Fast Particle Beams Research Articles

Kinetic Phenomena in a Semiconductor Excited by an Oriented Beam of Fast Particles

Kinetic Phenomena in a Semiconductor Excited by an Oriented Beam of Fast Particles

The Use of Synchrotron Radiation for Studying Superconducting Nb3Sn Samples Irradiated by Fast Particle Beams

The influence of proton irradiation on the structure of Nb3Sn superconducting materials has been investigated. The irradiation to different proton doses was performed on a cyclotron at the National Research Centre “Kurchatov Institute” (NRC KI). The Nb3Sn samples were placed successively pairwise on the beam axis to obtain different degrees of radiation-induced damage at a specified dose. X-ray diffraction (XRD) analysis of irradiated and unirradiated samples was carried out on a synchrotron radiation source at the NRC KI. The results of studies revealed that the diffraction peaks broaden and become weaker with an increase in the concentration of radiation-induced defects. The phase compositions of the samples and dynamics of changes in the found NbO phase under irradiation have been investigated. The change in the lattice period for the irradiated samples is studied. It is shown that the lattice period increases with an increase in concentration of radiation-induced defects.

The In-Vessel Protection Components for ITER First Plasma Operation

ITER plasma operations begin with “first plasma,” with the main purpose of concluding integrated tokamak commissioning. This first plasma is targeted to last at least 100 ms with a 100-kA minimum plasma current. As in other tokamaks, the first plasma occurs prior to full installation of the baseline in-vessel components. For example, the blanket and divertor are installed only after the first plasma experiments are completed. Thus, several temporary components are installed to protect the vacuum vessel (VV) and other already-installed high-value components (in-vessel coils and cable looms) from the potentially adverse effects of startup plasma interaction. This protection is in part provided by temporary limiters (TLs) and divertor replacement structures (DRSs), which together create poloidal and toroidal guides to shelter the VV from the plasma and possible fast particle beams. The 72 TL segments are positioned at the first wall contour in four poloidal loops of 18 segments each. For engineering safety, the system is capable of maintaining plasma pulses of up to 3 s, tolerating up to 30 MJ of thermal energy and 1 MA of plasma current. DRSs complete the bottom periphery of the poloidal loop to interrupt any potential downward plasma movement. The vessel also requires protection from the electron cyclotron resonance heating (ECRH) beam, which injects 6.7 MW of microwave energy across the plasma null region to help energize and assist the breakdown of first plasma. To protect the VV, an inboard mounted mirror reflects the ECRH beam energy from its upper port origin into an outboard equatorial port, where a second component (a beam dump) absorbs the radiation.

Hybrid Fourier–Vlasov simulation in non-inertial reference frames

We present some useful extensions of the spectral Fourier–Vlasov algorithm for simulations of interactions of collisionless plasmas with ion beams. For many practical applications the relative drifts of various particle populations require high resolution of particle distribution functions (PDFs) or the use of large phase space domain, which makes the simulations extremely memory- and time-consuming. We propose using non-inertial reference frames moving in the velocity dimensions for the beam particle distribution functions. As a result, it is possible to simulate plasma–beam interactions at a much lower resolution. This method is particularly suitable for simulations of fast particle beams and plasmas with heavy ion species or cold particle populations. In addition, for simulations of strongly nonlinear instabilities which cause strong plasma heating, the adaptive mesh refinement and phase space reduction are proposed. Contrary to the Vlasov simulation in the real velocity space, plasma heating in the Fourier-inverted space leads to the PDF profile shrinking. Thus, instead of having to extrapolate the PDF into the regions where it was previously undefined, in the Fourier-space, it is sufficient to interpolate the pre-existing solution.

