R. F. Radel

Overview of University of Wisconsin Inertial-Electrostatic Confinement Fusion Research

Abstract In Inertial Electrostatic Confinement (IEC) devices, a voltage difference between concentric, nearly transparent spherical grids accelerates ions to fusion-relevant velocities. The University of Wisconsin (UW) operates two IEC devices: a cylindrical aluminum chamber and a spherical, water-cooled, stainless-steel chamber, with a power supply capable of 75 mA and 200 kV. The research program aims to generate fusion reaction products for various applications, including protons for creating radioisotopes for nuclear medicine and neutrons for detecting clandestine materials. Most IEC devices worldwide, including the UW devices, presently operate primarily in a pressure range (1-10 mtorr) that allows ions to make only a few passes through the core before they charge exchange and lose substantial energy or they collide with cathode grid wires. It is believed that fusion rates can be raised by operating at a pressure where neutral gas does not impede ion flow, and a helicon ion source has been developed to explore operation at pressures of ~0.05 mtorr. The UW IEC research group uses proton detectors, neutron detectors, residual gas analyzers, and spectroscopic diagnostics. New diagnostic techniques have also been developed, including eclipse disks to localize proton production and chordwires to estimate ion fluxes using power balance.

Detection of highly enriched uranium using a pulsed D-D fusion source

Abstract This paper overviews the work that has been done to date towards the development of a compact, reliable means to detect Highly Enriched Uranium (HEU) and other fissile materials utilizing a pulsed Inertial Electrostatic Confinement (IEC) D-D fusion device. To date, the UW IEC device has achieved 115 kV pulses in excess of 2 ampere, with pulsed neutron rates of 1.8x109 n/s during a 0.5 ms pulse at 10 Hz. MCNP modeling indicates that detection of samples of U-235 as small as 10 grams is achievable at current neutron production rates, and initial pulsed and steady-state HEU detection experiments have verified these results.

Recent Progress in Steady State Fusion Using D-3He

Abstract The University of Wisconsin (UW) inertial electrostatic confinement (IEC) facility has made significant progress since 2000. The operating voltage has doubled to 160 kV. The neutron production rate has increased by a factor of 2, from 4.9 x 107/s to 1.1 x 108s-1. The D-3He proton production rate has increased by, a factor of over 40. In addition new diagnostics have been developed, including a method to determine the spatial distribution of fusion reactions A new water cooled stainless steel chamber for higher power and lower pressure has been put into operation. Medical isotopes have been produced in an IEC device for the first time.

Optimizing Neutron Production Rates from D-D Fusion in an Inertial Electrostatic Confinement Device

Abstract Detection of explosives has been identified as a near term commercial opportunity for using a fusion plasma. Typical explosive compositions contain low Z material (C, N, O) which are not easily detected using conventional x-rays or metal detectors. However, 2.45 MeV neutrons produced in a D-D fusion reaction can be used for detection of explosives or other clandestine materials in suitcases, packages, or shipping containers. Steady-state D-D operation is possible using an Inertial Electrostatic Confinement (IEC) fusion device. The University of Wisconsin IEC device has produced D-D neutrons at 1.8 × 108 neutrons/second at a true cathode voltage of 166 kV and a meter current of 68 mA. These neutron production rates are approaching the levels required for the detection of explosives. In order to increase and optimize the neutron production rate in the IEC device, experiments were performed altering the cathode’s size (diameter), geometry, and material composition. Preliminary results indicate that significant differences in neutron production rates are not achieved by altering the geometry or material composition of the cathode. However, the neutron production rate was found to increase approximately 20% by doubling the cathode’s diameter from 10 cm to 20 cm. In addition, increasing the cathode voltage from 34 kV to 94 kV at a meter current of 30 mA increased the neutron production rate from 1.24 × 106 n/s to 2.83 × 107 n/s.

Production of 13N Via Inertial Electrostatic Confinement Fusion

Abstract This paper describes a proof of principle experiment to produce 13N using an inertial electrostatic confinement (IEC) fusion device. This radioisotope is often used in positron emission tomography scans to image the heart. The 10-minute half-life of 13N limits its use to those areas and clinics that possess an accelerator. A portable IEC device could be brought to remote locations, however, and produce short-lived PET isotopes on-site. Using the 14.7 MeV protons produced from the D-3He fuel cycle, the University of Wisconsin IEC device was used to produce approximately 4 - 8 Bq of 13N during two separate experiments.

