Anode plasma ionization due to sheath heating in magnetically insulated ion diodes

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Abstract
We show that anode plasma sheath heating can cause undesirable in situ ionization of the anode plasma in magnetically insulated ion diodes. We find that extremely high electron energies will occur in the anode plasma sheath due to an increasing magnetic field at the anode surface during the diode pulse. These high‐energy electrons can collisionally ionize the plasma ions to an unwanted ionization state. Multiply ionized anode plasma ions represent a beam contaminant that may cause undesired preheat in inertial confinement fusion (ICF) capsules. The fraction of unwanted higher ionization states increases with anode plasma density. Therefore, this effect places an upper limit on the anode plasma density that can be used in an ICF application. As an example, we estimate the fraction of Li++ generated by sheath heating in the PBFA‐II Applied‐B ion diode as a function of the initial Li+ anode plasma density and obtain an upper limit for a lithium anode plasma density of approximately 1×1016 cm−3. At anode plasma densities of 1×1016 cm−3 or less there will be substantial motion of the ion emitting surface during the diode pulse. We discuss the advantages and disadvantages of this plasma motion.