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

INITIAL EXPERIMENTAL STUDIES OF ELECTRON ACCUMULATION IN A HEAVY-ION BEAM* A.W. Molvik 1,2,* , D. Baca 1,3 , F.M. Bieniosek 1,3 , R.H. Cohen 1,2,* , A. Friedman 1,2 , M.A. Furman 3 , E.P. Lee 1,3 , S.M. Lund 1,2 , L. Prost 1,3 , A. Sakumi 4 , P.A. Seidl 1,3 , J.-L. Vay 1,3 Virtual National Laboratory for Heavy Ion Fusion Lawrence Livermore National Laboratory, Livermore, CA 94550 Lawrence Berkeley National Laboratory, Berkeley, CA 94720 RIKEN, (Now at CERN) than the flattop duration. This enables exploration of unique electron trapping regimes: multipactor trapping will not occur during the flattop, however secondary electrons will be trapped during the rise time. Ionization of gas by the beam generates deeply trapped electrons, the ions from gas are expelled in ≤1 µs. Trailing edge multipacting, if it occurs, will be at the end of the fall time when the bounce time of electrons between walls grows to ≥25 ns as the beam potential falls below 100 V. Then electrons gain ≥40 eV on each transit. But, all electrons should be lost before the next pulse, 10 s later in HCX, 0.2 s later in a future power plant driver. On PSR, electrons are observable for surprisingly long times, but still only until 1 µs after a pulse [5] HCX provides an opportunity to search for subtle elec- tron trapping mechanisms. To elaborate – an electron emitted from the beam tube with a few eV is accelerated by the beam potential to ~10 3 higher energy, then decelerates towards the opposite wall. An irreversible conversion of only ~10 -3 of the peak radial energy to axial or azimuthal energy will trap the electron, preventing it from reaching the opposite wall. We have used the Gas-Electron Source Diagnostic (GESD) on the HCX to measure the flux of electrons and gas evolved from a target, whose angle to the beam can be varied between 78° and 88° from normal incidence. The results will be discussed in subsequent sections. We have installed a variety of charged particle diagnostics in quadrupole magnets to characterize electron production and trapping: (1) Electrodes, flush with the beam tube wall, are to measure the beam halo loss plus the resulting secondary electron emission. Using the electron- emission coefficient measured with the GESD, we can infer the beam-halo loss. (2) Capacitive probes measure the net beam charge from which we can infer electron densities if they exceed a few percent of the beam density. (3) Grids shield collectors from the 3-orders-of-magnitude larger capacitive signal, to enable measurement of the current of expelled ions from ionization of gas. This ion current will be calibrated against an ion gauge, varying the pressure by leaking in a known gas. Then, we can determine the time dependence of gas density in the beam. It also directly measures the production rate of electrons from gas (when corrected by the ratio of the ionization cross section to the sum of ionization and charge-exchange cross sections). Slit scanners and beam profile diagnostics Abstract Accelerators for heavy-ion inertial fusion energy (HIF) have an economic incentive to fit beam tubes tightly to beams, putting them at risk from electron clouds produced by emission of electrons and gas from walls. Theory and PIC simulations suggest that the electrons will be radially trapped in the ≥1 kV ion-beam potential. We are beginning studies on the High-Current Experiment (HCX) with unique capabilities to characterize electron production and trapping, the effects on ion beams, and mitigation tech- niques. We are measuring the flux of electrons and gas evolved from a target, whose angle to the beam can be varied between 78° and 88° from normal incidence. Quad- rupole magnets are operating with a variety of internal charged particle diagnostics to measure the beam halo loss, net charge, electron ionization rate, and gas density. INTRODUCTION Electron cloud effects (ECEs) are increasingly recog- nized as important, but incompletely understood, dyna- mical phenomena, which can severely limit the perfor- mance of colliders, the next generation of high-intensity rings, or future high-intensity heavy ion accelerators such as envisioned in Heavy Ion Inertial Fusion (HIF) [1]. Accelerators for HIF have an economic incentive to fit beam tubes tightly to beams. 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]. HCX CAPABILITIES 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. On HCX we are studying the transport of a 1 MeV, 180 mA, K + ion beam. (HCX has also operated at 1.8 MeV, 500 mA.) It has a beam potential of ~1.5 kV, rise and fall times of 1 µs, and a flattop duration of 4 µs, repeated at 10 s intervals. Electron transit times between walls are in the range of 7 ns, almost 3 orders of magnitude shorter *molvik1@llnl.gov