Performance demonstration of vacuum microwave components critical for the operation of the ITER low-field side reflectometer.

Final design studies in preparation for manufacturing have been performed for functional components of the vacuum portion of the ITER Low-Field Side Reflectometer (LFSR). These components consist of an antenna array, electron cyclotron heating (ECH) protection mirrors, phase calibration mirrors, and vacuum windows. Evaluation of these components was conducted at the LFSR test facility and DIII-D. The antenna array consists of six corrugated-waveguide antennas for simultaneous profile, fluctuation, and Doppler measurements. A diffraction grating, incorporated into the plasma-facing miter bend, provides protection of sensitive components from stray ECH at 170 GHz. For in situ phase calibration of the LFSR profile reflectometer, an embossed mirror is incorporated into the adjacent miter bend. Measurements of the radiated beam profile indicate that these components have a small, acceptable effect on mode conversion and beam quality. Baseline transmission characteristics of the dual-disk vacuum window are obtained and are used to guide ongoing developments. Preliminary simulations indicate that a surface-relief structure on the window surfaces can greatly improve transmission. The workability of real-time phase measurements was demonstrated on the DIII-D profile reflectometer. The new automated real-time analysis agrees well with the standard post-processing routine.

Development of a narrow stopband filter for spectroscopic fast ion deuterium-alpha measurements.

A new technique to attenuate the unshifted deuterium Balmer-alpha (D-alpha) emission is developed and tested for the fast ion D-alpha (FIDA) diagnostic. The unshifted D-alpha emission, at λ = 656.1 nm, is around three orders of magnitude higher than the desired FIDA emission. Blocking the strong emission feature is essential to prevent blooming and light smearing on the CCD chip and scattered light contamination. The new method is a notch filter approach that utilizes the reflection from ultra-narrow bandpass filters to block the saturating signal before it enters the spectrometer. Collimated light from the fibers is reflected off the filter at a 15° angle of incidence. Measurements show that a center wavelength transmission of 0.006 and a blocking full width at half maximum of ∼1 nm are achieved by using a 200 µm fiber and a 20 mm focal length collimator with two filters.

Diagnosis of fast ions produced by negative-ion neutral-beam injection with fast-ion deuterium-alpha spectroscopy.

Negative-ion neutral-beam injection (NNBI) is an important source of heating and current drive for next-step fusion devices where the injected energy can range from hundreds of keV to 1 MeV. Few diagnostics are suitable for phase-space resolved measurements of fast ions with energy in excess of 100 keV. A study to assess the feasibility of fast-ion deuterium-alpha (FIDA) spectroscopy to diagnose high-energy ions produced by NNBI is presented. Case studies with the Large Helical Device (LHD) and JT-60SA illustrate possible solutions for the measurement. The distribution function of fast ions produced by NNBI is calculated for both devices, and the FIDA spectrum is predicted by synthetic diagnostic simulation. Results with 180 keV NNBI in LHD show that, with a judicious choice of viewing geometry, the FIDA intensity is comparable to that obtained with the existing FIDA system. The measurement is more challenging with the 500 keV NNBI in JT-60SA. Simulations predict the FIDA intensity to be about 1% of the background bremsstrahlung, which is small compared to existing FIDA implementations with positive neutral-beam injection where signal levels are an order of magnitude larger. The sampling time required to extract the small FIDA signal is determined using a probabilistic approach. Results indicate that long averaging periods, from ones to tens of seconds, are needed to resolve the FIDA signal in JT-60SA. These long averaging times are suitable in long-pulse (∼100 s), steady-state devices like JT-60SA where an important measurement objective is the spatial profile of the slowing-down distribution of fast ions.

A heterodyne dispersion interferometer for wide bandwidth density measurements on DIII-D.

In order to improve both the density and particularly the temporal resolution beyond previous dispersion interferometers (DIs), a heterodyne technique based on an acousto-optic (AO) cell has been added to the DI. A 40 MHz drive frequency for the AO cell allows density fluctuation measurements into the MHz range. A CO2 laser-based heterodyne DI (HDI) installed on DIII-D has demonstrated that the HDI is capable of tracking the density evolution throughout DIII-D discharges, including disruption events and other rapid transient phenomena. The data also show good agreement with independent density measurements obtained with the existing DIII-D two-color interferometer. The HDI line-integrated density resolution sampled over a 1 s interval is ∼9 × 1017 m-2. Density fluctuations induced by MHD instabilities are also successfully measured by the HDI.

