David S. Walsh

Comparison of the thermal stability of the codeposited carbon/hydrogen layer to that of the saturated implant layer

Abstract This paper presents the results of an experimental study of the thermal stability in air and vacuum of the codeposited carbon/hydrogen layer formed in a carbon-lined fusion reactor. Results are also presented for the stability of the saturated layer formed by the implantation of energetic hydrogen ions into a graphite surface. For both films, the hydrogen isotope release occurs at a much lower temperature in air than it does in vacuum. At temperatures above 600 K, the hydrogen isotope release in air is very rapid and is emitted in a condensible form. It is speculated that isotopic exchange with water present in the air is responsible for this release. In vacuum, temperatures in excess of 1000 K are required to produce a rapid release from either film. The implications of these results to the safety of tritium in carbon-lined fusion reactors are discussed.

Single-event upset and snapback in silicon-on-insulator devices and integrated circuits

The characteristics of ion-induced charge collection and single-event upset are studied in silicon-on-insulator (SOI) transistors and circuits with various body tie structures. Impact ionization effects, including single-event snapback, are shown to be very important. Focused ion microbeam experiments are used to find single-event snapback drain voltage thresholds in n-channel SOI transistors as a function of device width. Three-dimensional device simulations are used to determine single-event upset and snapback thresholds in SOI SRAMs, and to study design tradeoffs for various body-tie structures. A window of vulnerability to single-event snapback is shown to exist below the single-event upset threshold. The presence of single-event snapback in commercial SOI SRAMs is confirmed through broadbeam ion testing, and implications for hardness assurance testing of SOI integrated circuits are discussed.

Codeposition of deuterium with beryllium

Abstract We have measured the codeposition and/or coimplantation of deuterium with beryllium. Beryllium, sputtered from a target disk, was collected on a heated silicon catcher plate where it was simultaneously bombarded by reflected deuterons. Oxygen to beryllium ratios in the layer varied between 0.03 and 0.13. Deuterium to beryllium ratios dropped from 0.15 at 373 K to 0.02 at 573 K.

Single event upset imaging with a nuclear muprobe

Abstract An entirely new ion beam analysis technique is described: single event upset (SEU) imaging. SEU-imaging utilizes the scanning of a mu-focused MeV ion beam across an integrated circuit. This beam generates both electrons and logic state changes which are monitored by a computer. The data is collected in a way that permits the generation of visual images which depict both the physical appearance of the scanned region (through the ion-induced electron signals) and the areas of the IC which are susceptible to upset (through detection of chip malfunctions). Comparison of these images with the chip design facilitates matching the individual transistor components with the upset-sensitive region. While our initial results with 1 μm resolution ion beams have demonstrated the viability of this new technique in directly identifying the sources of upset in mun-scale integrated circuits, the trend toward submun feature size will necessitate higher-resolution muprobes and improved appearance-imaging systems in future applications of this new technique.

Flux and fluence dependence of H+ trapping in graphite

Abstract Retention of 1 keV hydrogen and deuterium ions in carbon was measured as a function of temperature (300–900 K), fluence (1017 to 1021 ions/cm2) and flux density (−1014 to 1016 ions/cm2 s) for different types of graphite: pyrolytic, pseudo-monocrystal and fine grain graphite. The amount of hydrogen retained in the specimens was measured by thermal desorption spectrometry (TDS) and nuclear reaction analysis (NRA). NRA measurements for pyrolytic graphite indicate that the amount of hydrogen trapped in the near-surface region initially increases with fluence and then reaches a saturation level. While our NRA and TDS results are in good agreement for fluences below which surface saturation sets in, the TDS measurements continue to increase with increasing incident fluence. The extent of this increase was, however, found to be sensitive to the type of graphite used. Similar fluence dependence was observed for different graphite temperatures, although the absolute level of retained hydrogen dropped an order of magnitude as the temperature increased from 300 to 900 K. Within the errors of the experiment, no dependence on the incident flux density was observed for the amount of retained hydrogen.

