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A mobile heater for ionospheric modification studies requires a new megawatt (MW) class radio frequency (RF) source operating with an antenna array 1/20 the area of the High-Frequency Active Auroral Research Program (HAARP). To deliver an effective power density comparable to HAARP, the total source power must be in the range of 16 MW, thus demanding highly efficient sources. While the development of a whole multi-megawatt system for mobile ionospheric heaters is a complex engineering problem, in the present paper we describe only the work of our group on studying main features of a prototype MW-class vacuum electronics RF source for such system. The source design we are currently pursuing assumes class D operation using a modified version of the inductive output tube. The electron beam is a thin annular beam, switched on and off by a mod-anode as opposed to a grid. The beam is then passed through a decelerating gap, and its kinetic energy is extracted using a tunable resonant circuit that present...

The University of Maryland Electron Ring (UMER) group is in the process of an experiment to test a new accelerator theory that mitigates beam loss from resonances, yet still supports stable beam orbits [1]. A nonlinear magnet element (an octupole channel consisting of several separate flexible printed circuit boards (PCBs)) has been constructed with preliminary measurements [2]. We present work done to characterize the channel. We use a Velmex stage with a Hall probe to generate gridded magnetic field measurements. This method gives a direct way to achieve fine resolution of the longitudinal dependence of the focusing field, which is a fundamental component of the nonlinear focusing theory. We concluded that the needed octupole strength profile is achievable and that given our assumptions of the measurement error, the magnetic center through the length of the channel remains within an acceptable tolerance.

The University of Maryland Electron Ring (UMER) is a dedicated research accelerator facility for studying the physics of high-intensity charged particle beams. Advancing the capabilities of the longitudinal confinement system paves the way for multi-bunch electron confinement, allowing UMER to extend its performance and modes of operation for various experiments on longitudinal dynamics. The research reported here employs an existing induction cell [1], and a radiofrequency (RF) amplifier to mimic traditional RF cavity confinement. By applying sinusoidal RF waves at the revolution frequency with synchronous phase angle equal to zero, we can confine an electron beam in the wave's linear regions. To additionally combat particle loss, we calculated the separatrices-the areas of phase space within which the beam is longitudinally confined-with a central synchronous particle.

We present results of longitudinally compressing a beam bunch by inwardly displacing both the head and tail end focusing fields used to contain the beam. This method of compression is different from the conventional velocity tilt method that rotates the beam in phase space. Simulation and experimental results are presented at numerous compression rates. The results described below assume an injected longitudinal current profile into the University of Maryland Electron Ring (UMER) that is uniform both in current and velocity.

The power extraction circuit in an lOT (Inductive Output Tube) is a resonant circuit made of actual inductors and capacitors at low frequencies (3–10 MHz) due to the size limitation of a cavity. To build a mobile, low frequency range, high power sources for ionospheric heating with high efficiency and tunability, we need a constant impedance to decelerate the beam at all frequencies [1]. This paper discusses the design and construction of pi-circuit for two different gun designs: 70kV - 30A & 20 kV - 1A guns [2]. While the tunability is achieved by variable capacitors, the high efficiency is limited by the internal resistance of the inductor in the pi-circuit.

The ionosphere plays a prominent role in the performance of critical civilian and military communication systems. The properties of the ionosphere can be modified by Ionospheric Modification (IM). The key instrument in IM research is a powerful, ground-based, High Frequency (HF) source of electromagnetic waves known as a heater. Existing heaters operate with large, fixed location antenna arrays. With a mobile heater, investigators would be able to conduct IM research at different latitudes without building a costly permanent installation. A mobile heater for ionospheric modification studies will require a new megawatt class RF source. Our design incorporates a grid-less electron gun in which the beam is turned on and off by a mod-anode in class-D mode [1].

