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A new tool for the exploration and diagnosis of the internal magnetic field of plasmas in the DIII-D tokamak in the form of a constraint on the EFIT (Equilibrium Fitting) Grad–Shafranov code based on the Faraday-effect Radial Interferometer-Polarimeter (RIP) diagnostic is presented, including description, verification, and sample application. The physics underlying the diagnostic and its implementation into EFIT are discussed, and the results showing the verification of the model are given, and the model’s limitations are discussed. The influence of the diagnostic’s input on the resulting equilibrium parameters is characterized. The effect of electron density profile refinement is evaluated and found to be negligible. A sample application of the diagnostic is shown, indicating that the RIP constraint has similar effects on the equilibrium as motional Stark effect constraints do.

Vertical position stability plays a crucial role in maintaining safe and reliable plasma operation for long-pulse fusion devices. In general, the vertical position is measured by using inductive magnetic coils installed inside the vacuum vessel; however, the integration drift effects are inherent for steady-state or long-pulse plasma operation. Developing a non-magnetic approach provides a fusion reactor-relevant steady-state solution that avoids the negative impact of integration drift. In this paper, we compare the non-inductively determined vertical position achieved by line-integrated interferometer and polarimeter measurements to that employing an inductive flux loop for a 1056 s discharge recently achieved on EAST (Experimental Advanced Superconducting Tokamak). Experimental results show that the non-inductive measurement is more robust than flux loops after 300 s if the integrator is not reset to suppress integrator drift. Real-time vertical position control using the non-inductive system is proposed for the next EAST experimental campaign.

Toroidal current profile measurements in the tokamak plasma edge are critical for fusion plasma physics research and model validation. A three-wave Faraday-effect polarimeter-interferometer with a sub-centimeter spatial resolution is proposed on the DIII-D tokamak to determine the edge current profile via Abel inversion. By using probe beams with 316 µm wavelength, a low-field-side, vertical-view, single-pass optical layout covering the plasma edge region (R = 2.15–2.27 m) is assessed. Measurements with no greater than 0.1° polarimetric systematic uncertainty, no greater than 0.01° polarimetric root-mean-square noise (1 kHz bandwidth), and a 0.8 cm radial chord spacing are considered feasible based on the achieved performance of existing systems using similar wavelengths on fusion devices. Synthetic diagnostic calculations taking various factors into account, such as diagnostic uncertainty and quality of magnetic flux surfaces, find that the edge current profile can be determined with up to 0.12 MA/m2 uncertainty, or about 10% of the peak current density in the pedestal of an investigated high-confinement plasma.

DIII-D research is addressing critical challenges in preparation for ITER and the next generation of fusion devices through focusing on plasma physics fundamentals that underpin key fusion goals, understanding the interaction of disparate core and boundary plasma physics, and developing integrated scenarios for achieving high performance fusion regimes. Fundamental investigations into fusion energy science find that anomalous dissipation of runaway electrons (RE) that arise following a disruption is likely due to interactions with RE-driven kinetic instabilities, some of which have been directly observed, opening a new avenue for RE energy dissipation using naturally excited waves. Dimensionless parameter scaling of intrinsic rotation and gyrokinetic simulations give a predicted ITER rotation profile with significant turbulence stabilization. Coherence imaging spectroscopy confirms near sonic flow throughout the divertor towards the target, which may account for the convection-dominated parallel heat flux. Core-boundary integration studies show that the small angle slot divertor achieves detachment at lower density and extends plasma cooling across the divertor target plate, which is essential for controlling heat flux and erosion. The Super H-mode regime has been extended to high plasma current (2.0 MA) and density to achieve very high pedestal pressures (~30 kPa) and stored energy (3.2 MJ) with H 98y2 ≈ 1.6–2.4. In scenario work, the ITER baseline Q = 10 scenario with zero injected torque is found to have a fusion gain metric independent of current between q 95 = 2.8–3.7, and a lower limit of pedestal rotation for RMP ELM suppression has been found. In the wide pedestal QH-mode regime that exhibits improved performance and no ELMs, the start-up counter torque has been eliminated so that the entire discharge uses ≈0 injected torque and the operating space is more ITER-relevant. Finally, the high- (⩽3.8) hybrid scenario has been extended to the high-density levels necessary for radiating divertor operation, achieving ~40% divertor heat flux reduction using either argon or neon with P tot up to 15 MW.

