bluuebunny/crossref_metadata_2025 huggingface dataset:
KSTAR edge probe diagnostics
Authors: Bak J. G. , Lee S. G. ,   the KSTAR Project Team
Published: March 2003

Edge probe diagnostics (EPDs) for the KSTAR consists of fixed Langmuir probe arrays, movable and reciprocating Langmuir probe assemblies. The probe array is designed to measure plasma basic parameters and their poloidal profiles in the plasma-surface interaction regions such as an inboard limiter and divertors. The movable and the reciprocating probe assemblies are designed to measure spatial profiles of plasma parameters in the scrape-off layer region and near the X point, respectively. In this work, the current engineering design activities for the KSTAR EPDs, and technical issues from sample fabrications and their performance tests are presented.
Application of fast reflectometer density profile measurements to investigate plasma instabilities in DIII-D
Authors: Zeng L. , Doyle E. J. , Rhodes T. L. , Wang G. , Peebles W. A. , Burrell K. H.
Published: March 2003

The high spatial (⩾2 mm depending upon plasma conditions) and temporal (10 μs) resolution possible with the DIII-D solid state profile reflectometer system makes it possible to investigate profile modifications associated with plasma instabilities. In order to take full advantage of this fast sweep and high resolution capability, multiple issues had to be addressed, including fast data acquisition, large data acquisition memory depth, improved signal to noise, accurate profile start location, and finally fast accurate automatic data analysis. Improvements in all these areas make the profile reflectometer ready for plasma instability studies. In a demonstration of this capability, fast, high spatial resolution reflectometer measurements have been successfully used to investigate the edge harmonic oscillation (EHO) in the quiescent double barrier regime, as well as the edge localized modes (ELMs) on DIII-D. It is found that the scrape-off layer density profile is modulated at the EHO fundamental frequency. The edge density profile evolution is also tracked during ELMs, showing that the density profile expands to the vessel wall at the onset of Type 1 ELMs.
Improved reflectometer electron density profile measurements on DIII-D
Authors: Wang G. , Zeng L. , Doyle E. J. , Rhodes T. L. , Peebles W. A.
Published: March 2003

Details are presented of recent improvements to hardware and analysis code components of the DIII-D density profile reflectometer system. The improvements to the hardware increased the system signal-to-noise ratio and reduced spurious reflections. An improved automated method is developed to accurately determine the zero density plasma start position. With these improvements the uncertainty in the zero density plasma start position is reduced from ∼7 to ∼2 mm, the position uncertainty with a fixed mirror target is reduced from ∼4 to ∼2 mm, and the time resolution is improved to 10 μs (previously ∼100 μs).
Design of an X-mode fast-scanning reflectometry for edge density profile measurement on HT-7 tokamak
Authors: Zhang S. Y. , Qin G. B. , Zhuravlev V. , Poznyak V. , Ploskirev E.
Published: March 2003

An amplitude modulation reflectometry system with fast-scanning microwave source is designed for edge density profile measurement on the HT-7 superconducting tokamak (R0=1.22 m, a=0.28 m, Bt: 1.8–2.3 T, plasma pulses of 1–20 s). The reflectometer launches microwave power from a very fast-scanning backward wave oscillator in 53–78 GHz to probe the plasma and receive the reflection by the density cutoff layer. It is an extraordinary mode reflectometer, which can measure the edge density profiles in normalized radius r/a from scrape-off layer up to 0.5 when Bt=2.2 T, Ne0=3×1019 m−3 in typical HT-7 operation parameters. The microwave amplitude modulation frequency is 200 MHz. A 2 MHz reference signal and the probing signal are obtained by mixing a 198 MHz local oscillator power with the demodulated microwave signals. Phase comparison is then accomplished at 2 MHz by I/Q phase detector. The scanning time over the full band of 53–78 GHz is about 0.5 ms. The reflectometry will fill in the space left by the five-chord interferometer, which measures in the core plasma region and the Langmuir probes, which work in the scrape-off layer; especially the reflectometry is used to study the lower hybrid wave coupling into plasmas for the current drive and the radio frequency wave coupling problems for heating on HT-7. The reflectometry is also considered to be useful on the future HT-7U superconducting tokamak.
Ultrashort pulse reflectometry for density profile and fluctuation measurements on SSPX
Authors: Roh Y. , Domier C. W. , Luhmann N. C.
Published: March 2003

A broadband, multichannel ultrashort pulse reflectometry (USPR) diagnostic on the Sustained Spheromak Physics eXperiment device has recently undergone a number of system upgrades, which has resulted in significant improvements in the signal-to-noise ratio of the USPR signals and a dramatic reduction in the number of “lost signals” in which the amplitude of the reflected wave form drops below detection threshold. This has greatly enhanced the ability of USPR to study relatively fast density profile modifications, and allows the simultaneous monitoring of multiple density layers with as short as a 3 μs pulse repetition rate. This article provides details of the upgraded USPR system together with density profiles and fluctuation data.
Design of a single-channel millimeter-wave interferometer system for Korea Superconducting Tokamak Advanced Research
Authors: Nam Y. U. , Cheon M. S. , Kwon M. , Hwang Y. S.
Published: March 2003

