Experimental determination of critical threshold in electron transport on Tore Supra

In Tore Supra plasmas with fast wave electron heating, a critical threshold in the electron temperature gradient (inverted DeltaT(e)) is clearly observed, i.e., a finite value of inverted DeltaT(e) for which the turbulent heat diffusivity vanishes. The radial profile of this critical gradient is experimentally determined from a set of discharges characterized by similar plasma parameters with fast wave powers ranging from 0.75 to 7.4 MW. The dependence of the electron heat flux on the gradient length is found to be offset linearly. The offset term increases linearly with the ratio of the local magnetic shear to the safety factor.     

Measurement and simulation of edge plasma flow in the Tore Supra tokamak

A simple fluid model is used to interpret two edge flow phenomena that are observed in the scrape-off layer of the Tore Supra tokamak. (1) The Mach number of the parallel flow midway between the two sides of the toroidal limiter is always a significant fraction of the ion sound speed, typicallyM _?=?0.3÷?0.6. directed towards the high field side. In Tore Supra theE×B drift is relatively unimportant, so the esixtence of alarge parallel flow implies a significant poloidal aysmmetry of the SOL source. (2) Massive injection of neutral gas leads to a recycling source that is poloidally localized near the nozzle. The parallel Mach number changes during injection. The sign of the change is positive for injection on the high field side, and negative for injection on the low field side. The ionization source that is needed to produce the change is about 5÷10% of the total amount of injected gas, consistent with atomic physics calculations.     

Giant growth of quantum oscillations in an inhomogeneous 2D electron system

It is detected experimentally for the first time that the connection of 2D electron systems with different electrochemical potentials results in long-range (τ 50 μm) electron density disturbances. When a gated region of a Corbino sample is strongly depleted, the amplitude of the magnetoresistance oscillations caused by high density ungated regions is found to increase in direct proportion to the sample resistance, which is dominated by low-density regions with small conductivity. Experiments on samples with an artificial potential profile (antidots and etched rings) below the gate show that the observed effects are not due to contact effects. Pis’ma Zh. Eksp. Teor. Fiz. 63, No. 1, 55–60 (10 January 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Plasma oscillations in fullerene molecules during electron capture

The anomalously wide energy range for the formation of long-lived negative molecular ions during electron capture by fullerene molecules is explained by the excitation of collective electron (plasma) oscillations in these molecules. A model for such excitations for fullerenes C₆₀ and C₇₀ is proposed on the basis of the Thomas-Fermi model. This model provides good correlation between the experimental curves of resonant electron capture and the theoretical energy dependences of the density of plasma oscillation modes.     

Low energy electron oscillations during epitaxial growth of thin films

We have measured the specularly reflected intensity of low energy (12 eV) electrons as a function of the deposition time during the growth of Cu-thin films on fcc-Co(100) and Co-thin films on Cu(100). The measured curves show pronounced periodical oscillations. The origin of these oscillations is identified as arising from the interference of the electron waves reflected at the vacuum-film and film-substrate boundaries.     

About bursty behaviour, coherent structures, wide scrape-off layer and large parallel flows in the edge of the Tore Supra tokamak

We present the measurements of plasma characteristics in the scrape-off layer (SOL) of the Tore Supra tokamak performed by means of reciprocating Langmuir probe. The probe is inserted into the machine from top. As the radial distance from last closed flux surface (LCFS) increases, ion saturation current exhibits stronger bursty character and its probability density function becomes increasingly skewed towards positive values. At the same time, burst duration and inter-burst time increase dramatically. We explain this phenomenon by radial propagation and dynamics of the ensemble of coherent turbulent structures of different size. The results of two-dimensional fluid modelling based of flux-driven interchange instability mechanism are in excellent agreement with experimental results. We obtained clear experimental evidence that most of the coherent structures are formed in poloidally localized region of the SOL around the outboard midplane. If the probe is magnetically connected to this region, the SOL is very wide and we detect bursty behaviour in the far SOL. On the other hand, if the probe is not magnetically connected to the outboard midplane region (magnetic field lines are intercepted by the outboard limiter), then the SOL is very thin and bursty behaviour is much less prominent. Detection of bursty behaviour in the far SOL is correlated with existence of wide SOL pointing on important role of bursty transport by means of coherent turbulent structures in establishing the width of the SOL in tokamaks. The measurements of parallel flow in the SOL shows that plasma particle radial flux coming from confinement region to the SOL is mostly poloidally localized around the outboard midplane. Our estimations show that more than 80% of plasma particle radial flux is coming from confinement region to the SOL in poloidally localized region — approximately ±15° — around the outboard midplane. Presented at the Workshop “Electric Fields, Structures and Relaxation in Edge Plasmas”, Roma, Italy, June 26–27, 2006.     

