Session PO4: C-Mod and KSTAR and Other Tokamaks

Overview of Recent Results from Alcator C-Mod

We have extended the I-mode regime to high power and plasma performance. I-mode yields strong edge ion and electron temperature barriers, excellent energy confinement, and low collisionality, with no need for ELMs to maintain particle control. Detailed studies of ICRF-induced flow drive on C-Mod reveal that the efficiency depends strongly on He3 concentration in the D(He$^{3})$ mode conversion regime, with driven core toroidal rotation up to 110 km/s. Studies of intrinsic rotation show that central toroidal rotation, observed in the absence of external momentum input, scales with edge temperature gradient, and the relationship to fluctuation-induced residual stress is under investigation. For n$_{e}>$1x10$^{20}$ m$^{-3}$ LHCD efficiency drops off more rapidly than expected theoretically, and mechanisms of anomalous absorption in the edge plasma are under investigation. Results from a new, advanced Lower Hybrid launcher, aimed at low-loss and high power density ($\sim $100 MW/m$^{2})$ will be reported. Detailed studies of SOL heat footprints reveal a two zone structure: a narrow channel at the strike point locations, and a tail feature that extends into the far scrape-off layer.   

Measurements of heat-flux profiles on the divertor targets of Alcator C-Mod

Acceptable power handling is one of the primary functions - and most challenging problems - for a tokamak divertor. A section of C-Mod's outer divertor has been modified and instrumented in order to measure the incident heat-flux there. Surface temperatures are measured using IR thermography, and the heat-flux ``footprints'' are derived from those measurements. Peak surface-normal heat fluxes greater than 15 MW/m$^{2}$, corresponding to parallel heat-fluxes $>$ 300 MW/m$^{2}$, are typical in both EDA H-modes and RF-heated L-modes. In EDA H-modes widths (FWHM) of the main peak of the heat-flux profiles are in the range 1.5-4 mm (magnetically mapped to the midplane), and these values help to constrain the major radius and magnetic field dependencies of multi-machine empirical scaling relations for SOL heat-flux widths. Also evident in the heat-flux profiles is a far-SOL ``tail'' with constant or slowly decreasing heat-flux. Scalings of the ``footprint'' profile widths with various ``engineering'' parameters, as well as with the SOL pressure profile widths of the main plasma, will be presented.   

Experiments on the Alcator C-Mod Tokamak Utilizing a Novel Lower Hybrid Wave Launcher

A new, novel launcher for coupling rf power into lower hybrid waves has been installed on the Alcator C-Mod tokamak. The launcher is composed of 4 rows of 16 active waveguides. This launcher retains the full phase flexibility of the original launcher (1 $< \quad \vert $n$_{\vert \vert }\vert \quad <$ 4) with high directivity but significantly reduces ohmic power losses in the launching structure. A novel 4-way vertical power splitter allows recirculation of reflected power. The launcher achieved 700 kW net power coupled for 0.5 s on the second day of operation without any conditioning. Added launcher diagnostics will aid in exploring the previously observed density limit and help determine the expected performance of the launcher in advanced tokamak discharge parameters. Results on operation, including current drive efficiency, current profile modification and plasma rotation effects in a variety of plasma configurations will be presented.   

Coupling of Lower Hybrid waves with four-way-splitter antenna on Alcator C-Mod

We present the first experimental wave coupling results obtained with the new Lower Hybrid (LH) launcher of Alcator C-Mod which is based on the 4-way-splitter concept. A diagnostic based on the microwave probes concept has been installed to verify the LH2 design and study the physics of LH wave coupling. A total of 32 dedicated probes measure the forward and reflected power in a carefully selected set of the active and passive waveguides of the LH2 grill. Validity of LH coupling theory at low power (linear regime) is assessed and experimental observations of coupling at high power (nonlinear regime) are presented.   