Internal magnetic fluctuation measurements are utilized to identify turbulence associated with micro-tearing modes (MTM) in the DIII-D Edge-Localized-Mode (ELM)-y H-mode pedestal. Using a Faraday-effect polarimeter, magnetic turbulence (150–500 kHz) is directly observed with a typical line-averaged fluctuation amplitude of ∼0.8 G at peak frequency (250 kHz) and ∼15 G integrated over the spectrum from 150 to 500 kHz. Frequency, poloidal wavenumber, and propagation direction of the magnetic turbulence all serve to identify as MTM. Magnetic turbulence amplitude non-monotonically correlates with collision frequency, peaks off mid-plane, and correlates with electron temperature gradient evolution between ELMs, consistent with MTM features identified from theory and gyro-kinetic simulation. The magnetic turbulence growth correlates with confinement degradation in ELMy H-mode plasmas during a slow density ramp. These internal measurements provide unique constraints toward developing physics understanding and validating models of the H-mode pedestal for future devices.

An 11-channel Far-InfaRed (FIR) three-wave POlarimeter-INTerferometer (POINT) system has been successfully operated in 2015 EAST experimental campaign. For high accuracy measurement of POINT system, optimized optical system to reduce the stray light and crosstalk is very important. Optical design is done and improved by using ZEMAX software, in which spot size and energy distribution can be calculated in any position. The crosstalk and stray light can be reduced by optimized design of optical components and putting high extinction ratio materials in some key positions. Vibration isolation coefficient of optical platform is set to 90%. The optical platform and vibration isolation system are about 5 and 20 tons in weight respectively. To reduce vibration caused by the EAST hall, a more than 30 tons in weight stainless steel tower, filled with sand and mounted independent of the EAST machine, is constructed to ensure the stability of optics. Based on the optimized opto-mechanical design, the POINT system resolutions for Faraday rotation and line integral electron density measurements are 0.1° and 1 × 10 16 m −2 , respectively.

Magnetic turbulence is directly observed internally in the pedestal of ELMy H-mode tokamak plasmas using a newly developed Faraday-effect polarimetry measurement. Fluctuation amplitude is δ b r ≥ 15 G (150–500 kHz), with a ratio of magnetic to density fluctuation | δ b r / B | / | δ n / n | ≥ 0.15. Magnetic turbulence is identified as resulting from micro-tearing-instability and mode growth accompanied by degraded plasma confinement is observed.

A Faraday-effect-based radial-interferometer-polarimeter diagnostic has been developed to explore fast magnetic dynamics in high-performance DIII-D plasmas. The instrument measures radial magnetic field perturbations using three chords positioned near the magnetic axis. Newly developed solid-state sources operating at 650 GHz provide phase noise down to 0.01°/kHz and tunable bandwidth up to 10 MHz. Various systematic errors which can contaminate the polarimetric measurement have been investigated in detail. Distortion of circular polarization due to non-ideal optical components is calibrated using a rotating quarter wave plate technique. The impact of perpendicular magnetic field, i.e., the Cotton-Mouton effect, is evaluated. The error due to non-collinearity of probe beams is minimized to less than 0.5° for electron density up to 7 × 1019 m−3 by alignment optimization. Optical feedback, due to multiple reflections induced by the double-pass configuration, is identified and reduced. Coherent and broadband high-frequency magnetic fluctuations for DIII-D H-mode plasmas are observed.A Faraday-effect-based radial-interferometer-polarimeter diagnostic has been developed to explore fast magnetic dynamics in high-performance DIII-D plasmas. The instrument measures radial magnetic field perturbations using three chords positioned near the magnetic axis. Newly developed solid-state sources operating at 650 GHz provide phase noise down to 0.01°/kHz and tunable bandwidth up to 10 MHz. Various systematic errors which can contaminate the polarimetric measurement have been investigated in detail. Distortion of circular polarization due to non-ideal optical components is calibrated using a rotating quarter wave plate technique. The impact of perpendicular magnetic field, i.e., the Cotton-Mouton effect, is evaluated. The error due to non-collinearity of probe beams is minimized to less than 0.5° for electron density up to 7 × 1019 m−3 by alignment optimization. Optical feedback, due to multiple reflections induced by the double-pass configuration, is identified and reduced. Coherent and broadband...

Correlation techniques have been successfully utilized for plasma diagnostics like electron cyclotron emission to reduce measurement noise. We present the first application of such a technique to Faraday-effect polarimetry measurements on the Madison Symmetric Torus (MST). The MST far infrared (FIR) interferometer-polarimeter diagnostic utilizes 11 vertical chords with a chord separation of 7-8 cm and a heterodyne detection system for fluctuation measurements up to several hundred kHz. The planar-diode mixers viewing each chord represent independent noise sources; modifying the optical setup so that two different mixers view the same chord allows cross correlation between the two independent signals to reduce the noise floor in fluctuation measurements. In this manner, the noise floor in both interferometry and polarimetry measurements in reversed-field pinch discharges has been reduced by a factor of 20-30. The correlation polarimeter provides a sensitive measurement of broadband magnetic fluctuations.

Vertical instability control in an elongated plasma is highly desirable for a tokamak reactor. A multi-channel 694 GHz far-infrared laser-based polarimeter-interferometer system has been used to provide a non-inductive vertical position measurement in the long-pulse EAST tokamak. A detailed comparison of vertical position measurements by polarimetry and external inductive flux loops has been used to validate Faraday-effect polarimetry as an accurate high-time response vertical position sensor.

