### Session GO3: NSTX and General Spherical Torus

Chair: Jef Ongena, LPP/ERM-KMS Brussels
Room: Adam's Mark Hotel Governor's Square 15

 Tuesday, October 25, 2005 2:00PM - 2:12PM GO3.00001: Progress toward fully non-inductive operation in NSTX Jonathan Menard Several machine and operational upgrades have been implemented on NSTX to improve MHD stability limits and increase pulse duration. H-modes induced during the plasma current ramp increase the central safety factor and decrease the internal inductance li. Reduced li allows stable operation with high elongation which can increase the poloidal beta and bootstrap fraction. Newly installed divertor coils now allow for high triangularity $>$ 0.7 to be achieved at high elongation $>$ 2.5, and this enhanced plasma shaping allows operation with small ELMs at high elongation. The combination of these improvements has resulted in record discharge pulse-lengths $>$ 1.5s in NSTX with normalized beta above 4 sustained for over 1s. In these scenarios, the peak non-inductive current fraction exceeds 70\% with 60\% of the current driven by the plasma pressure gradient. MHD stability, transport, and current profile evolution characteristics of these long-pulse plasmas will be described. Tuesday, October 25, 2005 2:12PM - 2:24PM GO3.00002: Development of Reversed Shear and Monotonic q-profiles in NSTX Fred Levinton , Howard Yuh This work describes the implementation of the MSE-CIF diagnostic on NSTX. Due to the low magnetic field on NSTX the implementation of the MSE diagnostic requires a new approach for the viewing optics and spectral filter. This has been accomplished with a novel optical design that reduces the geometric Doppler broadening, and a high throughput, high resolution spectral filter to optimize signal-to-noise. This MSE diagnostic presently has eight of a possible 19 sightlines operating, providing measurements of the magnetic field line pitch from the plasma center to near the outboard edge of the plasma. The system operates well at low magnetic field, $\geq$0.3~T, using collisionaly induced fluorescence (CIF) from a deuterium heating beam operating at about 90 keV. MSE data has been obtained in several regimes, including L-mode, H-mode, and reversed shear. The measurements reveal the development of both monotonic and reversed shear q- profiles depending on the discharge evolution. The unique aspects of this diagnostic will be discussed as well as the results of analysis now underway to determine the effect of the q-profile in these plasmas on their transport and stability properties. Tuesday, October 25, 2005 2:24PM - 2:36PM GO3.00003: Energy Confinement Scaling in the Low Aspect Ratio National Spherical Torus Experiment (NSTX) S.M. Kaye , M.G. Bell , R.E. Bell , E.D. Fredrickson , B.P. LeBlanc , K.C. Lee , S. Lynch , S.A. Sabbagh Studies have been conducted to develop an understanding of the parametric dependence of the energy confinement time at low aspect ratio in high power NSTX discharges. Systematic scans of plasma current and heating power indicate parametric trends similar to those observed at higher aspect ratio, but statistical analyses indicate that the NSTX data exhibit a weaker current, but stronger magnetic field, dependence than at higher R/a. While confinement values are enhanced over those given by conventional aspect ratio scalings, there is a great deal of shot-to-shot variability that originates from differences in ELM activity, low frequency magnetic fluctuation levels and plasma shaping. Analyses using standard linear regression, Bayesian and principal component techniques indicate a range of fits to the data for both sets of engineering and physics regressor variables. In particular, the dependence of confinement on $\beta$ varies from being slightly unfavorable ($\beta^{-0.2}$) to being more strongly favorable ($\beta^{0.4}$). Tuesday, October 25, 2005 2:36PM - 2:48PM GO3.00004: Resonant Field Amplification and Resistive Wall Mode Stability in NSTX S.A. Sabbagh , A.C. Sontag , W. Zhu , J. Bialek , R.E. Bell , M.G. Bell , D.A. Gates , B.P. LeBlanc , J.E. Menard , D.J. Battaglia The resistive wall mode (RWM) is passively stabilized in NSTX by sufficient plasma flow at values of normalized beta exceeding 6: a factor of 1.5 above the ideal MHD no-wall beta limit. The RWM stability of wall-stabilized plasmas has been examined by active techniques. Using a set of six coils located at the device midplane, magnetic fields with toroidal mode number, n, up to three are used to slow plasma rotation below the RWM critical rotation frequency, and to examine the stable RWM response. Resonant field amplification (RFA) has been observed in plasmas exceeding the no-wall beta limit. Fixed frequency, n=1 external fields phased to propagate either with, or against the direction of plasma toroidal flow yield a different RFA response. RWM growth rate and rotation frequency are examined from these data using a single mode model. Neoclassical tearing modes have been triggered by RWMs when the applied field follows the plasma flow. Standing wave fields with n=3 have slowed plasma rotation, yielding growing n=1 RWMs that rotate in the direction of the plasma flow. Tuesday, October 25, 2005 2:48PM - 3:00PM GO3.00005: Physics and Control of Toroidal Rotation Damping in NSTX W. Zhu , S.A. Sabbagh , A.G. Sontag , J. Bialek , R.E. Bell , J.E. Menard , D.A. Gates , B.P. LeBlanc , C.C. Hegna , K.C. Shaing , D. Battaglia Understanding plasma rotation damping mechanisms at high beta and low aspect ratio is important to sustain passive stabilization of MHD modes. Recent NSTX experiments use the new resistive wall mode (RWM) stabilization coils to control plasma toroidal rotation, and RWM rotation and growth. Plasma rotation damping/control was examined both below and above the ideal MHD no-wall beta limit. Non-resonant plasma rotation damping by n=1-3 applied fields was demonstrated. Rotation recovery was observed after the applied~field was reduced. Quantitative comparison is made between neoclassical toroidal viscosity and other applicable theories and experiment to determine the physics and parameter dependence of the rotation evolution by the applied field and the observed instabilities. Applied n=1 fields propagating in the direction of plasma flow led to a longer period of RWM rotation. When~the applied field propagated against the plasma flow, the RWM grew unabated and~quenched the plasma. Tuesday, October 25, 2005 3:00PM - 3:12PM GO3.00006: Toward High I$_{N}$ Operation in the Pegasus Toroidal Experiment E.A. Unterberg The present experimental goal of the Pegasus experiment is to delineate the external kink boundary, which determines the low-q, high-I$_{N}$ operational space for an ultra-low-A ST. Equilibrium and stability modeling projects stable equilibria approaching I$_{p}$/I$_{TF} \quad \sim$ 3 (I$_{N} \quad \sim$ 20). Upgrades to the facility added improved position and shape control, increased and time-variable toroidal field, and programmable loop voltage. These upgrades allow for greater flexibility in q(r,t) tailoring and should provide access to I$_{p}$/I$_{TF} \quad >$ 1 and the external kink boundary. The Phase I operating space has been recovered, with discharges characterized by I$_{p,max} \quad \sim$ 150 kA, $<$n$_{e}> \quad \le$ 0.6 n$_{GW}$, and large 2/1 tearing modes. Experiments to date have focused on resistive MHD mode suppression by using the expanded capabilities to tailor plasma startup. Tearing mode mitigation has been demonstrated with plasma-current ramp-rate control (from 5-30 MA/s) and improved gas handling with $<$n$_{e}> \quad \ge$ 0.6 n$_{GW}$. Electrostatic plasma guns have been installed to increase the effective V-s and to provide plasma startup without a central solenoid. Tuesday, October 25, 2005 3:12PM - 3:24PM GO3.00007: Final results from the CDX-U lithium program R. Majeski , T. Gray , R. Kaita , H. Kugel , D. Mansfield , J. Spaleta , J. Timberlake , L. Zakharov , R. Maingi , V. Soukhanovskii In the latest and last phase of experiments with liquid lithium plasma