Neutralized ion-beam drift compression for short-pulse target heating experiments

The axial compression is achieved with an induction bunching module (IBM) inserted after the matching section. Operating at ±0.1 MV, a ±12-15% velocity ramp is imparted to a 0.6 μs subset near the front of the several-microsecond beam pulse. The beam then drifts through a neutralizing plasma in a drift compression section three meters in length. A ferro-electric plasma source establishes a neutralizing plasma along most of the length of the drift compression section, and cathodic arc plasma sources inject a high-density plasma near the focal plane where the beam density is greatest. A fundamental limit to the current amplification and pulse duration is the longitudinal energy spread of the injected beam. The measured energy spread of 170 eV is adequate for achieving nanosecond-duration bunches. Other limits are set by the uniformity and density of the background neutralizing plasma, and imperfections of the bunching module waveform. Incomplete neutralization of the ion beam space charge can limit the beam intensity. We are developing plasma injection improvements to establish a plasma density always greater than the beam density, expected to be >10¹³ cm^-3. Measurements suggested that the plasma density might be lower than the beam density just upstream of the target, and also in parts of a high-field focusing solenoid. Regarding the plasma distribution in the solenoid, subsequent analytic estimates and particlein-cell simulations support the experimental observations and we are testing compact plasma sources to inject plasma along solenoid field lines where the density may be too low. Bunching waveform errors have systematic and random (changing on each pulse) components. Methods to correct them are being developed, including fast inductive correction modules and circuits capable of programmable small amplitude corrections with fast rise and fall times. Chromatic aberrations increase the rms focal spot size by about a factor of two, so time-dependent compensation for the large and limiting chromatic aberration in the bunched beam is desirable. A possible approaches is to correct the envelope with a time-varying series of einzel lenses. Modeling the beam dynamics through einzel lenses shows the required waveforms and lattice parameters may be feasible for the NDCX, but becomes challenging for higher intensity and higher energy accelerators. The peak intensity on target of the uncompressed beam is ≈ 0.4 MW/cm², with 50% of the total beam power within a radius of 1 mm. This enables interesting experiments where thin targets are quickly heated to melting temperatures and the temperature and radiation distribution is measured with a fast pyrometer, a streak camera and a spectrometer. The bunched beam, with about 5 mJ in 3 ns (FWHM) incrementally heats the target by ∼500 C. This timescale is relevant because it is similar to the hydrodynamic expansion time of the targets. These techniques will be applied to warm dense matter experiments with the much higher intensity beam of NDCX-II, presently under construction. Some of the beam dynamics are of particular interest for fusion energy research due to their relevance to beam manipulations required for heavy-ion driven inertial fusion energy.