Stray-electron accumulation and effects in HIF accelerators

Abstract

Stray electrons can be introduced in positive-charge accelerators for heavy ion fusion (or other applications) as a result of ionization of ambient gas or gas released from walls due to halo-ion impact, or as a result of secondaryelectron emission. Electron accumulation is impacted by the ion beam potential, accelerating fields, multipole magnetic fields used for beam focus, and the pulse duration. We highlight the distinguishing features of heavy-ion accelerators as they relate to stray-electron issues, and present first results from a sequence of simulations to characterize the electron cloud that follows from realistic ion distributions. Also, we present ion simulations with prescribed random electron distributions, undertaken to begin to quantify the effects of electrons on ion beam quality.

Parametic Study of the current limit within a single driver-scaletransport beam line of an induction Linac for Heavy Ion Fusion

“…Moreover, the secondary electrons may not be as problematic in an electrostatic quadrupole, where they will be swept out of the beam path due to the large electric fields that exist, as in a magnetic quadrupole, where they can be trapped in the beam potential [79].…”

“…Though the field quality of magnetic [170] and electrostatic lattices may be similar in an integrated sense, field nonlinearities are different in spatial distribution at the lead and return ends. The Gaussian charge density distribution useful fill factor in the magnetic lattice will also be influenced by trapped secondary electrons [171], and gas desorption. The influence of gas desorption is mitigated by shorter pulse length at higher ion kinetic energy.…”

Initial experimental studies of electron accumulation in a heavy-ion beam

“…Theory and PIC simulations suggest that the electrons will be radially trapped in the ≥1 kV ion-beam potential [2,3], and can be detrapped by drifting into an upstream acceleration gap [2]. We have installed four quadrupole magnets on the High-Current Experiment (HCX) [4] at LBNL with internal electron diagnostics.…”

“…(1) Halo ions in quadrupole magnets tend to be lost where the magnetic field is tangent to the beam tube, which confines secondary electrons to remain harmlessly near the wall [2]. But ion scattering allows energetic ions to impinge the beam tube at other azimuths where the field lines pass through the beam.…”

“…This places them at risk from gas desorption runaway, and from electron clouds produced by secondary electrons and ionization of gas. We have initiated an experimental and theoretical program to measure, understand, and model these effects in heavy-ion accelerators [2].…”