Session NO1: NSTX

Research directions and highlights from the 2006 NSTX experiments.

NSTX research in toroidal magnetic confinement at low aspect ratio has focused this year on experiments making use of new capabilities in five areas. First, experiments have investigated coating the plasma facing surfaces with evaporated lithium as a means to reduce wall recycling and benefit confinement. Second, highly elongated shapes were developed to produce MHD stable plasmas with a high fraction of bootstrap current and low resistive dissipation of poloidal flux. Third, external non-axisymmetric field correction coils were used both to reduce intrinsic field errors and to provide active feedback control of the resistive-wall mode in high-beta plasmas. Fourth, a tangentially viewing small-angle scattering diagnostic for millimeter microwaves was used to measure density fluctuations with a radial component of their wavenumber comparable to the inverse of the electron gyro-radius. Lastly, the technique of coaxial helicity injection was extended to generate record plasma startup current without reliance on induction by a central solenoid. Other experiments have been conducted to address research of high priority to the ITPA and ITER.   

NSTX Experiments with Evaporated Lithium Coatings on Plasma-Facing Surfaces

An evaporator has been used to apply lithium to the carbon tiles on the center column and lower divertor in NSTX. Twelve depositions, ranging from about 10mg to 5g of lithium, were performed. In 1MA L-mode, lower single-null divertor, deuterium plasmas with 1MW of NBI, the volume-average electron density decreased by up to 25\% after depositing about 400mg of lithium, compared to a similar shot before coating, and the electron and ion temperatures both increased about 20\% on axis. The reduction in density lasted only for one discharge, although a residual effect on the temperature persisted for several shots and the line emission from oxygen decreased dramatically for several days of operation. In similar H-mode plasmas with 5MW NBI, there was an increase of about 25\% in the central electron and ion temperatures after applying 440mg of lithium, but there was no reduction in the average density, although there were effects on its profile shape. Increasing the amount of lithium beyond about 400mg did not appear to increase or prolong its effectiveness. Examination of the divertor tiles at the conclusion of the experiments showed migration of the lithium away from the strike-points after its deposition.   

Observation of the Enhanced High-k Spectra during H-mode Phase on NSTX.

A multi-channel collective scattering system was commissioned on NSTX to investigate the anomalous electron transport physics. A high power microwave source ($\sim $100 mW) at $\sim $ 1 mm wavelength was employed as the probe beam. The system consists of 5 discrete channels which primarily measure five radial wave-numbers up to k$_{r} \quad \sim $ 20 cm$^{-1}$ which corresponds to k$_{\bot }\rho _{e} \quad \sim $ 0.2 and $\sim $0.7 for the edge and core for the NSTX parameters, respectively. Initial tests were performed at the edge of the quiescent OH and NBI heated plasmas. In OH and L-mode plasmas, the variation of the scattered power spectra at each wave-number was monotonic. However, significant changes of the scattered power were observed during the L/H transition phase. Relative decrease of the scattered power spectra at lower wave-numbers is largely attributed to the refraction effect due the edge density gradient, but the increase of the power spectra at the high-k channels is under investigation. The details of the scattered spectra from two types of H-mode plasmas (double null and single null) and spatial dependence will be discussed with the relevant TRANSP calculations.   

The Enhanced Pedestal H-mode in NSTX

Typical H-mode pedestal temperatures in NSTX range from 100-300 eV. A new operating regime has been observed in which the pedestal ion temperature increases to $\sim $ 600 eV in about 50ms, or one energy confinement time, resulting in a global confinement improvement. Ion temperature gradients as high as 30 keV/m are observed. The regime is correlated with a braking of the edge toroidal rotation, in which case the pressure gradient term in the radial force balance becomes dominant over the toroidal rotation term. Present analysis indicates that these discharge scenarios have a very low central current density, possibly leading to a reduced normalized beta limit $\sim $ 4.5 ({\%}-m-T/MA). The observed characteristics of this scenario will be presented.   