Alternate Applications of Fusion - Production of Radioisotopes

Abstract A major effort to find near-term, non-electric applications of fusion energy has shown that the production of radioisotopes is attractive. The use of the D3He fusion reaction to produce Positron Emission Tomography (PET) isotopes is described. An Inertial Electrostatic Confinement (IEC) device is particularly well suited to produce low levels of high-energy (14.7 MeV) protons, which in turn, can produce short-lived PET isotopes. The IEC device at the University of Wisconsin has been modified to investigate the potential of this process to be commercially attractive.

Effects of High Temperature Pulsed Helium Implantation on Tungsten Surface Morphology

Abstract The durability and lifetime of thin tungsten or refractory metal coatings on the first walls of inertial and magnetic confinement fusion reactors is a key issue for the feasibility of such devices. Past studies at UW-Madison have indicated that tungsten, when subjected to He+ fluences in excess of 4 × 1017 He+/cm2, shows extensive pore formation at 800 °C.1,2 The current study attempts to produce more realistic results by simultaneously irradiating tungsten samples with helium and hydrogen species and by investigating the effects of pulsed helium ion irradiation on tungsten.

Implantation of D and He in W-Coated Refractory Carbides

Abstract The effect of high temperature (700-1200°C) implantation of deuterium and helium in candidate fusion first wall materials was studied in the University of Wisconsin Inertial Electrostatic Confinement (IEC) device. Tungsten coated TaC and HfC ’’foam’’, single crystal tungsten, and high-emissivity tungsten coated ’’foam’’ were compared to previous tungsten powder metallurgy samples studied in the IEC device for the High Average Power Laser (HAPL) program. Scanning electron microscopy was performed to evaluate changes in surface morphology for various ion fluences at temperatures comparable to first wall temperatures. Single crystal tungsten was shown to exhibit less damage than polycrystalline samples at a fluence of 4×1016 He+/cm2. It was found that no significant deformations occur with deuterium implantation up to ~1018 D+/cm2 at 800°C on W-coated TaC and HfC foam samples. However, helium fluences in excess of 6×1017 He+/cm2 show extensive pore formation at 800°C and higher. These changes may have an impact on the lifetime of tungsten coatings on the first walls of inertial and magnetic confinement fusion reactors.

Special Nuclear Materials Detection Using IEC Fusion Pulsed Neutron Source

Special Nuclear Materials (SNM) detection efforts have largely been divided into two main groups: active and passive. Passive techniques are highly desirable in that a radiation source need not be employed in order to detect fissile materials which broadcast a clear radiative signature. However, disadvantages can be seen in HEU (Highly Enriched Uranium) detection, for example, where the system’s efficacy is limited by its ability to detect a weak self-radiative signature from U. Active interrogation provides a catalyst for amplifying HEU’s presence vis-a-vis fission event inducement, which in turn yields a starker signature which can be discerned through an understanding of fissile materials and neutron transport in various media. Ongoing work in the Fusion Technology Institute’s Inertial Electrostatic Confinement (IEC) Experiment has focused on using the pulsed D-D neutrons from an IEC to interrogate the presence of HEU in an enclosed space. The paper begins with a brief description of the neutron-based detection schemes of Delayed Neutron Analysis (DNA) and Differential Die-Away (DDA). Experimental delayed neutron counts of ninety above the background at an interrogating neutron flux of 5.5x104 n/cm2-s are seen to confirm MCNP modeling results. MCNP is also utilized to probe future concepts in neutron-based active interrogating SNM detection systems using DDA analysis.

An Improved Near Term 14 MeV Neutron Test Facility for Fusion Power Plant Materials

Abstract A near term, low cost 14 MeV neutron materials test facility has been designed that allows significant radiation damage (dpa, appm He, etc.) levels to be achieved typical of those that will be experienced in DT Demonstration or commercial DT power plants. The design described in this paper produces peak damage levels of ≈4–6 dpa/fpy in 15 cm3 and has ≈600 cm3 test volume covering the damage range from 1 to 6 dpa/fpy. The total active tritium inventory in the test facility is less than 1 g and the overall construction costs are also roughly unchanged from an earlier (2015) design. The time to initial operation remains at ≈4 years from the start of construction because it builds on an on-going project for radioisotope production already under construction. This latest facility design has the possibility to provide a 2 MW-y/m2, 14 MeV neutron exposure to first wall materials in less than 4 fpy’s of operation.