A heterodyne detection scheme is combined with a 10.59 μm CO

The operation of ITER requires high-quality estimates of the plasma electron density over multiple regions in the plasma for plasma evaluation, plasma control and machine protection purposes. Although the density regimes of ITER are not very different from those of existing tokamaks (1018–1021 m−3), the severe conditions of the fusion plasma environment present particular challenges to implementing these density diagnostics. In this paper we present an overview of the array of ITER electron density diagnostics designed to measure over the entire ITER domain: plasma core, pedestal, edge, scrape-off layer and divertor. It will focus on the challenges faced in making these measurements, and the technical solutions of the current designs.

A double-pass, radially viewing, far-infrared laser-based POlarimeter-INTerferometer (POINT) system utilizing the three-wave technique has been implemented for diagnosing the plasma current and electron density profiles in the Experimental Advanced Superconducting Tokamak (EAST). POINT has been operated routinely during the most recent experimental campaign and provides continuous 11 chord line-integrated Faraday effect and density measurement throughout the entire plasma discharge for all heating schemes and all plasma conditions (including ITER relevant scenario development). Reliability of both the polarimetric and interferometric measurements is demonstrated in 25 s plasmas with H-mode and 102 s long-pulse discharges. Current density, safety factor (q), and electron density profiles are reconstructed using equilibrium fitting code (EFIT) with POINT constraints for the plasma core.

Motivated by the need to measure fast equilibrium temporal dynamics, non-axisymmetric structures, and core magnetic fluctuations (coherent and broadband), a three-chord Faraday-effect polarimeter-interferometer system with fast time response and high phase resolution has recently been installed on the DIII-D tokamak. A novel detection scheme utilizing two probe beams and two detectors for each chord results in reduced phase noise and increased time response [δb ∼ 1G with up to 3 MHz bandwidth]. First measurement results were obtained during the recent DIII-D experimental campaign. Simultaneous Faraday and density measurements have been successfully demonstrated and high-frequency, up to 100 kHz, Faraday-effect perturbations have been observed. Preliminary comparisons with EFIT are used to validate diagnostic performance. Principle of the diagnostic and first experimental results is presented.

Measuring high-frequency fluctuations (above tearing mode frequencies) is important for diagnosing instabilities and transport phenomena. The Madison Symmetric Torus interferometer-polarimeter system has been upgraded to utilize improved planar-diode mixer technology. The new mixers reduce phase noise and allow more sensitive measurements of fluctuations at high frequency. Typical polarimeter rms phase noise values of 0.05°–0.07° are obtained with 400 kHz bandwidth. The low phase noise enables the resolution of fluctuations up to 250 kHz for polarimetry and 600 kHz for interferometry. The importance of probe beam alignment for polarimetry is also verified; previously reported tolerances of ≤0.1 mm displacement for equilibrium and tearing mode measurements minimize contamination due to spatial misalignment to within acceptable levels for chords near the magnetic axis.

A full-scale ITER toroidal interferometer and polarimeter (TIP) prototype, including an active feedback alignment system, has been installed and tested on the DIII-D tokamak. In the TIP prototype, a two-color interferometry measurement of line-integrated density is carried out at 10.59 μm and 5.22 μm using a CO2 and quantum cascade laser, respectively, while a separate polarimetry measurement of the plasma-induced Faraday effect is made at 10.59 μm. The TIP prototype is equipped with a piezo tip/tilt stage active feedback alignment system that minimizes noise in the measurement and keeps the diagnostic aligned throughout DIII-D discharges. The measured phase resolution for the polarimeter and interferometer is 0.05° (100 Hz bandwidth) and 1.9° (1 kHz bandwidth), respectively. The corresponding line-integrated density resolution for the vibration-compensated interferometer is δnL = 1.5 × 1018 m-2, and the magnetic field-weighted line-integrated density from the polarimeter is δnBL = 1.5 × 1019 Tm-2. Both interferometer and polarimeter measurements during DIII-D discharges compare well with the expectations based on calculations using Thomson scattering measured density profiles and magnetic equilibrium reconstructions. Additionally, larger bandwidth interferometer measurements show that the diagnostic is a sensitive monitor of core density fluctuations with demonstrated measurements of Alfven eigenmodes and tearing modes.

We report on current profile evolution in EAST NBI driven plasmas where two neutral beams are injected, one during the current ramp phase and the second during flattop. At the end of the current ramp phase, it is found that a flat q profile with q 0 ~ 1 is achieved with low magnetic shear in the core. It is observed that plasma current and density both relax much faster than resistive time, even in the absence of sawtooth activity when H–L transition occurs. Density fluctuations associated with magnetic perturbations (3/2) as a precursor to the H–L transition are observed. It is likely that these modes play a role in fast current transport.

