Inductive current density perturbations to probe electron internal transport barriers in tokamaks

Improved electron energy confinement in tokamak plasmas, related to internal transport barriers, has been linked to nonmonotonic current density profiles. This is difficult to prove experimentally since usually the current profiles evolve continuously and current injection generally requires significant input power. New experiments are presented, in which the inductive current is used to generate positive and negative current density perturbations in the plasma center, with negligible input power. These results demonstrate unambiguously for the first time that the electron confinement can be modified significantly solely by perturbing the current density profile.     

A transport model for alcator scaling in tokamaks

A theoretical model is proposed to explain the tokamak energy confinement time. With no adjustable numerical coefficients, the model predicts experimentally observed values to within a level of uncertainty consistent with the intrinsic spread of the experimental data and the necessity of calculating the confinement time without precise knowledge of the temperature profile.     

New scaling laws for trapped-particle anomalous transport in tokamaks

An improved formula for anomalous transport caused by the dissipative trapped-ion instability (without shear effect) is derived numerically and by two independent analytical methods. The new shearless result is also used to derive the anomalous diffusion with shear effect by a general method already published.     

Density profiles in tokamaks from electron cyclotron radiation spectra

Measurement of the line shape of optically thick and optically thin lines in the electron cyclotron radiation spectrum emitted by a tokamak plasma may yield both electron temperature and density profiles. Currently temperature profiles are routinely extracted from optically thick lines. Consequently, this paper is addressed to the density profile problem. Algorithms for extracting density profiles are outlined in the case of uncontrolled reflection and controlled reflection of the cyclotron radiation within the tokamak vacuum chamber.     

Nature of the isotope effect on transport in tokamaks

The reduction of energy and particle losses with the increasing mass of the hydrogen isotope is more pronounced under conditions of improved confinement when the dominant ion temperature gradient instability is suppressed and other channels of anomalous transport are of importance. In this Letter, we reconsider the dissipative trapped electron (DTE) instability by taking into account finite Larmor radius effects in the analysis of the ion response to perturbations. By applying the improved mixing length approximation in order to estimate the transport coefficients, it is demonstrated that DTE contribution is intrinsically dependent on the isotope mass and provides a plausible explanation for the isotope effect. Contrary to the common belief, it is shown that the DTE turbulence may be of importance for reactor plasmas of low collisionality.     

Measurements of electron thermal transport due to electron temperature gradient modes in a basic experiment

Production and identification of electron temperature gradient modes have already been reported [X. Wei, V. Sokolov, and A.?K. Sen, Phys. Plasmas 17, 042108 (2010)]. Now a measurement of electron thermal conductivity via a unique high frequency triple probe yielded a value of χ(⊥e) ranging between 2 and 10 m(2)/s, which is of the order of a several gyrobohm diffusion coefficient. This experimental result appears to agree with a value of nonlocal thermal conductivity obtained from a rough theoretical estimation and not inconsistent with gyrokinetic simulation results for tokamaks. The first experimental scaling of the thermal conductivity versus the amplitude of the electron temperature gradient fluctuation is also obtained. It is approximately linear, indicating a strong turbulence signature.     

Continuum self-organized-criticality model of turbulent heat transport in tokamaks

A simple generic one-dimensional continuum model of driven dissipative systems is proposed to explain self-organized bursty heat transport in tokamaks. Extensive numerical simulations of this model reproduce many features of present day tokamaks such as submarginal temperature profiles, intermittent transport events, 1/f scaling of the frequency spectra, propagating fronts, etc. This model utilizes a minimal set of phenomenological parameters, which may be determined from experiments and/or simulations. Analytical and physical understanding of the observed features has also been attempted.     

Modelling of start-up assist by electron cyclotron waves for large-size tokamaks

Summary This paper presents a model describing the plasma production in a tokamak by means of electron cyclotron microwave power and the initial ohmic current ramp-up. A zero-dimensional code is applied to the case of large-size, high magnetic field tokamaks and, in particular, to the case of the INTOR/NET tokamak. It is shown that the r.f. plasma can be produced and heated to temperatures of a few hundred eV with power densities of the order of 50 kW/m³. Detailed results are given for a case in which 4 MW of electron cyclotron power is used to create a plasma whose steady-state temperature is well above the impurity radiation barriers. For the plasma inductance assumed and a current ramp-up rate of 0.625 MA/s, the ohmic current can be initiated in such a plasma with a total-loop voltageV _L<20 V. The resistive contribution to the loop voltage is of the order of 1 V.

