[en] A second fast ion D-alpha (FIDA) installation is planned at NSTX to complement the present perpendicular viewing FIDA diagnostics. Following the present diagnostic scheme, the new diagnostic will consist of two instruments: a spectroscopic diagnostic that measures fast ion spectra and profiles at 16 radial points with 5-10 ms resolution and a system that uses a band pass filter and photomultiplier to measure changes in FIDA light with 50 kHz sampling rate. The new pair of FIDA instruments will view the heating beams tangentially. The viewing geometry minimizes spectral contamination by beam emission or edge sources of background emission. The improved velocity-space resolution will provide detailed information about neutral-beam current drive and about fast ion acceleration and transport by injected radio frequency waves and plasma instabilities.

No abstract available

[en] Full text: We report on ELM triggering and pacing experiments in NSTX-U, including comparisons to pellet ablation models. Multiple sizes and types of solid impurity granules are injected into the low field side of the plasma to determine their ELM triggering and pacing capability. Examining the penetration depths, mass deposition locations, and ELM triggering efficiencies of sub-millimetre lithium, boron carbide (B4C) and carbon granules, we assess the optimal size and composition for minimally perturbative high frequency ELM pacing. By utilizing a neutral gas shielding model, benchmarked with lithium granule ablation experiments performed on DIII-D, the pedestal atomic deposition characteristics for the three different species of granule have been modelled for NSTX H-mode discharges with low natural ELM frequencies. Variations in the depositional barycentre can range from 5 cm for lithium to 17 cm for the same size and velocity carbon granule. We estimate that these penetration depths will be reduced by a factor proportional to q_s∼n_eT^3/2_e as the full NSTX-U capabilities are realized. In addition, by reducing the rotation speed of the impeller, the mass deposition location is translated closer to the top of the pedestal allowing further tuning of the pressure perturbation. At this location the pressure profile generated by the granule can be added to the preexisting pedestal pressure gradient, leading to a set of characteristics advantageous for ELM triggering while affecting a minimal perturbation to the core plasma. Using multiple high-speed cameras to precisely track the granule injections and monitor the ablation duration and penetration depths in NSTX-U, a fractional mass deposition location can be extrapolated. Fast infrared camera measurements are used to characterize the variations between triggered ELMs and the inter-ELM period. In addition, comparisons are also made between stimulated and spontaneously occurring ELMs. These measurements provide a comparison of the ELM peak heat flux mitigation factor, as well as variations in the ELM footprint due to the triggering mechanism. The results of ELM pacing and comparisons with the constructed ablation model in NSTX-U will be reported. (author)

[en] The interaction between neutrals outside tokamak plasmas and fast ions that are generated by neutral beam injection and pass outside the plasma is studied. It is shown that at beam energies up to ~100 keV fast-ion losses are significant, up to 20% in the studied case, because of charge exchange reactions between the neutrals and the fast ions. About half of the fast ions that neutralize outside the plasma get lost to the wall while the other half re-enters the plasma and ionizes again, often on better confined orbits, which is visible as an enhanced beam power deposition in the plasma core ($\mathrm{\rho <!--\; ??\; -->}\lesssim <!--\; ???\; -->0.7$). As a result, the plasma performance as measured from beam-plasma fusion reactions is hardly changed despite the significant fast-ion losses toward the plasma edge. (paper)

[en] Bulk plasma toroidal rotation is observed to invert spontaneously from counter to cocurrent direction in TCV (Tokamak a Configuration Variable) Ohmically heated discharges, in low confinement mode, without momentum input. The inversion occurs in high current discharges, when the plasma electron density exceeds a well-defined threshold. The transition between the two rotational regimes has been studied by means of density ramps. The results provide evidence of a change of the balance of nondiffusive momentum fluxes in the core of a plasma without an external drive

[en] By employing a neutral gas shielding (NGS) model to characterize impurity granule injection, the ablation rates for three different species of granule: lithium, boron, and carbon, are determined. Utilizing the duration of ablation events recorded on experiments performed at DIII-D to calibrate the NGS model, we quantify the ablation rate with respect to the plasma density profile. The species-specific granule shielding constant is then used to model granule ablation within NSTX-U discharges. Simulations of 300, 500 and 700 micron diameter granules injected at 50 m s⁻¹ are presented for NSTX-U L-mode type plasmas, as well as H-mode discharges with low natural ELM frequency. Additionally, ablation calculations of 500 micron granules of each species are presented at velocities ranging from 50–150 m s⁻¹. In H-mode discharges these simulations show that the majority of the injected granule is ablated within or just past the edge steep gradient region. At this radial position, the perturbation to the background plasma generated by the ablating granule can lead to conditions advantageous for the rapid triggering of ELM crashes. (paper)

