Material damage in graphite by run-away electrons

The importance of radiation damage in materials for use in nuclear power reactors is emphasized and an account is given of various types of radiation damage in such materials. The displacement of atoms by nuclear radiation is briefly discussed and examples are shown of recent electron microscopical photographs of radiation damage in materials. A discussion is given of radiation damage in graphite and the importance of stored energy as a reactor problem. The effect of displaced atoms in producing radiation hardening in metals and radiation embrittlement in structural steels is emphasized and discussed in relation to reactor pressure-vessels. Problems of growth, wrinkling, creep, and inert fission gases in uranium are considered in relation to the life of reactor fuel elements.

An application study of modern pixel semiconductor detectors for characterization of runaway electron events in tokamaks is presented. Characterization techniques utilizing both spectroscopic measurements and monitoring of the intensity of secondary X-rays produced by the runaway electrons were used. Energy spectra of X-rays and time evolutions of their intensity on two tokamaks (Golem and Compass) were measured under different conditions and compared with results of standard runaway diagnostics. The energy spectra measured on both tokamaks have similar exponential shapes but with a significant variation in numbers of events per shot. The time evolutions of the X-ray intensity during several discharges on the tokamak Golem were measured using both the Timepix3 device and scintillation detectors (NaI:Tl and YAP:Ce). On a microsecond time scales, the signal time evolution measured by the TimePix3 device shows patterns in a form of unexpected or periodic-like increases of the intensity. We have also observed significant differences in number of events of the detected X-rays generated by the runaway electrons flying frontward and backward with respect to a limiter of the tokamak Golem. This fact declares that the runaway electrons have relativistic velocities. The experiments on the tokamak Compass provide similar results. Measurements in the immediate vicinity of tokamak Compass were impossible to perform because of a rapid change of the tokamak magnetic field. Measurements performed in the distance of at least 0.5 m from a diagnostic port of the tokamak Compass gave millions of correctly measured events per shot and an unknown number of events affected by pileups. The correctly measured events were used for construction of energy spectra and the time evolutions of the X-ray intensity.

Studies of runaway electrons in present day tokamaks are essential to improve theoretical models and to support possible avoidance or suppression mechanisms in future large-scale plasma devices. Some of the phenomena associated with the runaway electrons take place at faster time scales, and thus it is essential to probe the runaway electrons to investigate underlying physics. The present article reports a few experimental observations of runaway electron associated events, at fast time scales, using a state-of-the-art multi-detector system developed at the Ioffe Institute and recently deployed on the TUMAN-3M tokamak. The system is based on the high-performance scintillation gamma-ray spectrometers for measurements of bremsstrahlung generated during the interaction of accelerated electrons with plasma and materials of the tokamak chamber. It includes a total three detectors configured in the spectroscopic mode having different lines of sight. Along with this hardware, dedicated algorithms were developed and validated that enables the separation of piled-up pulses, maximize the dynamic range of the detector and provides a counting rate as high as 107 counts per second. The inversion code, DeGaSum, has been used for the reconstruction of a runaway electron energy distribution function from the measured gamma-ray spectra. Using this tool, experimental analysis of the runaway electron beam generation and evolution of their energy distribution in the TUMAN-3M representative plasma discharges is performed. The effect on gamma-ray count rate during the magnetohydrodynamic activities and possible changes in the runaway electron energy distribution function during sawtooth oscillations is discussed in detail. Possible maximum limit of the runaway electron energy in TUMAN-3M is investigated and compared with the numerical analysis. In addition, the probability of the runaway electron generation throughout the plasma discharge is estimated analytically and compared with the experimental observation that suggests a balance between production and loss of the runaway electrons.

It has been found necessary to perform a series of first-wall conditioning steps prior to successful high power plasma operation in the Tokamak Fusion Test Reactor (TFTR). This series begins with glow discharge cleaning (GDC) and is followed by pulse discharge cleaning (PDC). During machine conditioning, the production of impurities is monitored by a Residual Gas Analyzer (RGA). PDC is made in two distinct modes: (1) Taylor discharge cleaning (TDC), where the plasma current is kept low (15--50 kA) and of short duration (50 ms) by means of a relatively high prefill pressure, and (2) aggressive PDC, where lower prefill pressure and higher toroidal field result in higher current (200--400 kA) limited by disruptions at q(a){approx}3 at {approx}250 ms. At a constant repetition rate of 12 discharges/minute, the production rate of H{sub 2}O, CO, or other impurities has been found to be an unreliable measure of progress in cleaning. However, the ability to produce aggressive PDC with substantial limiter heating, but without the production of X-rays from runaway electrons, is an indication that TDC is no longer necessary after {approx}10{sup 5} pulses. During aggressive PDC, the uncooled limiters are heated by the plasma from the bakeout temperature of 150 {degree}Cmore » to about 250 {degree}C over a period of three to eight hours. This limiter heating is important to enhance the rate at which H{sub 2}O is removed from the graphite limiter.« less

The material of limiter in HT-7 tokamak was changed from graphite to molybdenum in the last experimental campaign. The pitch angle scattering of runaway electrons due to anomalous Doppler resonance effects was observed. The experimental results agree very well with the stable boundary condition expected from the linear resistive theory but only agree with that from the nonlinear evolutionary of runaway-electron distribution theory in low electric field region. The current carried by runaway electrons is the same under different limiter conditions.

