D. Baca

The high current experiment: First results

The High Current Experiment (HCX) is being assembled at Lawrence Berkeley National Laboratory as part of the US program to explore heavy-ion beam transport at a scale representative of the low-energy end of an induction linac driver for fusion energy production. The primary mission of this experiment is to investigate aperture fill factors acceptable for the transport of space-charge dominated heavy-ion beams at high spacecharge intensity (line-charge density {approx} 0.2 {micro}C/m) over long pulse durations (>4 {micro}s). This machine will test transport issues at a driver-relevant scale resulting from nonlinear space-charge effects and collective modes, beam centroid alignment and beam steering, matching, image charges, halo, lost-particle induced electron effects, and longitudinal bunch control. We present the first experimental results carried out with the coasting K{sup +} ion beam transported through the first 10 electrostatic transport quadrupoles and associated diagnostics. Later phases of the experiment will include more electrostatic lattice periods to allow more sensitive tests of emittance growth, and also magnetic quadrupoles to explore similar issues in magnetic channels with a full driver scale beam.

Experiments Studying Desorbed Gas and Electron Clouds in Ion Accelerators

Electron clouds and gas pressure rise limit the performance of many major accelerator rings. We are studying these issues experimentally with ∼ 1 MeV heavy-ion beams, coordinated with significant efforts in self-consistent simulation and theory. The experiments use multiple diagnostics, within and between quadrupole magnets, to measure the sources and accumulation of electrons and gas. In support of these studies, we have measured gas desorption and electron emission coefficients for potassium ions impinging on stainless steel targets at angles near grazing incidence. Our goal is to measure the electron particle balance for each source – ionization of gas, emission from beam tubes, and emission from an end wall – determine the electron effects on the ion beam and apply the increased understanding to mitigation. We describe progress towards that goal.

Initial Results on Neutralized Drift Compression Experiments (NDCX-IA) for High Intensity Ion Beam

Ion beam neutralization and compression experiments are designed to determine the feasibility of using compressed high intensity ion beams for high energy density physics (HEDP) experiments and for inertial fusion power. To quantitatively ascertain the various mechanisms and methods for beam compression, the Neutralized Drift Compression Experiment (NDCX) facility is being constructed at Lawrence Berkeley National Laboratory (LBNL). In the first neutralized drift compression experiment, a 280 KeV, 25 mA, K+ion beam is longitudinally 50-fold compressed using an induction core to produce a velocity tilt. This compression ratio is measured using various diagnostics.

Fabrication of large diameter alumino silicate K+ sources

Alumino-silicate sources that are used for Heavy Ion Fusion (HIF) drivers can produce high current beams, and have a long lifetime, typically many months of normal operation. The usual ion source diameter is about 10 cm with a curved emitting surface. Recently we achieved reliable fabrication of such large diameter sources with high quality alumino-silicate coating on tungsten substrates. The effects of substrate porosity, powder size distribution, coating technique, particle packing density, drying, and heat firing temperature have been investigated.

Absolute measurement of electron cloud density

Beam interaction with background gas and walls produces ubiquitous clouds of stray electrons that frequently limit the performance of particle accelerator and storage rings. Counterintuitively we obtained the electron cloud accumulation by measuring the expelled ions that are originated from the beam-background gas interaction, rather than by measuring electrons that reach the walls. The kinetic ion energy measured with a retarding field analyzer (RFA) maps the depressed beam space-charge potential and provides the dynamic electron cloud density. Clearing electrode current measurements give the static electron cloud background that complements and corroborates with the RFA measurements, providing an absolute measurement of electron cloud density during a 5 mus duration beam pulse in a drift region of the magnetic transport section of the High-Current Experiment (HCX) at LBNL.