S. L. Prunty

Poloidal polarimeter for current density measurements in ITER

One of the systems envisaged for measuring the current density profile in the ITER is a 118 μm poloidal polarimeter system. The proposed system has two independent views: one fan of chords observes the plasma via an equatorial port and a second fan views down from an upper port. This article will present the status of the on-going work and will address issues as sensitivity and accuracy, refraction, Gaussian beam ray-tracing, alignment, and calibration as well as some specific design details.

Enhanced edge Thomson scattering on MAST

A new edge Thomson scattering diagnostic has been implemented at MAST to complement an existing high spatial resolution ruby laser system and the high time sampling core Nd:YAG system. The Nd:YAG system comprises of four independently controllable lasers. Scattered light from these lasers is viewed at large scattering angle (153°) by a special optical arrangement in the new edge system. The Nd:YAG lasers are viewed at 16 contiguous spatial locations separated by ∼1cm each, located at the plasma outboard pedestal and scrape-off layer region. Here the use of a low f-number lens for the collection of a large solid angle of scattered light is particularly beneficial due to low plasma density (ne). The spectrum of scattered light is significantly broader at large scattering angles, allowing diagnosis of lower plasma temperatures (Te) while using the same spectrometer design as the core system. The four Nd:YAG lasers follow two separate slightly offset (<1∕3 of a spatial channel) optical paths through the vesse...

Experimental investigation into ELM filament formation on MAST

Radial profiles of electron temperature and density through type I ELM filaments have been obtained from a new edge Thomson scattering diagnostic at MAST. The lasers were fired in burst mode, 5 µs apart, to study profile evolution of a single filament as it moves toroidally past the laser beams. The plasma particle and energy loss due to each filament can be deduced from these profiles. As the filaments move out of the plasma, the ne pedestal is seen to collapse locally inwards by as much as 7.5% of the plasma minor radius. Insight into the toroidal structure of the perturbation has been obtained from high time resolution interferometry data. The interferometer data show excursions in line integral density through the midplane of the plasma occurring from 150 µs before the onset of the ELM particle loss. These excursions are due to the evolution of the spatial structure of the plasma during the ELM and indicate that the filaments may develop from broader structures. By combining the toroidal structure information from the interferometer and the radial structure information from the TS system with other diagnostic data on MAST, a two dimensional picture of the ELM phenomenon is obtained.

An analytic formula for the relativistic Thomson scattering spectrum for a Maxwellian velocity distribution

A derivation of the exact scattered spectrum including depolarization effects is obtained for the LIDAR scattering of light by electrons from an incoherent relativistic plasma and an analytical formula for this spectrum is derived with an error of <0.1% for temperatures as high as 35 keV. The results for exact backscattering are compared with those expected under real measurement conditions, where the backscattered radiation is collected within a small scattering angle about the probing direction. It is shown that the exact backscattering results deviate from real measurement conditions by ~0.09–0.6% in the temperature range 10–50 keV, which must be taken into account in LIDAR Thomson scattering systems. Note that the assumption of a Maxwellian velocity distribution has been utilized throughout this paper, believing that the straightforward derivation and subsequent analytical formulae shown may prove useful.

Detecting non-Maxwellian electron velocity distributions at JET by high resolution Thomson scattering

The present work is motivated by a long standing discrepancy between the electron temperature measurements of Thomson scattering (TS) and electron cyclotron emission (ECE) diagnostics for plasmas with strong auxiliary heating observed at both JET and TFTR above 6–7 keV, where in some cases the ECE electron temperature measurements can be 15%–20% higher than the TS measurements. Recent analysis based on ECE results at JET has shown evidence of distortions to the Maxwellian electron velocity distribution and a correlation with the TS and ECE discrepancies has been suggested. In this paper, a technique to determine the presence of non-Maxwellian behavior using TS diagnostics is outlined. The difficulties and limitations of modern TS system designs to determine the electron velocity distribution are also discussed. It is demonstrated that small deviations such as those suggested by previous ECE analysis could be potentially detected, depending on the spectral layout of the TS polychromators. The spectral layout of the JET high resolution Thomson scattering system is such that it could be used to determine these deviations between 1 and 6 keV, and the results presented here indicate that no evidence of non-Maxwellian behavior is observed in this range. In this paper, a modification to the current polychromator design is proposed, allowing non-Maxwellian distortions to be detected up to at least 10 keV.