Energetic-particle driven instabilities have been extensively observed in both tokamaks and stellarators. In order for such devices to ultimately succeed as D-T fusion reactors, the super-Alfvenic 3.5 Mev fusion-produced alpha particles must be sufficiently well confined. This requires the evaluation of losses from classical collisional transport processes as well as from energetic particle-driven instabilities. An important group of instabilities in this context are the discrete shear Alfven modes, which can readily be destabilized by energetic particles (with velocities of the order of vAlfven) through wave-particle resonances. While these modes in three-dimensional systems have many similarities to those in tokamaks, the detailed implementation of modeling tools has required development of new methods. Recent efforts in this direction will be described here, with an emphasis on reduced models (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A comprehensive understanding of the impact of the Cotton–Mouton effect on Faraday polarimetry measurements using counter-rotating circular polarization (CP) probe beams has been developed. By using the Jones theory, an analytic study shows that the Cotton–Mouton effect cancels to first order with coupling into the Faraday measurement only at higher orders. A Jones-based numerical study shows the coupling effect strongly depends on the Cotton–Mouton effect, Faraday effect, and wavelength chosen for the measurement. For realistic DIII-D plasma conditions and far-infrared wavelength, numerical calculation suggests the measurement is dominated by the Faraday effect while the coupling effect leads to a small but finite correction. By statistical comparison between the experimental measurement and the Jones-based numerical calculation under various plasma parameters, the impact of the Cotton–Mouton effect has been verified. Proper treatment of the coupling effect is essential in the data analysis under certain conditions for polarimetric measurement using CP in present devices and beyond.

A double-pass radially view 11 chords polarimeter-interferometer system has been operated on the experimental advanced superconducting tokamak and provides important current profile information for plasma control. Stray light originating from spurious reflections along the optical path (unwanted reflections from various optical components/mounts and transmissive optical elements such as windows, waveplates, and lens as well as the detectors) and also direct feedback from the retro-reflector used to realize the double-pass configuration can both contribute to contamination of the Faraday rotation measurement accuracy. Modulation of the Faraday rotation signal due to the interference from multiple reflections is observable when the interferometer phase (plasma density) varies with time. Direct reflection from the detector itself can be suppressed by employing an optical isolator consisting of a λ/4-waveplate and polarizer positioned in front of the mixer. A Faraday angle oscillation during the density ramping up (or down) can be reduced from 5°-10° to 1°-2° by eliminating reflections from the detector. Residual modulation arising from misalignment and stray light from other sources must be minimized to achieve accurate measurements of Faraday rotation.

Density fluctuations in the large-density-gradient region of improved confinement Madison Symmetric Torus reversed field pinch (RFP) plasmas exhibit multiple features that are characteristic of the trapped-electron mode (TEM). Core transport in conventional RFP plasmas is governed by magnetic stochasticity stemming from multiple long-wavelength tearing modes. Using inductive current profile control, these tearing modes are reduced, and global confinement is increased to that expected for comparable tokamak plasmas. Under these conditions, new short-wavelength fluctuations distinct from global tearing modes appear in the spectrum at a frequency of f ∼ 50 kHz, which have normalized perpendicular wavenumbers k⊥ρs≲0.2 and propagate in the electron diamagnetic drift direction. They exhibit a critical-gradient threshold, and the fluctuation amplitude increases with the local electron density gradient. These characteristics are consistent with predictions from gyrokinetic analysis using the Gene code, including ...

For future burning plasma experiments, all diagnostics must be re-evaluated in terms of their measurement capabilities and robustness in the harsh, high-temperature environment. This is certainly true for interferometry measurements where conventional approaches may not be ideal and interpretation may require modification due to high plasma temperatures. Optimizing these systems to provide maximum information is crucial to understanding burning plasma dynamics. This paper explores a variety of phase measurement techniques for the main body and divertor regions that can be utilized on fusion plasma experiments like ITER and beyond.

Recent results are summarized for the Helically Symmetric Experiment (HSX), which has the capability of running as a quasi-helically symmetric stellarator or as a more conventional, nonsymmetric stellarator. From X-ray measurements, we have demonstrated improved confinement of energetic particles. With central electron cyclotron heating, the density profiles in the quasi-symmetric configuration are peaked, in contrast to the hollow or flat profiles when the symmetry is broken. The difference in profiles is attributed to the lowering of the neoclassical thermodiffusive flux when the symmetry is present. The central electron temperature is ∼200 eV higher for the quasi-symmetric configuration over the nonsymmetric case. The power deposition profiles are similar for the two cases, implying that the neoclassical electron thermal conductivity is reduced with quasi-symmetry. Related to the good confinement characteristics in the quasi-symmetric mode of operation, fluctuations in the density and magnetic field, consistent with that of a global Alfven eigenmode (GAE), are observed. While the neoclassical characteristics of the quasi-symmetric and nonsymmetric configurations are very different, we have yet to find, under present operating conditions, any significant difference (other than the possible GAE mode) in turbulence characteristics or blob formation at the plasma edge.

