J. W. Coleman

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.

14 MeV neutron yields from D-T operation of the MIT Cockcroft-Walton accelerator

As part of our fusion-product diagnostic development program, we have begun a series of experiments with 14 MeV neutrons generated in a Cockcroft-Walton accelerator. Two different detectors have been used to measure the neutron yield: a silicon SBD and a Cu foil. The energy of the emitted neutrons has been determined by using two spectrometers: the SBD and a3He proportional counter. The reaction rate is monitored, with about ±5% accuracy, by detecting the α particles from D + T →n +α. The neutron yields obtained from the Si detector and the Cu activation had associated uncertainties of about ±15% and agreed well with the predicted values from α measurements.

Magnetic alignment of the Tara tandem mirror

Techniques developed for the alignment of high‐energy accelerators have been applied to the alignment of the Tara tandem mirror magnetic confinement device. Tools used were: a transit/laser surveyor’s system for establishing an invariant reference; optical scattering from ferromagnetic crystallites for establishing magnetic centers in the quadrupole anchor/transition modules; an electron‐optical circle‐generating wand for alignment of the solenoidal plug and central cell modules; and four differently configured electron emissive probes, including a 40‐beam flux mapping e gun, for testing the alignment of the coils under vacuum. Procedures are outlined, and results are given which show that the magnetic axes of the individual coils in the Tara set have been made colinear with each other and with the reference to within ±1.0 mm over the length of the machine between the anchor midplanes.

TIME RESPONSE OF A TYPE IIA NATURAL DIAMOND PHOTOCONDUCTOR TO LONG-PULSE LOW-INTENSITY SOFT X RAYS

For the study of soft x rays from plasmas and laser‐plasma interactions, type IIa diamond photoconductor detector (PCD) is an important diagnostic for emissions from z pinches, [R. B. Spielman, Rev. Sci. Instrum. 63, 5056 (1992)], synchrotons, and laser‐produced plasmas [D. R. Kania et al., Rev. Sci. Instrum. 61, 2765 (1990)]. Among diamond’s advantages are its time response (in the picosecond‐to‐nanosecond range for high‐intensity fast pulses of soft x rays [D. R. Kania et al., J. Appl. Phys. 68, 124 (1990)]), and its band gap of 5.45 eV (which reduces its visible and near‐uv sensitivity to near zero). We have investigated type IIa PCD using low‐intensity slow pulses generated by a pulsed e beam hitting a copper target, with repetitions up to 50 kHz. The time response in this case is presented, leading to a proposal for a new type of diamond PCD for tokamaks, where the single detector has an array capability for position sensing and energy analysis.

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)].

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.

MIT modular x‐ray source systems for the study of plasma diagnostics

Two new x‐ray source systems are now on line at our facility. Each provides an e‐beam to 25 kV. Targets are interchangeable between machines, and four x‐ray detectors may be used simultaneously with a target. The gridded e‐gun of the RACEHORSE system gives a 0.5–1.0‐cm pulsable spot on target. The nongridded e‐gun of the SCORPION system provides a 0.3‐mm or smaller dc microspot on target. RACEHORSE is being used to study and characterize type‐II diamond photoconductors for use in diagnosing plasmas, while SCORPION is being used to develop a slitless spectrograph using photographic film. Source design details and some RACEHORSE results are presented.

γ‐ray and neutron calibration facility (abstract)

High‐temperature inertial and magnetically confined plasmas are copious emitters of γ rays and neutrons. Some γ‐ray and neutron reactions of interest are D+He3→Li5+γ (16.7), D+P→He3+γ (5.5 MeV), D+T→He5+γ (16.6 MeV), D+D→He4+γ (23 MeV), and D+D→He3+n (2.45 MeV). In order to test and absolutely calibrate γ‐ray and neutron diagnostics for such reactions, we are investigating the feasibility of renovating a Cockcroft–Walton accelerator which resides in our laboratory. We will report on the status of this work. This work was supported by U. S. Department of Energy contract No. DE‐AC02‐78ET51013.

Multiple filament plug‐in module for use on the Tara neutral beam source

A plug‐in module bearing 14–20 tungsten filament emitters has been developed for use with the field‐free neutral beam sources to be employed in the Tara tandem mirror experiments. Eight such modules constitute the total distributed cathode for one source. Each module has its own plug‐in power connector, and may be removed from the source assembly as a unit, independently of the outer modules. The plug‐in approach to construction results in a reduction both in the cost to build the source and in the time required for routine servicing and filament replacement. Plasma profiles are the same as in conventionally engineered sources of the same geometry.