David Donovan

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.

Plasma source development for fusion-relevant material testing

Plasma-facing materials in the divertor of a magnetic fusion reactor have to tolerate steady state plasma heat fluxes in the range of 10 MW/m2 for ∼107 s, in addition to fusion neutron fluences, which can damage the plasma-facing materials to high displacements per atom (dpa) of ∼50 dpa. Materials solutions needed for the plasma-facing components are yet to be developed and tested. The material plasma exposure experiment (MPEX) is a newly proposed steady state linear plasma device designed to deliver the necessary plasma heat flux to a target for testing, including the capability to expose a priori neutron-damaged material samples to those plasmas. The requirements of the plasma source needed to deliver the required heat flux are being developed on the Proto-MPEX device which is a linear high-intensity radio-frequency (RF) plasma source that combines a high-density helicon plasma generator with electron- and ion-heating sections. The device is being used to study the physics of heating overdense plasmas i...

A multi-technique analysis of deuterium trapping and near-surface precipitate growth in plasma-exposed tungsten

In this work, we examine how deuterium becomes trapped in plasma-exposed tungsten and forms near-surface platelet-shaped precipitates. How these bubbles nucleate and grow, as well as the amount of deuterium trapped within, is crucial for interpreting the experimental database. Here, we use a combined experimental/theoretical approach to provide further insight into the underlying physics. With the Tritium Plasma Experiment, we exposed a series of ITER-grade tungsten samples to high flux D plasmas (up to 1.5 × 1022 m−2 s−1) at temperatures ranging between 103 and 554 °C. Retention of deuterium trapped in the bulk, assessed through thermal desorption spectrometry, reached a maximum at 230 °C and diminished rapidly thereafter for T > 300 °C. Post-mortem examination of the surfaces revealed non-uniform growth of bubbles ranging in diameter between 1 and 10 μm over the surface with a clear correlation with grain boundaries. Electron back-scattering diffraction maps over a large area of the surface confirmed th...

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.

Heat flux estimates of power balance on Proto-MPEX with IR imaging

The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) at Oak Ridge National Laboratory (ORNL) is a precursor linear plasma device to the Material Plasma Exposure eXperiment (MPEX), which will study plasma material interactions (PMIs) for future fusion reactors. This paper will discuss the initial steps performed towards completing a power balance on Proto-MPEX to quantify where energy is lost from the plasma, including the relevant diagnostic package implemented. Machine operating parameters that will improve Proto-MPEXs performance may be identified, increasing its PMI research capabilities.

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.

Measuring D(d,p)T fusion reactant energy spectra with Doppler shifted fusion products

Deuterium fusion reactant energy spectra have been measured using a diagnostic that records the Doppler shift imparted to charged particle fusion products of the D(d,p)T reaction by the center-of-mass velocity of the deuterium reactants. This diagnostic, known as the fusion ion Doppler shift diagnostic (FIDO) measures fast deuterium energy spectra in the inertial electrostatic confinement (IEC) experiment at the University of Wisconsin–Madison {Santarius et al. [Fusion Sci. Technol. 47, 1238 (2005)]}, a device to confine high energy light ions in a spherically symmetric, electrostatic potential well. This article details the first measurements of the fusion reactant energy spectra in an IEC device as well as the design and principles of operation of the FIDO diagnostic. Scaling of reactant energy spectra with a variety of experimental parameters have been explored.

Optimization of an IEC Fusion Device to Increase Steady-State D-D Neutron Generation Rates

The University of Wisconsin-Madison Inertial Electrostatic Confinement (IEC) Fusion Research Group has been performing experiments on an IEC device known as HOMER. This device is a 65cm high, 91cm diameter cylindrical aluminum vacuum chamber that contains two concentric spherical wire grids, the outer grid acting as the anode and the inner grid as the cathode. The potential difference between the anode and cathode drives ions towards the center of the grids. Using this device, steady-state D-D fusion reactions are created in order to produce 2.45 MeV neutrons. With the goal of achieving maximum neutron production rates, the following parameters have been varied: cathode voltage, ion current, operating pressure, and the separation distance between the anode and cathode. The studies on pressure, voltage, and current have led to the discovery of trends that allow for the extrapolation of neutron rates at various conditions. The cathode/anode separation studies have offered valuable insight into how the distance between the electrodes effects the concentration of deuterium molecular ions and the ion energy spectra, and has led to the implementation of a configuration that better maximizes neutron production rates.

Measuring time of flight of fusion products in an inertial electrostatic confinement fusion device for spatial profiling of fusion reactions

A new diagnostic has been developed that uses the time of flight (TOF) of the products from a nuclear fusion reaction to determine the location where the fusion reaction occurred. The TOF diagnostic uses charged particle detectors on opposing sides of the inertial electrostatic confinement (IEC) device that are coupled to high resolution timing electronics to measure the spatial profile of fusion reactions occurring between the two charged particle detectors. This diagnostic was constructed and tested by the University of Wisconsin-Madison Inertial Electrostatic Confinement Fusion Group in the IEC device, HOMER, which accelerates deuterium ions to fusion relevant energies in a high voltage (∼100 kV), spherically symmetric, electrostatic potential well [J. F. Santarius, G. L. Kulcinski, R. P. Ashley, D. R. Boris, B. B. Cipiti, S. K. Murali, G. R. Piefer, R. F. Radel, T. E. Radel, and A. L. Wehmeyer, Fusion Sci. Technol. 47, 1238 (2005)]. The TOF diagnostic detects the products of D(d,p)T reactions and determines where along a chord through the device the fusion event occurred. The diagnostic is also capable of using charged particle spectroscopy to determine the Doppler shift imparted to the fusion products by the center of mass energy of the fusion reactants. The TOF diagnostic is thus able to collect spatial profiles of the fusion reaction density along a chord through the device, coupled with the center of mass energy of the reactions occurring at each location. This provides levels of diagnostic detail never before achieved on an IEC device.

Characterization of He-Induced Bubble Formation in Tungsten due to Exposure from an Electron Cyclotron Resonance Plasma Source

Abstract A compact electron cyclotron resonance plasma source has been utilized at Sandia National Laboratory to expose heated W samples (1270 K) to 50–75 eV He ions at fluxes on the order of 1019 m−2 s−1 and fluences on the order of 1024 m−2. Scanning electron microscopy (SEM) analysis of the surface has indicated bubbles up to 150 nm in diameter that exhibit signs of bursting near the surface. Comparisons have been made between W samples prepared from warm-rolled W sheet stock and ITER-Grade W rod stock. Focused ion beam (FIB) cross sectioning has been used with SEM and transmission electron microscopy (TEM) to identify large sub surface bubbles (100 nm diameter) at depths up to one micron as well as a dense layer of smaller bubbles (<10 nm diameter) within the first 100 nm of the surface, similar to bubble layers observed on higher flux experiments. SEM-Electron Backscatter Diffraction (EBSD) analysis has identified a unique surface morphology feature associated with the exposed ITER-Grade W as well as features similar to previous EBSD studies of rolled W stock. Thermal desorption spectroscopy (TDS) analysis has identified that pre-existing He bubbles found in the Sandia He-ion exposed samples do alter the D trapping and desorbing behavior in W. The findings from these preliminary characterization studies are presented and discussed in context with results from similar plasma exposure stages at other facilities around the world.