Tests of a full-scale ITER toroidal interferometer and polarimeter (TIP) prototype on the DIII-D tokamak (invited).

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 Alfvén eigenmodes and tearing modes.

Quasistationary Plasma Predator-Prey System of Coupled Turbulence, Drive, and Sheared E×B Flow During High Performance DIII-D Tokamak Discharges.

A new, long-lived limit cycle oscillation (LCO) regime has been observed in the edge of near zero torque high performance DIII-D tokamak plasma discharges. These LCOs are localized and composed of density turbulence, gradient drives, and E×B velocity shear damping (E and B are the local radial electric and total magnetic fields). Density turbulence sequentially acts as a predator (via turbulence transport) of profile gradients and a prey (via shear suppression) to the E×B velocity shear. Reported here for the first time is a unique spatiotemporal variation of the local E×B velocity, which is found to be essential for the existence of this system. The LCO system is quasistationary, existing from 3 to 12 plasma energy confinement times (∼30-900 LCO cycles) limited by hardware constraints. This plasma system appears to contribute strongly to the edge transport in these high performance and transient-free plasmas, as evident from oscillations in transport relevant edge parameters at LCO time scale.

Far-infrared tangential interferometer/polarimeter design and installation for NSTX-U.

The Far-infrared Tangential Interferometer/Polarimeter (FIReTIP) system has been refurbished and is being reinstalled on the National Spherical Torus Experiment-Upgrade (NSTX-U) to supply real-time line-integrated core electron density measurements for use in the NSTX-U plasma control system (PCS) to facilitate real-time density feedback control of the NSTX-U plasma. Inclusion of a visible light heterodyne interferometer in the FIReTIP system allows for real-time vibration compensation due to movement of an internally mounted retroreflector and the FIReTIP front-end optics. Real-time signal correction is achieved through use of a National Instruments CompactRIO field-programmable gate array.

Bench testing of a heterodyne CO2 laser dispersion interferometer for high temporal resolution plasma density measurements.

A heterodyne detection scheme is combined with a 10.59 µm CO2 laser dispersion interferometer for the first time to allow large bandwidth measurements in the 10-100 MHz range. The approach employed utilizes a 40 MHz acousto-optic cell operating on the frequency doubled CO2 beam which is obtained using a high 2nd harmonic conversion efficiency orientation patterned gallium arsenide crystal. The measured standard deviation of the line integrated electron density equivalent phase resolution obtained with digital phase demodulation technique, is 4 × 1017 m-2. Air flow was found to significantly affect the baseline of the phase signal, which an optical table cover was able to reduce considerably. The heterodyne dispersion interferometer (DI) approach is found to be robustly insensitive to motion, with measured phase shifts below baseline drifts even in the presence of several centimeters of retroreflector induced path length variations. Plasma induced dispersion was simulated with a wedged ZnSe plate and the measured DI phase shifts are consistent with expectations.

Technical overview of the millimeter-wave imaging reflectometer on the DIII-D tokamak (invited).

The two-dimensional mm-wave imaging reflectometer (MIR) on DIII-D is a multi-faceted device for diagnosing electron density fluctuations in fusion plasmas. Its multi-channel, multi-frequency capabilities and high sensitivity permit visualization and quantitative diagnosis of density perturbations, including correlation length, wavenumber, mode propagation velocity, and dispersion. The two-dimensional capabilities of MIR are made possible with 12 vertically separated sightlines and four-frequency operation (corresponding to four radial channels). The 48-channel DIII-D MIR system has a tunable source that can be stepped in 500 µs increments over a range of 56 to 74 GHz. An innovative optical design keeps both on-axis and off-axis channels focused at the cutoff surface, permitting imaging over an extended poloidal region. The integrity of the MIR optical design is confirmed by comparing Gaussian beam calculations to laboratory measurements of the transmitter beam pattern and receiver antenna patterns. Measurements are presented during the density ramp of a plasma discharge to demonstrate unfocused and focused MIR signals.

