concept, we are developing a small (4-5-m diameter) prototype recirculator which will accelerate a space-charge-dominated beam of K{sup +} ions through 15 laps, from 80 to 320 keV and from 2 to 8 mA. Transverse beam confinement is effected via permanent-magnet quadrupoles; bending is via electric dipoles. This ``Small Recirculator`` is being developed in a build-and-test sequence of experiments. An injector, matching section, and linear magnetic channel using seven half-lattice periods of permanent-magnet quadrupole lenses are operational. A prototype recirculator half-lattice period is being fabricated. This paper outlines the research program, and presents initial experimental results.

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.

The 1990 International Symposium on Heavy Ion Inertial Fusion (HIF) was the fifth in a series, following meetings in Darmstadt (1982), Tokyo (1984), Washington, DC (1986) and Darmstadt (1988). The meeting was sponsored by the American Physical Society and the United States Department of Energy (USDOE). It was hosted by the Lawrence Berkeley Laboratory (LBL). The Proceedings of the symposium will be published as a refereed special issue of Particle Accelerators, edited by D. Judd (LBL).

A energy analyzer system is being built for the DARHT-II accelerator similar to the energy analyzer used on the Astron accelerator. This system consists of a scattering wire, magnetic bend, and null signal detector. The wire thickness of 40 mil carbon and the scattering angle of 11 degrees is chosen for good signal to noise ratio. The dipole bend angle is 60 degrees, with a 30 cm radius of curvature. The image-plane focal distance is chosen for the required energy resolution. The energy resolution and acceptance are 0.1% and /spl plusmn/5% with a time response of 10 nsec. The wire must survive the 2 usec 2 kA, 18.4 MeV DARHT-II beam. The MCNP code was used to study the wire scattered properties. The scattered beam fills the available 1/spl times/2 cm dipole aperture. The dispersion normal to the beam direction is 0.43 cm/%. The detector is a PIN diode array which determines the beam position on the chip. This array consists of 40 2.5/spl times/0.1/spl times/0.25 mm bins with a gain in excess of 10000. The system will be installed in the space between the debris blocker and the cruncher solenoid up-stream from the shuttle dump.

LBLs Lawrence Berkeley Laboratory's Heavy Ion Fusion Accelerator Research group has completed the engineering study of the Induction Linac Systems Experiment (ILSE). ILSE will address nearly all accelerator physics issues of a scaled heavy-ion induction linac inertial-fusion pellet driver. Designed as a series of subsystem experiments, ILSE will accelerate 16 parallel carbon ion beams from a 2-MeV injector presently under development to 10 MeV at 1 mu s. The physics and engineering requirements are presented, and conceptual design approaches for building ILSE are described. Major ILSE subsystems comprise electrostatic focusing quadrupole matching and accelerating sections, a sixteen- to four-beam transverse combining section, a four-beam magnetic focusing quadrupole accelerating section, a single-beam 180 degrees bend section, a drift compression section, and a final focus and target chamber. Also discussed are vacuum and alignment, diagnostics/data acquisition, and controls. Plans for further development are considered. >

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. >

Experiments conducted on MBE-4, a four-beam Cs/sup +/ induction linac, are discussed. The space-charge dominated beams of MBE-4 are focused by electrostatic quadrupoles while they are accelerated from nominally 200 keV up to approximately 1 MeV by 24 accelerating gaps. Initially the beams have currents of typically 4 mA to 10 mA per beam. Early experimental results showed a growth of the normalized emittance by a factor of 2 while the beam current was amplified by up to nine times its initial value. The results of recent experiments are discussed in which a mild bunch length compression rate, more typical of that required by a fusion driver, has shown that the normalized emittance can be maintained at its injection value (0.03 mm-mr) during acceleration. >

The US Inertial Fusion Energy (IFE) Program is developing induction accelerator technology toward the goal of electric power production using heavy-ion beam-driven inertial fusion (HIF). The recirculating induction accelerator promises driver cost reduction by repeatedly passing the beam through the same set of accelerating and focusing elements. We present plans for and progress toward a small (4.5-m diameter) prototype recirculator, which will accelerate K/sup +/ ions through 15 laps, from 80 to 320 keV and from 2 to 8 mA. Beam confinement is effected via permanent-magnet quadrupoles; bending is via electric dipoles. Scaling laws, and extensive particle and fluid simulations of the space-charge dominated beam behavior, have been used to arrive at the design. An injector and matching section are operational. Initial experiments are investigating intense-beam transport in a linear magnetic channel; near-term plans include studies of transport around a bend. Later experiments will study insertion/extraction and acceleration with centroid control.

