Thomson Scattering System Research Articles

Electron Temperature Measurement on QUEST Spherical Tokamak by Thomson Scattering System

On QUEST, we are carrying out the steady state operation by RF current drive. A Thomson scattering (TS) system has been constructed to measure the plasma electron temperature. This system consists of a Nd:YAG laser of energy 1.65 J, a spherical mirror for collection of the scattering light, and a polychromator for spectroscopy. Because the Thomson scattering signal is weak for the low-density RF-sustained plasma, many scattering pulse signals were accumulated from steady-state plasmas. Electron temperature profiles were obtained for the steadystate spherical tokamak (ST) plasma for the first time. Six spatial points were measured by moving the fiber position, and the plasma electron temperature was in the range 10–500 eV.

Development of a Laser Timing Controller for the High Time-Resolution Nd:YAG Thomson Scattering System in Heliotron J

A new laser timing controller for the high time-resolution Nd:YAG Thomson scattering system with two Nd:YAG lasers has been developed to study improved confinement physics in Heliotron J. A PIC-based timing controller synchronizes the timings of laser oscillations with plasma discharges and enables the measurement of plasma profiles with a precision of 80 ns), which is crucial for transport physics studies including spontaneous transitions.

Development of polarization-controlled multi-pass Thomson scattering system in the GAMMA 10 tandem mirror

In the GAMMA 10 tandem mirror, the typical electron density is comparable to that of the peripheral plasma of torus-type fusion devices. Therefore, an effective method to increase Thomson scattering (TS) signals is required in order to improve signal quality. In GAMMA 10, the yttrium-aluminum-garnet (YAG)-TS system comprises a laser, incident optics, light collection optics, signal detection electronics, and a data recording system. We have been developing a multi-pass TS method for a polarization-based system based on the GAMMA 10 YAG TS. To evaluate the effectiveness of the polarization-based configuration, the multi-pass system was installed in the GAMMA 10 YAG-TS system, which is capable of double-pass scattering. We carried out a Rayleigh scattering experiment and applied this double-pass scattering system to the GAMMA 10 plasma. The integrated scattering signal was made about twice as large by the double-pass system.

The ITER divertor Thomson scattering system: engineering and advanced hardware solutions

A divertor Thomson scattering (TS) system being developed for ITER has incorporated proven solutions from currently available TS systems. On the other hand any ITER diagnostic has to operate in a hostile environment and very restricted access geometry. Therefore the operation in an environment of intensive stray light, plasma background radiation, the necessity meet the requirement using only a 20 mm gap between divertor cassettes for plasma diagnosis as well as to measure plasma temperatures as low as 1 eV severely constrain the divertor TS diagnostic design. The challenging solutions of this novel diagnostic system which has to ensure its steady performance and also the operability and maintenance are the focus of this report. One of the most demanding parts of the in-vessel diagnostic equipment development is the design assessment using different engineering analyses. The task definition and first results of thermal, e/m and seismic analyses are provided. The process of further improving of the design involves identification of susceptible areas and multiple iterations of the design, as needed. One of the key points for all Thomson scattering diagnostics are the laser capabilities. A high-performance and high-power laser system using a steady-state and high-repetitive mode Nd:YAG laser (2J, 50–100Hz, 3ns) has been developed. The reduced laser pulse duration matched with high-speed low-noise APD detector can be very important under high background light level. For diagnostics such as Thomson scattering and Raman spectroscopy, a high-degree of discrimination against stray light at the laser wavelength is required for successful detection of wavelength-shifted light from the laser-plasma interaction region. For this case of high stray light level, a triple grating polychromator characterized by high rejection and high transmission has been designed and developed. The novel polychromator design minimizes stray light while still maintaining a relatively high transmission.

Coaxial Multiple Laser Beam Combiner for the LHD Thomson Scattering System

We developed a polarization-based beam-combining method for multiple lasers to improve the time resolution and accuracy of a large helical device (LHD) Thomson scattering system. We combined two or more beams from different apertures of the laser heads in series into a coaxial beam line with the use of polarization optics. We have implemented this beam-combining method in an LHD Thomson scattering system from the 15th experimental campaign. We also propose a method of combining three or more laser beams by the use of a Pockels cell. c

Fiber interferometer for calibration of electron density profile measured by Thomson scattering in LHD

An electron density (ne) calibration method using a fiber-based interferometer (FI) for Thomson scattering (TS) diagnostics is proposed. An interferometer probe laser is injected into the plasma from the back face of a TS final bend mirror that has a transparency at the wavelength of the FI probe beam. The coaxial line electron density is measured by the FI, making it possible to calibrate neL from TS without either misalignment or optical loss in the collective optics of the TS system. In this paper, we report the conceptual design, wavelength selection of FI and potential issues in calibration of the fiber-based interferometer system.

Edge and core Thomson scattering systems and their calibration on the ASDEX Upgrade tokamak

A new 10 channel Thomson scattering (TS) system was installed on the ASDEX Upgrade tokamak to measure radial profiles of electron density and temperature at the plasma edge with high radial resolution. Together with the already existing TS system, which is now used for the core plasma, electron density and temperature profiles extending from the edge to the core are now obtained in a single discharge. The TS systems are relatively calibrated by an optical parametric oscillator.