Results of a scaled experiment using the electron beam produced by a gridded thermionic cathode from an Inductive Output Tube electron gun to drive an external lumped element circuit for high efficiency radio frequency generation are reported. The IOT gun produces an electron beam bunched at the driving frequency that is then collected by an external circuit for resonant impedance matching to the load. Results showed that effects such as the internal resistance of the inductor and deflection of beam electrons by the induced RF voltages on the beam collector are important considerations to be included in the design of a practical device using this configuration for high efficiency, MW-class RF sources for Mobile Ionospheric Heaters (MIHs).

A mobile heater for ionospheric modification studies will require a new megawatt class RF source operating with an antenna array that is 1/20 the area of HAARP [1]. In order to deliver an effective power density comparable to that of HAARP, the total source power must be in the range of 16 MW, thus demanding highly efficient sources [2]. The source design we are currently pursuing utilizes a grid-less electron gun that uses a mod-anode to turn a thin annular beam on and off in class D operation. The beam is passed through a decelerating gap and its kinetic energy is extracted using a tunable resonant circuit that presents a constant impedance over the frequency range of 3–10 MHz, such that the beam is almost decelerated at all frequencies.

The resonant circuit in an Inductive Output Tube (IOT) extracts kinetic energy of the modulated electron beam converting it ti electromagnetic energy. For the application considered here, mobile low-frequency sources (3–10 MHz) for ionospheric heating, high efficiency is important, and thus class D operation is desired. The broad frequency range requires the circuit to be tunable, and the need for a constant decelerating voltage requires constant impedance [1]. This paper discusses the design and construction of a resonant circuit with a highly coupled transformer that has the above features. The design and optimization of the circuit was performed with Orcad Spice and the transformer design was designed using Cadence Autodesk Inventor and a finite element electromagnetic field solver, Maxwell 3D and HFSS simulator software.

Our research is motivated by the desire to build a transmitter for modification of the ionosphere that can operate in the mid-latitudes, be moveable, and have comparable effective radiated power to stationary, large land-based, fixed facilities such as HAARP 1. Our current design incorporates a grid-less electron gun in which the beam is turned on and off by a mod-anode. The three main electron gun models have the following key features: Model A has a uniform magnetic field (1kG) and no beam compression, Model B has a tapered guiding magnetic field (peak field of 1kG) and provides beam compression, and Model C has no guiding magnetic field. These simulations were performed with the beam optics code Michelle 2. The solenoid power consumption in the case of Model A is roughly 2kW, while for model B it is just under 5kW.

We report progress on the development of a high-power, high-efficiency source for a moveable ionospheric heater. The source is based on the Inductive Output Tube concept. The application requires operation in the 3–10 MHz range, thus opening the possibility to achieve true class — D operation and with that high efficiency. Our designs incorporate a solid-state driver, a grid-less gun, and a tunable output circuit. Preliminary results on a scaled device will be presented.

We consider the development of a highly efficient, gridless tetrode as a megawatt-level RF source in the 3 to 10 MHz range for application in mobile ionospheric heaters. Such a heater has potential advantages over the stationary facilities, such as High-Frequency Active Auroral Research Program, found at high latitudes. The considered device operates in class D mode with an annular electron beam allowing realization of high efficiency. The present study, based on numerical simulations using the Particle in Cell code Michelle [Petillo et al., IEEE Trans. Electron Devices Sci. 52, 742 (2005)], examines the optimization of device geometry. In particular, the dependence of efficiency on spacing between electrodes is studied. In addition, the role of secondary electrons emitted at the collector is examined. Both static and time dependent operations are simulated. In the time dependent case, it is found that during the portion of the RF cycle when the beam current is on, secondaries emitted from the collector are driven back into the collector by the incoming primary beam. When the beam is switched off, secondaries can stream back into the tetrode and have a small negative impact on efficiency. We present a design in which the secondary electrons are eventually absorbed at the collector rather than at the cathode or anode.