Study of the Relationship between Type I ELM Severity and Perturbed Electron Transport in NSTX

Global T$_{e}$ profile crashes of 10-30{\%} amplitude are observed following large Type I ELMs in some H-mode NSTX discharges. While the soft X-ray (SXR) data indicates that the ELM itself is causing only a peripheral T$_{e}$ perturbation, the propagation of the cold pulse initiated by the ELM is anomalously fast ($\sim $ms timescale) and can extend to the core of the plasma. The estimated perturbed $\chi _{e}$ is a few hundred m$^{2}$/s for $\rho \quad >$ 0.4, and a few tens of m$^{2}$/s for $\rho \quad <$ 0.4. This behavior suggests a link between the severity of Type I ELMs and the perturbed electron thermal transport on NSTX. We produced controlled perturbations at the plasma edge by injecting small low-Z pellets into ELMy H-mode plasmas, and compared the ELM and pellet induced cold pulse using multi-color SXR imaging. In plasmas with large Type I ELMs the pellet perturbation has a similarly large effect on the global T$_{e}$ profile. In recently developed high triangularity regimes with smaller ELM perturbations, the pellet induced perturbations are likewise reduced.   

Developments in plasma shaping experiments on NSTX

The spherical torus concept relies heavily on the bootstrap current in order to maintain plasma current in the absence of a large transformer. In addition, the cost of fusion power is strongly affected by the recirculating power fraction due to the normally conducting toroidal field coil. This additional constraint necessitates operating with high plasma toroidal \textit{$\beta $}$_{t}$, where \textit{$\beta $}$_{t}$\textit{ = 2$\mu $}$_{0}$\textit{$<$p$>$/B}$_{t}^{2}$ with $<$\textit{p}$>$ the volume averaged plasma pressure, and $B_{t}$ the toroidal field at the plasma geometric center. A useful figure of merit that combines these 2 parameters is $f_{bs}$\textit{$\beta $}$_{t}$ where $f_{bs}=I_{bs}/I_{p}$ is the bootstrap current fraction. $f_{bs}$\textit{$\beta $}$_{t}$ has been referred to as the sustained \textit{$\beta $}. Progress in this parameter requires either improved plasma shaping or increases in $C_{T}$, the Troyon coefficient ($C_{T}=\beta _{max}$aB$_{t}$/I$_{p})$. To this end, NSTX is investigating pathways to stronger plasma shaping. Progress to date is reported, including achievement of world record sustained plasma shape factor and world record controlled plasma elongation of $\sim $3.1. An interesting result of this campaign was the achievement of extremely low sustained normalized internal inductance $l_{i} \quad \sim $ 0.3. The effect of improved plasma shaping on plasma performance as determined by the sustained $\beta $ will also be presented.   

Error field identification and correction in NSTX high beta-N plasmas

Toroidal rotation damping and mode locking near the plasma boundary has been identified as responsible for inducing plasma disruptions in some high-normalized-beta scenarios in NSTX. This edge rotation damping has previously been shown to be correlated with magnetic error fields from toroidal field coil motion induced by the ohmic solenoid. Several techniques have been utilized to attempt to reduce this error field including: ``predictive'' correction of the time-evolving estimated error field and active feedback control of the plasma response to the residual error field utilizing newly commissioned real-time mode identification and feedback control software. The best plasma performance is obtained when the two techniques are used in concert. Use of the long-time-average of the active-feedback-controlled correction current results in similar plasma performance as the unfiltered current. The measured plasma rotation damping is decreased by error field correction, thereby increasing passive RWM stabilization relative to plasmas without error field correction. Details of the error field identification and correction algorithms will be described.   

RWM Stabilization Physics in NSTX

Leading theories of resistive wall mode (RWM) stabilization invoke energy dissipation related to the plasma rotation as key stabilization physics. Experiments focused on examining the parametric dependence of the minimum toroidal rotation required for passive stability (critical rotation profile) were performed to examine the dissipation physics allowing stabilization. Consistency between NSTX critical rotation data and stabilization models dependent on Alfven speed and ion collisionality is evaluated. Discharges with higher ion collisionality and similar Alfven speed profiles appear to have lower critical rotation profiles. Experimental results are inconsistent with models predicting instability when rotation is slowed to 1/2 of a steady-state value. The RWM appears to stabilize when faster rotating plasma modes (internal kink or tearing modes) exist. After unstable RWM growth has begun, the appearance of a faster rotating mode can stabilize the RWM before a rapid beta collapse. Discharges at marginal RWM stability and very low plasma rotation have exhibited RWM growth when a faster rotating mode is stabilized.   

