T. Bernat

Response of SBDs to MeV protons, tritons, and alphas: Evidence that the charged‐particle sensitive depth is not generally the depletion depth

As part of an on‐going effort to develop diagnostics for energetic charged particles from laboratory and space experiments, we examined the possibility that particle identification could be expedited by varying the applied bias voltage on silicon surface barrier detectors (SBDs). Using MeV protons, tritons, and alphas, we performed spectroscopy experiments whereby we observed changes of the energy spectrum as a function of the bias voltage. These particles were either generated via a Cockcroft–Walton linac as fusion products, or emitted from radioisotopes. The results indicate that, contrary to commonly held belief, the detector sensitive depth is not generally the depletion depth. Indeed for partially depleted SBDs the performance is not greatly degraded even for zero bias.

A fusion‐product source

A Texas Nuclear Cockcroft–Walton neutron generator was refurbished for use as a general fusion‐product source. This well‐calibrated source is now used routinely for characterizing energetic charged‐particle detectors, for the development of nuclear fusion diagnostics, for studying radiation damage, and for calibrating x‐ray detectors for laboratory and space plasmas. This paper is an overview of the facility. We describe the main accelerator operating systems, the primary fusion reactions studied, and several diagnostics used to characterize the fusion‐product source.

X‐ray imaging of uniform large scale‐length plasmas created from gas‐filled targets on Nova

We report on the production and characterization of large scale‐length plasmas created by illuminating gas‐filled thin‐walled balloonlike targets using the Nova laser. The targets consisted of a 5–6000 A skin surrounding 1 atm of neopentane which when ionized becomes a plasma with 1021 electrons/cm3. Results are presented from x‐ray imaging used to evaluate the uniformity of the plasma. The most uniform plasmas were produced by illuminating the target with large converging beams that overlapped to cover most of the surface of the gas bag. An alternate focus geometry using small beam spots resulted in a less uniform plasma with low density holes in it.

PIXE x rays: From Z=4 to Z=92

A high‐intensity, charged‐particle‐induced x‐ray (PIXE) source has been developed for the purpose of characterizing x‐ray detectors and optics, and measuring filter transmissions. With energetic proton beams up to 165 keV, intense line x radiations (0.5 A≤λ≤111 A) have been generated from the K, L, M, and N shells of elements 4≤Z≤92. The PIXE spectrum has orders‐of‐magnitude lower background continuum than a conventional electron beam or radioactive α‐fluorescence source [C. K. Li, R. D. Petrasso, K. W. Wenzel et al. (to be published)].

The National Direct-Drive Program: OMEGA to the National Ignition Facility

Abstract The goal of the National Direct-Drive Program is to demonstrate and understand the physics of laser direct drive (LDD). Efforts are underway on OMEGA for the 100-Gbar Campaign to demonstrate and understand the physics for hot-spot conditions and formation relevant for ignition at the 1-MJ scale, and on the National Ignition Facility to develop an understanding of the direct-drive physics at long scale lengths for the MJ Direct-Drive Campaign. The strategy of the National Direct-Drive Program is described; the requirements for the deuterium-tritium cryogenic fill-tube target being developed for OMEGA are presented; and preliminary LDD implosion measurements of hydrodynamic mixing seeded by laser imprint, the target-mounting stalk, and microscopic surface debris are reported.

A proton activation diagnostic to measure D–3He reaction yields

We are developing activation diagnostics for monitoring energetic charged-particle fluxes in space and laboratory plasmas. More immediately, we plan to use activation to measure the time-integrated proton flux from D--{sup 3}He fusion reactions in Alcator C-MOD, providing a measure of the time-averaged D--{sup 3}He fusion rate. We demonstrated the techniques feasibility by inducing significant gamma activity in a titanium sample exposed to D--{sup 3}He protons created in our Cockcroft--Walton generator. The titanium target received a fluence of 5.5{times}10{sup 9} protons at 14.7 MeV (of order what a 3-cm{sup 2} target should receive from one shot in Alcator C-MOD) and became activated by the{sup 48}Ti({ital p},{ital n}){sup 48}V reaction. The activitys spectrum from a high-purity germanium (HPGe) detector showed the characteristic 0.984- and 1.312-MeV lines of {sup 48}V. The measured activity agreed reasonably well with theory. An absence of activity at those energies before D--{sup 3}He activation eliminated background or D--D product-induced activity as the gamma source. We intend to repeat the experiment with a chromium target to evaluate that materials diagnostic potential.

Measurements of charged-particle fusion yields and ratios from Omega (abstract)

We have recently conducted a series of experiments on Omega in which we utilized a charged-particle spectrometer. It consists of a 7.6 kG magnet, CR-39 track for particle detection, and an assortment of range filters to either discriminate against or emphasize certain particles. Because of excellent particle selectivity and energy resolution, the spectrometer can accurately measure the ratios of various fusion products from a single shot. Among other uses, these ratios will be used to determine ion temperatures.