Confinement in the reversed field pinch (RFP) has been shown to increase strongly with current profile control. The MST RFP can operate in two regimes: the standard regime with a naturally occurring current density profile, robust reconnection and dynamo activity; and the improved confinement regime with strong reduction in reconnection, dynamo and transport. New results in standard plasmas include the observation of a strong two-fluid Hall effect in reconnection and dynamo, the determination that the m = 0 edge resonant mode is nonlinearly driven, and the determination that tearing modes can lock to the wall via eddy currents in the shell. New results in improved confinement plasmas include observations that such plasmas are essentially dynamo-free, contain several isolated magnetic islands (as opposed to a stochastic field) and contain reduced high frequency turbulence. Auxiliary current drive and heating is now critical to RFP research. In MST, a programme to apply auxiliary systems to the RFP is underway and progress has accrued in several techniques, including lower hybrid and electron Bernstein wave injection, ac helicity injection current drive, pellet injection and neutral beam injection.

Far-infrared laser polarimetry with time response up to ∼1 μs and spatial resolution ∼8 cm has been successfully implemented on the Madison Symmetric Torus reversed-field pinch. Internal magnetic field and current density fluctuations are nonperturbatively measured. This is accomplished by taking parallel polarimetry chords which measure the line-integrated magnetic field fluctuations via Faraday rotation. With suitable analysis we are able to obtain information on spatial profiles of magnetic field fluctuations and current density fluctuations. Coherent interaction between these fluctuations is also measured and observed to generate an electromagnetic fluctuation-induced torque 〈δJ×δB〉.

The current profile and core magnetic field fluctuation amplitudes in a reversed-field pinch are measured by using a high-resolution polarimetry–interferometry system. This paper presents data showing a redistribution of the current during a sawtooth crash. Also, the core magnetic field fluctuation amplitude is observed to increase at a sawtooth crash consistent with the idea of nonlinearly driven dynamo current. In addition, the parallel current density increases in the outer region of the plasma during auxiliary pulsed parallel current drive. This was expected, as the external application of an edge parallel electric field is designed to flatten the current profile providing an equilibrium closer to the minimum energy Taylor state. However, the current density also increases in the core, relative to standard plasmas. This increase can be explained by a reduction of the dynamo (anti)current drive in the core that should accompany the measured reduction of magnetic fluctuations and by a drop in resistivity caused by the increased confinement of fast electrons.

Energy confinement comparable with tokamak quality is achieved in the Madison Symmetric Torus (MST) reversed field pinch (RFP) at a high beta and low toroidal magnetic field. Magnetic fluctuations normally present in the RFP are reduced via parallel current drive in the outer region of the plasma. In response, the electron temperature nearly triples and beta doubles. The confinement time increases ten-fold (to ~10 ms), which is comparable with L- and H-mode scaling values for a tokamak with the same plasma current, density, heating power, size and shape. Runaway electron confinement is evidenced by a 100-fold increase in hard x-ray bremsstrahlung. Fokker–Planck modelling of the x-ray energy spectrum reveals that the high energy electron diffusion is independent of the parallel velocity, uncharacteristic of magnetic transport and more like that for electrostatic turbulence. The high core electron temperature correlates strongly with a broadband reduction of resonant modes at mid-radius where the stochasticity is normally most intense. To extend profile control and add auxiliary heating, rf current drive and neutral beam heating are in development. Low power lower-hybrid and electron Bernstein wave injection experiments are underway. Dc current sustainment via ac helicity injection (sinusoidal inductive loop voltages) is also being tested. Low power neutral beam injection shows that fast ions are well-confined, even in the presence of relatively large magnetic fluctuations.

For several reasons, improved-confinement achieved in the reversed field pinch (RFP) during the last few years can be characterized as 'tokamak-like'. Historically, RFP plasmas have had relatively poor confinement due to tearing instability which causes magnetic stochasticity and enhanced transport. Tearing reduction is achieved through modification of the inductive current drive, which dramatically improves confinement. The electron temperature increases to >1?keV and the electron heat diffusivity decreases to ~5?m2?s?1, comparable with the transport level expected in a tokamak plasma of the same size and current. This corresponds to a 10-fold increase in global energy confinement. Runaway electrons are confined, and Fokker?Planck modelling of the electron distribution reveals that the diffusion at high energy is independent of the parallel velocity, uncharacteristic of stochastic transport. Improved-confinement occurs simultaneously with increased beta ~15%, while maintaining a magnetic field strength ten times weaker than a comparable tokamak. Measurements of the current, magnetic, and electric field profiles show that a simple Ohm's Law applies to this RFP sustained without dynamo relaxation.

