Measurements of charged particle dynamics in high accelerating fields

A high-voltage power pulse applied to a magnetically-insulated ion accelerating gap results in a complex evolution of the electron and ion distributions within the gap. Divergence-inducing electromagnetic instabilities and the ion current both depend on the charged-particle distributions. Thus, understanding and control of these distributions is extremely important for applications, such as Inertial Confinement Fusion, that require high-brightness ion beams. This work describes time- and space-resolved spectroscopic measurements of the charged particle distributions in the acceleration gap of an applied-B ion diode. The Particle Beam Fusion Accelerator II supplies a 20 TW, 40 nsec power pulse to a 15 cm radius, cylindrically-symmetric diode. A ∼3 T magnetic field applied parallel to the symmetry axis insulates the anode-cathode (AK) gap against electron losses. The Li+ ion beam is accelerated radially from a LiF source to ∼ 10 MV across the ∼ 2-cm-wide AK gap. The authors determine the charged-particle distributions using measurements of the electric field, obtained from Stark-shifted Li I 2s-2p emission that arises from Li neutrals injected into the AK gap when Li ions undergo charge exchange near the anode surface. The diagnostic system is presently capable of measuring the field to within about ±4% with ∼ 1 nsec time resolution and ∼ 2 mm spatial resolution at 18 locations within the AK gap. The electric field and measured Li+ ion current density are used to determine the space- and time-resolved ion and electron densities in the AK gap