Victor Jozef Harper-Slaboszewicz

Use of a radial self-field diode geometry for intense pulsed ion beam generation at 6 MeV on Hermes III

We investigate the generation of intense pulsed focused ion beams at the 6 MeV level using an inductive voltage adder (IVA) pulsed-power generator, which employs a magnetically insulated transmission line (MITL). Such IVA machines typical run at an impedance of few tens of Ohms. Previous successful intense ion beam generation experiments have often featured an “axial” pinch-reflex ion diode (i.e., with an axial anode-cathode gap) and operated on a conventional Marx generator/water line driver with an impedance of a few Ohms and no need for an MITL. The goals of these experiments are to develop a pinch-reflex ion diode geometry that has an impedance to efficiently match to an IVA, produces a reasonably high ion current fraction, captures the vacuum electron current flowing forward in the MITL, and focuses the resulting ion beam to small spot size. A new “radial” pinch-reflex ion diode (i.e., with a radial anode-cathode gap) is found to best demonstrate these properties. Operation in both positive and negat...

Coarse spectral characterization of warm x-rays at the Z facility using a filtered thermoluminescent dosimeter array

A new collimated filtered thermoluminescent dosimeter (TLD) array has been developed at the Z facility to characterize warm x-rays (hν > 10 keV) produced by Z pinch radiation sources. This array includes a Kapton debris shield assembly to protect the TLDs from the source debris, a collimator array to limit the field of view of the TLDs to the source region, a filter wheel containing filters of aluminum, copper and tungsten up to 3 mm thick to independently filter each TLD, and a hermetically sealed cassette containing the TLDs as well as tungsten shielding on the sides and back of the array to minimize scattered radiation reaching the TLDs. Experimental results from a krypton gas puff and silver wire array shot are analyzed using two different functional forms of the energy spectrum to demonstrate the ability of this diagnostic to consistently extend the upper end of the x-ray spectrum characterization from ∼50 keV to >1 MeV.

Use of pulsed bremsstrahlung excitation on hermes-iii for investigation of active detection of fissionable material

Summary form only given. We investigate the use of pulsed bremsstrahlung as a standoff active interrogation tool for the production of neutron and gamma signatures in fissionable materials.1 Photons are generated by an electron beam striking a Ta converter on the 16 MV Hermes III facility at Sandia National Laboratories, and propagate to a target location 19 m from the electron beam diode, at which various materials are placed. X-rays produce (γ,f) and (γ,n) reactions in fissionable material, while nonfissionable materials such as Pb will result in competing (γ,n) reactions. The intense prompt 30 ns x-ray burst from the diode produces γ-rays and neutrons from materials in the environment, all of which must be discriminated by a successful detector set.

Polarity inversion on Saturn

Saturn is a pulsed power accelerator consisting of 36 parallel modules producing 10 MA at 1.7 MV in a 40 ns power pulse[1]. Saturn is built to operate in negative polarity. Two methods of inverting the polarity in vacuum and driving up to 2.5 MA into a triplate MITL with a low impedance load have been built and demonstrated. Both rely on the use of a ballast inductance to invert the polarity[2]. The first method uses a dual post-hole convolute while the second method uses no convolutes.

Efficient injection of electron beams into magnetic guide fields

Propagation of pulsed electron beams in grad B channels has been studied extensively theoretically and experimentally as a method for guiding such beams at high energy densities onto a converter target to generate intense bremsstrahlung radiation. A key problem in making this approach practical is the efficient injection of the beam from the diode into the drift tube. A method for achieving nearly 100% injection from the diode into the drift channel was developed at Kharkiv State University (KhSU) for 50 kA beams. This method uses an injection of the beam from a ring diode into the grad B drift tube with an appropriate transition of the magnetic field from the diode into the drift tube. Scaling experiments have been performed at up to 100 kA level at KhSU and are being extended to >500 kA level on Gamble II generator at the Naval Research Laboratory.