A simple single-channel horizontal millimeter-wave interferometer has been designed for plasma electron density measurements on the Korea Superconducting Tokamak Advanced Research (KSTAR). To measure line integrated plasma densities of 2×1019 m−2 in the initial phase of the KSTAR, Gunn oscillator frequency of 280 GHz has been chosen to optimize errors due to both vibration on the beam path and refraction in the plasma. To reduce the free propagation length of the probing beam and to obtain small beam width on the vacuum windows, a retractable cassette system for deep positioning of the diagnostic system has been designed, where microwave parts are located as close as possible to the tokamak with a shielding box. A beam focusing system with concave reflecting mirrors has been designed on the cassette and on the inner wall of the tokamak to reduce beam losses and to minimize beam width in the plasma. The estimated total transmission loss is about 25 dB, and beam widths are reduced significantly in the range of 20–50 mm.
Herriott cell interferometry for millimeter-scale plasma measurements
Authors: Antonsen Erik L. , Burton Rodney L. , Spanjers Gregory G. , Engelman Scott F.
Published: March 2003

A Herriott cell consists of two concave mirrors positioned on opposite sides of plasma so as to create multiple laser paths through the plasma. Added to a traditional interferometry diagnostic, the Herriott cell multiplies the effective path length through the medium and thereby increases instrument resolution. Previous work used a planar geometry to validate the use of Herriott cells in interferometer applications where the numerous mirror reflections will significantly degrade the phase front quality. The current work extends the Herriott cell capability to a point configuration. In this geometry the multipass beams converge near a single point within the plasma, useful for performing density measurements in very small scale length plasmas. Ray tracing analysis is used to illustrate example measurement geometries attainable with the instrument. It is shown that the configuration results in two convergence points for the laser paths, which somewhat complicates the interpretation of the experimental data. In addition, the multitude of passes through the high density, small scale length passes increases the likelihood for deleterious refractive effects. The diagnostic capability is demonstrated with measurements of the electron and neutral densities in the plasma exhaust of a Micro Pulsed Plasma Thruster. The measurements are validated with similar, lower resolution measurements, obtained using a single-pass interferometer.
Emissive probe measurements of plasma potential fluctuations in the edge plasma regions of tokamaks
Authors: Balan P. , Schrittwieser R. , Ioniţă C. , Cabral J. A. , Figueiredo H. F. C. , Fernandes H. , Varandas C. , Adámek J. , Hron M. , Stöckel J. , Martines E. , Tichý M. , Van Oost G.
Published: March 2003

The plasma potential Φpl and its fluctuations Φ̃pl were measured by electron emissive probes in the edge plasma regions of two fusion experiments: the Instituto Superior Técnico Tokamak (ISTTOK) (Lisbon, Portugal), and the Czech Academy of Sciences Torus (CASTOR) tokamak (Prague, Czech Republic). Into ISTTOK, three emissive probes were inserted outside the last closed flux surface (LCFS) on different minor radii. In CASTOR, two emissive probes, poloidally separated, and two cold cylindrical probes, mounted on the same shaft, were used, which could be radially shifted outside and inside the LCFS. The advantages of a sufficiently emissive probe are that in principle Φpl and Φ̃pl can be measured directly, without being affected by electron temperature fluctuations or drifting electrons.
Measurements of coherent structures and transport in the full cross section of a magnetized plasma
Authors: Fredriksen Å. , Riccardi C. , Cartegni L.
Published: March 2003

A systematic analysis of electrostatic fluctuations and of the transport has been performed in the full cross section of the simple magnetized torus BLAAMANN applying the conditional sampling method in order to discriminate the coherent structures from the turbulent background. The analysis was carried out considering also the contribution of the electron temperature fluctuations.
Measurements of the radial profile of magnetic field in the Gas-Dynamic Trap using a motional Stark effect diagnostic
Authors: Bagryansky P. A. , Deichuli P. P. , Ivanov A. A. , Korepanov S. A. , Lizunov A. A. , Murakhtin S. V. , Savkin V. Ya. , Den Hartog D. J. , Fiksel G.
Published: March 2003

We have implemented a spectral motional Stark effect diagnostic for spatially localized measurements of magnetic field on the Gas-Dynamic Trap (GDT) magnetic mirror device. A 7 A, 40 keV diagnostic neutral hydrogen beam was focused to a current density of 0.25 A/cm2 in the measurement volume, allowing achievement of a radial resolution of 4.5 cm by viewing the beam at 45° with a well-defined light collection chord. The entire Stark spectrum was recorded by a spectrometer with a charge-coupled device detector. Analysis of the splitting of the components of the Stark spectrum directly results in a measurement of local |B|. When operated with deuterium heating neutral beam injection into a hydrogen target plasma, the GDT axial magnetic field is strongly perturbed by plasma diamagnetic effects. Thus, measurement of the radial profile of the perturbed magnetic field, combined with equilibrium modeling, will yield an estimate of the plasma beta.
Optical design for Li beam Zeeman polarimetry measurements on DIII–D
Authors: Carlstrom T. N. , Thomas D. M. , Brewis G.
Published: March 2003

