The poloidal magnetic field distribution in a tokamak can be determined by probing the plasma with an array of FIR laser beams perpendicular to BT and measuring the Faraday rotation ψ of their plane of polarization. A concomitant detection of the phase shifts Δφ provides sufficient information to take the electron density profile properly into account. For the TEXTOR tokamak (R=1.75 m, a=0.5 m, BT =2 T) a nine‐channel Veron‐type interferometer has been upgraded to allow simultaneous measurements of Δφ and ψ with sensitivities of about 0.01×2π and 0.15° and time resolutions of 0.1 and 3 ms, respectively. The number of channels and their distribution are sufficient to reconstruct the local poloidal field and hence the current density profile with an accuracy of about ±5%–10% at half the plasma radius. For the central current density an accuracy of ±15%–20% is achieved by slowly moving the plasma across the probe beams and thereby multiplying the number of chords especially near the magnetic axis. For rapid ...

Current distribution measurement in a tokamak by FIR polarimetry (invited)

Recent progress in magnetically confined plasma research has brought the goal of controlled thermonuclear fusion within reach. Simultaneously, the increased size, temperature, and density of these devices has resulted in rapid changes in diagnostic techniques. In this article, recent developments in instrumentation are reviewed together with trends for the future. The topics discussed include far‐infrared laser interferometry and polarimetry, ruby laser television Thomson scattering, excimer and Nd‐glass laser scattering, ion temperature determination via large‐α scattering with FIR and CO2 lasers, collective scattering, Schottky diode mixer technology, synchrotron radiation diagnostics and imaging, ion beam probes, x‐ray diagnostics and imaging, neutron diagnostics, resonance fluorescence scattering, ultraviolet diagnostics, and internal magnetic field measurement.

Instrumentation for magnetically confined fusion plasma diagnostics

New developments in Faraday rotation polarimetry have provided the first measurements of current density profile and core magnetic fluctuations in the core of a high-temperature reversed field pinch. This has been achieved by a fast-polarimeter system with time response up to 1 μs and phase resolution <1 mrad. Recent experiments on Madison Symmetric Torus have directly measured radial magnetic field fluctuations in the plasma interior with amplitude 33 G, ∼1%. A broad spectrum of magnetic fluctuations is observed up to 100 kHz. Relaxation of the current density profile at the sawtooth crash occurs on the timescale of 100 μs. Reversed-field pinch behavior is determined in large part by magnetic fluctuations driven by the radial gradient in the parallel current density. Hence, measurement of magnetic fluctuations and the current density profile is essential to understand the link between the current density profile, fluctuations, and transport.

Laser polarimetric measurement of equilibrium and fluctuating magnetic fields in a reversed field pinch (invited)

A multichannel far-infrared laser-based POlarimeter-INTerferometer (POINT) system utilizing the three-wave technique is under development for current density and electron density profile measurements in the EAST tokamak. Novel molybdenum retro-reflectors are mounted in the inside wall for the double-pass optical arrangement. A Digital Phase Detector with 250 kHz bandwidth, which will provide real-time Faraday rotation angle and density phase shift output, have been developed for use on the POINT system. Initial calibration indicates the electron line-integrated density resolution is less than 5 × 1016 m−2 (∼2°), and the Faraday rotation angle rms phase noise is <0.1°.

Faraday-effect polarimeter-interferometer system for current density measurement on EAST.

A high-performance Faraday-effect polarimeter-interferometer system has been developed for the J-TEXT tokamak. This system has time response up to 1 μs, phase resolution 35 mW per cavity) and sensitive Planar Schottky Diode mixers are used, providing stable intermediate-frequency signals (0.5-3 MHz) with S/N > 50. The collinear R- and L-wave probe beams, which propagate through the plasma poloidal cross section (a = 0.25-0.27 m) vertically, are expanded using parabolic mirrors to cover the entire plasma column. Sources of systematic errors, e.g., stemming from mechanical vibration, beam non-collinearity, and beam polarization distortion are individually examined and minimized to ensure measurement accuracy. Simultaneous density and Faraday measurements have been successfully achieved for 14 chords. Based on measurements, temporal evolution of safety factor profile, current density profile, and electron density profile are resolved. Core magnetic and density perturbations associated with MHD tearing instabilities are clearly detected. Effects of non-axisymmetric 3D RMP in ohmically heated plasmas are directly observed by polarimetry for the first time.

W. Kunz

Papers

A far-infrared ‘polari-interferometer’ for simultaneous electron density and magnetic field measurements in plasmas

Theoretical considerations and preparatory experiments for poloidal field measurements in tokamaks by far-infrared polarimetry