Samuel J. Zenobia

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.

Retention and Surface Pore Formation in Helium Implanted Tungsten as a Fusion First Wall Material

Single- and polycrystalline tungsten samples were implanted with 30 keV3He ions to fluences of 5e16, 4e17 and 5e18 He/cm2 at temperatures ranging from ˜850 - 1000 °C. After implantation tungsten’s retention characteristics were studied using 3He(d,p)4He nuclear reaction analysis (NRA) and 3He(n,p)T neutron depth profiling (NDP). Morphological analyses included scanning electron microscopy (SEM), focused ion beam (FIB) milling, and X-ray diffraction on the single crystalline W samples (XRD). SEM analysis showed that the threshold forsurface pore formation occurs in both single-crystalline tungsten (SCW) and polycrystalline tungsten (PCW) between ˜5e16 - 4e17 He+/cm2. Both surface and sub-surface pore formation is observed to increase with higher implant fluences. Focused ion beam (FIB) milling revealed a sub-surface porous layer in both SCW and PCW, which increased in depth with implanted fluences. NRA measured the retained He fluence in SCW between 1.1e16 - 1.1e17 He/cm2 and in PCW between 1.3e17 - 1.5e17 He/cm2. NDP analysis measured the retained He fluence in SCW between 2.0e16 - 2.7e17 He/cm2 and in PCW between 4.1e16 - 3.2e17 He/cm2. Both of these analysis techniques reveal that the retained helium saturates in both single and polycrystalline W at ˜4e17 cm-2. The NDP analysis showed that the peak helium concentration shifted deeper into the specimens as the dose was increased, indicating a decrease in the effective density of the surface layer with an increased dose. Average retained helium concentrations were found to range from 0.7 - 8.6 at% in SCW and from 1.3 - 11.4 at% in PCW.

Near Term Applications of Inertial Electrostatic Confinement Fusion Research

For the past 15 years, the Inertial Electrostatic Confinement (IEC) fusion group at the University of Wisconsin-Madison has been conducting experiments to demonstrate that there can be many near term applications of fusion research long before the production of electricity in commercial fusion power plants. This research has concentrated on three fuel cycles: DD, D3He, and 3He3He. Some of the major accomplishments are listed below: a. The production of > 108 DD neutrons per second on a steady state basis b. The production of pulsed DD neutrons to over 1010 per second in 10Hz, 100 μs bursts. c. The production of 14.7 MeV protons at > 108 per second (steady state) from the D3He reaction. d. Demonstrated the detection of the explosive C-4 with steady state DD neutrons. e. Demonstrated the detection of Highly Enriched U (HEU) with pulsed DD neutron fluxes. f. Production of the positron emission tomography (PET) isotopes, 94mTc and 13Nusing D3He protons. g. Production of the first measured 3He3He fusion reactions in an IEC device. h. Development of unique diagnostic techniques to measure the rate, spectrum, and location of fusion reactions in IEC devices. i. Use of an IEC device to study the behavior of materials at high temperature during charged particle bombardment. The accomplishments above were carried out in 3 devices HOMER, 3HeCTRE, and HELIOS that have operated up to 180 kV and meter currents of 65 mA. New applications are currently being explored and expanded roles for the IEC device will be described.

The materials irradiation experiment for testing plasma facing materials at fusion relevant conditions

The Materials Irradiation Experiment (MITE-E) was constructed at the University of Wisconsin-Madison Inertial Electrostatic Confinement Laboratory to test materials for potential use as plasma-facing materials (PFMs) in fusion reactors. PFMs in fusion reactors will be bombarded with x-rays, neutrons, and ions of hydrogen and helium. More needs to be understood about the interactions between the plasma and the materials to validate their use for fusion reactors. The MITE-E simulates some of the fusion reactor conditions by holding samples at temperatures up to 1000 °C while irradiating them with helium or deuterium ions with energies from 10 to 150 keV. The ion gun can irradiate the samples with ion currents of 20 μA-500 μA; the typical current used is 72 μA, which is an average flux of 9 × 10(14) ions/(cm(2) s). The ion gun uses electrostatic lenses to extract and shape the ion beam. A variable power (1-20 W), steady-state, Nd:YAG laser provides additional heating to maintain a constant sample temperature during irradiations. The ion beam current reaching the sample is directly measured and monitored in real-time during irradiations. The ion beam profile has been investigated using a copper sample sputtering experiment. The MITE-E has successfully been used to irradiate polycrystalline and single crystal tungsten samples with helium ions and will continue to be a source of important data for plasma interactions with materials.

New Insight into Gridded Inertial Electrostatic Confinement (IEC) Fusion Devices

Abstract Gridded inertial electrostatic confinement (IEC) devices use a 10-200 kV voltage difference to accelerate ions through a 0.1-10 mTorr background gas in a spherical or cylindrical geometry. The detailed investigation of a gridded IEC device using DD fuel has resulted in several surprises that have greatly altered our perception of how these systems operate. It was found that there are at least 4 major misconceptions that have been in place for over 15 years on how such IEC systems operate. These misconceptions range all the way from what energetic ion is causing the majority of fusions, to the energy and charge state of the reacting ions. Experimental results will illustrate some of the surprising reactions that are taking place in DD gridded system.

Surface Pore Formation in Helium Implanted Fine-Grain Tungsten and Tungsten Needles as Engineered First Wall and Divertor Plate Materials

Abstract Surface morphology changes of sub-micron tipped tungsten needles (W.N.) and an engineered fine-grain tungsten (FGW) were studied after implantation with He ions at reactor relevant conditions. Surface and subsurface pore formation was observed on all of the samples by using scanning electron microscopy (SEM) and focused ion beam (FIB) milling. Additionally, helium retention analysis was performed on the FGW and compared to several previously studied W materials. Three samples of FGW were irradiated with 30 keV 3He ions to 3×1017 He+/cm2 at 700 °C, 9×1017 He+/cm2 at 850 °C, and 1×1019 He+/cm2 at 1050 °C. SEM analysis revealed that the threshold for visible pore formation was below ˜1018 He+/cm2. The sample irradiated to the highest fluence showed “coral-like” morphology on the surface, and FIB analysis showed that the sub-surface semi-porous layer extended almost one micron below the surface. The percentage of implanted helium retained in the samples ranged from 4.5-40%. Two W.N. were implanted with 100 keV 4He ions to conditions of 3×1018 He+/cm2 at 700 °C and 1.3x1019 He+/cm2 at 1000 °C. Extensive pore formation was observed on both specimens. FIB analysis revealed that a sub-surface semi-porous layer developed after ion implantation that extended ˜300 nm in the W.N. implanted to the lower dose, and over 1500 nm in the needle implanted to the higher dose. This second needle also exhibited a drastic morphology change, which appears to be a result of the unraveling of the grains at the tip.