Author: Ekdahl, C.
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THPAK046 The Ion-Hose Instability in High-Current Multi-Pulse Induction Linacs 3320
  • C. Ekdahl
    LANL, Los Alamos, New Mexico, USA
  The ion-hose instability has long been considered a danger for long-pulse, high-current electron linear induction accelerators (LIAs)*. This instability is enabled by beam-electron ionization of residual background gas in the accelerator. The space-charge of the high-energy beam ejects low-energy electrons from the ionized channel, leaving a positively-charged ion channel that attracts the electron beam. The beam can oscillate in the potential well around the channel position. Likewise, the electron beam attracts the ions, which can oscillate about the beam position. Because of the vast differences in particle mass, the oscillations are out of phase, and the amplitudes grow unstably. The number of instability e-foldings is proportional to the channel ion density*, which in turn is proportional to the background pressure and pulse length. This scaling of the instability growth was demonstrated on the long-pulse DARHT-II linear induction accelerator (LIA) at Los Alamos**. The ion-hose instability is also problematic for high-current multi-pulse LIAs, because ion recombination times are so very long at typical background pressures. Moreover, because of low ion channel ion densities, and massive ions, channel expansion is too slow to reduce the instability growth by very much. In particular, the ion channel is expected to persist and its density to increase during the 3-microsecond duration of a four-pulse burst from the 2-kA, 20-MeV Scorpius LIA now being developed. Recent simulations with an experimentally validated code that was used to predict DARHT-II growth rates have shown that the magnetic focusing field designed for Scorpius will be strong enough to inhibit ion-hose instability if the background pressure is kept below a value that is readily attainable with the present designs of induction cells and other accelerator components. Details and results of these calculations are the subject of this presentation.
*H. L. Buchanan, Phys. Fluids, vol. 30, pp. 221 - 231, 1987
**C. A. Ekdahl, et al., IEEE Trans. Plasma Sci., vol. 34, pp. 460-466, 2006
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