Hole-clump Pair Suppression with HHFW on NSTX

Hole-Clump-like behavior has been observed for fast ion driven modes at frequencies $<$ f$_{ci}$/4 on NSTX. In experiments with combined High Harmonic Fast Wave heating (HHFW) and Neutral Beam Injection (NBI), it is often observed that during HHFW the hole-clump chirping is suppressed. HHFW is known to accelerate the beam fast ions, based on Neutral Particle Analyzer measurements of the fast ion distribution function, as well as an increase in the neutron rate during HHFW. The heating of the fast ion population has been predicted to decrease the lifetime of hole-clump pairs by diffusing the fast ion phase space holes and clumps. However, the apparent suppression of non-chirping modes, as well suggests that fundamental reorganization of the fast ion distribution function could eliminate the bump-on-tail needed to drive the instability.   

New beam driven modes below the geodesic acoustic frequency in NSTX

New global MHD eigenmode solutions are found associated with the gap in the Alfv\'en/acoustic continuum. The modes are at frequencies below the geodesic acoustic modes (GAM). In contrast to the mostly electrostatic polarization of GAMs the new modes contain electromagnetic component due to the interaction with the Alfv\'en branch. Indeed a new continuum branch emerges with a modified shear Alfv\'en wave dispersion relation. The new modes associated with this shear Alfv\'en branch have phase velocity significantly above the background ion thermal velocity and thus are expected to be insensitive to kinetic effects. These new modes may be associated with so-called BAE modes that have been found (but not explained theoretically) in numerical MHD codes [A.Turnbull, et.al. Phys. Fluids {\bf B 5} 2546 (1993)] and kinetic codes [N.N. Gorelenkov, et.al., Nucl.Fusion {\bf 42} (2002) 150]. We speculate that new modes are being driven by beam ions in NSTX at roughly half of the TAE frequency.   

HHFW Propagation and Damping Properties on NSTX versus B$_{\Phi }$ and Antenna k$_{\vert \vert }$

HHFW RF power delivered to the core plasma of NSTX is strongly reduced as the launched wavelength is increased -- for B$_{\Phi }$ = 4.5 kG, heating is $\sim $ 1/2 as effective for k$_{\vert \vert }$ at -7m$^{-1}$ as at 14m$^{-1}$ and $\sim $ 1/10 as effective at -3m$^{-1}$. Measured edge ion heating, attributable to parametric decay (PDI), increases with wavelength as well but does not increase fast enough to account for the observed power loss. Surface fast waves (FW) may enhance both PDI and also losses in sheaths and structures around the machine -- FW fields propagate closer to the wall with decreasing B$_{\Phi }$ and k$_{\vert \vert }$ (n$_{onset}\propto {\rm B}_{\Phi }$* k$_{\vert \vert }^{2})$. Experiments at several k$_{\vert \vert }$ and B$_{\Phi }$ values suggest strong propagating FW effects on heating. A dramatic increase in core heating was observed at -7m$^{-1}$ when B$_{\Phi }$ was increased to 5.5 kG -- central Te near 4 keV at P$_{RF}$ = 2 MW -- and reduced core heating was observed at B$_{\Phi }$= 3 kG. Also, the far-field RF poloidal magnetic field increased a factor of $\sim $ 3 when k$_{\vert \vert }$ was reduced from 14m$^{-1}$ to -3m$^{-1}$, suggesting a large increase in wall/sheath power loss. The results of these experiments and their implications for the role of surface wave losses will be discussed.   

