MBE-4 experiments with bright Cesium+ beams

Since 1985 the Heavy Ion Fusion Accelerator Research program at the Lawrence Berkeley Laboratory has been studying current amplification and emittance variations in MBE-4, a four-cesium-beam ion induction linac. This experiment models much of the accelerator physics of the electrostatically focused section of a fusion driver. Four space-charged Cs{sup +} beams, initially about one meter in length at currents of 5--10 mA, are focused by electrostatic quadrupoles and accelerated in parallel from approximately 200 keV up to one MeV by 24 accelerating gaps. Final currents of 20--40 mA per beam are typical. Recent experiments with extremely low emittance beams ({var epsilon}{sub n}=0.03 {pi}mm-mRad) have investigated variations of transverse and longitudinal normalized emittance for drifting and accelerating beams. Experiments show that very cold ({sigma}{sub o}=72{degrees},{sigma}=6{degrees}), off-axis or poorly matched beams increase transverse emittance when drifted or accelerated through these MBE-4 apparatus. Only by carefully centering and matching the beams can acceleration at constant normalized emittance be achieved. Warmer beams with less tune depression exhibit little to no emittance growth and show smaller emittance fluctuations when off axis or mis-matched. 10 refs., 10 figs