A compact pellet injection system that was recently developed at the Oak Ridge National Laboratory has been installed on the Madison Symmetric Torus (MST) at University of Wisconsin and used in initial plasma fueling experiments. The system, referred to as a ''pellet injector in a suitcase,'' is a pipe gun device with a four-barrel capability (presently equipped with two 1.0-mm-bore barrels), and it uses a cryogenic refrigerator for in-situ hydrogen pellet formation (typically, D{sub 2} pellets). This new, portable, stand-alone pellet injection system was developed to provide a flexible means of plasma fueling on a wide variety of magnetic confinement devices, with relatively low costs for installation and operation. The injector has already been used to produce useful results with pellets on MST plasmas, including significant and rapid increases (almost 100%) in the line average density, and effectively depositing fuel in the plasma core (central densities of [approximately equal to] 1.4 x 10{sup 19} m{sup -3}). In this paper, the injection system, its performance, and reliability will be described, and results from some initial MST pellet experiments will be highlighted.

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

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 [ne>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.

Experimental and modeling investigations on the DIII-D and EAST tokamaks show the attractive transport and stability properties of fully noninductive, high poloidal-beta (β P ) plasmas, and their suitability for steady-state operating scenarios in ITER and CFETR. A key feature of the high-β P regime is the large-radius (ρ > 0.6) internal transport barrier (ITB), often observed in all channels (ne, Te, Ti, rotation), and responsible for both excellent energy confinement quality and excellent stability properties. Experiments on DIII-D have shown that, with a large-radius ITB, very high β N and β P values (both ≥ 4) can be reached by taking advantage of the stabilizing effect of a nearby conducting wall. Synergistically, higher plasma pressure provides turbulence suppression by Shafranov shift, leading to ITB sustainment independent of the plasma rotation. Experiments on EAST have been used to assess the long pulse potential of the high-β P regime. Using RF-only heating and current drive, EAST achieved minute-long fully noninductive steady state H-mode operation with strike points on an ITER-like tungsten divertor. Improved confinement (relative to standard H-mode) and steady state ITB features are observed with a monotonic q-profile with q min ~ 1.5. Separately, experiments have shown that increasing the density in plasmas driven by lower hybrid wave broadens the q-profile, a technique that could enable a large radius ITB. These experimental results have been used to validate MHD, current drive, and turbulent transport models, and to project the high-β P regime to a burning plasma. These projections suggest the Shafranov shift alone will not suffice to provide improved confinement (over standard H-mode) without rotation and rotation shear. However, increasing the negative magnetic shear (higher q on axis) provides a similar turbulence suppression mechanism to Shafranov shift, and can help devices such as ITER and CFETR achieve their steady-state fusion goals.

A full-scale 120 m path length ITER toroidal interferometer and polarimeter (TIP) prototype, including an active feedback alignment system, has been constructed and undergone initial testing at General Atomics. In the TIP prototype, two-color interferometry is carried out at 10.59 μm and 5.22 μm using a CO2 and quantum cascade laser (QCL) respectively while a separate polarimetry measurement of the plasma induced Faraday effect is made at 10.59 μm. The polarimeter system uses co-linear right and left-hand circularly polarized beams upshifted by 40 and 44 MHz acousto-optic cells respectively, to generate the necessary beat signal for heterodyne phase detection, while interferometry measurements are carried out at both 40 MHz and 44 MHz for the CO2 laser and 40 MHz for the QCL. The high-resolution phase information is obtained using an all-digital FPGA based phase demodulation scheme and precision clock source. The TIP prototype is equipped with a piezo tip/tilt stage active feedback alignment system responsible for minimizing noise in the measurement and keeping the TIP diagnostic aligned indefinitely on its 120 m beam path including as the ITER vessel is brought from ambient to operating temperatures. The prototype beam path incorporates translation stages to simulate ITER motion through a bake cycle as well as other sources of motion or misalignment. Even in the presence of significant motion, the TIP prototype is able to meet ITER's density measurement requirements over 1000 s shot durations with demonstrated phase resolution of 0.06° and 1.5° for the polarimeter and vibration compensated interferometer respectively. TIP vibration compensated interferometer measurements of a plasma have also been made in a pulsed radio frequency device and show a line-integrated density resolution of m−2.

At anticipated high electron temperatures in ITER, the effects of electron thermal motion on Thomson scattering (TS), toroidal interferometer/polarimeter (TIP), and poloidal polarimeter (PoPola) diagnostics will be significant and must be accurately treated. The precision of the previous lowest order linear in τ = Te/mec(2) model may be insufficient; we present a more precise model with τ(2)-order corrections to satisfy the high accuracy required for ITER TIP and PoPola diagnostics. The linear model is extended from Maxwellian to a more general class of anisotropic electron distributions that allows us to take into account distortions caused by equilibrium current, ECRH, and RF current drive effects. The classical problem of the degree of polarization of incoherent Thomson scattered radiation is solved analytically exactly without any approximations for the full range of incident polarizations, scattering angles, and electron thermal motion from non-relativistic to ultra-relativistic. The results are discussed in the context of the possible use of the polarization properties of Thomson scattered light as a method of Te measurement relevant to ITER operational scenarios.