limiting surfaces in CDX-U, prior to construction of the Lithium Tokamak eXperiment, two new techniques for producing wall coatings of lithium have been tested -- a resistively heated evaporator (poster by D. Mansfield) and an electron beam heated system. The resistive system continuously deposits lithium on the walls and center stack limiter of CDX-U, while the e-beam system deposits lithium films of up to 1000 {\AA} thickness between discharges. The use of these systems has resulted in the highest edge plasma pumping rate ever reported in a magnetically confined plasma, with $\tau _{p}$* values of $<$ 2 msec, and recycling coefficients $<$ 50{\%}. The e-beam experiments also simulate conditions of high localized power deposition (power density up to 50 MW/m$^{2 }$ in a 0.3 cm$^{2}$ spot) during evaporation. Surprisingly, convective transport of heat away from the beam spot is so effective that it is necessary to heat the entire lithium inventory (140 g), rather than just the area under the beam spot, to the point of evaporation (T $>$ 400--500 \r{ }C). These results are promising for the implementation of lithium plasma-facing components in reactor scale devices. Tuesday, October 25, 2005 3:24PM - 3:36PM GO3.00008: Recycling Measurements Following Repeated Lithium Pellet Injection H.W. Kugel , M. Bell , T. Gray , D. Mueller , B. Leblanc , R. Kaita , T. Stevenson , C.H. Skinner , A.L. Roquemore , C. Bush , R. Maingi , V. Soukhanovskii , R. Raman Repeated Li pellet injection into ohmic He discharges was used to deposit 30 mg on the Center Stack Limiter (CSL) immediately after pre-conditioning this surface with a series of He discharges. A CSL, NBI, D fiducial discharge, then exhibited a reduction in the volume-average density by a factor of about 4 and a peaked density profile. The density reverted to the pre-Li level after two further similar discharges. After another 24 mg of Li was deposited, almost the identical density behavior was observed in the CSL NBI fiducial discharges. These results are similar to aspects of the experience with Li deposition in TFTR. Diverted, LSN, He discharges were then used to condition the Lower Divertor target in NSTX and, similarly, Li pellets injected into a sequence of discharges were used to deposit about 25 mg of Li on the Lower Divertor. As this deposition progressed, the neutral Li line emission from the Lower Divertor increased. Finally, a LSN, NBI, D fiducial was applied. The density exhibited a factor of about 5 reduction from a similar reference discharge at the beginning of the experiment and the density profile was again peaked. These results demonstrated edge pumping of diverted plasmas and increased peaking of the density profile. Tuesday, October 25, 2005 3:36PM - 3:48PM GO3.00009: Dissipative divertor experiments in NSTX V.A. Soukhanovskii , R. Maingi , C.J. Lasnier , C.E. Bush , R. Raman , S. Sabbagh , R.E. Bell , R. Kaita , H.W. Kugel , B.P. Leblanc , J.E. Menard , D. Mueller , S.F. Paul , A.L. Roquemore High divertor heat loads and material erosion are of particular concern for the spherical torus because of its compact divertor design. Experiments in NSTX with 1 - 6 MW NBI-heated lower single null plasmas reveal that, as in conventional tokamaks operating without actively pumped divertors, the inner divertor leg is naturally detached throughout most of the operational space in density and input power. The outer divertor strike point, with the peak heat flux up to 10 MW/m$^{2}$, however, remains attached even at the density approaching the Greenwald density. Dedicated experiments aimed at developing dissipative divertor scenarios in 3-4 MW NBI heated L- and H-mode plasmas using $D_2$ and neon injections will be described. High divertor neutral density and SOL collisionality $\nu^{*} > 60$ accessible only with $D_2$ injections into the private flux region lead to detachment of the outer strike point (OSP). The detachment is evidenced by a four-fold reduction of the peak heat flux and spectroscopic signatures of volume