Experimental studies of electron and gas sources in a heavy-ion beam - eScholarship (2004)

LBNL-55046 HIFAN-1321 EXPERIMENTAL STUDIES OF ELECTRON AND GAS SOURCES IN A HEAVY-ION BEAM * A. W. Molvik # , R. H. Cohen, A. Friedman, M. Kireeff Covo , S. M. Lund, Lawrence Livermore National Laboratory, Heavy Ion Fusion Virtual National Laboratory, Livermore, CA 94550, USA. P. A. Seidl, F. M. Bieniosek, A. Faltens, L. Prost, Lawrence Berkeley National Laboratory, Heavy Ion Fusion Virtual National Laboratory, Berkeley CA 9472-82010, USA Abstract We use the High Current Experiment (HCX) to study three mitigation measures: a rough surface to reduce electron emission and gas desorption from ions striking walls near grazing incidence, a suppressor electrode after the magnets to block beam-induced electrons off the end structures from drifting upstream, and clearing electrodes to remove electrons from drift regions between magnets. We find that each technique performs as intended. On HCX we are using a 1 MeV, 180 mA, K + ion beam to study transport [7], beam induced electron emission and gas desorption [6], and electron cloud and gas effects in magnetic quadrupoles [4]. The beam has a space- charge potential of ~2 kV, rise and fall times of 1 µs, and a flattop duration of 4 µs, repeated at 10 s intervals. An aperture can be inserted at the D2 diagnostic region, immediately preceding the magnetic quadrupoles, to reduce the beam current to 25 mA and ~300 V beam potential. Electron transit times between walls are in the range of 4 ns (11 ns if apertured) for an unneutralized beam, almost 3 orders of magnitude shorter than the flattop duration. This enables exploration of unique electron trapping regimes: beam-induced multipactor trapping, that is frequently observed with in rf accelerators, will not occur during the flattop, and trailing edge multipactor is not an issue because any electrons generated will be lost before the next pulse in ~10 s. However electrons, emitted from a beam-tube wall under beam bombardment, will be trapped during the current rise at the beam head. Electrons from an end wall will be pulled into the positive beam potential, and can be transported upstream in quadrupole magnets by electron drifts. Ionization of gas by the beam generates electrons that are deeply trapped; the associated ions from gas are expelled in ≤1 µs. INTRODUCTION Electron cloud effects (ECEs) [1] and beam-induced pressure rises [2], that are frequently observed to limit the performance of colliders and high-intensity rings, are also a concern for future high-intensity heavy ion linear accelerators such as envisioned in Heavy Ion Inertial Fusion (HIF) [3]. Electron clouds have thus far been a problem only in rings where the beam passes the same location many times. However, HIF linacs have characteristics that may cause ECEs to be a concern: (1) The cost of accelerators for HIF can be reduced by fitting beam tubes more tightly to beams. This places them at risk from gas desorption runaway, and from electron clouds produced by secondary electrons and ionization of gas. (2) Beam space-charge potentials exceed 1 kV, more than sufficient to electrostatically trap electrons. (3) Beam flat-top durations in our conceptual power-plant drivers range from ~30 µs near injection to 0.2 µs at high energies; the longer durations are sufficient for desorbed gas to reach the beam. (4) Electrons emitted from an end wall of a linac that is bombarded by ions can backstream along the beam. We are engaged in an experimental and theoretical program to measure, understand, and model these effects in heavy-ion accelerators [4,5]. We have found that 1 MeV K + ions, incident on a surface near grazing incidence, generate the order of 10 2 electrons (extrapolating as 6/cos(θ) to about 6 at normal incidence) and 10 4 gas molecules [6]. The beam will also generate electrons where it dumps on a metal end wall. In this paper, we discuss the use of a suppressor electrode to switch, or control, the flow of electrons from the end wall, measuring the electron flow with clearing electrodes, intended to remove electrons from the drift regions between quadrupole magnets, and using an electrode that lines the beam tube in the last quadrupole magnet as a diagnostic. MITIGATION OF EMISSION Heavy-ion beams impinging on surfaces near grazing incidence (to simulate the loss of halo ions) generate copious amounts of electrons and gas that can degrade the beam. Electron emission and gas desorption coefficients η e and η 0 respectively, due to ion bombardment of metal surfaces near grazing incidence, have been measured with the Gas-Electron Source Diagnostic (GESD) [6]. The electron emission coefficient, η e , is measured from the ratio of the emission current off a target electrode to the beam current into the GESD that is measured with a Faraday cup. The gas desorption coefficient, η 0 , is measured from the pressure rise after a pulse compared with the time-integral of the beam current into the GESD Faraday cup. The GESD pumps out through the 0.3 by 2.5 cm entrance aperture, plus a 1 cm diameter hole, giving a pump-out time constant of 0.3 sec, long enough for an ion gauge to determine the peak pressure, but short compared with the 10 s before the next pulse. molvik1@llnl.gov