Spiky Fine Structure of Type III-like Radio Bursts in Absorption

An uncommon fine structure in the radio spectrum consisting of bursts in absorption was observed with the Chinese Solar Broadband Radiospectrometer (SBRS) in the frequency range of 2.6 – 3.8 GHz during an X3.4/4B flare on 13 December 2006 in active region NOAA 10930 (S05W33). Usual fine structures in emission such as spikes, zebra stripes, and drifting fibers were observed at the peak of every new flare brightening. Within an hour at the decay phase of the event we observed bursts consisting of spikes in absorption, which pulsated periodically in frequency. Their instantaneous frequency bandwidths were found to be in the 75 MHz range. Moreover, in the strongest Type III-like bursts in absorption, the spikes showed stripes of the zebra-pattern (ZP) that drifted to higher frequencies. All spikes had the duration as short as down to the limit of the instrument resolution of ≈8 ms. The TRACE 195 A images indicate that the magnetic reconnection at this moment occurred in the western edge of the flare loop arcade. Taking into account the presence of the reverse-drifting bursts in emission, in the course of the restoration of the magnetic structures in the corona, the acceleration of the beams of fast particles must have occurred both upward and downward at different heights. The upward beams will be captured by the magnetic trap, where the loss-cone distribution of fast particles (responsible for the emission of continuum and ZP) were formed. An additional injection of fast particles will fill the loss-cone later, breaking the loss-cone distribution. Therefore, the generation of continuum will be quenched at these moments, which was evidenced by the formation of bursts in absorption.

Peculiarities of induced radiation and controlled dechanneling of relativistic particles in crystals under the Cherenkov—Vavilov extreme condition

Peculiarities of induced radiation and absorption of hard electromagnetic radiation are considered on the basis of the channeling of relativistic particles in crystals under the Vavilov—Cherenkov extreme condition. It is shown that the consistent inclusion of quantum recoil in the relativistic region makes it possible to produce a number of anomalous effects having no analogs in optics. In particular, it is predicted that it is possible to simultaneously cool the beam of fast particles during radiation and absorption and to create a tandem laser for which successive radiation of quasicharacteristic-radiation quanta at the normal and anomalous Doppler effect leads to the restoration of the initial state of the quantum system and to the multiple repetition of the radiation cycle. It is shown that the main difficulties preventing these effects from being produced directly in the X-and gamma-ray spectral regions are related to the necessity to use media with positive dispersion. It is considered that such dispersion can be produced by changing the effective susceptibility of the medium in the case of radiation diffraction.

MHD processes during the cascade development of the neck and hot spot in an X-pinch

Results are presented from two-dimensional MHD simulations of X-pinch implosion. The simulations were performed in the (r, z) and (x, y) geometries for homogeneous (dense plasma) and heterogeneous (core-corona) loads. The formation of a minidiode, the development of a neck and an X-radiating hot spot, and the influence of the plasma corona on the implosion dynamics of the dense X-pinch plasma were investigated. For through simulations, the conical neck model was used, whereas a detailed analysis of the X-ray burst was performed in the parabolic neck model. The MHD processes occurring during the implosion of oblique shock waves and the onset of instability of the plasma column were examined. It is found that, due to the quasi-periodic character of these processes, the neck compression proceeds in a cascade fashion. The plasma state in a hot spot just before the break of the neck is analyzed, and the possibility of generating fast particle beams is considered.

Bright spots generated by lower hybrid waves on JET

Observations of bright spots on the JET divertor aprons during lower hybrid current drive experiments are described. These bright spots are important because they can potentially cause damage to large tokamaks. The bright spots arise due to the impact of a fast particle beam. This beam originates from the front of the lower hybrid launcher, where thermal particles are accelerated according to theory by interaction with the high spatial harmonics of the lower hybrid wave. The bright spots are clearly related to the lower hybrid power as they disappear when the lower hybrid power is switched off. According to the analysis versus various parameters, the brightness of the spots clearly decreases with increasing plasma–wall distance, i.e. the distance between the last closed flux surface and the poloidal limiter. This is clearly beneficial for ITER, as it is designed to operate at a large plasma–wall distance.