Tritium detection in plasma facing component by imaging plate technique

Tritium imaging plate technique (TIPT) has been successfully applied to measure the tritium areal distribution on various graphite tiles used as limiters in TEXTOR. It is observed that tritium distribution on the ALT-II tile is quite homogeneous and different from deuterium distribution and the tritium in redeposited layer is rather small. Such tritium distribution on the graphite tiles in TEXTOR behaves different compared to those in JET and TFTR where tritium was used as fueling gas or NBI injection. In JET and TFTR the tritium is part of the fuel and is co-deposited and retained in a similar manner as the deuterium. In a device like TEXTOR, the high-energy tritons are decoupled from the thermalized deuterons and show different behavior of retention; the main retention mechanism is deep implantation rather than co-deposition with eroded carbon on redeposition-dominated areas. It is also found that the tritium distribution measurements give useful new information on plasma-wall interactions.

The Si surface yield as a calibration standard for RBS

Abstract The Rutherford backscattering spectroscopy (RBS) surface height of a pure bulk material can be used as an absolute standard value to calibrate the detector solid angle. This work presents the results of an international collaboration started at the beginning of 1998 to define the surface height of the RBS spectrum ( H 0 ) of Si, amorphized by ion implantation to avoid channeling. The analyses were performed with 1–3 MeV He beams and 170° scattering angle. The detector solid angle was estimated in the different laboratories either by geometrical measurement or by a calibrated standard. The agreement of the experimental H 0 values is of the order ±2%, the claimed accuracy for RBS. The results are also consistent at 2% level with both the stopping power measurements of Konac et al. (1998), and the measurements of Lennard et al. (1999).

Runaway electron damage to the Tore Supra Phase III outboard pump limiter

Operation of the Phase III outboard pump limiter (OPL) in Tore Supra in 1994 was terminated prematurely when runaway electrons during the current decay following a disruption pierced leading edge tube on the electron side and caused a water leak. The location, about 20 mm outside the last closed flux surface during normal operation, and the infrared (IR) images of the limiter indicate that the runaways moved in large outward steps, i.e. tens of millimeters, in one toroidal revolution. For plasma (runaway) currents in the range of 155 to 250 kA, the drift orbits open to the outside. Basic trajectory computations suggest that such motion is possible under the conditions present for this experiment. Activation measurements made on sections of the tube to indicate the area of local damage are presented here. An understanding of this event may provide important guidance regarding the potential damage from runaways in future tokamaks.

The structure, properties and performance of plasma-sprayed beryllium for fusion applications

Plasma-spray technology is under investigation as a method for producing high thermal conductivity beryllium coatings for use in magnetic fusion applications. Recent investigations have focused on optimizing the plasmaspray process for depositing beryllium coatings on damaged beryllium surfaces. Of particular interest has been optimizing the processing parameters to maximize the through-thickness thermal conductivity of the beryllium coatings. Experimental results will be reported on the use of secondary H2 gas additions to improve the melting of the beryllium powder and negative transferred-arc cleaning to improve the bonding between the beryllium coatings and the underlying surface. Information will also be presented on thermal cycle tests which were done on beryllium coated ISX-B beryllium limiter tiles using 10s cycle times with 60s cooldowns using a heat flux slightly in excess of 5 MW/m2.

Ion microbeam studies of cadmium zinc telluride radiation detectors by IBICC

Abstract Ion beam induced charge collection (IBICC) and time resolved IBICC (TRIBICC) techniques were used for imaging electronic properties of cadmium zinc telluride (CZT) room temperature radiation detectors. The detectors were bombarded with a scanned 5.4 MeV He microbeam and the detector response was analyzed at each point. The electron mobility ( μ e ) and lifetime ( τ e ), and charge collection efficiency maps were calculated from the data. In order to determine the radiation damage to the detectors, the signal deterioration was measured as the function of dose.