recombination. Similar heat flux reduction and $P_{rad} \simeq 0.3 \times P_{in}$, however without the OSP detachment, were obtained in H-mode plasmas with neon injections. This work is supported by the U.S. DOE under contract No. W-7405-Eng-48. Tuesday, October 25, 2005 3:48PM - 4:00PM GO3.00010: Time resolved measurements of deposition and dust in NSTX C.H. Skinner , H.K. Kugel , A.L. Roquemore , W. Davis , T.M. Biewer , N. Nishino , C.V. Parker Tritium codeposition and dust accumulation will impact the operation of next-step devices such as ITER and measurements in contemporary tokamaks are important to gain a predictive understanding that can help mitigate the associated risk. We will discuss results from three diagnostics that address these issues. Incandescent particles have been observed by fast cameras moving at 10-100 m/s in some NSTX plasmas. The particle trajectories appear to be complex including velocity reversal, and particle breakup. We also have developed a novel electrostatic device to detect dust on remote surfaces[1,2]. Recent laboratory work with ultra-fine 25 µm trace spacing has shown 1 $\mu$g/cm2 sensitivity with information on the particle size in the detected waveform. Quartz crystal microbalances have been deployed in NSTX to measure pulse-by-pulse deposition at various locations[3] and observations from the current campaign will be reported. [1] A. Bader et al., Rev. Sci. Instrum., 75, (2004) 370. [2] C. Voinier et al., J. Nucl. Mater. in press (2005) [3] C. H. Skinner et al., J. Nucl. Mater., 337-339 (2005) 129. Tuesday, October 25, 2005 4:00PM - 4:12PM GO3.00011: Transient CHI Solenoid-free Plasma Startup in NSTX R. Raman , T.R. Jarboe , B.A. Nelson , M. Bell , D. Mueller A new method of non-inductive startup, referred to as \textit{transient }coaxial helicity injection (CHI), has been successfully developed on the HIT-II experiment to produce 100kA of closed-flux toroidal current [R. Raman et al., Nucl. Fusion \textbf{45} (2005) L15-L19]. The plasma current is produced by discharging a capacitor bank across the lower divertor plates in the presence of toroidal and poloidal magnetic fields. The initial static magnetic flux is chosen such that this rapidly expanding plasma has a tendency to detach from the lower divertor electrodes. Rapidly reducing the injector current during this phase eases the detachment process, which results in the production of a robust closed field line equilibrium. In recent on-going discharges in NSTX, discharges with up to 140 kA of toroidal current have produced with current multiplication factors approaching 170 towards the end of the discharge, approaching conditions necessary for the observation of toroidal current persistence after the injector current is reduced to zero. DOE Contract No. DE-AC02-76CH03073 and DE-FG03-99ER54519 supports this work. Tuesday, October 25, 2005 4:12PM - 4:24PM GO3.00012: CHI during ohmic discharges in spherical tori D. Mueller , M.G. Bell , W.T. Hamp , B.A. Nelson , R.J. Redd , S.A. Sabbagh Coaxial Helicity Injection (CHI) has been used on the National Spherical Torus Experiment (NSTX), the Helicity Injected Torus (HIT) and HIT-II to initiate plasma and to drive up to 400 kA of toroidal current. The primary goal of the CHI systems is to provide a start-up plasma with substantial toroidal current that can be heated and sustained with other methods. We have investigated the use of CHI systems to add current to an established, inductively driven plasma. The persistence of toroidal current after termination of the injector current has been observed on HIT-II [accepted for publication]. We will present the results of similar experiments on NSTX, in which a richer set of diagnostics is available. This technique may be an attractive method to add edge current that could modify the stability characteristics of the discharge or modify the particle and energy transport in a spherical torus. Use of an ignitron to terminate the current on open field lines will make possible comparison of before and after CHI using