Measurements of the magnetic field pitch angle are obtained from the polarization characteristics of the σ component of the Zeeman triplet of Li emission at 670.8 nm. A four element optical system images a horizontal Li beam on an array of 3×32, 1 mm diam optical fibers, providing 32 spatial channels with 5 mm radial resolution in the plasma. Low Verdet constant glass is used for all optical elements near the DIII–D vessel to minimize the effects of Faraday rotation caused by stray magnetic fields. Before entering the fiber optics, the light passes through two crossed (45°) photoelastic modulators (PEM) and a linear polarizer, which convert the various polarization states into an intensity modulated signal at the fundamental and second harmonic of the PEM frequencies. For each spatial channel, light from a three fiber bundle is collimated and passes through a temperature tuned etalon (free spectral range, FSR=0.3 nm; finesse, F=5.7) in order to select only one σ line of the triplet. The FSR is large enough to adequately cover the expected Zeeman triplet and small enough to achieve a low bandwidth (0.06 nm) at reasonably low F. A 1.0 nm wide interference filter is used to block all but four to five peaks of the etalon. The light is then focused on GaAs photomultiplier detectors. Details of the design and performance are presented.
Simulation of amplitude and phase variations induced by magnetic islands with turbulence on reflectometry signals
Authors: da Silva F. , Heuraux S. , Hacquin S. , Manso M.
Published: March 2003

Reflectrometry is influenced by plasma perturbations and turbulence, making the understanding of how signals are affected by such phenomena of prime importance. In this work we look at the effect magnetic islands with turbulence have on the amplitude and phase of reflectometry signals. For this purpose we resort to a two-dimensional finite difference time domain code and use a turbulence model based on experimental data. We investigate the possibility of occurrence of destructive interference due to a chain of islands and the role turbulence plays. The simulation of an island with turbulence present at the resonant surface q=2, with dimensions of the same order of magnitude as the phenomena observed on ASDEX Upgrade on Tore Supra, is also done with results which are in agreement with experimental observations.
Ultrashort-pulse reflectometry for steady-state plasmas
Authors: Kogi Y. , Mase A. , Ohsako S. , Yamamoto A. , Yasuda T. , Bruskin L. G. , Hojo H.
Published: March 2003

An ultrashort-pulse reflectometry (USRM) has been applied to the inductively coupled steady-state plasma (ICP) for density profile measurement. A reflected wave from ICP is directly recorded into a digitizing sampling scope and is analyzed by the signal record analysis (SRA) method. From repetition of the measurement, we have obtained the time evolution of the density profile. In order to apply the USRM and SRA method to high density plasmas such as core region of the Large Helical Device plasma, generation of the pulse with higher frequency up to 90 GHz is essential. We have utilized a frequency upconverter and an active doubler for this purpose. We confirmed the generation of the upconverted pulse in the range of 60–90 GHz and the second-harmonic frequency in the range of 26–40 GHz. Time of flight measurements have been preliminarily performed with a metal target.
A Stark-tuned laser application for interferometry and polarimetry on the National Spherical Torus Experiment
Authors: Lee K. C. , Domier C. W. , Deng B. H. , Johnson M. , Nathan B. R. , Luhmann N. C. , Park H.
Published: March 2003

Interferometer/polarimeter systems based on far infrared lasers have suffered from lack of frequency modulation techniques that can allow higher intermediate frequency (IF) frequency for a fast time resolution together with stable operation including low frequency drift. This is mainly due to the intrinsic narrow gain profile of the lasers. It is known that the Stark effect can broaden the laser gain profile as has been demonstrated in the laboratory. A Stark-tuned optically pumped far infrared (FIR) CH3OH laser together with two additional FIR lasers at 119 μm have been successfully implemented in the far infrared tangential interferometer/polarimeter system which will provide temporally and radially resolved two-dimensional electron density profile [ne(r,t)] and toroidal field profile [BT(r,t)] data for the National Spherical Tokamak Experiment. The characteristic frequencies of the IF system are ∼3, ∼4, and ∼7 MHz and a phase lock system was utilized for tracking the drift of the IF frequencies. In this article, the characteristics and operation of the Stark-tuned laser are described in detail. The measured electron density and Faraday rotation for various physics operation regimes are compared to the line integral of Thomson scattering measurement and EFIT results, respectively.
First results from the multichannel interferometer system on HSX
Authors: Deng C. , Brower D. L. , Ding W. X. , Almagri A. F. , Anderson D. T. , Anderson F. S. B. , Gerhardt S. P. , Probert P. , Talmadge J. N.
Published: March 2003

Measuring the equilibrium electron density distribution and its response to perturbations provides important information to magnetically confined plasma research. A multichannel interferometer system is now routinely operating on the new quasihelically symmetric stellator (HSX) to measure the equilibrium profile and electron density dynamics. The interferometer system has nine viewing chords with 1.5 cm spacing. The source is a bias-tuned Gunn diode at 96 GHz with passive solid-state tripler providing output at 288 GHz (8 mW). The HSX plasma is produced by 28 GHz electron cyclotron resonance heating and first results of the interferometer measurement are reported. The density spatial distribution is reconstructed from the measured line-integrated density. At high density [n̄e>2×1012 cm−3], an m=1 density oscillation with frequency of 1–2 kHz is observed. Plans to determine the radial particle flux and transport coefficients will be discussed.
Advances of the density profile reflectometry on TORE SUPRA
Authors: Clairet F. , Bottereau C. , Chareau J. M. , Sabot R.
Published: March 2003

