Field In Tokamak Plasmas Research Articles

The three-wave laser polarimeter-interferometer on J-TEXT tokamak

Motivated by increasing demands on high-quality measurement of interior magnetic field in tokamak plasma, a far-infrared laser-based polarimeter-interferometer system has been developed on J-TEXT. Three formic acid lasers separately pumped by three CO2 lasers are used as sources, providing more than 90 mW output power in total. High laser power along with usage of newly developed planar Schottky diode mixer enable high phase resolution < 1 mrad. Collinearity and polarization calibrations have been carefully done to improve the measurement reliability. Meanwhile, real-time feedback control of three-wave laser source has been realized for the first time, to fulfill the system stability. Based on three-wave technique, Faraday angle and integrated density phase along the laser path are simultaneously measured with high temporal resolution. In addition, the laser beam is expanded to cover the entire cross-section of the plasma to provide high spatial resolution measurement. With this system, MHD equilibrium of the J-TEXT plasma has been reconstructed. Obscure perturbations on magnetic topology and electron density associated with MHD instabilities, e.g. sawteeth and tearing modes have also been observed. In particular, some interesting features of disruptions in high-density discharges are identified by carefully interpreting the measured polarimeter-interferometer data. In the density ramp-up phase of a high density discharge, asymmetry in both electron density and current density profiles between the Low-Field-Side (LFS) edge (r > 0.8a) and the High-Field-Side (HFS) edge (r < −0.8a) would appear and extend gradually toward the center region. At the same time, a low-frequency (< 1 kHz) density perturbation suddenly occurs at the HFS edge and also gradually propagates into the center region. The disruption takes place when the electron density asymmetry/perturbation reaches the location nearly the m/n = 2/1 (where m and n are the toroidal mode number and the poloidal one, respectively) resonant surface. Evolution of the reconstructed electron density and current density profiles present the details on the asymmetrical behaviors and provide a possible explanation for the high density disruption.

Radial Electric Field in Tokamak Plasmas as a Physical Consequence of Ehrenfest’s Paradox

A simplified form and some possible theoretical resolutions so-called Ehrenfest's Paradox is described. A relation between physical consequences of this relativistic Paradox and the charge density of a tokamak plasma is shown. Plasma experiments which can resolve the Ehrenfest's Paradox is presented.

A Comparative Study of Electron Heat Transport in a Stochastic and a Non‐Stochastic Magnetic Field

AbstractWe studied the electron heat transport across a non‐local stochastic magnetic field and a non‐local non‐stochastic magnetic field in tokamak plasmas. Analytical results and numerical simulation results were compared with conditions of a stochastic and a non‐stochastic magnetic field respectively in this article. Parameter of stochasticity in perturbed magnetic field was found not tobe a key factor in influencing effective radial heat conductivity in radial direction of tokomak when Chirikov parameter is close to one (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Study of Heat Transport Across a Quasi-Stochastic Magnetic Field in Magnetic Confined Plasma Physics

Heat diffusion across a non-local quasi-stochastic magnetic field in tokamak plasma is numerically studied. The perturbed magnetic field is found to be a key factor in influencing effective radial heat conductivity whether the magnetic field is stochastic or not. Being different from previous work, a non-local perturbed magnetic field is used. Analytical results and numerical simulation results are compared between the conditions with a full and a quasi-perturbed stochastic field. The analytical results are found to be still consistent with numerical simulation results when the perturbed field is quasi-stochastic.

Heat Diffusion across a Strong Stochastic Magnetic Field in Tokamak Plasmas

We investigate heat diffusion across a local strong stochastic magnetic field by using eleven low-m perturbed magnetic islands. A maximum stochasticity of 38.82 between two neighboring rational surfaces is attained. The correlation between the effective radial heat conductivity χr and the ratio of the parallel heat diffusion coefficient to the perpendicular coefficient, χ|/χ⊥, numerically studied and compared with earlier work.

&lt;i&gt;&amp;delta;f&lt;/i&gt; Simulation and the Radial Electric Field

The neoclassical radial electric fields in a tokamak are calculated for the shifted Maxwellian distribution function by using a δf Monte Carlo particle simulation code ”FORTEC-FSM”. The calculation results agree well with the analytical estimations. The present method will be useful for the determination of the neoclassical radial electric field in tokamak plasmas with input momentum or torque by, e.g., NBI heating.

Development of popup Langmuir probe system for the JET MkIIa divertor

The successful operation of a popup Langmuir probe system, which was installed in the JET MkIIa divertor, is described. The system utilises the ambient magnetic field in tokamak plasmas to act on a current carrying coil and pop up a rail containing Langmuir probes. Measurements were made using pin-plate probes which, owing to their relatively large exposed area, are ideally suited for use with such a system. Details of the design, testing, measurements and potential applications of JET’s popup system are given.