MHD effects triggered by beams of fast particles in magnetic tubes and their possible relation to plasma heating and dynamics during solar flares

We study the magnetohydrodynamic response of a plasma in the low solar atmosphere to a changing current system of a flaring magnetictube, which contains a beam of fast non-thermal electrons. The local disturbances of a current system of a magnetic tube when the beam is injected into it are estimated using the classical idea of a return current. According to this idea, after injection of abeam, the total current density in a magnetic tube, which includes as well the current density of the beam jb, should not change compared to the current density in the tube before the injection. In order to keep constant the total current density j=j'+ jb, the current density of the magnetic tube j'in fact changes. This change is due to a return current jr.c.= –jb, which compensates the current density of the injected beam of fast electrons. At the same time, any changes of the current density in the magnetic tube change the Joule heating and disturb the thermodynamic equilibrium of the system. Changing of the plasma temperature destroys also the force balance and starts the process of complex dynamics of the whole plasma-magnetic structure. The impulsive character of a beam injection causes two stages in the dynamic behavior of the tube. During the first stage, characterized by the presence of a beam, the preliminary equilibrium state of a magnetic tube is disturbed and complex dynamics of the plasma start in the region of the beam propagation. During the second stage, when the injection of the beam is already over, the plasma and magnetic field continue to evolve from the disturbed state and gradually relax to an equilibrium state. Various types of magnetic tube response onto injection of a beam of energetic electrons are studied using the dynamic models of the magnetic tube (Khodachenko, 1996a; 1996b) built on the basis of known self-similar solutions of plasma MHD. The model results are applied to the interpretation of observed flaring and burst phenomena.

Ion-sound model of microwave spikes with fast shocks in the reconnection region

A new model for solar spike bursts is considered based on the interaction of Langmuir waves with ion-sound waves: l+s→t. Such a mechanism can operate in shock fronts, propagating from a magnetic reconnection region. New observations of microwave millisecond spikes are discussed. They have been observed in two events: 4 November 1997 between 05:52–06:10 UT and 28 November 1997 between 05:00–05:10 UT using the multichannel spectrograph in the range 2.6–3.8 GHz of Beijing AO. Yohkoh/SXT images in the AR and SOHO EIT images testify to a reconstruction of bright loops after the escape of a CME. A fast shock front might be manifested as a very bright line in T e SXT maps (up to 20 MK) above dense structures in emission measure (EM) maps. Moreover one can see at the moment of spike emission (for the 28 November 1997 event) an additional maximum at the loop top on the HXR map in the AR as principal evidence of fast shock propagation. The model gives the ordinary mode of spike emission. Sometimes we observed a different polarization of microwave spikes that might be connected with the depolarization of the emission in the transverse magnetic field and rather in the vanishing magnetic field in the middle of the QT region. Duration and frequency band of isolated spikes are connected with parameters of fast particle beams and shock front. Millisecond microwave spikes are probably a unique manifestation of flare fast shocks in the radio emission.

Small-scale electron density and magnetic-field structures in the wake of an ultraintense laser pulse.

We investigate the interaction of a high intensity ultrashort laser pulse with an underdense collisionless plasma in the regime where the Langmuir wake wave excited behind the laser pulse is loaded by fast particle beams, formed during the wake wave breaking. The beam loading causes the deterioration of the central part of the wake wave near the pulse axis, and the formation of bunches of sharply focalized ultrarelativistic electrons. The bunches of electrons generate a fast propagating magnetic field, which we interpret in terms of the magnetic component of the Lienard-Wiechert potential of a moving electric charge.

Precision lifetime measurements on alkali atoms and on helium by beam–gas–laser spectroscopy

The results of a new series of precision lifetime measurements on alkali atoms by means of beam–gas–laser spectroscopy are reported and discussed. The lifetimes of the first excited states in lithium, sodium, potassium and rubidium were measured with uncertainties ranging from ±0.14% to ±0.25%. The results are discussed in comparison with other recent experimental and theoretical work. The longstanding discrepancy between ab initio line strength calculations and the measurements of Gaupp et al. [Phys. Rev. A26, 3351 (1982)] for the resonance transitions in lithium and sodium is resolved by the new measurements in favour of the theoretical calculations. Finally the role of the beam divergence in lifetime measurements on fast particle beams is discussed and a method to correct for this effect is presented. The importance of the beam divergence effect is illustrated by additional recent lifetime measurements on helium and on lithium.