Osteoinductive potential and bone-bonding ability of ProRoot MTA, MTA Plus and Biodentine in rabbit intramedullary model: Microchemical characterization and histological analysis

Erosion of metals and carbon based materials during disruptions — simulation experiments in plasma accelerators

Growth, characterization and photoconduction properties of Sb0.1Mo0.9Se2 single crystals grown by DVT technique

The mechanisms of propellant gas chemical erosion on gun barrel materials are discussed creatively from a microscopic perspective of molecular diffusion and intermolecular interaction. The influence of high-temperature and high-pressure propellant gas atoms on the shallow surface microstructure of the barrel material is theoretically analyzed using the molecular dynamics simulation method, which can be used as a new idea for further study of the gun barrel erosion mechanism. The results show that the diffusion of the propellant gas atoms into the barrel material can be characterized by the significant longitudinal distribution characteristics of propellant gas atoms and the normal stress distribution characteristics in the surface layer of the barrel. Moreover, the diffusion ability of the propellant gas atoms in the surface of the barrel material decreases gradually with the increasing diffusion depth. Under the action of propellant gas atoms, the dislocation structure dominated by face-centered cubic structure (FCC) forms in the shallow surface of the barrel material. The production of initial erosion in the shallow surface of barrel materials can be predicted by the continual growth of the new structure and the stress distribution characteristics of the material. In addition, the temperature variation of the propellant gas contributes significantly to barrel material erosion.

We report on a direct spatial profile measurement of optical field intensity issuing from subwavelength slits flanked by periodic grooves and fabricated on a thin metallic layer. This type of structure is of interest for the manipulation of cold atoms by optical potentials near surfaces.

Nonlinear damage in the composite materials is developed with the growth of damages in the material under fatigue loading. Nonlinear ultrasonic techniques are sensitive to early stage damages such as, fiber breakages, matrix micro-cracking, and deboning etc. Here, in this work, early stage damages are detected in Unidirectional (UD) carbon fiber composite under fatigue loading. Specimens are prepared according to American Society for Testing and Materials (ASTM) standard. Specimens are subjected to low cycle high load (LCHL) fatigue loading until 150,000 cycles. Sensors are mounted on the specimen used for actuation and sensing. A five count tone burst with low frequency (fc =375 kHz) followed by high frequency (fc =770 kHz) signal, was used as actuation signal. Pitch-catch experiments are collected at the interval of 5,000 cycles. Sensor signals are collected for various excitation voltage (from 5V to 20V, with 5V interval). First Fourier Transform (FFT) of the sensor signals are performed and side band frequencies are observed at around 770 kHz. Severity of damages in the material is quantified from the ratio of amplitude of side band frequencies with the central frequency. Nonlinearity in the material due to damage development is also investigated from the damage growth curve obtained at various excitation amplitude. Optical Microcopy imaging were also performed at the interval of 5,000 to examine developments of damages inside the material. This study has a good potential in detection of early stage damages in composite materials.

Magnetic field effects on runaway electron energy deposition in plasma facing materials and components have been investigated using the Integrated TIGER Series. A 3D computational model of 100-MeV electrons incident on a graphite block was used to simulate runaway electrons striking a plasma facing component at the edge of a tokamak. An embedded magnetic field of 1, 3, and 5 T intersecting the front face of the graphite block at 1 degrees and 5 degrees was studied. Analysis shows that a plasma facing component in the presence of a magnetic field will have more energy deposited in it than it would in no field. Most of the increased energy is deposited near the surface with a higher energy density. This is due primarily to a reduction in the reflection of electrons from the surface, resulting in a substantial increase in surface energy deposition. A second, but smaller effect is that knock-on electrons born in the material attempt to orbit the embedded magnetic field lines, resulting in a localized energy deposition (closer to the surface). The higher energy deposition and higher energy density will cause higher temperatures which are expected to increase material damage.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Comparisons of single pulse discharge crater geometries in EDM and EAM

A plasma drag hypervelocity particle accelerator (HYPER)

Large-grained (111)-oriented CdTe thin films grown by “quasi-rheotaxy” on steel substrates