Transport Of Intense Ion Beams With Z-Discharge Plasma Channels

Light ion beams are a promising driver for Inertial Confinement Fusion (ICF). An isolation distance of several meters between the ion source and the target is required to protect the ion source and to allow for time-of-flight power compression of the ion beam. A Z-discharge plasma channel is a candidate for the ion beam transport mechanism. A pinch-reflex ion diode is used on the NRL Gamble II accelerator to generate a 1.2-MeV, 500-kA focused proton beam. The entrance aperture of the 2.5-cm diameter, 1.2-m long Z-discharge transport system is placed near the beam focus. The beam ions are charge- and current-neutralized in the discharge plasma, and execute betatron-like orbits in the magnetic field of the discharge. Typical discharge parameters are 40-kA peak current and 2-Torr air fill. Diagnostics of the ion beam include shadowboxes to sample the phase-space of transported ion beam and prompt- γ radiation from LiF targets
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to measure ion beam energy and transport efficiency. The
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Li(p, γ) reaction yields a time-integrated response, while the
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F(p, α γ) reaction yields a time-dependent measurement. The experiments are analyzed using detailed calculations of ion trajectories and ion energy losses in the gas fill. The beam energy transport is shown to be highly efficient, with the only significant energy loss attributed to classical slowing down in the channel gas. The beam ion distribution measured by the shadowbox is consistent with the assumption of uniform current distribution in the discharge, and full charge- and current-neutralization of the ion beam by the discharge plasma.