Use of laser-ablated fast particle beams for the measurement of n e and T e profiles in the TEXTOR boundary layer

We have developed a source of suprathermal atoms consisting of a laser system, which ablates small portions of coating materials from the rear side of a glass substrate. The coating can be either a single species or a multilayer of several-thousand-angstrom thickness. Neutral particle beams produced by this method can have energies in the range of about 10 eV and penetrate into the plasma up to line densities of about nel=1×1013 cm−2. For the determination of the electron temperature we have chosen two elements, the ionization and excitation cross sections of which differ in the range up to 100 eV. For TEXTOR a two-component target with a C-LiF coating is used, which delivers a Li and C beam with about 5- and 15-eV energy, respectively, ablated by a ruby laser with 1-J energy. The beam attenuation is determined from the emitted light by using a camera with a Si-diode array system for spatial recording. The different attenuations and emission characteristics then allow the determination of radial electron density and temperature profiles over the spatial range from the liner into a few centimeters beyond the limiter.

Cellular Fast-Mixing Techniques: Possible Applications with Particle Beams

In the past cellular fast-mixing techniques have been used to investigate the time resolution of radiation processes that lead to modification of radiation response in bacterial and mammalian cellular systems. So far, published studies have been confined to effects with low-LET electron beams. The brief for this paper was to discuss where, and under what conditions, such a technique could be used to advantage with high-LET particle beams. Criteria for the experimental design, including conditions of flow rate, dose rate, and mixing times, are discussed. Radiobiological problems appropriate for applications of fast-particle beams are also discussed. These include studies to reveal possible multicomponents in cellular sensitization by oxygen and electron-affinic radiation sensitizers, studies designed to assist in the resolution of direct and indirect effects, and resolution of intracellular DNA damage.

Measurement and direct regulation of the velocity of a fast beam using doppler-tuned laser photon-atom interactions

An optical method is described that uses the Doppler shift to measure the velocity of a fast particle beam merged with a photon beam from a laser. The Doppler-tuned resonance signal is used in a feedback loop to regulate directly the velocity of the beam. The method is demonstrated using a beam of 4He atoms pumped by CO 2 laser photons into high Rydberg pure quantum states. Applications to other areas of fast beam physical research are discussed.

Strahlabtastsystem fu¨r teilchenbeschleuniger

A beam-scanning device is described which is able to measure the position, current and the profile of a beam of fast particles. The scanning is done by a rotating ellipsoidal-formed tungsten-wire which scans alternatively the x- and y-coordinate of the beam. The probe-current is amplified and used to demonstrate the beam profiles in both coordinates on a normal oscilloscope. Out-of-center position of the beam shows up in double peaks. By using two auxiliary electrodes on different potentials, placed before and behind the rotating probe, the beam position is clearly determined. The set-up used for the beam positioning of our Van de Graaff generator consists of three probes located at different places of the beam tube and rotating synchronously with the mains frequency of 50 Hz and a phase-shift of 120° to each other in order to avoid “shadow effects”.

STUDIES OF SHORT-RANGE INTERMOLECULAR FORCES

This review presents the fundamentals of the method of determining the short-range interaction potentials of atoms and molecules by means of scattering of beams of fast particles. Original results obtained in recent years from studying more than 50 different systems are presented. The obtained potentials are analyzed and compared with independent semiempirical and theoretical calculations. The outlook for future studies is discussed.

The dipole spectrum and properties of helium

Analytical representations of the dipole spectrum of the ground state of helium are derived. From them are calculated the Verdet constant, the mean excitation energies I and I1 characterizing stopping and straggling of a beam of fast particles absorbed in helium and the leading coefficient C(a, b)/R6 of the long-range interaction between atoms a and b a distance R apart. I is 42.0 ev, I1 is 73 ev, C(He, He) is 1.471 A.U. and C(He, H) is 2.830 A.U.

Power Calorimetry of Fast-Particle Beams

Equations are developed for the temperature behavior of a target struck by a particle beam. The temperature change is obtained as a function of time for given beam power, heat capacity, and conductivity. Using thermistors to measure temperature difference, a calorimetric target was developed with a sensitivity of about 5 μW. Such a detector is capable of comparing ion and neutral beams.