Significant improvements have been achieved in the performances of the broadband reflectometry for the density profile measurements on Tore Supra. The frequency range of the former 50–75 GHz X-mode reflectometer has been extended with an additional device operating between 75 and 110 GHz. Both setups have ultrafast sweeping capabilities allowing full band measurements in 20 μs. The heterodyne detection and the intermediate frequency system have been upgraded to record beat frequencies up to 100 MHz with a signal to noise ratio of about 40 dB. The in phase and quadrature-type phase detection system (IQ detector) provides sine and cosine components of the received signal for a separate measurement of the phase and the amplitude. A new fast data acquisition system in Versa Module Europa format has been developed. It allows sampling frequency up to 200 MHz with 32 Mo memory capabilities per channel and 10 bit resolution digitizers. Edge profile measurements are now available for a wide range of toroidal magnetic fields (from 3 to 4 T).
Plasma position measurements from ordinary FM-CW reflectometry on ASDEX Upgrade
Authors: Santos J. , Manso M. , Varela P. , Neuhauser and the J. ,   ASDEX Upgrade Team
Published: March 2003

Microwave reflectometry has been proposed as an alternative/backup approach to the usual magnetic systems in long pulse operation on the International Thermonuclear Experimental Reactor (ITER) for plasma position and shape control. This new application of reflectometry must be fully demonstrated in present machines in view of its importance for ITER. First experiments performed in ASDEX Upgrade with frequency modulated continuous wave with reflectometry diagnostic showed that radial variations of the plasma column occurring during preprogrammed horizontal as well as vertical plasma displacements could be followed with sufficient accuracy (⩽1 cm) in phases with approximately constant density. In this article we determine the plasma edge position simultaneously in the inner and outer midplane, of ohmic L-mode density limit and ELMy H-mode discharges in ASDEX Upgrade, both with significant density variations. It is shown that if normalized density layers are selected, at a certain percentage of the average line density, the results from reflectometry are in good agreement with the corresponding data, separatrix position, derived from the magnetics. Further work aiming at the full demonstration of plasma position and shape measurements from reflectometry is discussed.
High resolution edge density measurements in ASDEX upgrade H-mode discharges with broadband reflectometry
Authors: Varela P. , Manso M. E. , Conway G. D. , Suttrop W. , Zohm H. ,   ASDEX Upgrade Team
Published: March 2003

In high-confinement (H-mode) plasmas with edge localized modes (ELMs), the energy stored at the edge pedestal region directly affects the global plasma confinement. The measurement of the edge pedestal evolution and the study of the collapse mechanism of the pedestal structure due to ELMs is, therefore, of great importance to understand plasma confinement. In Axially Symmetric Divertor Experiment (ASDEX) Upgrade, information about the edge density profile has been obtained in H-mode plasmas both with Thomson scattering and by combining lithium beam and interferometry measurements. However, information around the edge pedestal region is not routinely available. Here we use microwave reflectometry to obtain high resolution measurements of the edge transport barrier and pedestal region, namely the pedestal position and density, the pedestal width, and the edge density gradient. The evolution of the pedestal density is obtained automatically with an algorithm applied directly to the group delay. The other parameters are derived from the corresponding density profiles. Measurements performed in the presence of type I ELMs reveal that the edge density profile modifications occur in the same time scale of the Dα radiation at the divertor. The collapse and recovery of the pedestal structure during each ELM is resolved. The values of the pedestal width (around 4–5 cm) measured in-between ELMs are similar to those obtained previously in similar ASDEX Upgrade discharges. Also, the evolution of the pedestal density given by reflectometry is found to be in good agreement with the changes of the line integrated density measured by the interferometer.
BXO mode-converted electron Bernstein emission diagnostic (invited)
Authors: Volpe F. , Laqua H. P. ,   W7-AS Team
Published: March 2003

Electron temperature profiles at densities above the electron cyclotron emission (ECE) cutoff are measured at the W7-AS stellarator by a novel diagnostic based on black body emission and Bernstein-extraordinary-ordinary mode conversion of electron Bernstein waves (EBWs). The radiation is collected along a special oblique line of sight by an antenna with gaussian optics. This was optimized for maximal conversion efficiency and minimal Doppler broadening by means of EBW ray tracing calculations in full stellarator geometry. The elliptical O-mode polarization detected along the oblique line of sight is changed into a linear polarization by a broadband quarter wave shifter, namely an elliptical waveguide. The signal is spectrum analyzed by an heterodyne radiometer and temperature profiles are derived from spectra by means of ray tracing. The diagnostic was applied to measurements of edge-localized modes to illustrate its advantages in terms of spatial and temporal resolution. Moreover, for the first time, the heat wave propagation method for the determination of local heat transport coefficients was extended beyond the ECE cutoff density by combining EBW emission measurements at the first harmonic (f=66–78 GHz) with modulated EBW heating at the second harmonic (140 GHz).
Correlation reflectometry for turbulence and magnetic field measurements in fusion plasmas (invited)
Authors: Kramer G. J. , Nazikian R. , Valeo E.
Published: March 2003

