Injection of high power e-beams into a magnetic cusp with and without electrostatic end plugs

The injection of relativistic electron beams into a magnetic cusp through the ring cusp has previously been investigated at a power level of 10¹⁰ watts.¹ Energy was transferred from a ring-shaped beam inside the diode to a target in the zero-field region of the cusp with a maximum efficiency of 30%. Subsequent experiments² using Angara I with 10-15 kJ, 60 ns pulses confirmed that such beams can be transported into the magnetic cusp at power levels of a few tenths terrawatt. The present investigation extends the data base to power levels up to 0.5 × 10¹² watts. The losses through the point cusps are substantially reduced by introducing foils which separate the controlled pressure within the magnetic cusp from vacuum outside the cusp. Electrons are no longer lost out the point cusps, but pass through and reflect from a virtual cathode back into the magnetic cusp. Thus, the cusp leaks are electrostatically plugged by foils, or by the diode at the ring cusp. Plugging of the point cusps increased the beam-target coupling efficiency by a factor of two.