An attractive feature of the inertial fusion energy (IFE) approach to commercial energy production is that the fusion driver is well separated from the fusion confinement chamber. This ''standoff'' feature means the driver is largely isolated from fusion reaction products. Further, inertial confinement fusion (ICF) target ignition (with modest gain) is now scheduled to be demonstrated at the National Ignition Facility (NIF) using a laser driver system. The NIF program will, to a considerable extent, validate indirectly-driven heavy-ion fusion (HIF) target designs for IFE. However, it remains that HIF standoff between the final focus system and the fusion target needs to be seriously addressed. In fact, there now exists a timely opportunity for the Office of Fusion Energy Science (OFES) to experimentally explore the feasibility of one of the attractive final transport options in the fusion chamber: the self-pinched transport mode. Presently, there are several mainline approaches for HIF beam transport and neutralization in the fusion chamber. These range from the (conservative) vacuum ballistic focus, for which there is much experience from high energy research accelerators, to highly neutralized ballistic focus, which matches well to lower voltage acceleration with resulting lower driver costs. Alternatively, Z-discharge channel transport and self-pinched transport in gas-filled chambers may relax requirements on beam quality and final focusing systems, leading to even lower driver cost. In any case, these alternative methods of transport, especially self-pinched transport, are unusually attractive from the standpoint of chamber design and neutronics. There is no requirement for low chamber pressure. Moreover, only a minuscule fraction of the fusion neutrons can escape from the chamber. Therefore, it is relatively easy to shield sensitive components, e-g., superconducting magnets from any significant neutron flux. Indeed, self-pinched transport and liquid wall protection endow DT fusion with many of the advantages of aneutronic fusion. The question is: will self-pinched transport work? Early theoretical studies indicated that self-pinched transport was not an option because net currents established in gas during beam injection were too small to cause beam pinching. However, recent numerical simulations using the 3D hybrid code IPROP3, including the effects of non-local ionization, indicate that self-pinched transport may be possible. The capability to test the concept exists today in scaled experiments using a high-current focused proton beam produced by the Gamble II pulsed-power accelerator at the Naval Research Laboratory. This White Paper describes the implications of the self-pinched transport approach to HIF power plant design and the relevance of proton experiments designed to test the concept. Near-term experiments and analysis are also suggested.

We review the status of the experimental campaign being carried out at Lawrence Livermore National Laboratory, involving scaled investigations of the acceleration and transport of space‐charge dominated heavy ion beams. The ultimate goal of these experiments is to help lay the groundwork for a larger scale ion driven inertial fusion reactor, the purpose of which is to produce inexpensive and clean electric power.

As part of the small induction recirculator development at LLNL, the authors are testing an injector and transport line that delivers 4 {micro}s beams of potassium with repetition rates up to 10 Hz at a nominal current of 2 mA. The normalized K-V equivalent emittance of the beams is near 0.02 {pi} mm-mrad and is mostly determined by the temperature of the source (0.1 eV). K{sup +} ions generated at 80 keV in a Pierce diode are matched to an alternating gradient transport line by seven electric quadrupoles. Two additional quads have been modified to serve as two-axis steerers. The matching section is followed by a transport section comprised of seven permanent magnet quadrupoles. Matching to this section is achieved by adjusting the voltages on the electric quadrupoles to voltages calculated by an envelope matching code. Measurements of beam envelope parameters are made at the matching section entrance and exit as well as at the end of the permanent magnet transport section. Beam current waveforms along the experiment are compared with results from a one-dimension longitudinal dynamics code. Initial experiments show particle loss occurring at the beam head as a result of overtaking. The apparatus is also being used for the development of non or minimally intercepting diagnostics for future recirculator experiments. These include capacitive monitors for determining beam line-charge density and position in the recirculator; flying wire scanners for beam position; and gated TV scanners for measuring beam profiles and emittance.

A recirculating induction accelerator requires accelerating waveforms that produce current amplification and provide bunch length control throughout the acceleration process. Current amplification occurs because of both an increase in the beam velocity and a shortening of the length of the beam bunch. The pulsed acceleration and control waveforms seen by the beam change as the pulse duration shortens. For one acceleration cycle of the small recirculator, each accelerating gap is driven by a burst of 15 pulses. As the beam gains velocity, the time interval between pulses shortens from approximately 20 to 10 /spl mu/sec. A zero-dimensional design code REC is used to develop the accelerator waveforms. An envelope/fluid code CIRCE and a 3-D particle code WARP3d are used to confirm the REC design and study the effects of errors. We find that acceleration errors can lead to space-charge waves launched at the bunch ends that strongly affect or even destroy the current pulse shape. The relation between the rate of longitudinal compression and the velocity of space charge waves is studied.