The foremost challenge for Heavy-Ion Fusion (HIF) is achieving sufficiently low emittances and small energy spreads in the presence of intense space-charge, to achieve the high deposition densities necessary for pellet ignition. The University of Maryland Electron Ring (UMER) uses intense low-energy electron beams to access the scaled physics of HIF drivers. In particular, the long path-length propagation in UMER presents an opportunity to study, at realistic scales, the longitudinal beam dynamics and manipulations required for such a driver. With the use of induction modules, as in the ion machines such as NDCX-II, the resulting bunch dynamics show evidence of space-charge waves excited by an initial mismatch between the detailed initial beam distribution at the bunch ends and the applied focusing waveforms, persisting with multiple damped reflections propagating along the bunch flat-top. Using the induction module we are able to suppress space-charge waves with great accuracy, at amplitudes that include wave steepening prior to the formation of solitary wave trains. The longitudinal dynamics largely dominates when no containment fields are applied, coupling through the natural chromaticity of the ring even within the first turn. After subsequent turns, the bunch elongates and wraps the circumference of the machine multiple times; eventually reaching a point of instability that has also been shown through simulation, obtaining excellent agreement when the detailed longitudinal dynamics of the experiment are carefully incorporated into the model.

The ionosphere plays an important role in the performance of critical civilian and defense communication systems. Perturbation studies of the ionosphere allow us to investigate the ionosphere and improve the performance of communications that rely on the ionosphere as well as develop new applications. A key instrument in ionospheric modification is the ionospheric heater, a powerful high frequency transmitter that perturbs the properties of the ionospheric plasma by modulating the electron temperature at preselected altitudes. The latest technology uses stationary facilities, among which HAARP is the largest. There are numerous benefits from using mobile ionospheric heaters (MIHs) [1]. Such a mobile system would allow investigators to conduct the research at different latitudes without building a costly permanent installation. These mobile sources would be substantially smaller than HAARP and located on a barge-size platform.

The resonant circuit in an Inductive Output Tube (IOT) extracts the kinetic energy of the modulated electron beam converting it to electromagnetic energy. For the application considered here, mobile low-frequency sources (5–10 MHz) for ionospheric heating, high efficiency is important, and thus class D operation is desired. The broad frequency range requires the circuit to be tunable, and the need for a constant decelerating voltage requires constant impedance. This paper discusses the design and construction of a resonant circuit with a highly coupled transformer that has the above features. The design and optimization of the circuit was performed with Orcad Spice and the transformer design was designed using Autodesk Inventor and a finite element electromagnetic field solver, Maxwell 3D and HFSS Simulator software.

The University of Maryland is developing a high power Radio Frequency (RF) source for use as a Mobile Ionospheric Heating Source (MIHS). This prototype will utilize Inductive Output Tube (IOT) technology operating in class-D amplifier mode using a grid-less electron gun. This paper will present a novel grid-less electron gun design, capable of producing annular beams with high currents in the range of 15A that utilizes a mod-anode to bring the gun from cut-off to saturation

Development of Mobile Ionospheric Heating sources (MIHs) would allow investigators to conduct needed research at different latitudes without building permanent and costly installations. As part of an Air Force Multi-University Research Initiative (MURI), the University of Maryland is designing a prototype of a powerful Radio Frequency (RF) source utilizing Inductive Output Tube (IOT) technology operating in class-D with a mod-anode controlled electron gun [1]. This technology was chosen because it has the potential to operate at efficiencies exceeding 90% [2].

The University of Maryland Electron Ring (UMER) and the Long Solenoid Experiment (LSE) are two electron machines that were designed explicitly to study the physics of space-charge-dominated beams. The operating parameters of these machines can be varied by choice of apertures and gun operating conditions to access a wide range of parameters that reproduce, on a scaled basis, the full nonlinear time-dependent physics that is expected in much costlier ion systems. Early operation of these machines has demonstrated the importance of the details of beam initial conditions in determining the downstream evolution. These machines have also been a convenient tested for benchmarking simulation codes such as WARP, and for development of several novel diagnostic techniques. We present our recent experience with multi-turn operation as well as recent longitudinal and transverse physics experiments and comparisons to simulation results. Development of novel diagnostic techniques such as time-dependent imaging using optical transition radiation and tomographic beam reconstruction are also described.

