L. Roquemore

Edge turbulence measurements in NSTX by gas puff imaging

Turbulent filaments in visible light emission corresponding mainly to density fluctuations at the edge have been observed in large aspect ratio tokamaks: TFTR, ASDEX, Alcator C-Mod, and DIII-D. This article reports on similar turbulent structures observed in the National Spherical Torus Experiment (NSTX) using a fast-framing, intensified, digital visible camera. These filaments were previously detected mainly in high recycling regions, such as at limiters or antennas, where the line emission from neutral atoms was modulated by the fluctuations in local plasma density. However, by introducing controlled edge gas puffs, i.e., gas puff imaging, we have increased the brightness and contrast in the fluctuation images and allowed the turbulent structure to be measured independently of the recycling. A set discrete fiber-optically coupled sight-lines also measured the frequency spectra of these light fluctuations with a 200 kHz bandwidth. Initial results in NSTX show that the turbulent filaments are well aligned...

Wave driven fast ion loss in the National Spherical Torus Experiment

Spherical tokamaks have relatively low toroidal field which means that the fast-ion Larmor radius is relatively large (ρfi>0.04 ap) and the fast ion velocity is much greater than the Alfven speed (Vfi>2 VAlfven). This regime of large Larmor radius and low Alfven speed is a regime in which fast ion driven instabilities are potentially virulent. It is therefore an important goal of the present proof-of-principle spherical tokamaks to evaluate the role of fast ion driven instabilities in fast ion confinement. This paper presents the first observations of fast ion losses in a spherical tokamak resulting from energetic particle driven modes. Two classes of instabilities are responsible for the losses. Multiple, simultaneously bursting modes in the toroidal Alfven eigenmode frequency gap cause neutron drops of up to 15%. A bursting, chirping mode identified as precession and/or bounce resonance fishbone also causes significant neutron drops. Both modes are usually present when the losses are observed.

EUV spectroscopy on NSTX

Spectroscopy in the extreme ultraviolet region is important to magnetic fusion research as well as to astrophysics. We report on XEUS (X-ray and Extreme Ultraviolet Spectrometer) and LoWEUS (Long-Wavelength and Extreme Ultraviolet Spectrometer), which operate in the 5–400 A region on the NSTX (National Spherical Tokamak Experiment) tokamak. The instruments are being used to survey impurities, both for intrinsic elements present in the plasma and for metal impurities resulting from damage to various components. In addition, we have used XEUS and LoWEUS to investigate density-dependent and temperature-dependent emission lines for diagnostic use.

Edge transport studies in the edge and scrape-off layer of the National Spherical Torus Experiment with Langmuir probes

Transport and turbulence profiles were directly evaluated using probes for the first time in the edge and scrape-off layer (SOL) of NSTX [Ono et al., Nucl. Fusion 40, 557 (2000)] in low (L) and high (H) confinement, low power (Pin∼ 1.3 MW), beam-heated, lower single-null discharges. Radial turbulent particle fluxes peak near the last closed flux surface (LCFS) at ≈4×1021 s−1 in L-mode and are suppressed to ≈0.2×1021 s−1 in H mode (80%–90% lower) mostly due to a reduction in density fluctuation amplitude and of the phase between density and radial velocity fluctuations. The radial particle fluxes are consistent with particle inventory based on SOLPS fluid modeling. A strong intermittent component is identified. Hot, dense plasma filaments 4–10 cm in diameter, appear first ∼2 cm inside the LCFS at a rate of ∼1×1021 s−1 and leave that region with radial speeds of ∼3–5 km/s, decaying as they travel through the SOL, while voids travel inward toward the core. Profiles of normalized fluctuations feature levels o...

Grazing-incidence spectrometer for soft x-ray and extreme ultraviolet spectroscopy on the National Spherical Torus Experiment

A compact grazing-incidence spectrometer has been implemented on the National Spherical Torus Experiment for spectral measurements in the 6–65A spectral region. The spectrometer employed a 2400l∕mm grating designed for flat-field focusing and a cryogenically cooled charge-coupled device camera for readout. The instrument was tested by recording the K-shell lines of boron, carbon, nitrogen, and oxygen, as well the L-shell lines from argon, iron, and nickel that fall into this spectral band. The observed linewidth was about 0.1A, which corresponds to a resolving power of 400 for the CV lines. A temporal resolution as fast as 50ms was obtained.