Since 1985 the Heavy Ion Fusion Accelerator Research program at the Lawrence Berkeley Laboratory has been studying current amplification and emittance variations in MBE-4, a four-cesium-beam induction linac. This experiment models much of the accelerator physics of the electrostatically focused section of a fusion driver. One of the initial concerns was possible interactions between the beams as they were accelerated or transported through the accelerator. With the exception of a weak interaction in the injector where the beams are at low energy and see each other for some distance, no such effect was observed. At low energy the beams interact primarily through their radial space-charge electric fields. These are easily controlled by radial shorting planes along the accelerator which are present for other reasons. >

This report contains a discussion of the diagnostics required for the beam propagation experiment to be done with the ATA accelerator. Included are a list of the diagnostics needed; a description of the ATA experimental environment; the status of beam diagnostics available at Livermore including recent developments, and a prioritized list of accelerator and propagation diagnostics under consideration or in various stages of development.

The Lawrence Livermore Laboratory is constructing an induction linac with the following parameters 10kA, 50ns FWHM pulse width, 5MeV, and 5PPS. This paper describes the design features of the 2.5 MeV injector and it's associated pulsed power system.

At normal incidence, the back scattered, forward scattered and 90/sup 0/ scattered cross sections of the beam were measured at 24 GHz for air and nitrogen at 500 torr pressure. For a single pulse, the measured radius of the beam was 4.3 mm at Z = 80 cm, whereas in a burst the radius decreased to 2.5 mm for the fifth pulse. This corresponds to a density reduction of a factor of 5 and corresponds to a channel temperature of 1200/sup 0/C. Within experimental error, the observed cross sections were those to be expected from a metal rod of these dimensions. There was no evidence of any enhancement in cross section at long times as might be expected from a hydrodynamic channel instability. However, receiver noise would have masked any fluctuations present. The cross sections were also measured with the microwave beam forming an angle of 10/sup 0/ with the electron beam. The backscatter cross section and the bistatic cross section to a second antenna forming an angle of 10/sup 0/ with the beam was also measured. In air, the bistatic and monostatic cross sections were approximately 10 db higher than expected from the beam size measurement, i.e, the returnsmore » observed were those one would expect from a good conducting rod approximately 5 cm radius. In nitrogen, the bistatic cross section was the same as air, but the monostatic cross section was 30 db larger than expected. This result is probably due to sausaging induced in the channel by an axial magnetic field. It could, however, be an indication of a fast growing hydrodynamic instability.« less

This report contains a description of the initial ETA propagation experiments in air at a beam current of 4.5 kA. The beam was observed to propagate at the pressures anticipated on the basis of previous theory and experiment. A comparison of measured net current waveforms with predictions of the PHOENIX code showed good agreement over the pressure range 0.1 to 200 torr. However, the beam was observed to expand with Z at a faster rate than theory predicts. Excessive transverse beam modulation at injection complicated the experiments and limited their comparison with theory.

This work reports initial tests of a carbon brush or tuft cathode intended for use by the Beam Research Program. It was found that electric fields of approximately 100 kV/cm were required to produce current densities above 20 A/sq cm. The beam extracted from the cathode consisted of many beamlets - one for each tuft. The beamlets were found to be quite uniform in peak current density and the cathode operation was microscopically repeatable. The turn-on time was estimated to be 200 ns.

Recirculating induction accelerators have been studied as a potential low-cost driver for inertial fusion energy. At LLNL, we are developing a small (4.5-m diameter), scaled, experimental machine which will demonstrate many of the engineering solutions of a full scale driver. The small recirculator will accelerate singly ionized potassium ions from 80 to 320 keV and 2 to 8 mA, using electric dipoles for bending and permanent magnet quadrupoles for focusing in a compact periodic lattice. While very compact, and low cost, this design allows the investigation of most of the critical physics issues associated with space-charge-dominated beams in future IFE power plant drivers. Additionally, a straight magnetic-transport experiment is being carried out to test diagnostics and magnetic transport in preparation for the recirculator.

Work is underway at the Lawrence Livermore National Laboratory to design and build a small-scale, heavy ion recirculating induction accelerator. An essential part of this design work is the development of small non-intercepting diagnostics to measure beam current and position. This paper describes some of this work, with particular emphasis on the development of a small capacitive probe beam position monitor to resolve beam position to the 100 /spl mu/m level in a 6 cm diameter beam pipe. Initial measured results with an 80 keV potassium ion beam are presented.