For mobile ionospheric heaters, it is necessary to develop highly efficient RF sources capable of delivering radiation in the frequency range from 3 to 10 MHz with an average power at a megawatt level. A promising source, which is capable of offering these parameters, is a grid-less version of the inductive output tube (IOT), also known as a klystrode. In this paper, studies analyzing the efficiency of grid-less IOTs are described. The basic trade-offs needed to reach high efficiency are investigated. In particular, the trade-off between the peak current and the duration of the current micro-pulse is analyzed. A particle in the cell code is used to self-consistently calculate the distribution in axial and transverse momentum and in total electron energy from the cathode to the collector. The efficiency of IOTs with collectors of various configurations is examined. It is shown that the efficiency of IOTs can be in the 90% range even without using depressed collectors.

In this paper we describe two methods that demonstrate an energy evolution at the ends of a long bunch. The first method is a nondestructive diagnostic that extends the response matrix technique, to provide insight into the time-sliced transverse behavior of a coasting bunch in a ring. This method uncovers intense beam physics from the resulting longitudinal energy variations at the ends of a rectangular bunch. Measurements of the tune profile, using this method, agree with analytical predictions resulting in a percent error of 0.32% and 2.45% (head and tail respectively). Measurements and calculations presented in this paper are presented for a beam with an intensity of 0.901 (beam current of 21 mA). The head and the tail tunes, averaged over four ring chambers, also illustrate the intensity variation axially along the bunch. This is exhibited through tune-shift measurements at the ends of the bunch. As confirmed through calculations, the measured tune shift at the head is smaller than the tail. This results from the sum of both the transverse coherent tune shift from image forces with the component from longitudinal space charge. The sum of the two components produces a smaller tune shift at the head of the beam and a larger tune shift at the tail. The second method presented in this paper optically illustrates the centroid ``cork-screwing'' motion within the beam ends, at a single location on the ring. The motion is resolvable using this diagnostic since it is capable of axially time-slicing regions of the beam with different energies. The difference between the two methods presented is that the first is a nondestructive diagnostic that can be extended to multiple turns whereas the second method is a destructive first turn imager.

The first experimental observation of a Korteweg\char21{}de Vries-type soliton wave train in intense electron beams is reported. A narrow, large-amplitude perturbation on a long-pulse beam is observed to steepen and spawn a soliton wave train. The pulse width and amplitude of each peak remain unchanged over a long propagation distance, and the amplitude is inversely proportional to the square of the width. Two such pulses are seen to pass through each other, emerging from the collision unchanged. The experimental results are reproduced by particle-in-cell simulations.

With ever increasing demands for intensities in modern accelerators, the understanding of space-charge effects becomes crucial. Herein are presented measurements of optically shaped picosecond-long electron beams in a superconducting L-band linac over a wide range of charges, from 0.2 nC to 3.4 nC. At low charges, the shape of the electron beam is preserved, while at higher charge densities, modulations on the beam convert to energy modulations. Energy profile measurements using a spectrometer and time profile measurements using a streak camera reveal the dynamics of longitudinal space-charge on MeV-scale electron beams.

For numerous applications, it is desirable to develop electron beam driven efficient sources of electromagnetic radiation that are capable of producing the required power at beam voltages as low as possible. This trend is limited by space charge effects that cause the reduction of electron kinetic energy and can lead to electron reflection. So far, this effect was analyzed for intense beams propagating in uniform metallic pipes. In the present study, the limiting currents of intense electron beams are analyzed for the case of beam propagation in the tubes with gaps. A general treatment is illustrated by an example evaluating the limiting current in a high-power, tunable 1–10 MHz inductive output tube (IOT), which is currently under development for ionospheric modification. Results of the analytical theory are compared to results of numerical simulations. The results obtained allow one to estimate the interaction efficiency of IOTs.