EFIT analysis of the data. Tuesday, October 25, 2005 4:24PM - 4:36PM GO3.00013: Experiments with HHFW on NSTX J.R. Wilson , R. Bell , S. Bernabei , J.C. Hosea , C. Kessel , B. LeBlanc , J. Menard , C.K. Phillips , P.M. Ryan , D.W. Swain , J. Wilgen Experiments on NSTX utilizing HHFW have concentrated on those elucidating the conditions required for effective HHFW heating and current drive and those utilizing HHFW for solenoidal free start-up and discharge sustainment. Past experiments have established that power absorption depends on antenna phasing. Decreased heating efficiency and increased edge ion heating are observed as the fast wave toroidal wave-number decreases. Differences in absorption are seen for waves excited in the co-current and counter-current toroidal direction. These differences will be explored by reversing the direction of the magnetic fields to differentiate between geometric and plasma physics effects. Edge ion heating due to parametric decay is further explored by direct digitization of an edge probe signal and an edge microwave reflectometer signal. The reflectometer signal should allow better spatial localization of the decay waves. HHFW has been used for solenoidal-free startup experiments where the rf power is used to breakdown and sustain a plasma that than has toroidal current induced by the outboard poloidal field windings. To date, the most effective plasma production is with the antenna phased for the longest waveguide excitation. In experiments with loop voltage clamping, HHFW in conjunction with NBI can extend the current duration. Tuesday, October 25, 2005 4:36PM - 4:48PM GO3.00014: Electron Bernstein Wave Physics in NSTX G. Taylor , S. Diem , P.C. Efthimion , R.A. Ellis , E. Fredd , J. Hosea , J.R. Wilson , T.S. Bigelow , M.D. Carter , J.B. Caughman , F. Jaeger , D.A. Rasmussen , J.B. Wilgen , R.W. Harvey , A.P. Smirnov , N.M. Ershov , J. Urban , J. Preinhaelter , A. Bers , J. Decker , A.K. Ram Numerical modeling [1] and emission diagnostics [2] are used to study electron Bernstein wave (EBW) coupling, propagation and damping physics in NSTX. This research supports both the design of a 1 MW, 28 GHz EBW current drive (EBWCD) system and the development of an EBW Te(R) diagnostic. Off-axis EBWCD may be critical to sustaining solenoid-free high $\beta$ plasmas in NSTX. Modeling and emission measurements support efficient EBW coupling via obliquely launched O-mode polarized electromagnetic waves. Off-axis 28 GHz EBWCD appears viable on NSTX, however intrinsic Doppler broadening of EBWs in the plasma, combined with significant cyclotron harmonic overlap, may preclude core access for 28 GHz EBWs. [1] R.W. Harvey and G. Taylor, Phys. Plasmas 12, 051509 (2005) [2] G. Taylor, et al., Phys. Plasmas 12, 052511 (2005) Tuesday, October 25, 2005 4:48PM - 5:00PM GO3.00015: High-resolution measurements in the EUV on NSTX P. Beiersdorfer , M. Bitter , L. Roquemore , J.K. Lepson , M.-F. Gu , S.M. Kahn The extreme ultraviolet (EUV) wavelength band is rich in lines useful as plasma diagnostics. This fact is being used by the Chandra and XMM-Newton satellites for studying stellar coronae and galactic nuclei. We have installed a new grating spectrometer on the NSTX tokamak that allows us to study emission lines in the EUV with similar spectral resolution. We have observed the K-shell lines of heliumlike and hydrogenlike boron, carbon, and oxygen. Moreover, we have measured the L-shell spectra of neonlike Ar, Fe, and Ni. All elements except argon were intrinsic to NSTX plasmas. Many of these spectra are of great interest to astrophysics. Our measurements provide line lists and calibrate density-sensitive line ratios in a density regime not accessible by other laboratory sources. Moreover, we were able to measure the temperature dependence of several iron lines needed to address puzzling results from stellar flare plasmas. \\ This work was performed under the auspices of the U.S. DOE by UC-LLNL under contract W-7405-Eng-48 and by PPPL under contract DE-AC02-76CHO3073.