2D microwave imaging reflectometer electronics.

A 2D microwave imaging reflectometer system has been developed to visualize electron density fluctuations on the DIII-D tokamak. Simultaneously illuminated at four probe frequencies, large aperture optics image reflections from four density-dependent cutoff surfaces in the plasma over an extended region of the DIII-D plasma. Localized density fluctuations in the vicinity of the plasma cutoff surfaces modulate the plasma reflections, yielding a 2D image of electron density fluctuations. Details are presented of the receiver down conversion electronics that generate the in-phase (I) and quadrature (Q) reflectometer signals from which 2D density fluctuation data are obtained. Also presented are details on the control system and backplane used to manage the electronics as well as an introduction to the computer based control program.

Phase-locking of magnetic islands diagnosed by ECE-imaging.

Millimeter-wave imaging diagnostics identify phase-locking and the satisfaction of 3-wave coupling selection criteria among multiple magnetic island chains by providing a localized, internal measurement of the 2D power spectral density, S(ω, kpol). In high-confinement tokamak discharges, these interactions impact both plasma rotation and tearing stability. Nonlinear coupling among neoclassical tearing modes of different n-number, with islands not satisfying the poloidal mode number selection criterion ⟨m, m('), m - m(')⟩, contributes to a reduction in core rotation and flow shear in the vicinity of the modes.

Optics design for J-TEXT ECE imaging with field curvature adjustment lens.

Significant progress has been made in the imaging and visualization of magnetohydrodynamic and microturbulence phenomena in magnetic fusion plasmas. Of particular importance has been microwave electron cyclotron emission imaging (ECEI) for imaging Te fluctuations. Key to the success of ECEI is a large Gaussian optics system constituting a major portion of the focusing of the microwave radiation from the plasma to the detector array. Both the spatial resolution and observation range are dependent upon the imaging optics system performance. In particular, it is critical that the field curvature on the image plane is reduced to decrease crosstalk between vertical channels. The receiver optics systems for two ECEI on the J-TEXT device have been designed to ameliorate these problems and provide good performance with additional field curvature adjustment lenses with a meniscus shape to correct the aberrations from several spherical surfaces.

Process to generate a synthetic diagnostic for microwave imaging reflectometry with the full-wave code FWR2D.

A synthetic microwave imaging reflectometer (MIR) diagnostic employing the full-wave reflectometer code (FWR2D) has been developed and is currently being used to guide the design of real systems, such as the one recently installed on DIII-D. The FWR2D code utilizes real plasma profiles as input, and it is combined with optical simulation tools for synthetic diagnostic signal generation. A detailed discussion of FWR2D and the process to generate the synthetic signal are presented in this paper. The synthetic signal is also compared to a prescribed density fluctuation spectrum to quantify the imaging quality. An example is presented with H-mode-like plasma profiles derived from a DIII-D discharge, where the MIR focal is located in the pedestal region. It is shown that MIR is suitable for diagnosing fluctuations with poloidal wavenumber up to 2.0 cm(-1) and fluctuation amplitudes less than 5%.

Calibration techniques for fast-ion Dα diagnostics.

Fast-ion D(α) measurements are an application of visible charge-exchange recombination (CER) spectroscopy that provide information about the energetic ion population. Like other CER diagnostics, the standard intensity calibration is obtained with an integrating sphere during a vacuum vessel opening. An alternative approach is to create plasmas where the fast-ion population is known, then calculate the expected signals with a synthetic diagnostic code. The two methods sometimes agree well but are discrepant in other cases. Different background subtraction techniques and simultaneous measurements of visible bremsstrahlung and of beam emission provide useful checks on the calibrations and calculations.

Evaluation of the operating space for density fluctuation measurements employing 2D imaging reflectometry.