Deconvolution of Thomson scattering temperature profiles

Deconvolution of Thomson scattering (TS) profiles is required when the gradient length of the electron temperature (T(e)) or density (n(e)) are comparable to the instrument function length (Δ(R)). The most correct method for deconvolution to obtain underlying T(e) and n(e) profiles is by consideration of scattered signals. However, deconvolution at the scattered signal level is complex since it requires knowledge of all spectral and absolute calibration data. In this paper a simple technique is presented where only knowledge of the instrument function I(r) and the measured profiles, T(e, observed)(r) and n(e, observed)(r), are required to obtain underlying T(e)(r) and n(e)(r). This method is appropriate for most TS systems and is particularly important where high spatial sampling is obtained relative to Δ(R).

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.

Near-infrared detectors for ITER LIDAR Thomson scattering

In this paper we discuss strategies for the development of fast photodetectors suitable for operation in the λ > 850 nm near-infrared (NIR) spectral region in the ITER core LIDAR Thomson scattering (TS) system. Detection of this spectral range is necessary if a Nd:YAG laser operating at the fundamental wavelength ( λ = 1.06 μm) will be used as the input laser source. Different types of NIR photodetectors are potentially suitable for use in ITER LIDAR TS: the transferred electron (TE) InGaAsP/InP hybrid photodiodes and microchannel plate photomultipliers (MCP PMTs), the In x Ga 1− x As MCP image intensifiers and PMTs, and the detectors based on transmission Si photocathodes. But their characteristics of either sensitivity, active area or speed of response, do not match the ITER specifications and all devices require some developmental work. For each of these detector types we review the characteristics of devices presently available and suggest a realistic development strategy suitable to extend their performances to meet the ITER specifications. Finally the expected performance of the ITER LIDAR TS system for different detector choices are compared by calculating the expected signal-to-noise ratio of the measured plasma temperature and density.

Development of a Double-Pass Thomson Scattering System in the TST-2 Spherical Tokamak

A double-pass Thomson scattering system, in which a laser pulse makes a round trip through the plasma, was constructed. Using the same optics and a fast detection unit, we can resolve backward and forward scattering pulses in the signal. Because these scatterings reflect velocity distribution along different directions, electron temperature anisotropy can be estimated from the double-pass Thomson scattering system.

Upgrading of Thomson Scattering System for Measurements of Spatial Dynamics of Plasma Heating in GOL-3

Thomson scattering diagnostics has been updated to allow measurements at two instants of time for either of two spatial points in plasma during a single plasma discharge. Laser produces now two 15J laser pulses with variable (0.2–100μs) delay between them. It was made possible by integration into previous laser system a second independent laser oscillator. Multichannel spectral measurements of scattered radiation can be accomplished at two spatial locations simultaneously, at z1 = 4m from the input of electron beam into plasma (old location) and at z2 = 2m (new location). Three geometries of measurement are possible: a single radial point at each of plasma cross-sections at z1 and z2 or two radial points at either cross-sections at z1 or z2. Number of radial points is limited by number of available detector channels (15ch) now. We present first experimental results with 90° detection of scattered radiation with upgraded system.

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.

Edge Thomson Scattering in RFX-mod: Operation and first measures

During the 2009 experimental campaign the Edge Thomson Scattering System started the operations, after a commissioning period. The main modification has been the adoption of interferential filters spectrometers with APD detectors, similar to those used on the main TS. Other improvements were the optimization of the laser input path and of the collection optics, which increased the signal level of about 50 %.With the present setup, temperature and density measures show good compatibility with those of the main TS diagnostic; density data have been calculated after Rotational Raman Scattering calibration, a method which is not affected by impurity deposition on the collection window. Present operational limits of the diagnostic are discussed as well as short terms planned operations, focusing in particular on laser multi-pulse issues and laser sharing with the impurity injection system by laser blow-off.

Design of new Thomson scattering diagnostic system on COMPASS tokamak

Two new Thomson scattering (TS) diagnostic systems are being built on the COMPASS tokamak to get radial profiles of both electron temperature and electron density. The core TS system will measure at 24 spatial points with ≈10 mm resolution across the core plasma while the edge TS system will measure at 32 spatial points with ≈ (3–5) mm resolution across the edge plasma. Two independent Nd:YAG lasers (1.5 J, 30 Hz each) could be combined to produce a repetition rate of 60 Hz. The lasers will follow separate beam paths to the COMPASS vessel. Scattered light will be collected by two objectives: one for the core region, the second for the plasma edge. Both TS systems are based on Nd:YAG/APD (Avalanche photodiodes) technology. Scattered light is spectrally resolved in polychromators equipped with a set of spectral filters and APDs. The output signal is digitized by fast sampling (GS/s) 8 bit analog digital converters (ADCs).

