|Optical Diagnostics for HIF beam experiments
Optical imaging of the beams in HIF experiments, using glass and ceramic scintillators,has led to new diagnostic tools of great capability and flexibility [HIF News, Dec. 2001]. Such imaging enhances the speed and quality of dataacquisition by providing a complete high-resolution two-dimensional image of the beam in a single pulse, in contrast to the thousands of beam pulses required by a crossed-slit scan. We have implemented optical diagnostics on HCX and NTX, and are beginning to exploit their capabilities.
Time-resolved images of the beam striking the scintillator screen are captured with a gated and intensified CCD camera. A mesh
the path of the beam provides charge-neutralizing electrons to the scintillator material. The mesh may be grounded, or
biased negative to minimize undesired light emission from stray energetic electrons striking the scintillator. Detailed comparisons
show agreement with slit scanner measurements taken on HCX. A full time history of the beam pulse can be developed by gating
different time windows on multiple pulses.
Current work includes development of a compact optical diagnostic, and studies of the effects of optical emissions from beam-created
plasma on the front face of the scintillator. In addition, improved algorithms for data analysis and interpretation are being developed;
this research includes the development of capabilities for tomographic synthesis of the 4-D transverse distribution function from multiple
3-D views, to allow detailed discrete-particle simulation of the beam dynamics in the downstream system.
Experiments on electron and gas emission from a surface bombarded by heavy-ion beam
Accelerators for heavy-ion inertial fusion energy (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 use the Gas-Electron Source Diagnostic (GESD) on the High Current Experiment (HCX) at LBNL to measure the flux of electrons and gas evolved when a 1 MeV K+ beam impinges a stainless steel target, whose angle can be varied between 78° and 88° from normal incidence. The results show that electron emission yield (SEY) scales with 1/cos(θ) to within a few degrees of grazing incidence, consistent with emission from a thin layer near the surface. Gas desorption coefficients are larger, of order ten thousand, and vary more slowly with angle. The latter indicates that most desorption is not from beam interaction with multiple monolayers of gas on the surface. We are investigating other possible sources of gas, and are testing the applicability of models predicting that both electron and gas emission scale with electronic energy loss (dE/dx) of ions. This understanding will be applied to mitigating the effects of gas and electrons.
We apply the measured electron-emission coefficients, from the GESD, to infer beam-halo loss from electron emission current in four quadrupole magnets that are installed on the HCX. From this, with gas-desorption coefficients, we will infer the associated gas desorption. – Art Molvik