Inverse diode for combination of multiple modules and fusion driver-target standoff

A newly invented, multi-megampere inverse diode converts the currents in many electron beams to current in a single Magnetically Insulated Transmission Line (MITL) for driving a common load. Electrons are injected through a transparent anode, cross a vacuum gap, and are absorbed in the cathode of the inverse diode. The cathode current returns to the anode through a load and generates electric and magnetic fields in the anode-cathode gap. Counter streaming electron flow is prevented by self-magnetic insulation in most of the inverse diode and by self-electrostatic insulation where the magnetic field is insufficient. Two-dimensional simulations with a 40 MA, 4 MeV, 40 ns electron beam at 3.5 kA/cm2 current density, 5 degree beam divergence, and up to 60 degree injection angle show 85% of the injected electron beam current is captured and fed into the MITL. Exploratory experiments with a 2.5 MA, 2.8 MeV, 40 ns electron beam at 2 kA/cm2at injection normal to the anode gave 70+/−10% collection efficiency in an unoptimized inverse diode. The inverse diode appears to have the potential of coupling multiple pulsed power modules into a common load at rates of change of current ∼1.6× 1015 A/s required for a fusion energy device called the Plasma Power Station with a Quasi Spherical Direct Drive fusion target.

Dosimetry Experiments at the MEDUSA Facility (Little Mountain)

A series of experiments on the MEDUSA linear accelerator radiation test facility were performed to evaluate the difference in dose measured using different methods. Significant differences in dosimeter-measured radiation dose were observed for the different dosimeter types for the same radiation environments, and the results are compared and discussed in this report.

Z-discharge electron beam transport for Decade Quad

Summary form only given. The DTRA Decade Quad generator comprises four modules that collectively provide a large-area bremsstrahlung source. For small-area (large-dose) applications, it is necessary to compress the beam energy to an area at least four times smaller than that produced by the separate modules. This can be achieved by a combination of beam transport over roughly 30 cm, and either 2/spl times/ radial beam compression or beam overlap at the target. We are investigating two approaches to this problem: (1) grad-B-drift transport, followed by compression or overlap, and (2) radial beam compression using pinched-beam diodes followed by wall-confined, Z-discharge transport (ZDT) to the target. ZDT results are presented here.

The differential absorption hard x-ray (DAHX) spectrometer at the Z facility

The Differential Absorption Hard X-ray (DAHX) spectrometer has been developed to diagnose photon energies above 80 keV and provides a time-resolved measurement of the x-ray spectrum from the plasma driven by Z. The DAHX spectrometer is a revitalization of the Hard X-Ray Spectrometer (HXRS) fielded on Z prior to refurbishment1. DAHX consists of an array of seven collimated silicon PIN diodes with varying filters in a 2 to 4 inch thick tungsten cylinder that shields the diodes from the surrounding radiation environment. Analyzed data from DAHX will be presented for several wire array experiments. Development of DAHX is driven by the need to extend the present diagnostic suite on Z to energies above 25 keV to investigate the origin, intensity, and spectrum of warm and hard x-rays within the emitting plasma. Filtered PCDs2 and TLDs were also fielded. The PCDs are capable of measuring time resolved output up to a few tens of keV while TLDs are capable of measuring time integrated yield up to hundreds of MeV. These measurements pair well with the time resolved hard x-ray information that DAHX provides. To aid analysis of the silicon PIN diode signals, the diode performance was studied using the short pulse Z-Petawatt laser. Data and analysis from this study will be presented along with wire array measurements from TLDs, PCDs, and DAHX. The analysis of the DAHX data will be discussed, both in isolation and in conjunction with the additional diagnostics. The design of an updated version of DAHX with 15 diodes tailored to the current Z machine radiation environment will also be presented.

Refurbishment and enhancement of the saturn accelerator

The Saturn accelerator was commissioned in 1987 as a 5.4 MJ, 2.7 MV flash x-ray machine that delivers 750 kJ at 2.1 MV to a three-ring diode with peak power of 25 TW and FWHM pulse width of 40 ns.1 The machine has also been extensively used for wire-array z-pinch experiments2 with currents up to 8 MA, primarily prior to the startup of the Z accelerator. In the last few years the machine has not been fully used, and has not been fully maintained. But recently there has been interest in using it more regularly for x-ray testing of components. Therefore, a project has been initiated to refurbish the machine with emphasis on achieving better repeatability. This effort has involved rebuilding each of the 36 pulsed power lines, including Marx banks, 3 MV gas switches and trigger system, and water switches. In addition, there was considerable wear and mechanical misalignment in high-current joints in the vacuum magnetically-insulated transmission lines that has been corrected. Furthermore, timing and calibration of diagnostic monitors has been improved. In doing initial testing of the refurbished modules we realized that it is possible to transmit much more energy to the load than has been done regularly in the past by adjusting gas switch timing. This modification can increase downstream voltage by more than 50%. In this paper, along with describing the status of the machine refurbishment, we will describe tests done with this modification, its implications for other components in the machine, and expected improvement of load performance.