For the interpretation of correlation reflectometry data a fast two-dimensional full wave code in which realistic plasma geometries are used has been developed. Results of this code are compared with experiments from which turbulence correlation lengths and fluctuation levels are extracted with statistical optics methods. It is shown that in general the measured reflectometer correlation length is not equal to the turbulence correlation length. The code is also used to study the possibility of O–X correlation reflectometry in FIRE for the determination of the local magnetic field strength. It was found that this is only possible at very low fluctuation levels.
Microwave imaging reflectometer for TEXTOR (invited)
Authors: Munsat T. , Mazzucato E. , Park H. , Deng B. H. , Domier C. W. , Luhmann N. C. , Wang J. , Xia Z. G. , Donné A. J. H. , van de Pol M.
Published: March 2003

Understanding the behavior of fluctuations in magnetically confined plasmas is essential to the advancement of turbulence-based transport physics. Though microwave reflectometry has proven to be an extremely useful and sensitive tool for measuring small density fluctuations in some circumstances, this technique has been shown to have limited viability for large amplitude, high kθ fluctuations and/or core measurements. To this end, a new instrument based on two-dimensional imaging reflectometry has been developed to measure density fluctuations over an extended plasma region in the TEXTOR tokamak. This technique is made possible by collecting an extended spectrum of reflected waves with large-aperture imaging optics. Details of the imaging reflectometry concept, as well as technical details of the TEXTOR instrument, are presented. Data from proof-of-principle experiments on TEXTOR using a prototype system is presented, as well as results from a systematic off-line study of the advantages and limitations of the imaging reflectometer.
Design of a heterodyne electron cyclotron emission system on KSTAR
Authors: Jeong S. H. , Kim I. Y. , Hwang C. K.
Published: March 2003

This article presents the design of an electron cyclotron emission (ECE) diagnostic system to measure the electron temperature and its profiles in the KSTAR tokamak. First, we discuss the feasibility of ECE heterodyne radiometer diagnostic for the initial plasma (BT=1.5 T,Ro=1.6 m,a=0.3 m) of KSTAR. From an evaluation of the cutoff range, the optical thickness, and the harmonic overlap, it is shown that the second harmonic X mode is the most suitable choice. The designed E-band radiometer operates in a single side band and has an eight-channel power divider and filter banks to cover a frequency range of 72–84 GHz. The noise temperature and the sensitivity of the radiometer are estimated. A time resolution of 10 μs and a spatial resolution of <5 cm are achieved. The KSTAR is to be equipped with a fully superconducting magnet system, and thus the vacuum vessel is covered with a large cryostat, which results in experimentally long viewports to the outside. This makes the optimization of the optical system difficult. The optical system consists of a metal lens, a dielectric lens, and a scalar horn antenna. A metal lens has a focal length that is dependent upon frequency. This feature is one of the advantages of using a metal lens, since it places the focus approximately where the emission occurs. Radii of beam waists in the plasma region are 10–15 mm, corresponding frequency range of 72–84 GHz.
Electron Bernstein wave-X–O mode conversion and electron cyclotron emission in MAST
Authors: Preinhaelter Josef , Pavlo Pavol , Shevchenko Vladimir , Valovic Martin ,   MAST Team, Vahala Linda , Vahala George
Published: March 2003

Electron cyclotron emission (ECE) from overdense plasmas can only occur due to electron Bernstein waves (EBW) mode converting near the upper hybrid region to an electromagnetic wave. Experimental data of ECE observations on MAST are studied and compared with EBW-X–O mode conversion modeling results.
Electron cyclotron emission imaging on a large helical device
Authors: Mase A. , Kogi Y. , Ohashi M. , Ohsako S. , Nagayama Y. , Kawahata K. , Aoi S. , Sakata E.
Published: March 2003

In this article, we describe the electron cyclotron emission (ECE) imaging system applied to the Large Helical Device (LHD) at the National Institute for Fusion Science. The imaging system consists of focusing optics installed inside the vacuum chamber of LHD and planer-type detectors fabricated by monolithic microwave integrated circuit technology. The detector consists of the integration of a bowtie antenna, a down-converting mixer using a Schottky barrier diode, and heterojunction bipolar transistors (HBTs) on a GaAs substrate. The HBTs work as an intermediate frequency (IF) amplifier with a 10 GHz bandwidth and a 10 dB voltage gain. The ECE signal and local oscillator beam are irradiated from both sides of the detector. The ECE signals are down-converted at the mixers and the IF signal is fed to a filter bank with center frequencies of 1–8 GHz. The time evolution and the intensity of the ECE signals agree with those obtained by a conventional ECE heterodyne receiver. The cross-correlation spectra of the signals obtained with different IF frequencies (radial correlation) and different detectors (poloidal correlation) are obtained.
Broadband electron cyclotron emission radiometry for the large helical device
Authors: Kawahata K. , Nagayama Y. , Inagaki S. , Ito Y. ,   LHD Experimental Group
Published: March 2003

