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Abstract: Plasmas are a source of unbound electrons for charge neutralizing intense heavy ion beams to focus them to a small spot size and compress their axial length. The plasma source should operate at low neutral pressures and without strong externally applied fields. To produce long plasma columns, sources based upon ferroelectric ceramics with large dielectric coefficients have been developed. The source utilizes the ferroelectric ceramic BaTiO3 to form metal plasma. The drift tube inner surface of the Neutralized Drift Compression Experiment (NDCX) is covered with ceramic material. High voltage (∼8kV) is applied between the drift tube and the front surface of the ceramics. A BaTiO3 source comprised of five 20-cm-long sources has been tested and characterized, producing relatively uniform plasma in the 5×1010 cm−3 density range. The source was integrated into the NDCX device for charge neutralization and beam compression experiments, and yielded current compression ratios ∼120. Present research is developing multi-meter-long and higher density sources to support beam compression experiments for high-energy-density physics applications. [Copyright &y& Elsevier]

Abstract: Highly ionized plasmas are being employed as a medium for charge neutralizing heavy ion beams in order to focus to a small spot size. Calculations suggest that plasma at a density of 1–100 times the ion beam density and at a length ∼0.1–1m would be suitable for achieving a high level of charge neutralization. A radio frequency (RF) source was constructed at the Princeton Plasma Physics Laboratory (PPPL) in support of the joint Neutralized Transport Experiment (NTX) at the Lawrence Berkeley National Laboratory (LBNL) to study ion beam neutralization. Pulsing the source enabled operation at pressures ∼10−6 Torr with plasma densities of 1011 cm−3. Near 100% ionization was achieved. The plasma was 10cm in length, but future experiments require a source 1m long. The RF source does not easily scale to the length. Consequently, large-volume plasma sources based upon ferroelectric ceramics are being considered. These sources have the advantage of being able to increase the length of the plasma and operate at low neutral pressures. The source will utilize the ferroelectric ceramic BaTiO3 to form metal plasma. A 1m long section of the drift tube inner surface of NTX will be covered with ceramic. A high voltage (∼1–5kV) is applied between the drift tube and the front surface of the ceramic by placing a wire grid on the front surface. Plasma densities of 1012 cm−3 and neutral pressures ∼10−6 Torr are expected. A test stand to produce 20cm long plasma is being constructed and will be tested before a 1m long source is developed. [Copyright &y& Elsevier]

Abstract: Intense, space-charge-dominated ion beam pulses for warm dense matter and heavy ion fusion applications must undergo simultaneous transverse and longitudinal bunch compression in order to meet the requisite beam intensities desired at the target. The longitudinal compression of an ion bunch is achieved by imposing an initial axial velocity tilt on the drifting beam and subsequently neutralizing its space-charge and current in a drift region filled with high-density plasma. The Neutralized Drift Compression Experiment (NDCX) at Lawrence Berkeley National Laboratory has measured a sixty-fold longitudinal current compression of an intense ion beam with pulse duration of a few nanoseconds, in agreement with simulations and theory. A strong solenoid is modeled near the end of the drift region in order to transversely focus the beam to a sub-millimeter spot size coincident with the longitudinal focal plane. The charge and current neutralization provided by the background plasma is critical in determining the total achievable transverse and longitudinal compression of the beam pulse. Numerical simulations show that the current density of an NDCX ion beam can be compressed over a few meters by factors greater than 105 with peak beam density in excess of 1014 cm−3. The peak beam density sets a lower bound on the local plasma density required near the focal plane for optimal beam compression, since the simulations show stagnation of the compression when n beam>n plasma. Beam–plasma interactions can also have a deleterious effect on the compression physics and lead to the formation of nonlinear wave excitations in the plasma. Simulations that optimize designs for the simultaneous transverse and longitudinal focusing of an NDCX ion beam for future warm dense matter experiments are discussed. [Copyright &y& Elsevier]