[en] The first toroidal rotation measurements in TCV ohmic L-mode plasmas with no external momentum injection are presented. The toroidal velocity profile of the fully stripped carbon species is measured by active Charge eXchange Recombination Spectroscopy with a temporal resolution of typically 90 ms and a spatial resolution of 2.5 cm, about 1/10 of the plasma radius. The observed carbon velocity is of the order of the deuterium diamagnetic drift velocity and up to 1/5 of the deuterium thermal velocity. It is directed opposite to plasma current in the electron diamagnetic toroidal drift direction. The profile reverses when reversing the plasma current. The angular velocity profile is flat, or hollow, inside the sawtooth inversion radius and decreases quasi linearly towards the plasma edge. By vertically shifting the plasma magnetic axis within the TCV vessel the plasma edge velocity profile was measured with high resolution. Such experiments confirm that, close to the limiter, the stationary rotation velocity is close to zero or somewhat positive (co-current directed). This suggests that the angular momentum is not driven from the plasma edge. The maximum carbon velocity scales as v_φ,Max [km s^-1] = -12.5T_i/I_p [eV/kA] for a significant range of densities and values of the edge safety factor. Comparison with neoclassical predictions show that the TCV plasma rotation is mainly driven by radial electric fields, with a negligible contribution from the toroidal electric fields. The neoclassical theory of small toroidal rotation quantitatively and qualitatively disagrees with the experimental observation. An alternative empirical equation for the angular momentum flux, able to reproduce the measured stationary profile outside the inversion radius, is proposed

[en] Full text: The scaling of toroidal rotation has been studied over a wide range of in plasma and shape parameters on the TCV tokamak using a diagnostic neutral beam that injects a negligible momentum into the plasma. As on other Tokamaks, the toroidal rotation magnitude on TCV, for a given shape and plasma conditions, is observed to scale inversely with the plasma current. More specifically, this describes the maximum toroidal rotation in plasmas with strong sawteeth activity. The toroidal rotation gradient from the plasma edge was similar over a wide range of plasma currents and only diverged inside the sawtooth inversion radius. By extrapolating the maximum core rotation from the rotation profiles outside the sawtooth inversion radii and adding a correction for the observed ion temperature changes, the scaling of the intrinsic toroidal rotation with plasma current is no longer observed. The TCV CXRS diagnostic was configured to measure the toroidal rotation profile with a low spatial resolution (4 points across the radial co-ordinate) and a high temporal resolution (2 ms). Using ECH X2 power deposited close to the sawteeth inversion radius, the sawteeth repetition time was extended to over 12 ms. The measurement used a newly available Real-Time node to generate a sequence of trigger pulses from analysis of the soft X-ray such that each spectroscopic frame was taken at 2, 4, 6, 8, ... ms after each sawtooth. The radial toroidal rotation profile evolution over a sawtooth was determined using coherent resampling over many sawtooth events. Initial results indicate that, at the sawtooth crash, the plasma core accelerates in the co current direction and then the core rotation profile relaxes back to the counter current direction with a time constant ∼ 10 ms. This implies that the scaling of intrinsic rotation with plasma current may be better understood as a peaked rotation profile (or an inwards momentum pinch) in the plasma core region that is flattened by sawtooth activity. This change of viewpoint should be integrated into the theory of momentum generation and transport in tokamaks such as ITER in that these mechanisms are not as universally affected by the plasma current as was at first thought. (author)

[en] 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.

[en] Full text: DIII-D studies of high-frequency ELM pacing by pellet injection were extended to ITER scenarios at low beam torque, demonstrating ELM peak heat flux mitigation with D₂ pellets, and also with Li spheres, proving the concept of ELM pacing with nonfuel pellets, a technique that could potentially reduce the throughput to the pumping and fuel reprocessing systems in ITER. Injection of D₂ pellets at frequencies up to 90 Hz was performed in low-torque ITER baseline scenarios (q₉₅ = 3.1, β_N = 1.7, n_e = 4x 10¹⁹/m³, T_inj = 0.1N*m). High frequency injection resulted in ELM pacing at frequencies as much as 8 the natural ELM frequency (10 Hz). The resulting inner divertor peak heat flux was reduced by more than a factor of 10. During the high frequency injection, confinement remained similar to the reference discharge (H₉₈(y, 2)∼0.8) with reduced concentration of metal impurities (Fe, Ni, Mo). Injection frequencies up to 200 Hz were obtained with new Impurity Granule Injector (IGI), capable of injecting spherical pellets of nonfuel materials (e.g., Li, C, B4C), with controllable speed (60-120 m/s) and selectable pellet size. By using the IGI with Li spheres (0.3-1.0 mm) in ITER shaped plasmas at moderate torque of 3.0 N*m, ELM pacing was demonstrated for the full discharge length, with constant H₉₈(y, 2) ≈ 1.0, effective density control and reduced high-Z impurities. In this scenario the ELM frequency was increased by 3-5 over the natural ELM frequency (12 Hz), but the maximum ELM frequency appears to be limited only by the injection frequency of the larger granules. The increase of paced ELM frequency resulted in a lower qELM, at the outer strike point, where q_ELM∼1/f_ELM. Measurements of qELM at the inner strike point found q_ELM often higher than expected at high frequency of ELM pacing. Li spheres of 0.7-1.0 mm diameter were also used to pace ELMs in the low torque ITER baseline scenario (β_N = 1.9, T_inj = 0.75 N*m), achieving 100% pacing efficiency at f_inj ≤ 200 Hz. A broad distribution of triggered ELM size was observed, where 5% paced ELMs had q_ELM similar to natural ELMs. The combined dataset provides unique support of ITER mitigation research, both in terms of operational demonstrations and understanding of the physics of ELM pacing and mitigation. (author)