A crucial problem in high-current radial line electron accelerators is radial oscillations of the beam in the foilless diode injector and in the accelerating gaps. This problem is studied via 2-1/2-D particle simulations (quasistatic and electromagnetic) in cylindrical r,z geometry, with emphasis on contouring the applied magnetic field for optimum transport. The results are generally optimistic for future systems; however, further work is needed on designing the injector.
IBEX is a 4-MV, 100-kA, 20-ns cylindrical isolated Blumlein accelerator. In the experiments reported here, the accelerator is fitted with a specially designed foilless diode which is completely immersed in a uniform magnetic field. Several diode geometries have been studied as a function of magnetic field strength. The beam propagates a distance of 50 cm (approx. 10 cyclotron wavelengths) in vacuum before either striking a beam stop or being extracted through a thin foil. The extracted beam was successfully transported 60 cm downstream into a drift pipe filled either with 80 or 640 torr air. The main objectives of this experiment were to establish the proper parameters for the most quiescent 4 MV, 20 to 40 kA annular beam, and to compare the results with available theory and numerical code simulations.
Stable beam transport may be the limiting factor in the development of a new generation of high current linear induction accelerators. In this paper we analyze several important beam stability topics, including radial oscillations induced by an accelerating gap, the diocotron, resistive wall, and cyclotron maser instabilities, and the transverse beam breakup and image displacement instabilities. At present image displacement appears to represent the most serious limitation to high current beam transport in linear accelerator structures.
A 2-D electromagnetic PIC code (MAGIC) is used to study the design of Helia and Hermes-III electron diodes. Important issues include impedance vs. flow pattern, control of electron angles at the convertor, real-pulse and ion effects, and coupling to the MITL. This series of vugraphs presents the study.
Experiments are described pertaining to the development of very high-current pulsed linear ion accelerators utilizing electron neutralization. A novel magnetically insulated gap using radial magnetic fields has been tested. It provides stable electron cloud confinement over microsecond time scales with no detectable leakage current. The gap can act as an ion injector when used in conjunction with a plasma source. Control of the electron cloud dynamics allows the injector to operate in an enhanced current density mode (10–50 times the Child-Langmuir limit) with high efficiency and with plasma source control of the current flow. Currents up to 20 kA at 100 kV applied voltage resulted when using a light-ion flashboard plasma source. Carbon beams were produced by extraction from a flowing plasma from a gun array. A 3-kA beam with equal fractions of C+ and C++ was extracted over a microsecond time scale with little proton contamination. The use of active plasma injection into the high-intensity magnetically insulated diode had the advantages of ion species control, reduction of gap damage, operation at constant impedance, elimination of plasma closure effects, and a demonstrated ability to control the extracted beam optics. Observations were also made of beam propagation and compared to fast neutralization models. Agreement was good, and an upper limit of 0.2% was calculated for the imbalance of ion and electron space charge. When using the carbon injector, two-thirds of the beam reached a second magnetically insulated gap where it was postaccelerated. The second gap had an applied voltage in the range 150 –200 kV with beam currents typically 2 kA. Observations were made of electrostatic focusing in the postacceleration gap. These were in good agreement with theory based on the concept of virtual electrodes determined by the neutralizing electron dynamics.
We present a preconceptual design for a 500-TW pulsed power accelerator capable of delivering 15-MJ kinetic energy into an imploding plasma radiation source (PRS). The HERMES-III technology of linear inductive voltage addition in a self-magnetically insulated transmission line (MITL) is utilized to generate the 8-10 MV peak voltage required for an efficient plasma implosion. The 50- to 60-MA current is achieved by utilizing many accelerating modules in parallel. The modules are connected to a common circular convolute electrode system in the center of which is located an imploding foil plasma radiation source. This accelerator produces no electron beam since the total current from the voltage adders (IVAs) to the inductive load flows on the surface of metallic conductors or nearby in the form of electron sheath. In this paper we outline the accelerator's conceptual design with emphasis on the power flow and coupling to the inductive load of the center section of the device.
A pulsed linear accelerator assembly, RIIM (RADLAC II Module), composed of an injector plus a number of post accelerating gaps was built and successfully operated. The injector and the post accelerating gaps were powered by water strip pulse forming and transmission lines. A high-current, high-voltage, foilless diode injector was used and an annular 40-kA relativistic electron beam was produced and further accelerated through the post accelerating gaps. The final beam energy was close to the sum of injector and gap voltages and equal to 9 MeV.
Particle simulation codes in two and three dimensions have become a standard theoretical tool in the study of many aspects of high current (kA range) accelerators. In this paper, we review some of the recent results obtained with these codes in the area of linear induction acceleration of electron beams. In particular, considerable progress is reported in the understanding of beam generation, oscillation control, accelerating-gap-induced instabilities, and extraction through a foil from a guide magnetic field.
The light-ion microfusion driver design consists of multiple accelerating modules fired in coincidence and sequentially in order to provide the desired ion energy, power pulse shape and energy deposition uniformity on an Inertial Confinement Fusion (ICF) target. The basic energy source is a number of Marx generators which, through the appropriate pulse power conditioning, provide the necessary voltage pulse wave form to the accelerating gaps or feeds of each module. The cavity gaps are inductively isolated, and the voltage addition occurs in the center conductor of the voltage adder which is the positive electrode while the electrons of the sheath flow closer to the outer cylinder which is the magnetically insulated cathode electrode. Each module powers a separate two-stage extraction diode which provides a low divergence ion beam. In order to provide the two separate voltage pulses required by the diode, a triaxial adder system is designed for each module. The voltage addition occurs in two separate MITLs. The center hollow cylinder (anode) of the second MITL also serves as the outer cathode electrode for the extension of the first voltage adder MITL. The voltage of the second stage is about twice that of the first stage. The cavities aremore » connected in series to form the outer cylinder of each module. The accelerating modules are positioned radially in a symmetrical way around the fusion chamber. A preliminary conceptual design of the LMF modules with emphasis on the voltage adders and extension MITLs will be presented and discussed.« less