Thermal Electron Bernstein Wave Emission Measurements on NSTX

NSTX high beta plasmas operate in the overdense regime, allowing the electrostatic electron Bernstein wave (EBW) to propagate and to be strongly absorbed and emitted at the electron cyclotron resonances. As such, EBWs offer the potential for local electron temperature measurements and local electron heating and current drive. A critical challenge for these applications is to establish efficient coupling between the EBWs and electromagnetic waves outside the plasma. Recently, two remotely steered, quad-ridged antennas, measuring EBW emission via the oblique B-X-O mode conversion process have been installed on NSTX. These antennas are connected to absolutely calibrated dual-channel radiometers, measuring fundamental (8-18 GHz), second and third harmonic (18-40 GHz) EBW emission, respectively. This diagnostic has been successfully used to map the EBW mode conversion efficiency as a function of poloidal and toroidal angles for L- and H-mode plasmas. Experimental results from this new diagnostic and comparisons to modeling will be presented.   

MARFE structure and dynamics in NSTX

Traditionally, MARFEs (Multifaceted Asymmetric Radiation From the Edge) are seen in tokamaks as poloidally localized, toroidally symmetric bands of low temperature, high density, highly radiating plasmas surrounding the center column. In NSTX a new aspect of MARFEs has been observed using a fast framing camera at up to 120,000 frames/s during high density, double null, H-mode discharges. As a result of ELM activity, a toroidally localized (i.e. not symmetric) MARFE precursor is created from partial burn-through of a pre-existing MARFE. A hot component of this MARFE pre-cursor is also observed in ultra-soft X-rays. This precursor then moves upwards and away from the divertor following the local magnetic field line at speeds of $\sim $15~km/s. A conventional MARFE then re-forms from this precursor seed as its vertical motion slows down (and eventually stops) at and above the device midplane. The re-formed traditional MARFE then moves down towards its equilibrium position in the lower divertor where it interacts with the next ELM in the cycle. The dynamics of the ELM/MARFE cycle as well as the MARFE characteristics in NSTX will be presented.   

Supersonic gas jet fueling experiments in NSTX

A high-pressure supersonic deuterium jet (SDJ) characterized by Mach number 4 has been used for fueling of ohmic and 2-6 MW NBI-heated L- and H-mode plasmas in NSTX. Reliable H-mode access was obtained with steady-state low field side fueling from the SDJ at a flow rate up to $4.5 \times10^{21}$ s$^{-1}$. Good progress has been made toward a controlled density H-mode scenario with SDJ fueling: the flow rate of the uncontrolled high field side gas injector was reduced by up to 20. As a result, comparable or slightly higher core and pedestal densities were obtained, with 5-15 \% reduction of core and pedestal temperatures, and a change in the ELM regime from Type I and small, Type V ELMs to Type III ELMs. The SDJ fueling efficiency was found to be a function of the SDJ pressure (density) and the plasma - SDJ distance, typically held at 5-15 cm. Typical fueling efficiency values inferred from the plasma electron inventory analysis were in the range 0.1 - 0.35. Fast camera imaging of the SDJ indicated that the gas jet penetrated through the scrape-off layer and ionized before reaching the separatrix. This work is supported by U.S. DOE under Contracts No. W-7405-Eng-48 and DE-AC02-76CH03073.   

Dynamic retention and deposition in NSTX measured with quartz microbalances

Dynamic retention of deuterium has been observed in NSTX with three quartz crystal microbalances. These are deployed in plasma shadowed areas at the upper and lower divertor and outboard midplane and enable spatially localized measurements of the pulse-by-pulse material gain and loss with a sensitivity of a fraction of an atomic monolayer. At the time of a plasma discharge a transient increase in mass of order 0.1 $\mu$g/cm$^2$ is seen. This decays in the interpulse period to a level either higher, lower or similar to that prior to the discharge. Following a days plasma operations a loss in mass is observed over several hours that parallels the deuterium outgassing from hydrocarbon deposits on the plasma facing components. For the first discharge of the day, the relatively unsaturated hydrocarbon layer shows a step-up in mass independent of plasma conditions. Some correlations of mass gain with plasma duration, stored energy, and change in the plasma shape are observed. The results are interpreted in terms of dynamic retention and erosion/deposition.