A polarization modulation technique for far-infrared polarimetry in large plasmas

In future ITER tokamak experiments the poloidal magnetic field and the plasma electron density profiles will be simultaneously measured using a far-infrared polarimeter. This paper presents the bench testing of a new polarimetric technique in which these quantities can be obtained by phase measurements only. Its application will result in a simpler instrument, more robust and less affected by fringe jumps and vibration problems than the conventional interferometer–polarimeter systems in use today. In this experiment a polarization modulation of the input beam is generated by recombining two coherent linearly polarized components, produced by a CO2 pumped FIR laser (λ = 118.8 µm), one of which is frequency shifted by a rotating grating. The beam is then sent through a half-wave plate rotated mechanically at 125 Hz and a fixed quarter-wave plate. When such a beam traverses the plasma, a time dependent phase shift is observed whose Fourier components in phase and in quadrature with the rotating wave-plate are related to the Faraday rotation angle and to the Cotton–Mouton effect, respectively. For this experiment the effect of the plasma has been simulated by a combination of a half-wave and a quarter-wave plates. Simple but effective phase measuring electronics have been developed to recover the two polarimetric parameters from the phase of the detected signal. The experiment has shown that variations in the azimuth and ellipticity angles of the polarization ellipse of less than 1° can be measured with a time resolution of the order of 10 ms.

Far-infrared polarimetric characterization of metallic mirrors exposed to a tokamak plasma

We describe the polarimetric characterization in the far infrared (λ=118.8μm) of a set of metallic mirrors and of a corner cube retroreflector whose surface has been exposed to a tokamak plasma. The objective of the measurements was to investigate possible changes of the polarization state of the incident radiation due to plasma depositions and identify possible sources of errors for the measurement of the ITER q profile by far-infrared (FIR) polarimetry. Tests have been made on Mo and Cu mirrors exposed to the plasma in TEXTOR and Tore Supra. A corner cube retroreflector exposed in Tore Supra was also tested. The thickness and composition of the plasma depositions on these optical elements were known by previous surface analysis and visible ellipsometry studies. Our tests have been carried out in the FIR laboratory of the RFX experiment in Padova, Italy, and in the Laser Research Laboratory of the Department of Electrical and Electronic Engineering at University College Cork, Ireland, using a Stokes pola...

Design and implementation of a full profile sub-cm ruby laser based Thomson scattering system for MAST

A major upgrade to the ruby Thomson scattering (TS) system has been designed and implemented on the Mega-ampere spherical tokamak (MAST). MAST is equipped with two TS systems, a Nd:YAG laser system and a ruby laser system. Apart from common collection optics each system provides independent measurements of the electron temperature and density profile. This paper focuses on the recent upgrades to the ruby TS system. The upgraded ruby TS system measures 512 points across the major radius of the MAST vessel. The ruby laser can deliver one 10 J 40 ns pulse at 1 Hz or two 5 J pulses separated by 100-800 μs. The Thomson scattered light is collected at F/15 over 1.4 m. This system can resolve small (7 mm) structures at 200 points in both the electron temperature and density channels at high optical contrast; ∼50% modulated transfer function. The system is fully automated for each MAST discharge and requires little adjustment. The estimated measurement error for a 7 mm radial point is <4% of T(e) and <3% of n(e) in the range of 40 eV to 2 keV, for a density of n(e)=2×10(19) m(-3). The photon statistics at lower density can be increased by binning in the radial direction as desired. A new intensified CCD camera design allows the ruby TS system to take two snapshots separated with a minimum time of 230 μs. This is exploited to measure two density and temperature profiles or to measure the plasma background light.

Single chord far‐infrared polarimetry experiment on RFX

This paper summarises the work of the authors in the realisation and testing of a pilot single chord polarimetry experiment on the RFX (reversed field experiment) machine, at the far-infrared wavelength of 118.8 µm. The fundamental purpose of this experiment is to demonstrate the feasibility of a Faraday rotation angle measurement on RFX, with the required resolution, and to clarify all the aspects which may be useful for the final six-chord version of the diagnostic. The apparatus which has been developed is described, with emphasis on some of the modifications which proved essential in order to obtain a satisfactory measurement, in spite of the hostile environment of the machine. Typical results of Faraday angle measurements are presented and discussed, and the reliability of the measurement is demonstrated. In particular, the required +0.2o accuracy is achieved and the results are consistent with magnetic field profiles similar to the ones predicted by the Bessel function model. Suitable design considerations are formulated in order to guarantee the required performance in the six-chord version, even when the machine moves into full regime.

Improvement of interferometric measurements on far‐infrared polarimeter interferometer systems

On many tokamaks the reconstruction of the magnetic field structure in the plasma is supported by polarimetric measurements. Recent proposed and realized methods are based on a far-infrared laser beam with a rotating polarization ellipse. The same instrument usually performs as an interferometer measuring the line integrated plasma density. It has been shown that the rotating polarization ellipse disturbs the interferometric measurements. A method based on the principle of a rotating polarization in which the interferometric measurement is unaffected is proposed. Bench test results are presented which show the feasibility of this method