Overview of the beam emission spectroscopy diagnostic system on the National Spherical Torus Experiment

Conceptual design and performance predictions for 2D beam emission spectroscopy turbulence measurements at Wendelstein 7-X

A conceptual design for a 2D beam emission spectroscopy diagnostic system to measure ion gyro-scale plasma turbulence at Wendeslstein 7-X is described. The conceptual design identifies field-aligned viewing geometries and ports for cross-field turbulence measurements in the neutral beam volume. A 2D sightline grid covers the outer plasma region, and the grid configuration provides sufficient k-space coverage in radial and poloidal directions for ion temperature gradient and trapped-electron mode turbulence measurements. Emission intensity estimates, optical transmission losses, and detector noise levels indicate that the measurements will be sensitive to plasma density fluctuations as small as δn/n ≈ 0.5% with a bandwidth of 1 MHz. Implementation challenges include a small beam emission Doppler shift due to nearly radial heating beams and reduced optical throughput due to collection aperture limitations.

Scoping study of detecting high harmonic fast waves in NSTX-U hot core plasma directly using beam emission spectroscopy

In this paper, we used radio frequency (RF) full wave simulation to explore the feasibility of generating a density beat oscillation formed by high harmonic fast waves (HHFWs) in the National Spherical Tokamak Experiment-Upgrade (NSTX-U) and of measuring it numerically with 2D beam emission spectroscopy (BES). This oscillation is of interest as it can be used to determine the injected RF fields. We began by computing HHFW fields in NSTX-U under a variety of experimental conditions and antenna configurations with Petra-M, a 3D RF wavefield solver. These fields were then used to evaluate the amplitude of δn_e,beat/n_e0 in the volume of measurement, which we predict to be between 5 × 10^-10 and 10^-9, although it is highly dependent on plasma parameters. Trends in magnetic field, core temperature, core density, and antenna phasing are presented, and the parameters most suitable to this measurement are identified. Finally, we developed a synthetic BES diagnostic to compare the experimental BES signal against.

Initial beam emission spectroscopy diagnostic system on HL-2A tokamak

A beam emission spectroscopy system is being developed and deployed on the HL-2A tokamak to measure local low wavenumber (k _⊥ ρ _i < 1) density fluctuations by measuring the Doppler-shifted emission from a 50 kV deuterium heating neutral beam. High spatial resolution (Δr ≤ 1 cm, Δz ≤ 1.5 cm) measurements are achieved with customized in-vacuum optics. High frequency, high-gain preamplifiers sample the light intensity at a Nyquist frequency of 1 MHz and achieve a high S/N ratio via high optical throughput, low-noise preamplifiers, and high quantum efficiency photodiodes. A first set of 16 detector channels [configured in an 8 (radial) × 2 (poloidal) array] has been installed and tested at HL-2A, covering the radial range r/a = 0.8-1.1. The frequency and wavenumber spectra have been measured under different plasma conditions. Initial measurements have demonstrated the capability of measuring edge plasma density fluctuation spectra and the poloidal flow velocity fields with a high S/N ratio.

Beam emission spectroscopy with radially and poloidally elongated optical sightlines

Beam emission spectroscopy is widely used as local density fluctuation measurement of fusion plasmas. We have installed a beam emission spectroscopy system in the large helical device (LHD) with radially and poloidally elongated optical configuration to enhance both signal-to-noise ratio and wavenumber sensitivity in radial and poloidal directions. The calculation considering the integral effect of the line of sight showed the radially and poloidally elongated sightlines have increased sensitivity compared with the square shaped bundle design of the same sampling area.

Diagnostic performance of the beam emission spectroscopy system on the National Spherical Torus Experiment

The beam emission spectroscopy system on the National Spherical Torus Experiment measures localized density fluctuations on the ion gyroscale. Optical sightlines provide core to edge radial coverage, and the sightlines are aligned to typical pitch angles to maximize cross-field spatial resolution. Sightline images are 2-3 cm, and point spread function calculations indicate image distortion from pitch angle misalignment and atomic state finite lifetimes is minor with a 15% increase in the image size. New generation photodetectors achieve photon noise limited measurements at frequencies up to 400 kHz with refrigerant cooling at -20 °C. Measurements near the pedestal show broadband turbulence up to 100 kHz, and poloidal correlation lengths are about 10 cm. Plasma turbulence signals can be 2-3 orders of magnitude above photon noise and amplifier thermal noise.

Low-noise, high-speed detector development for optical turbulence fluctuation measurements for NSTX

A new beam emission spectroscopy (BES) diagnostic is under development. Photon-noise limited measurements of neutral beam emissions are achieved using photoconductive photodiodes with a novel frequency-compensated broadband preamplifier. The new BES system includes a next-generation preamplifier and upgraded optical coupling system. Notable features of the design are surface-mount components, minimized stray capacitance, a wide angular acceptance photodiode, a differential output line driver, reduced input capacitance, doubling of the frequency range, net reduced electronic noise, and elimination of the need for a cryogenic cooling system. The irreducible photon noise dominates the noise up to 800 kHz for a typical input power of 60 nW. This new assembly is being integrated into an upgraded multichannel optical detector assembly for a new BES system on the NSTX experiment.