A 32-channel heterodyne radiometer for the purpose of measuring the electron cyclotron emission (ECE) spectrum has been developed and installed on the large helical device (LHD). Two double-sideband mixers [bandwidth (BW=2–26 GHz) with image rejection filters (high-pass and low-pass filters)] are used to cover almost the full spectral range of the second harmonic LHD ECE. The spectral resolution of each sideband receiver is Δf=1.2 GHz, which corresponds to a spatial resolution of 1 cm at the position of the half plasma radius. A Gunn oscillator at 132 GHz is used as a local oscillator, so the frequency range covered by the system is 106–158 GHz, which corresponds to the second harmonic ECE for B(0)=2.82 T. It is found that the high temporal resolution (Δt∼1 μs) of the system with excellent signal-to-noise ratio is favorable for studying fast phenomena related to the magnetohydrodynamic activities and transport transitions. In this article, the properties of the diagnostic instrument and typical experimental results of the LHD ECE will be presented.
Electron Bernstein wave emission diagnostic assisted by reflectometry on TST-2 spherical tokamak
Authors: Shiraiwa S. , Nagashima Y. , Ushigome M. , Yamada T. , Taniguchi T. , Ohara S. , Yamagishi K. , Kasahara H. , Iijima D. , Kobori Y. , Nishi T. , Aramasu M. , Ejiri A. , Takase Y.
Published: March 2003

A new electron Bernstein wave emission diagnostic consisting of a heterodyne radiometer and a reflectometer was installed on TST-2. The instrument has a frequency range from 5 to 12 GHz which covers partially the fundamental and fully covers the second and third harmonic emissions. X-mode emission perpendicular to the flux surface, generated by B–X–FX mode-conversion scenario, is measured. The mode-conversion efficiency is calculated using a one-dimensional full-wave code with the density profile obtained simultaneously from the reflectometer, and the electron temperature (Te) is estimated. The levels corresponding to the 100–150 eV range are measured from the plasma core region, and the mode-conversion efficiency is calculated to be 0.5–0.8, leading to Te of approximately 200 eV. In addition, it is found that the reflectivity measured by the reflectometer is consistent with the calculated mode-conversion efficiency. This property can be used for a direct measurement of the mode-conversion efficiency.
Electron cyclotron emission radiometer upgrade on the DIII-D tokamak
Authors: Austin M. E. , Lohr J.
Published: March 2003

The electron cyclotron emission (ECE) heterodyne radiometer diagnostic on DIII-D has been upgraded with the addition of eight channels for a total of 40. The new, higher frequency channels allow measurements of electron temperature into the magnetic axis in discharges at maximum field, 2.15 T. The complete set now extends over the full usable range of second harmonic emission frequencies at 2.0 T, covering radii from the outer edge inward to the location of third harmonic overlap on the high-field side. Full coverage permits the measurement of heat pulses and magnetohydrodynamic fluctuations on both sides of the magnetic axis. In addition, the symmetric measurements are used to fix the location of the magnetic axis in tokamak magnetic equilibrium reconstructions. Also, the new, higher frequency channels have been used to determine central Te with good time resolution in low-field, high-density discharges using third harmonic ECE in the optically gray and optically thick regimes.
Evaluation of possible nuclear magnetic resonance diagnostic techniques for tokamak experiments
Authors: Zweben S. J. , Kornack T. W. , Majeski D. , Schilling G. , Skinner C. H. , Wilson R. , Kuzma N.
Published: March 2003

Potential applications of nuclear magnetic resonance (NMR) diagnostic techniques to tokamak experiments are evaluated. NMR frequencies for hydrogen isotopes and low-Z nuclei in such experiments are in the frequency range ≈20–200 MHz, so existing rf antennas could be used to rotate the spin polarization and to make the NMR measurements. Our tentative conclusion is that such measurements are possible if highly spin polarized H or He3 gas sources (which exist) are used to fuel these plasmas. In addition, NMR measurements of the surface layers of the first wall (without plasma) may also be possible, e.g., to evaluate the inventory of tritium inside the vessel.
Time-of-flight refractometry for robust line integral electron density measurements and control in ITER
Authors: Petrov A. A. , Petrov V. G.
Published: March 2003

A new method of electron density measurements in magnetic confinement plasmas—time-of-flight refractometry (TFR)—is proposed for application in the International Thermonuclear Experimental Reactor (ITER) in order to get robust line integral electron density information. The method provides unambiguous density measurements directly from the measurements of time delay of microwave pulses propagating through the plasma. Generally, both O-and-X probing modes are available for use in the measurements. At ITER conditions (BT∼5 T), X-mode probing in the equatorial plane seems to be attractive for application due to transparency of ITER plasma for electromagnetic emission between lower and upper cutoffs at approximately 60–105 GHz frequency band. When these relatively low frequencies are used for measurements, both electron cyclotron resonance absorption and first mirror problems should be avoided. Also, relatively simple implementation of the access to plasma and comparatively low sensitivity to vibrations and misalignment are expected. General approach to TFR is given in the article. The choice of probing frequencies of TFR for ITER has been conducted. Estimations of expected time delays for different density profiles have been performed. Parameters of TFR prototype used on the T-11M tokamak and results obtained are discussed. A conclusion of sufficiency of recently attained accuracy of the measurements for TFR application on large tokamaks has been done. Some ways of improving the measurement accuracy have been proposed and tested. Advantages and problems of TFR application are discussed.
Progress toward a practical magnetic field diagnostic for low-field fusion plasmas based on dual mode correlation reflectometry
Authors: Gilmore M. , Peebles W. A. , Kubota S. , Nguyen X. V. , Ejiri A.
Published: March 2003