Feasibility of a Multi-Pass Thomson Scattering System with Confocal Spherical Mirrors

Multi-pass Thomson scattering is an attractive method for measuring electron temperature and density in low density plasmas. The feasibility of multi-pass laser optics consisting of two confocal spherical mirrors has been studied, and analytic expressions for the path have been obtained. These can be used to optimize the design under the given conditions. The effects of aberration, as well as positioning and alignment errors, were found to be critical for obtaining large transit numbers. Beam expansion resulting from aberration, surface roughness of the mirrors and diffraction is considered. Under practical conditions, transit numbers of about 37 (i.e., 19 round trips) are possible.

Development of a Thomson Scattering System in the TST-2 Spherical Tokamak

Electron temperature is one of most important parameters in all fusion devices, including spherical tokamaks (STs). STs can generate high pressure with a smaller magnetic field compared with conventional tokamaks. However, STs are characterized by MHD events which cause significant distortion in the temperature profile[1]. Thomson scattering measurement is a highly reliable measurement of electron temperature, and many devices are equipped with a system. Recently the system in TST-2 was upgraded and more accurate and new results are reported in this paper. This system consists of the following three components, incident optics, light collection optics, spectroscopic system. We use a compact Newtonian mirror system consists of a primary spherical mirror of diameter 254 mm and a small secondary flat mirror for the collection optics[2]. The system has large solid angle (88 msrad). For incoherent Thomson scattering, we use Nd:YAG laser with a wavelength of 1064 nm, with a pulse width of 10 ns, with a repetition rate of 10 Hz. The former low energy (0.45 J) laser has been replaced by a new high energy (1.6 J) laser, and the signal to noise ratio was improved from 5 to 10. The Calibration method of the polychrometar was modified, and the wavelength resolution was improved from 2.5 nm to 0.24 nm as a result. Recently, we introduce an 8-channel high speed oscilloscope, and thus all 5 wavelength channels can be recorded simultaneously, and fitting becomes more accurate. Rayleigh scattering is used for the calibration of density, and multi spatial point density (and temperature) measurement is possible. We measured intensity of scattering light and stray light in the calibration. Typical results are show in the following. The typical central electron temperature and the electron density are 150 eV and 1.5 × 10 19 m −3 , respectively. The temperature profile has a maximum and flat near the center of the plasma, the density profile also has an maximum near the center. When instability is observed in the radiation signal, the temperature becomes low. Dependence of the temperature on other parameter will be reported.

Thomson scattering at Pilot-PSI and Magnum-PSI

A robust and sensitive Thomson scattering (TS) system has been developed for the high density low temperature plasma in the linear plasma generator Pilot-PSI, which routinely and reproducibly measures electron density and temperature profiles along a detection chord of 25 mm with a spatial resolution of 0.6 mm. The capabilities of the system are illustrated in this paper by a selection of new results from the research program at Pilot-PSI. TS data are presented that demonstrate the present plasma density record in Pilot-PSI: 5 × 1021 m−3 at a temperature of 3 eV. TS measurements in front of the target are combined with ion saturation current data to determine plasma velocities of 4–5 km s−1, which shows that heat convection is dominating over conduction. Single shot operation of TS is also possible, which is demonstrated by measurements revealing a rotating filamentary return current channel to the source anode. Finally, the TS system upgrade that will provide real time feedback of electron density and temperature in the larger plasma generator Magnum-PSI is discussed.

Thomson scattering system on the TEXTOR tokamak using a multi-pass laser beam configuration

The main challenge for the Thomson scattering (TS) diagnostic on the TEXTOR tokamak is the detailed study of fast plasma events at a high spatial resolution and a high repetition rate of the measurements. The diagnostic uses intra-cavity probing of the plasma with a repetitively pulsed ruby laser and a fast CMOS camera as detectors. Since 2004, the TS system on TEXTOR has been gradually and systematically enhanced for the measurements of fast plasma events. For that it has recently been upgraded to obtain a multi-pass configuration. Two spherical mirrors have been installed that force the laser beam to probe the plasma a specified number of times before it is directed back into the laser medium. The diagnostics with the upgraded probing system have achieved the measurement accuracy of 3% for the electron temperature and 1.5% for the electron density at <1 cm spatial resolution and 3 × 1019 m−3 plasma density and can measure at 5 kHz during an interval up to 8 ms. This makes it possible to detect, amongst others, fine structures of magnetic islands and variations of the edge pedestal in the ELMy limiter H-mode.

Design and engineering of optical diagnostics for COMPASS

The paper generally outlines a design of new multichannel optical diagnostics for the COMPASS (COMPact ASSembly) tokamak. The final design of a port plug for a multispectral diagnostic tool is introduced and its evolution taking into consideration spatial and other technical restrictions is reviewed. A similar solution of both focusing optics and optical signal diversion will be used for a new Thomson scattering (TS) system, which is specified in the second part of the article. Main parameters of the TS system, such as laser energy and regions of observation, were determined with respect to both electron temperature and density profiles and required ultrahigh spatial resolution at the plasma edge.