MICROCHARGE NEUTRALIZATION TRANSPORT EXPERIMENTS AND SIMULATIONS FOR ION-DRIVEN INERTIAL CONFINEMENT FUSION

Space charge neutralization for intense beams for inertial confinement fusion is usually assumed to be perfect. However. small charge clumps in the beam will not be totally charge neutralized, and the residual net minimum potential set by electron trapping (e phi approximate to 1 /2 m(e)v(i)(2) where m(e) is the electron mass and v(i) is the ion vel ocity) may lead to a substantial microdivergence. Experiments on the S ABRE accelerator and simulations with the IPROP computer code are bein g performed to assess this mechanism. We have successfully created a 5 mrad beam on the SABRE accelerator, by expanding the beam (a process consistent with Liouville's theorem) and, by passing the beam through a plate with pinholes, we have created low divergence beamlets to stud y this mechanism. Results clearly show: (1) at low pressures, trapping does neutralize the beamlets, but only down to e phi approximate to 1 /2 m(e)v(i)(2); and (2) at higher pressures (approximate to 0.1-1 Torr ), plasma shielding does remove the effect.