Microwave imaging reflectometry provides broad poloidal coverage as a density fluctuation measurement tool. 2D imaging systems are evaluated for DIII-D relevant conditions using a full-wave reflectometer code, FWR2D. Reasonable correlation of the synthetic diagnostic signal with density fluctuations at the plasma cutoff surface for a wide range of fluctuation parameters is evaluated and achieved for coherent oscillations; also the frequency spectra are compared for relevant fluctuations. The consequences of non-idealities inherent to imaging fluctuations away from the plasma midplane, where receiving antennas view the plasma cutoff at oblique angles, are evaluated for the optimization of these systems.

Modeling the response of a fast ion loss detector using orbit tracing techniques in a neutral beam prompt-loss study on the DIII-D tokamak.

A numerical model describing the expected measurements of neutral beam prompt-losses by a newly commissioned fast ion loss detector (FILD) in DIII-D is presented. This model incorporates the well understood neutral beam deposition profiles from all eight DIII-D beamlines to construct a prompt-loss source distribution. The full range of detectable ion orbit phase space available to the FILD is used to calculate ion trajectories that overlap with neutral beam injection footprints. Weight functions are applied to account for the level of overlap between these detectable orbits and the spatial and velocity (pitch) properties of ionized beam neutrals. An experimental comparison is performed by firing each neutral beam individually in the presence of a ramping plasma current. Fast ion losses determined from the model are in agreement with measured losses.

Scintillator-based diagnostic for fast ion loss measurements on DIII-D.

A new scintillator-based fast ion loss detector has been installed on DIII-D with the time response (>100 kHz) needed to study energetic ion losses induced by Alfvén eigenmodes and other MHD instabilities. Based on the design used on ASDEX Upgrade, the diagnostic measures the pitch angle and gyroradius of ion losses based on the position of the ions striking the two-dimensional scintillator. For fast time response measurements, a beam splitter and fiberoptics couple a portion of the scintillator light to a photomultiplier. Reverse orbit following techniques trace the lost ions to their possible origin within the plasma. Initial DIII-D results showing prompt losses and energetic ion loss due to MHD instabilities are discussed.

Extended fast-ion D-alpha diagnostic on DIII-D.

A fast-ion deuterium-alpha (FIDA) diagnostic, first commissioned on DIII-D in 2005, relies on Doppler-shifted light from charge-exchange between beam neutrals and energetic ions. The second generation (2G) system was installed on DIII-D in 2009. Its most obvious improvement is the spatial coverage with 11 active in-beam and three passive off-beam views; the latter allows for simultaneous monitoring of the background signal. Providing extended coverage in fast-ion velocity space, the new views possess a more tangential component with respect to the toroidal field compared to their first generation counterparts. Each viewing chord consists of a bundle of three 1.5 mm core fibers to maximize light gathering. For greater throughput, fast f/1.8 optical components are used throughout. The signal is transmitted via fiber optics to a patch panel, so the user is able to choose the detector. FIDA was originally installed with a spectrometer and charge-coupled device (CCD) camera to monitor the full D(α) spectrum for two spatial views. 2G adds another spectrometer and CCD that monitor the blue-shifted wing for six spatial views at 1 kHz. In addition, a photomultiplier tube and fast digitizer provide wavelength-integrated signals at 1 MHz for eight spatial views.

A new fast-ion D(alpha) diagnostic for DIII-D.

The fast-ion D(alpha) (FIDA) technique is a charge-exchange recombination spectroscopy measurement that exploits the large Doppler shift of Balmer-alpha light from energetic hydrogenic atoms to infer the fast-ion density. Operational experience with the first dedicated FIDA diagnostic on DIII-D is guiding the design of the second-generation instrument. In the first instrument, dynamic changes in background light associated with plasma instabilities usually dominate measurement uncertainties. Accordingly, the design of the new instrument minimizes scattering of cold D(alpha) light while monitoring its level. The first instrument uses a vertical view to avoid bright interference from the injected-neutral beams. The sightline of the new instrument includes a toroidal component but only measures blueshifted fast-ion light that is Doppler shifted away from the redshifted light of the injected neutrals. The new views are more sensitive to fast ions that circulate in the direction of the plasma current and less sensitive to the trapped-ion and countercirculating populations. Details of the design criteria and solutions are presented.