Previously, the proof of principle of measurement of magnetic field strength, |B| internal to a plasma by cross correlation of ordinary (O) and extraordinary (X) mode fluctuation reflectometer signals has been demonstrated in a linear plasma device. It was found that dual mode (O-X) reflectometry data could be interpreted by a one-dimensional numerical model to determine |B| in the vicinity of the reflectometer cutoff positions. Radial correlation properties of turbulence are also measured simultaneously with |B|. This technique is potentially well suited to measurement of |B| in low-field fusion devices (e.g., B<0.6 T) where standard magnetic field diagnostics are expected to be difficult to implement. However, transfer of this method to toroidal magnetic fusion devices presents a number of potential difficulties such as the effects of magnetic shear, steep density gradients, and limited time for cross correlations. In addition, recent simulations suggest that two-dimensional modeling may be required to interpret experimental data when density fluctuation levels are high. Dual mode correlation reflectometry experiments on the National Spherical Torus Experiment are presented, and progress toward the implementation of this technique as a practical diagnostic is discussed.
Automatic profile reconstruction for millimeter-wave frequency-modulated continuous-wave reflectometry on NSTX
Authors: Kubota S. , Peebles W. A. , Nguyen X. V. , Roquemore A. L.
Published: March 2003

UCLA operates a set of millimeter-wave/microwave reflectometers on the National Spherical Torus Experiment (NSTX) for routine measurements of electron density profiles and fluctuations. The system has a combined frequency coverage of 12 to 50 GHz (in the bands 12–18, 20–32, and 33–50 GHz) or a corresponding ordinary-mode cutoff range of 1.8×1012 to 3.1×1013 cm−3 to cover both the plasma core and edge. Profile measurements via frequency-modulated continuous-wave operation are typically made in O-mode reflectometry, with sweep times down to 50 μs over the full band. Automated profile analysis of the reflectometry data is available with limited between-shot analysis and full batch analysis capabilities. The reconstruction algorithm uses complex demodulation with the short-time Fourier transform for signal processing. The unknown portion of the edge profile below the lowest cutoff density is modeled by fitting a family of polynomial density profiles to the experimental data. Uncertainties due to edge profile modeling and comparisons to Thomson scattering measurements are discussed. The reconstructed profiles have documented fast events such as L–H transitions and edge-localized modes in NSTX.
CO
2
laser imaging interferometer for high spatial resolution electron density profile measurements on LHD
Authors: Akiyama T. , Tanaka K. , Vyacheslavov L. N. , Sanin A. , Tokuzawa T. , Ito Y. , Tsuji-Iio S. , Okajima S. , Kawahata K.
Published: March 2003

A CO2 laser (λ=10.6 μm) imaging interferometer was developed for detailed and precise electron density profile measurements and was installed on the large helical device in 2001. A CO2 laser is adopted to avoid fringe loss due to the short wavelength. An imaging scheme is employed that uses three slab-like beams and multichannel detector arrays to measure with fine spatial resolution. As an initial trial we used one slab-like beam that passes through the plasma edge. Phase distributions were measured without fringe loss even after pellet injection. Density profiles can be obtained by the slice and stack method with a spatial resolution of 16 mm (about 3% of the plasma minor radius). The electron density was found to rise to about 6×1019 m−3 just after pellet injection even near the last closed flux surface. Mechanical vibration effects would be negligible in such a case. A bench top experiment of a coaxial YAG laser (λ=1.06 μm) interferometer for vibration compensation was carried out.
Collective Thomson scattering based on CO2 laser for ion energy spectrum measurements in JT-60U
Authors: Kondoh T. , Miura Y. , Lee S. , Richards R. K. , Hutchinson D. P. , Bennett C. A.
Published: March 2003

A collective Thomson scattering (CTS) diagnostic system based on a pulsed CO2 laser has been developed and brought into operation to establish a measurement technique for ion temperature and the energy spectrum of fast ions. The pulsed CO2 laser (wavelength 10.6 μm, beam energy 15 J, pulse width 1 μs) and a heterodyne receiver were installed on the JT-60U tokamak. Calculation of the scattered power spectrum from high temperature plasma in JT-60U shows that a good signal-to-noise value is expected for the bulk-ion temperature measurement. Calibration of the heterodyne receiver system has been carried out using a large area (12×12 cm2) blackbody radiation source. Commissioning of the CTS system by injecting the CO2 laser into the plasmas has been accomplished. However, a scattered signal has not yet been detected due to electrical noise originating from the pulsed lasers discharge and stray signal coming from mode impurities in the pulsed laser. After the electrical and stray light reduction, ion temperature will be evaluated from the scattered spectrum using the CTS system.
Large area avalanche photodiode detector array upgrade for a ruby-laser Thomson scattering system
Authors: Biewer T. M. , Den Hartog D. J. , Holly D. J. , Stoneking M. R.
Published: March 2003

