Explorations of Electron Cloud Effects and the consequences

1. Optical scan Warp simulation Explorations of Electron Cloud Effects and the consequences for Heavy-Ion Drivers for HEDP and Inertial Fusion Energy * M. KIREEFF COVO, LLNL and UCB, A.W. MOLVIK, R. COHEN, A. FRIEDMAN, LLNL, J-L. VAY, F. BIENIOSEK, D. BACA, P. A. SEIDL, LBNL, J. VUJIC, C. LEISTER, B.E. ROSENBERG, UCB In order to satisfy the requirements of focusing high-power density for high-energy-density physics and inertial-fusion targets, we should be able to transport high-current, high-energy beams with low emittance growth. With this aim the US Heavy Ion Fusion program built the High Current Experiment (HCX), a driver scale single beam injector, with an electrostatic matching section and electrostatic and magnetic quadrupole transport sections, that provides a K+ ion beam current of 0.2-0.5 A for 5 µs. It constitutes a unique facility to study the maximum fill factor (ratio of beam radius to tube radius) allowable, keeping the cost of a power plant competitive, without degrading the beam quality. A deleterious effect when we increase the fill factor is the electron cloud effect, a recognized problem that limits the current and emittance on many large accelerators. Our goal here is to understand and mitigate this effect using new diagnostics that we developed coupled with state-of-the-art simulations. * This work was performed under the auspices of the U.S. Department of Energy by University of California, LLNL and LBNL under contracts W-7405-Eng-48, and DE-AC03-76F00098. HCX instrumented to carry out “fill factor” and electron cloud experiments Fusion power plant cost reduced by high fill factor beams • Cost of a Fusion Power Plant is function of fill factor At the range of interest (beyond 60%), the beam runs closer to the walls and starts to produce secondary electrons and desorbed gas, which could move to the beam path and be ionized. The electrons produced are trapped by the space charge beam potential. We start to lose control of the beam transport and produce more secondary electrons and desorbed gas. It is the beginning of the “electron cloud effect”. Physical mechanism of electron production • New diagnostics were designed and placed within and between the magnets Capacitive, flush and gridded probes were placed inside the magnets, clearing electrodes and a retarding potential analyzer were placed between magnets, allowing to measure sources of electrons and expelled or secondary ions and test mitigation methods. • Beam hitting gas or walls creates electrons and gas At grazing incidence, each ion of K+ ion with energy of 1MeV desorbs 10,000 molecules of gas and produces 100 electrons, which can multiply. WARP simulations show phase space similar to the experiments WARP simulations show electron density distributions inside the magnetic quadrupoles • Electron distribution depends of the NATURE of the electron source A novel interpolated mover has been successfully used, allowing electrons to be advanced with larger timestep, without compromising accuracy. Conclusions • The High-Current Experiment (HCX) instrumented with new diagnostics is an unique platform to explore sources of electron generation and accumulation in positively charged beams. • The simulations not only reproduce qualitative aspects of the experiments, but also show some unanticipated physical effects. 46th Annual Meeting of the Division of Plasma Physics November 15-19, 2004Savannah, GA 2005 High-Energy-Density-Physics (HEDP) Summer School