Time-resolved x-ray and extreme ultraviolate spectrometer for use on the National Spherical Torus Experiment

We describe upgrades to a compact grazing-incidence spectrometer utilized on the National Spherical Torus Experiment for monitoring light and heavy impurities. A fast-readout charge couple device camera has been implemented that allows the recording of spectra with up to 25 ms time integration. This capability is used to study the time evolution of the K-shell emission of hydrogenlike and heliumlike boron, carbon, nitrogen, and oxygen between 10 and 65 A. Different camera positioning pieces have been employed to extend the possible spectral range to as high as 140. Several lines that cannot be ascribed to the usual elements found in the plasma have been observed in this spectral range, although often only in a few isolated discharges.

Effect of plasma shaping on performance in the National Spherical Torus Experiment

The National Spherical Torus Experiment (NSTX) has explored the effects of shaping on plasma performance as determined by many diverse topics including the stability of global magnetohydrodynamic (MHD) modes (e.g., ideal external kinks and resistive wall modes), edge localized modes (ELMs), bootstrap current drive, divertor flux expansion, and heat transport. Improved shaping capability has been crucial to achieving βt∼40%. Precise plasma shape control has been achieved on NSTX using real-time equilibrium reconstruction. NSTX has simultaneously achieved elongation κ∼2.8 and triangularity δ∼0.8. Ideal MHD theory predicts increased stability at high values of shaping factor S≡q95Ip∕(aBt), which has been observed at large values of the S∼37[MA∕(m∙T)] on NSTX. The behavior of ELMs is observed to depend on plasma shape. A description of the ELM regimes attained as shape is varied will be presented. Increased shaping is predicted to increase the bootstrap fraction at fixed Ip. The achievement of strong shaping ...

Development and operation of a thin foil Faraday collector as a lost ion diagnostic for high yield d-t tokamak fusion plasmas

We are continuing our development of a radiation-hard, charged-particle detector consisting of a series of thin parallel conducting foils as a lost ion diagnostic for high yield d-t tokamak fusion plasmas. Advantages of this detector concept include economy, ability to operate in relatively intense neutron/gamma ray radiation backgrounds and at moderately high temperatures, and a modest degree of energy resolution. A detector consisting of four parallel foils of Ni, each of thickness 2.5 μm, was operated in the Joint European Torus during the recent DTE-1 experiment. During the highest yield pulses of this campaign, (16 MW), the flux of energetic alpha particles at the detector was measured to be less than about 2 nA/cm2. This upper limit is significantly greater than the expected flux assuming classical losses and given the geometry of the detector. During most of the nearly 2500 pulses of the DTE-1 experiment for which the detector response has been inspected, a relatively intense (up to 200 nA/cm2) flu...

Fast electron temperature measurements using a “multicolor” optical soft x-ray array

A fast (⩽0.1ms) and compact “multicolor” scintillator-based optical soft x-ray (OSXR) array has been developed for time- and space-resolved measurements of the electron temperature [Te(R,t)] profiles in magnetically confined fusion plasmas. The 48-channel tangential multicolor OSXR prototype was tested on the National Spherical Torus Experiment. Each sight line views the same plasma volume at the midplane (0⩽r∕a⩽1), in three distinct energy ranges determined by beryllium foils with different thicknesses. A tangential view of the toroidally (circular) symmetric plasma allows an Abel inversion of the line-integrated SXR brightness to obtain the x-ray emissivity profiles which are then used to constrain the reconstruction of the fast Te(R,t). The first assessment of the electron temperature is obtained by modeling the slope of the continuum radiation with the ideal double-foil method using both the line-integrated intensity measurements as well as the inverted SXR emissivity profiles.

Fast-wave power flow along SOL field lines in NSTX and the associated power deposition profile across the SOL in front of the antenna

Fast-wave heating and current drive efficiencies can be reduced by a number of processes in the vicinity of the antenna and in the scrape off layer (SOL). On NSTX from around 25% to more than 60% of the high-harmonic fast-wave power can be lost to the SOL regions, and a large part of this lost power flows along SOL magnetic field lines and is deposited in bright spirals on the divertor floor and ceiling. We show that field-line mapping matches the location of heat deposition on the lower divertor, albeit with a portion of the heat outside of the predictions. The field-line mapping can then be used to partially reconstruct the profile of lost fast-wave power at the midplane in front of the antenna, and the losses peak close to the last closed flux surface (LCFS) as well as the antenna. This profile suggests a radial standing-wave pattern formed by fast-wave propagation in the SOL, and this hypothesis will be tested on NSTX-U. Advanced RF codes must reproduce these results so that such codes can be used to understand this edge loss and to minimize RF heat deposition and erosion in the divertor region on ITER.