A low-cost upgrade has been implemented on the Madison Symmetric Torus (MST) ruby-laser Thomson scattering (TS) system to increase spectral coverage and substantially improve the signal-to-noise ratio (SNR). The spectral resolution has been increased from five channels on the long-wavelength side to an 11-channel system, which covers both sides of the laser line. Coverage on both sides of the spectrum allows for more accurate detection of subtle changes in the distribution function, particularly relativistic spectral shifts of high-temperature plasmas during auxiliary current drive. The previous microchannel plate (MCP) detector was replaced with an array of modular large area avalanche photodiode detectors, which have approximately 16 times the quantum efficiency of the MCP detector. Scattered light collection has also been upgraded, allowing the radial viewing location of the TS system to be easily changed between plasma discharges. Improved SNR and upgraded light collection hardware in the Thomson scattering system have facilitated first-time measurements of the evolution of the electron temperature profile in the MST under a variety of discharge conditions, leading to increased understanding of the underlying dynamics of reversed-field pinch plasmas.
Prospects for measurement of rapid equilibrium changes and electron fluctuations using a high repetition rate Thomson scattering diagnostic
Authors: Den Hartog D. J. , Holly D. J. , O'Connell R. , Beach R. J. , Payne S. A. , Carlstrom T. N.
Published: March 2003

Recent technological developments make a laser Thomson scattering diagnostic, operating in the incoherent electron scattering regime, a good candidate to accomplish measurements of fast electron dynamics (i.e., fast equilibrium changes, turbulence, and electron fluctuations) in high temperature plasmas. Pulse repetition rates for current generation Thomson scattering diagnostics have been limited to about 100 Hz, with the possible exception of “burst” modes in which multiple laser systems provide a limited sequence of closely spaced pulses. To overcome this limitation in laser capability, we propose that recent advances in compact, high power, diode-pumped solid state lasers be applied to a fast Thomson scattering diagnostic for fusion research. To illustrate the possibilities, we present an overview of a diagnostic system designed for the Madison Symmetry Torus reversed field pinch. The operational goal for this single-laser system is to measure T̃e, ñe, and p̃e with a measurement rate of at least 10 kHz and spatial resolution of 2 cm. If successful, the technique can be extended to higher temporal frequencies and much better radial resolution, and implemented on other fusion research devices.
Operation of the NSTX Thomson scattering system
Authors: LeBlanc B. P. , Bell R. E. , Johnson D. W. , Hoffman D. E. , Long D. C. , Palladino R. W.
Published: March 2003

The National Spherical Torus Experiment (NSTX) multipoint Thomson scattering system has been in operation for nearly two years and provides routine Te(R,t) and ne(R,t) measurements. The laser beams from two 30 Hz Nd:YAG lasers are imaged by a spherical mirror onto 36 fiber-optic bundles. In the present configuration, the output ends of 20 of these bundles are instrumented with filter polychromators and avalanche photodiode detectors. In this article, we discuss the laser implementation and the installed collection optics. We follow with examples of raw and analyzed data, and close with some comments about calibration.
Thomson scattering upgrades on Alcator C-Mod
Authors: Hughes J. W. , Mossessian D. , Zhurovich K. , DeMaria M. , Jensen K. , Hubbard A.
Published: March 2003

The Thomson scattering (TS) diagnostics set has been upgraded to deliver measurements of electron temperature (Te) and density (ne) with improved temporal and spatial resolution. Two 30 Hz, 1.3 J Nd-doped yttrium–aluminum–garnet (Nd:YAG) lasers share a vertical beam path through the tokamak plasma and are fired in an alternating fashion, allowing the collection of scattered light with a 60 Hz repetition rate. Eight compact polychromators have been added to the system, better than doubling the spatial resolution of core plasma TS measurements and allowing better diagnosis of core ne and Te gradients. Each fiber holder can be repositioned easily, making it possible to concentrate core measurements in the regions of particular interest with a spatial resolution of ∼1 cm. The newly designed mount can be positioned with submillimeter precision, and its alignment with respect to the Nd:YAG beam can be adjusted between discharges using remote controlled stepper motors. Cross calibration of TS ne measurements with electron cyclotron emission (ECE) diagnostics is performed using dynamic ramps of both plasma density and magnetic field to force ECE signal cutoffs at known values of ne. This technique supplements an absolute calibration based on Raman scattering in hydrogen and deuterium gas.
GaAs photomultiplier for LIDAR Thomson scattering
Authors: Pasqualotto R. , Nielsen P.
Published: March 2003

Edge light detection and ranging (LIDAR) Thomson scattering at Joint European Torus (JET) is used for measuring temperature and density gradients of the edge plasma. The accuracy of the present measurements is limited not only by the spatial resolution of the system but also by photon statistics. The accuracy of the measurements can be increased by at least a factor of 2 by improving the sensitivity of the detection system. The present detectors are fast, 18 mm diameter microchannel plate photomultipliers (MCP PMTs) with multialkali photocathodes. The quantum efficiency is 5% at 650 nm decreasing toward the red. We present the characteristics of a newly developed MCP PMT with a GaAs photocathode. Four 18 mm diameter photomultipliers with a V-stack MCP and a preliminary prototype unit have been produced and tested to fully equip the edge LIDAR diagnostic. The GaAs photocathode of the new detectors has a quantum efficiency in excess of 20% in the spectral range of 520–850 nm. The effective quantum efficiency has been measured and is at least four times that of the present detectors. Investigations of the other characteristics required for use in a LIDAR system show good results. The pulse response time is slightly faster than that of the current tubes, potentially offering the possibility of improving the spatial resolution to better than the present value of 12 cm. The effect of varying the gate voltage between 100–300 V has been investigated. Lower gate voltages cause less ringing on the output signal and only increase the effective noise slightly.