Three-dimensional particle-in-cell simulations of applied-B ion diodes

Abstract

The three-dimensional particle-in-cell code quicksilver [Seidel et al., Computational Physics, edited by A. Tenner (World Scientific, Singapore, 1991), p. 475] has been used to study applied-B ion diodes. The impedance behavior of the diode in these simulations is in good agreement with both analytic theory and experiments at peak power. The simulations also demonstrate the existence of electromagnetic instabilities which induce divergence in the ion beam. Early in time, there is an instability at high frequency relative to the ion transit time τi, and the resulting beam divergence is low. However, later in time, the system makes a transition to an instability with a frequency close to 1/τi, and the ion beam divergence rises to an unacceptably high value. The transition is associated with the build-up of electron space charge in the diode, and the resulting increase in the beam current density enhancement (J/JCL). Using different schemes to inhibit the electron evolution, the transition has both been postponed and permanently eliminated, resulting in Li+1 ion beams with a sustained divergence of ∼10 mrad at an energy of ∼10 MeV.

A feasibility study of space-charge neutralized ion induction linacs: Final report

“…Substitution of eq (12) into eq (10) At the peak density of each ion pulse this yields -= -E SZ for small values of LI This is the same result that we obtained from the previous model. and GI is less .than 0.004.…”

“…The ratio of beam potential with and without the plasma is then given by -9 shows the time dependent potential ratio given by eq (10). The maximum ratio of -is plotted as a function of $2 in Fig.…”

Analytic model of a magnetically insulated transmission line with collisional flow electrons

“…Maron [65] has observed this loss as electron bursts, which ''are accompanied by sharp bursts in microwave radiation,'' and proposes that these bursts may be ''initiated by processes in the cathode plasma.'' Desjarlais [66,67], Westermann and Schuldt [69], and Pointon and co-workers [70] have developed ion-diode models that include collisional broadening (due to field fluctuations) of the electron sheath; these models are in substantially better agreement with experiment than collisionless models.…”

“…1, the electron-flow current I f and flow impedance Z f are given by Eqs. (70) and (71), respectively.…”

“…(xii) Instability-induced fluctuations appear to play a significant role in the performance of ion-beam diodes insulated by an externally applied magnetic field [33,62,[65][66][67][68][69][70]. According to Sudan [62], an insulated ion-diode experiences ''electron loss to the anode through instabilities generated by the velocity shear and electron density gradient'' of the electron sheath.…”