A vibration compensation interferometer (wavelength at 0.532 μm) has been designed and tested for Experimental Advanced Superconducting Tokamak (EAST). It is designed as a sub-system for EAST far-infrared (wavelength at 432.5 μm) poloarimeter/interferometer system. Two Acoustic Optical Modulators have been applied to produce the 1 MHz intermediate frequency. The path length drift of the system is lower than 2 wavelengths within 10 min test, showing the system stability. The system sensitivity has been tested by applying a periodic vibration source on one mirror in the system. The vibration is measured and the result matches the source period. The system is expected to be installed on EAST by the end of 2014.

At anticipated high electron temperatures in ITER, the effects of electron thermal motion on Thomson scattering (TS), toroidal interferometer/polarimeter (TIP) and poloidal polarimeter (PoPola) diagnostics will be significant and must be accurately treated. We calculate electron thermal corrections to the interferometric phase and polarization state of an EM wave propagating along tangential and poloidal chords (Faraday and Cotton-Mouton polarimetry) and perform analysis of the degree of polarization for incoherent TS. The precision of the previous lowest order linear in τ = Te/mec2 model may be insufficient; we present a more precise model with τ2-order corrections to satisfy the high accuracy required for ITER TIP and PoPola diagnostics. The linear model is extended from Maxwellian to a more general class of anisotropic electron distributions that allows us to take into account distortions caused by equilibrium current, ECRH and RF current drive effects. The classical problem of degree of polarization o...

The origin of large scale density fluctuations and their role in electron transport has been investigated in the Madison Symmetric Torus reversed-field pinch [R. N. Dexter et al., Fusion Tech. 19, 131 (1991)]. At medium frequencies (10–30 kHz), n is associated with core-resonant magnetic fluctuations having poloidal mode number m=1 and toroidal mode numbers n=6–10. These chord-averaged density fluctuations are shown to be coherent with large scale radial flow fluctuations, permitting fluctuation-induced particle flux of the form 〈nṽr〉 to be studied. In the edge, although amplitudes are large, n and ṽr are out of phase, consistent with advection of the mean density gradient by perturbed magnetic field lines and no electron transport. During improved confinement discharges, produced by inductive current drive, both fluctuation amplitudes and the total radial electron flux are greatly reduced over the entire plasma cross section.The origin of large scale density fluctuations and their role in electron transport has been investigated in the Madison Symmetric Torus reversed-field pinch [R. N. Dexter et al., Fusion Tech. 19, 131 (1991)]. At medium frequencies (10–30 kHz), n is associated with core-resonant magnetic fluctuations having poloidal mode number m=1 and toroidal mode numbers n=6–10. These chord-averaged density fluctuations are shown to be coherent with large scale radial flow fluctuations, permitting fluctuation-induced particle flux of the form 〈nṽr〉 to be studied. In the edge, although amplitudes are large, n and ṽr are out of phase, consistent with advection of the mean density gradient by perturbed magnetic field lines and no electron transport. During improved confinement discharges, produced by inductive current drive, both fluctuation amplitudes and the total radial electron flux are greatly reduced over the entire plasma cross section.

A multichannel millimeter-wave interferometer system has been designed, fabricated and installed on the helically symmetric experiment (HSX), located at the University of Wisconsin, Madison. The interferometer system will view the plasma cross section along nine adjacent chords with 1.5 cm spacing. With this arrangement, coverage will span from the low-field side plasma scrape-off layer to well past the magnetic axis. For the plasma densities anticipated on HSX, a solid-state source operating at 288 GHz will be utilized. At this frequency refraction will be manageable, being less than the channel spacing. The source will be bias-tuned and modulated with a sawtooth wave form at 750 kHz in order to generate the intermediate frequency necessary for the heterodyne detection scheme. The signals will be measured using Schottky-diode corner-cube mixers. The interferometer will have sensitivity nedl≈8×1011 cm−2, being able to measure density changes

A recent study investigating the role of electron density fluctuations in particle transport has been conducted on the Madison Symmetric Torus reversed-field pinch. Four diagnostics enabled this experiment: a high-speed multichord far-infrared laser interferometer, a multichord Hα array, a 64-position magnetic coil array, and a Doppler spectrometer that measured impurity ion flow fluctuations. Correlation analysis is used to elucidate the relationship among density, magnetic, and impurity ion flow fluctuations. We observe that the electron density fluctuations are highly coherent with the magnetic fluctuations resulting from core-resonant resistive tearing modes. Moreover, the fluctuation-induced particle transport, obtained from the correlation between electron density and flow fluctuations, indicates that the core-resonant tearing modes do not drive significant particle transport in the plasma edge. We will address these four primary diagnostics, details of the analysis techniques, and principal results...