Summary form only given. With the ability to inject bright beams into the University of Maryland electron ring (UMER) comes the problem of longitudinal end erosion of both the head and tail from high space charge. It is important therefore to apply suitably-designed longitudinal focusing forces to confine the beam and prevent it from its normal expansion. This paper presents the design and prototyping of an induction cell and the technology used to pulse this cell. With successful operation of the induction cell, the confinement of the beam within UMER would increase the number of turns and also enable us to perform studies of the longitudinal physics of such highly space-charge beams. The pulsed voltage requirements for a confinement system on UMER would require ear-fields that switch 3 kV in about ~8 ns or so at a maximum rate of 5 MHz for the most intense flat-top rectangular beam injected into the ring. This places a considerable challenge on the noise-free delivery to a compact core.

The University of Maryland Electron Ring (UMER) is a low energy, high current recirculator for beam physics research. The electron storage ring has been closed and recent operations have been focused on achieving multi- turn transport. An entire suite of terminal diagnostics is available for time-resolved phase space measurements of the beam. These diagnostics have been mounted and tested at several points on the ring before it was closed and will complete the ring when mounted to the extraction section. UMER utilizes a unique injection scheme which uses the fringe fields of an offset quadrupole to assist a pulsed dipole in bending the beam into the ring. Similar concepts, along with a more traditional electrostatic method, are being considered for beam extraction. This presentation will focus on the recent efforts to design and deploy these major subsystems required for beam extraction.

This is a numerical and experimental study of the effects of skew quadrupoles on the beam used in University of Maryland Electron Ring (UMER). As this beam is space- charge dominated, we expect new phenomena to be present compared to the emittance-dominated case. In our studies we find that skew quadrupoles can severely affect the halo of the beam and cause emittance growth, even in the first turn of the beam. For our simulations we use the WARP particle-in-cell code and we compare the results with the experimental study of skew quadrupole effects.

We present strong evidence of the observation of optical transition radiation (OTR) from aluminized silicon targets intercepting the UMER 10 keV, 100 ns pulsed electron beam, using fast (300 ps and Ins rise time) photomultiplier tubes. An intensified gated (3 ns-lms) intensified CCD camera is used to image the beam using OTR and to study its time evolution throughout the beam pulse. A comparison of wave forms and time resolved OTR images is presented along with time integrated images obtained with phosphor screens for different initial conditions, i.e. beam currents and gun bias voltages.

Longitudinal perturbations can be generated in the space-charge dominated regimes in which most beams of interest are born. To study the modification of transverse beam distributions by longitudinal beam dynamics, we have conducted experimental studies using low energy electron beams by taking time resolved images of a beam with longitudinal density perturbations. Two different diagnostics are used: optical transition radiation (OTR) produced from an intercepting silicon based aluminum screen and a fast (< 5 ns decay time) phosphor screen. It is found that the beam is significantly affected by the perturbation. However the OTR signal is very weak and requires over 45 minutes of frame integration. The fast phosphor screen has much better sensitivity (~1000 times enhancement). In this paper, we also report on the time resolved measurement of a parabolic beam, showing interesting correlations between transverse and longitudinal distributions of the beam.

The University of Maryland Electron Ring (UMER) is a scaled storage ring using low-energy electrons to inexpensively model beams with high space-charge. With the ability to inject such beams comes the problem of longitudinal end erosion of both the head and tail. It is important therefore to apply suitably designed longitudinal focusing forces to confine the beam and prevent it from its normal expansion. This paper presents the design and prototyping of an induction cell for this purpose. Successful operation of the induction cell would push the achievable number of turns and also enable us to perform studies of the longitudinal physics of such highly space-charge dominated beams. The pulsed voltage requirements for such a system on UMER would require ear-fields that switch 3 kV in about 8 ns or so for the most intense flat-top rectangular beam injected into the ring. This places a considerable challenge on the electronics used to deliver ideal waveforms with a compact module. Alternate waveforms are also being explored for other various injected beam shapes into UMER.