Hot plasma effects on the polarization of electron cyclotron waves

The evolution of the wave polarization during the propagation of electron cyclotron waves in hot plasmas is investigated. The coupled mode equations are solved in slab geometry, including the relativistic corrections to the dielectric tensor. This formalism is applied to purely ordinary or extraordinary modes, propagating obliquely with respect to the magnetic field in tokamak plasmas, in order to determine the rate of change of the mode purity due to finite temperature effects. It is found that such an alteration of the mode purity can be one to three orders of magnitude larger than in a cold plasma. However, the overall effect remains tolerable for most applications and typical parameters of magnetically confined plasmas.

Feasibility experiments for electron ripple injection on current drive experiment-upgrade

In search of a method to generate a radial electric field in tokamak plasmas, an experimental study has been performed to investigate the possibility of inducing radial electrical current. An external coil array has been used to create a local magnetic ripple well and the electron cyclotron resonance heating (ECH) has been used to trap some electrons that will then be subject to rapid vertical drifts into the plasma. Using a simplified experimental arrangement with only a toroidal magnetic field, an ECH-driven radial electrical current has been observed. The ECH-driven elecron temperature anisotropy, which is necessary for ripple trapping and electron drifts, has been determined by several different methods. The perpendicular temperature can be shown to be as large as 11 times the parallel temperature, which should yield a significant amount of ripple trapping and radial current.

Direct Measurement of the Radial Electric Field in Tokamak Plasmas using the Stark Effect

Motional Stark effect polarimetry (MSE) is a well established technique for measuring the magnetic field pitch angle in tokamaks. By viewing the Stark emission spectrum from two different angles, this technique can also provide local measurements of the plasma radial electric field, ${E}_{r}$. Simultaneous measurements of the profiles of magnetic field pitch angle and ${E}_{r}$ are presented for the first time in a high-performance DIII-D tokamak plasma. Direct measurement of ${E}_{r}$ is of great importance in fusion research because the suppression of turbulence through ${\mathbf{E}}_{r}\ifmmode\times\else\texttimes\fi{}\mathbf{B}$ velocity shear provides a mechanism to improve energy confinement.

A polarimeter for localized continuous measurement of the poloidal magnetic field in a tokamak

A polarimeter for the circular polarization of Zeeman-split spectral line emissions has been developed to measure the poloidal magnetic field in tokamak plasmas. The Li I 6708 Å line originating from a monoenergetic lithium beam provided a localized spectral source. In the polarimeter, interference filters of narrow bandpass were set at the peak wavelength of the circular polarization, enabling the measurement to be continuous and sensitive.

Electron cyclotron wave absorption by the fast tail generated by the DC electric field in Tokamak plasmas

Wave damping near the electron gyrofrequency in a Tokamak plasma with the energetic tail generated by the electric field is investigated. The electron tail is computed by a Fokker-Planck initial value code as a function of the relevant parameter E/sub ////Ec=E/sub ///Te/(2 pi nee3 Lambda ). It is shown that strong damping of the e-mode occurs for oblique propagation. The results are of relevance for studies of ECRH in present-day Tokamaks and in future reactors where a mildly relativistic electron tail is naturally present for large Te. Special emphasis is therefore given to wave absorption for frequencies of significantly below the central electron gyrofrequency, and to the associated r.f.-driven current.

Electron-cyclotron heating in the presence of a dc electric field in tokamak plasmas

The combined effects of the dc electric field and electron-cyclotron wave absorption in tokamak plasmas are investigated. The case of the ordinary mode for nearly normal propagation from the low magnetic field side is studied by a two-dimensional Fokker–Planck code. It is found that the small fraction of the wave energy that goes into the superthermal electrons enhances the effect of the continuous pushing of the tokamak electric field.

Trapped Electron Effect on Turbulent Heating by Low Pulsed Electric Field in Tokamak Plasma

The effect of magnetically trapped electrons on the turbulent heating due to a relatively small pulsed electric field is investigated experimentally. Density fluctuations due to low frequency ion acoustic instability and input energy of ions from the turbulent heating increase as the number of trapped electrons decrease. This result is supported with analysis using a dispersion equation which incorporates the contribution of the trapped electrons.

On the measurement of poloidal field distributions in Tokamaks by far-infrared polarimetry

In 1972, De Marco and Segre showed, theoretically, that it should be possible to measure the poloidal magnetic field in Tokamak plasmas on the basis of the Faraday effect. Preliminary polarimetric model experiments with an HCN laser are not reported and the author is optimistic that the problems of measuring small rotation angles in the far IR can be solved. With the improvements suggested, at least a sufficiently precise measurement of small Faraday rotation angles in a Tokamak experiment should be possible. Besides the precision with which the Faraday rotation angles can be measured, the precision with which the poloidal field can finally be deduced from such measurements, depends on other factors such as the measuring precision of the electron density profile, the diameter of the probing beam and the deviation of the plasma from circular symmetry.