Production and postacceleration of intense ion beams in magnetically insulated gaps

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Abstract
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.