强流电子束加速器运行过程电磁辐射的实验研究

强流电子束加速器运行时,临近的计算机和示波器会发生黑屏,这是由于在该过程中产生了较强的电磁辐射干扰。采用实验研究的方法,对实验室研制的强流电子束加速器产生的电磁辐射进行了测量和分析。结果表明：强流电子束加速器产生的电磁辐射主要来源于初级气体开关触发、初级气体开关导通以及气体主开关自击穿导通3个过程。其中,初级气体开关导通时辐射的电磁波强度较大,其强度最大处在与开关相同高度的位置。此外,强流电子束加速器在运行过程中的电磁辐射为低频辐射,主频为21 MHz。     

Giant oscillations of energy levels in mesoscopic superconductors

The interplay of geometrical and Andreev quantization in mesoscopic superconductors leads to giant mesoscopic oscillations of energy levels as functions of the Fermi momentum and/or sample size. Quantization rules are formulated for closed quasiparticle trajectories in the presence of normal scattering at the sample boundaries. Two generic examples of mesoscopic systems are studied: (i) one-dimensional Andreev states in a quantum box and (ii) a single vortex in a mesoscopic cylinder.     

Giant quantum oscillations of acoustoelectric current in Bi

In single crystals of Bi with resistivity ratio R₂₉₃/R_4.2 ≈ 300 by a helium temperature at frequencies 500 MHz of longitudinal sonic wave detected and investigated the longitudinal acoustoelectric effect.In strong magnetic fields if the condition $\text{h}\text{?}$ Ω ? kT,ql ? 1 Ω — cyclotron frequency, k — Boltzman constant) for directions H 6 q 6 y (y— a bissector crystal axis) and H 6 q 6 z (z — a triagonal crystal axis) is fulfilled, giant quantum oscillations of longitudinal acoustoelectric current are detected. Sign of current oscillations is determined by the type of current carriers.     

Giant quantum oscillations of the skin layer in semimetals

We study theoretically the effect of electron energy quantization in a magnetic field on the penetration of radio waves into a semimetal in a geometry in which a constant magnetic field H is directed along the trigonal axis of a crystal. In this geometry, strong magnetic Landau damping in semimetals prevents wave propagation and is responsible for the skin effect. It is shown that quantization of the transverse energy of electrons considerably influences the effectiveness of collisionless absorption of the wave. As a result, the magnetic Landau damping and the skin depth experience giant oscillations upon a change in magnetic field H.     

Effect of surface plasmon oscillations on Auger electron emission

The integrated areas of the Al L₂₃VV and O KL₂₃L₂₃ Auger peaks and the Al surface plasmon energy ?ω_S are reported for the Al(001) surface as a function of exposure to O in the exposure range 0–114 L(1 L=1langmuir=10^?6Torr sec). It is shown that for exposures below a critical value of 15 L, ?ω_S is constant within experimental error while the O Auger peak area increases linearly. For exposures above 15 L, ?ω_S decreases linearly from 10.5 eV to 8.5 eV and the O Auger peak area undergoes relatively slow linear increases correspondingly. The Al Auger peak area decreases by 30% per 1 eV decrease of ?ω_S. The results are discussed with reference to theory relating Auger transition intensities to the spectral density function.     

On the question of short-period oscillations of the collector voltage during transverse electron focusing

Giant short-period oscillations during transverse electron focusing at high emitter voltages have been observed in bismuth samples with a superlattice on the surface. A model is proposed which explains the onset of the oscillations, their position on the magnetic-field scale, and their intensification and shift along the magnetic-field scale with increasing current (this shift depending on the direction of current flow) and which can also account for the absence of oscillations in fields which are multiples of the field of the first oscillation. In this model the oscillations are attributed to the appearance of resonant surface (edge) states and their contribution to the electron transport. Pis'ma Zh. éksp. Teor. Fiz. 68, No. 12, 887–892 (25 December 1998)     

Spin-plasmon oscillations of the two-dimensional electron gas

Interaction of the spins of 2D electrons with an alternating electric field in the plane of the system is considered. It is assumed that the double spin degeneracy is eliminated by the spin-orbit splitting. It is shown that transitions between different spin states produce a narrow absorption band in the degenerate electron gas. In the frequency domain corresponding to these transitions, those frequencies are combined with two-dimensional plasmons; as a result, the plasmon spectrum is modified, and a new type of oscillations occurs, namely, a spin-plasmon polariton. The dispersion law of these oscillations is derived. The problem of the excitation of spin-plasmon polaritons by an external electromagnetic field is solved.     

Effect of betatron oscillations on waveguide free electron laser gain

In this paper we analyse the spontaneous emission and gain spectrum of a waveguide free electron laser with betatron motion included in the analysis. The zero slippage condition and the small signal gain in the waveguide free electron laser is discussed.     

Transverse oscillations of a long-pulse electron beam on alaser-formed channel

An intense relativistic electron beam may be transported in low-pressure gas using an ion channel which focuses and guides the beam. The beam can be unstable to the growth of transverse oscillations caused by the electric force between the beam and channel-the ion hose instability. Beam propagation on channels created by photoionization of gas with an excimer laser is discussed. Ion hose oscillations have been recorded which have a betatron wavelength of approximately 1.5 m. The growth rate of the ion hose instability in the linear regime was measured as 1.67±0.45. At this level of growth, the amplitude of beam oscillations equals the channel radius after a period of one-third of an ion oscillation time