As a consequence of recent improvements to the far-infrared (FIR) interferometer time response, from bandwidth of 10 kHz up to 1 MHz, high-frequency density fluctuations can now be resolved on the Madison symmetric torus (MST) reversed-field pinch. The phase measurement allows absolute calibration of the fluctuation amplitude while eleven chords generate information on the spatial distribution. Density fluctuations up to 200 kHz are observed. A unique feature of the MST interferometer is that six chords are toroidally displaced 5° from the remaining five. Hence, besides information on the poloidal structure, i.e., m=1, toroidal information is also available. By computing the cross power, coherence, and cross phase between two toroidally displaced chords, one can determine the toroidal rotation speed, dispersion, and correlation length.

The far-infrared laser interferometer on the Madison Symmetric Torus has recently been upgraded to allow simultaneous interferometry and polarimetry measurements. Centered around a heterodyne laser system that operates at λ≃432 μm, the diagnostic has a frequency response of 1 kHz for the polarimeter and up to 1 MHz for the interferometer. The distribution of laser power through the 11 chord system is managed with wire mesh beamsplitters. Concurrent with the system upgrade to polarimetry, a detailed study of the effect of the wire mesh beamsplitters on beam polarization was conducted. The study shows that the mesh characteristics must be included to extract the polarimetry phase measurement from the raw data. The modifications to the data analysis as well as preliminary five chord polarimetry profile data are presented.

High spatial resolution electron cyclotron emission imaging (ECEI) has been employed on TEXT-U [Texas Experimental Tokamak Upgrade, G. Cima et al., Phys. Plasmas 2, 720 (1995)] to measure turbulent electron temperature fluctuations using an intensity interferometric technique. With the first dispersion relation measurements in the plasma confinement region, a broadband spectral feature is identified at poloidal wave numbers consistent with expectations for electron drift waves.

Experimental results are described from an integrated interferometer-reflectometer system implemented on TEXT-Upgrade. In plasma regions where the systems have spatial overlap, excellent agreement is observed between the independently measured electron density profiles. Combined interferometer-reflectometer data are used to obtain the optimal total density profile. By removing chords from the interferometer, one can examine how resolution is compromised as the channel number decreases and spacing varies. Depending on the expected profile shapes (i.e. flat or peaked), one can design an integrated interferometer-reflectometer system employing a minimum number of chords while still providing crucial profile information.

The electron heat diffusivity has been measured in the sawtoothing core of the TEXT-U tokamak plasma during electron cyclotron heating (ECH). ECH creates a non-steady-state electron temperature gradient on a time scale faster than a single sawtooth ramp, whose relaxation is measured with two ECE systems. The measured diffusivity is considerably smaller than that measured in the steady state during the ohmic phase of the discharge outside the sawtooth inversion radius. In these conditions, high-resolution ECE has also shown the presence of small scale, non-symmetric structures characterized by very large temperature gradients. These spatially localized, temporally cyclical structures, previously described as plasma filamentation, have been identified as plasma magnetic islands. If these small-scale gradients are taken into account then the reported measurements imply an electron heat diffusivity which approaches the neoclassical prediction.

A polarimetry capability has been added to the UCLA multichord far-infrared heterodyne interferometer system on TEXT-Upgrade. The polarimeter utilizes a phase technique in order to measure the Faraday rotation angle of the incident laser beam upon transmission through the plasma. From these data one derives information on the poloidal magnetic field, current density and safety factor profiles. The response of these profiles to electron cyclotron resonance heating and fast current ramps is investigated.

The principles of plasma electron cyclotron (EC) radiometry are reviewed. The aim is to lay the groundwork for discussion of new high performance electron cyclotron emission (ECE) detectors. One example is that of heterodyne arrays. Preliminary results are presented of data obtained with a novel ECE imaging system on TEXT-U based on such an array. A variety of preliminary experiments have already been performed with this system where the ultimate space and time resolutions of ECE are attained. As examples we cite average electron temperature profile measurements with 1 cm resolution, high frequency fluctuations and their vertical chord correlation, magnetohydro-dynamic mode reconstruction and magnetic island detection.

A novel wide bandwidth, low-cost, 20 channel hybrid Schottky diode mixer array-based imaging system has been developed, tested and implemented for electron cyclotron emission diagnostics on the Texas experimental tokamak upgrade (TEXT-U). The array has been successfully utilized to measure 1D and 2D electron temperature profiles, to study sawteeth and magnetohydrodynamic phenomena, and to make measurements of the poloidal/radial correlation lengths and wave-number spectra of electron temperature fluctuations. Fabrication and laboratory characterization results are presented, together with details and test results from the actual implementation on TEXT-U.