The University of Maryland Electron Ring (UMER) is built as a low‐cost testbed for intense beam physics for benefit of larger ion accelerators. The beam intensity is designed to be variable, spanning the entire range from low current operation to highly space‐charge‐dominated transport. The ring has been closed and multi‐turn commissioning has begun. One of the biggest challenges of multi‐turn operation of UMER is correctly operating the Y‐shaped injection/recirculation section, which is specially designed for UMER multi‐turn operation. It is a challenge because the system requires several quadrupoles and dipoles in a very stringent space, resulting in mechanical, electrical, and beam control complexities. Also, the Earth’s magnetic field and the image charge effects have to be investigated because they are strong enough to impact the beam centroid motion. This paper presents both simulation and experimental study of the beam centroid motion in the injection region to address above issues.

The University of Maryland Electron Ring (UMER) is a scaled model to investigate the physics of such intense beams. It uses a 10‐keV electron beam along with other scaled beam parameters that model the larger machines but at a lower cost. Recently, multi turn operation of the ring (3.6 m diameter) has been achieved. In order to have full current transport of the electron beam, and to increase the number of turns of the beam around the ring, injection of the beam from the straight section into the ring becomes crucial. In this work, we report experimental results from the injection and matching of a space charge dominated beam and an emittance dominated beam. We also describe and analyze two methods of injection of the electron beam from the straight section into the ring. In one of the methods, the two injection quads are fixed to a preset value, while in the other both the injection quads are switched off (Collins injection).

The University of Maryland Electron Ring (UMER), designed for studies of space-charge dominated beam transport in a strong focusing lattice, is nearing completion. UMER models, for example, the recirculator machine envisioned as a possible driver for heavy-ion inertial fusion. The UMER lattice consists of 36 FODO periods distributed among 18, 20/spl deg/-bending sections containing two dipole magnets each. The main diagnostics are phosphor screens and capacitive beam position monitors placed at the center of each bending section. In addition, pepper-pot and slit-wire emittance meters, as well as an energy analyzer are in operation. We present here results of beam matching and characterization for a range of currents extending from about 1 mA to 100 mA, all at 10 keV and 100 ns pulse duration. With typical focusing given by /spl sigma//sub 0/=76, the zero-current betatron phase advance per period, the range of currents corresponds to tune depressions of 0.8 to 0.2. This range covers both the emittance dominated and extreme space-charge dominated regimes, which is unprecedented for a circular machine.

Photoelectric emission from dispenser cathodes has been studied earlier. Photoelectric emission in conjunction with themionic emission can provide a convenient and flexible means of modulating emission from thermionic cathodes with programmable formats and high bandwidth. This may be useful for experimental studies the beam dynamics of space charge dominated beams, such as in the University of Maryland electron ring (UMER). We have studied combined photoelectric and thermionic emission from a dispenser cathode using a nitrogen laser operating at 337 nm. Results will be presented for the effect on emission of laser intensity, cathode temperature, and accelerating voltage.

An induction cell has successfully been demonstrated to longitudinally confine a space-charge dominated bunch for over a thousand turns (>11.52 km) in the University of Maryland Electron Ring [Haber et al., Nucl. Instrum. Methods Phys. Res. A 606, 64 (2009) and R. A. Kishek et al., Int. J. Mod. Phys. A 22, 3838 (2007)]. With the use of synchronized periodic focusing fields, the beam is confined for multiple turns overcoming the longitudinal space-charge forces. Experimental results show that an optimum longitudinal match is obtained when the focusing frequency for containment of the 0.52 mA beam is applied at every fifth turn. Containment of the beam bunch is achievable at lower focusing frequencies, at the cost of a reduction in the transported charge from the lack of sufficient focusing. Containment is also obtainable, if the confinement fields overfocus the bunch, exciting multiple waves at the bunch ends, which propagate into the central region of the beam, distorting the overall constant current beam...

The University of Maryland Electron Ring (UMER) group is currently exploring the physics of space-charge dominated beams. Seventeen Beam Position Monitors (BPMs) will be used to determine the beam centroid for steering correction purposes to within 0.5 mm. Since the pulse length is relatively long (100 ns), the BPMs can also be used for temporal beam profiling. These features are extremely useful for perturbation and longitudinal dynamics studies. For these uses the BPM needs a temporal resolution better than 2 ns. We report on the final design and testing as well as other unique features of this device.