---

Riassunto Si descrive un modello di produzione del plasma e dello stadio iniziale di crescita della corrente ohmica in un tokamak, per mezzo di potenza elettromagnetica alla frequenza ciclotronica degli elettroni. Il codice a zero dimensioni che riassume il modello è applicato al caso di tokamak di grandi dimensioni e grandi campi magnetici toroidali ed, in particolare, al caso del tokamak INTOR/NET. Si mostra che il plasma dovuto alle microonde può essere prodotto e portato a temperature elettroniche di qualche centinaio di eV, in questa macchina, con una densità di potenza applicata dell'ordine di 50 kW/m³. Si danno risultati dettagliati, per un caso nel quale una potenza di 4 MW viene applicata per produrre un plasma con temperature di equilibrio molto al di sopra delle barriere di radiazione delle impurezze. Si mostra anche che, per la particolare formula assunta per definire l'induttanza del plasma e per un tasso di salita della corrente ohmica di 0.625 MA/s, la corrente ohmica stessa può salire, in questo tipo di plasma, con una tensione totale di giro inferiore a 20 V. La parte resistiva della tensione di giro si mantiene attorno ad 1 V.

---

Резюме В этой статье описывается модель образования плазмы и начальная стадия нарастания омического тока в токамаке с помощью электромагнитной мощности на циклотронной частоте электронов. Код нулевой размерности применяется в случае токамака больших размеров, больших магнитных тороидальных полей, и частности, в случае токамака INTOR/NET. Показывается, что образуется высокочастотная плазма и температура повышается до нескольких сотен эВ при плотности мощности порядка 50 кВТ/м³. Приводятся подробные результаты для случая, когда для образования плазмы исползуется мощность 4 мВт на электронной циклотронной частоте, причем равновесная температура плазмы много выше радиационных барьеров примесей. Показывается, что для принятой индуктивности плазмы и величины наростания омического тока 0.625 MA/c, в плазме этого типа омический ток может быть возбужден при полном напряжении в контуреV _L <20 В. Вклад активного сопротивления в напряжение в контуре составляет порядка 1 В.

---

     

飞秒激光-固体靶相互作用中超热电子输运的实验研究

报道了在100TW超短脉冲掺钛宝石激光装置上,完成的飞秒激光-固体靶相互作用中超热电子在靶内输运的实验研究结果.获得了超热电子的产额、注量和总能量.结果表明,超热电子的注量和总能量随靶厚的增加而减少,超热电子约80%的能量主要沉积在靶内的前10(m,对以上形成的原因进行了分析指出,是由于静电场对超热电子输运影响所致.     

Experimental researches on quantum transport in semiconductor two-dimensional electron systems

The author reviews contribution of Gakushuin University group to the progress of the quantum transport in semiconductor two-dimensional electron systems (2DES) for forty years from the birth of the 2DES in middle of the 1960s till the finding of temperature dependent collapse of the quantized Hall resistance in the beginning of this century.     

Experimental evidence for a reduction in electron thermal diffusion due to trapped particles

New high time resolution measurements of the electron thermal diffusion χ(e) throughout the sawtooth cycle of the Madison Symmetric Torus reversed-field pinch have been made by utilizing the enhanced capabilities of the upgraded multipoint, multipulse Thomson scattering system. These measurements are compared to the χ(e) due to magnetic diffusion predicted by using information from a new high spectral resolution zero-β nonlinear resistive magnetohydrodynamic simulation performed, for the first time, at the Lundquist number of high current Madison Symmetric Torus plasmas (S≈4×10(6)). Agreement between the measured and predicted values is found only if the reduction in thermal diffusion due to trapped particles is taken into account.     

Radial transport and electron-cyclotron-current drive in the TCV and DIII-D tokamaks

Calculation of electron-cyclotron-current drive (ECCD) with the comprehensive CQL3D Fokker-Planck code for a TCV tokamak shot gives 550 kA of driven toroidal current, in marked disagreement with the 100-kA experimental value. Published ECCD efficiencies calculated with CQL3D in the much larger, higher-confinement DIII-D tokamak are in excellent agreement with experiment. The disagreement is resolved by including in the calculations electrostatic-type radial transport at levels given by global energy confinement in tokamaks. The radial transport of energy and toroidal current are in agreement.     

Experimental evidence for electron channeling in Fe /Aau (100) superlattices.

We present transport and structural data from epitaxial (100) and (111) Au/Fe superlattices grown by molecular beam epitaxy. From their analysis, we conclude that an electron channeling mechanism, due to strong specular reflection of the minority spin carrier at the Au/Fe interfaces, is responsible for the high conductivity in the (100) superlattices.     

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.