Plasma source development for the NDCX-I and NDCX-II neutralized drift compression experiments

Abstract

Compressed ion beams are being studied as a driver for inertial confinement fusion energy and for the creation of matter in the high-energy-density regime. In order to facilitate compression of a positive ion charge bunch longitudinally and transversely beyond the limit determined by the space-charge field of the bunch, a source of charge-neutralizing electrons must be provided. Plasma sources have been developed for the NDCX-I and NDCX-II experimental facilities, both for the 2-m-long, field-free drift regions, and for the small-diameter interior of the multi-Tesla final focus solenoid. Barium titanate based cylinders with a high dielectric coefficient are used to line the wall of the 2-m-long drift region and by applying a 9 kV pulse between the inner and outer surfaces of the cylinders, plasma with a density in the 1010 cm−3 range is formed. Results are presented from experiments using this plasma source on NDCX-I. A compact plasma source 5.1 cm long and 3.8 cm in diameter, also made using the barium titanate based material, has been developed for use in the bore of the final focus solenoid. Plasma generated near the wall of the plasma source will follow the fringing magnetic field lines of the solenoid and help to fill the bore of the magnet with plasma. Improved designs for the barium titanate plasma sources are being considered that use different inner-surface electrode materials and structures, and also use a modified electrical driver employing a spark gap crowbar switch. In addition, plasma source designs using so-called flashboard technology have been developed. In the flashboard plasma source, high density plasma is formed when the applied high voltage pulse causes a series of breakdowns between isolated copper patches aligned in rows along the surface of the 0.2 mm thick flashboard.

Characterization of inductively coupled plasma generated by a quadruple antenna

“…´-1.25 10 s ´-6 10 s . The estimated values of the skin depths according to equations (12) and ( 13) are close to each other and are in the range 1.5-2.5 cm, which is significantly smaller than the distance (∼20 cm) between the arms of the quadruple antenna. Therefore, the main RF energy deposition into the plasma occurs near the antenna wires.…”

“…This range of frequency is comparable with the ICP driving frequency w =´-1.25 10 s 1 and is significantly smaller than the plasma frequency w =´-6 10 p 9 -6 10 s . 10 1 In these conditions, the RF discharge has features of collisional δ c and anomalous δ e skin effect, for which the skin depth is defined as [39]: The estimated values of the skin depths according to equations (12) and (13) are close to each other and are in the range 1.5-2.5 cm, which is significantly smaller than the distance (∼20 cm) between the arms of the quadruple antenna. Therefore, the main RF energy deposition into the plasma occurs near the antenna wires.…”

“…The same concern applies to ferroelectric plasma sources [11], which can be used for generating plasma that is uniform in the axial direction with a rather broad range of density (10 11 -10 14 cm −3 ). Such plasma, however, is not uniform in the radial direction [12].…”

Irradiation of materials with short, intense ion pulses at NDCX-II

“…In the final drift section, the bunch has a head-to-tail velocity ramp that further compresses the beam by an orderof-magnitude. The space-charge forces are sufficiently high at this stage to require that dense plasma (n plasma > n beam ), generated prior to the passage of the beam pulse, neutralizes the beam self-field and enables focusing and bunching of the beam to the millimeter and nanosecond range (Gilson et al, 2013;Gilson et al, 2012). The beam perveance:…”