The Role of Plasma Rotation in C-Mod Internal Transport Barriers

ITBs in Alcator C-Mod featuring highly peaked density and pressure profiles are induced by injecting ICRF power with the second harmonic of the resonant frequency for minority hydrogen off-axis at the plasma half radius. These ITBs are formed in the absence of particle or momentum injection, and with monotonic q profiles with q$_{min} \quad <$ 1. In C-Mod a strong co-current toroidal rotation, peaked on axis, develops after the transition to H-mode. If an ITB forms, this rotation decreases in the center of the plasma and forms a well, and often reverses direction in the core. This indicates that there is a strong EXB shearing rate in the region where the foot in the ITB density profile is observed. Preliminary gyrokinetic analyses indicate that this shearing rate is comparable to the ion temperature gradient mode (ITG) growth rate at this location and may be responsible for stabilizing the turbulence. Gyrokinetic analyses of recent experimental data obtained from a complete scan of the ICRF resonance position across the entire C-Mod plasma will be presented.   

Rotation Reversals in Alcator C-Mod L-mode Plasmas

Abrupt rotation reversals have been observed in Alcator C-Mod L-mode plasmas. The magnitude of the reversals is typically 10s of km/s, and can be triggered by slight changes in the electron density or plasma current. The critical density for the reversal is itself a strong function of plasma current. There are no other changes in macroscopic parameters such as the electron temperature or energy confinement time. Subtle differences in the turbulence characteristics are sometimes observed. The reversals occur inside of the q=2 surface.   

Using modulated on-axis ICRH to control fluctuations in an internal transport barrier

These recent experiments demonstrate apparent success in modulating core transport and fluctuations in an internal transport barrier (ITB) with ICRF heating, serving to localize phase contrast imaging measurements of density fluctuations. With well-resolved profile measurements for both ions and electrons, including flows, this provides a validation testbed for gyrokinetic simulations of electron transport. Modulated electron temperature swings of 40\% were accompanied by strong bursts of density fluctuations on phase contrast imaging (PCI), while edge fluctuations from reflectometry and Mirnov coils diminished. Previously, we observed strong density fluctuations during steady on-axis heating of C-Mod ITB's. Nonlinear gyrokinetic simulations of TEM turbulence [1] in the ITB reproduced the shape of the measured fluctuation wavelength spectrum during on-axis heating, using a synthetic PCI diagnostic in GS2, while matching the particle flux [2]. \newline [1] D. R. Ernst et al., 20th IAEA Fusion Energy Conference (2006), Chengdu, China, paper IAEA-CN-149/TH/1-3, http://www-pub.iaea.org/MTCD/Meetings/FEC2006/th\_1-3.pdf \newline [2] D. R. Ernst et al., Phys. Plasmas 11 (2004) 2637.   

Poloidal variation of Ar$^{16+}$ impurity density in Alcator C-Mod plasmas

Vertical poloidal asymmetries in impurity density have been observed in a number of tokamaks but have yet to be quantitatively explained by neoclassical theory. This up/down asymmetry, thought to be driven by a combination of ion-impurity friction and inertial forces, is being investigated in the wider context of verifying parallel impurity force balance theory necessary to utilize trace impurity flow measurements to calculate main-ion flows. The recent installation of an x-ray crystal imaging spectrometer allows simultaneous measurement of the up/down asymmetry in Ar$^{16+ }$density as well as radial profiles of the impurity temperature and both the poloidal and toroidal flows. The up/down density ratio was measured in Ohmic and ICRF-heated L-mode plasmas and shown to vary from 0.5 to 3.0 over a range of electron density, 0.2 $<$ n$_{e} <$ 2.0 10$^{20}$ [m$^{-3}$] and plasma current, 0.4 $<$ I$_{p}<$ 1.2 [MA]. The impurity density, n$_{z}$, is typically higher in the direction away from the $\nabla $B drift, regardless of x-point location and the asymmetry is localized to r/a $>$ 0.75. At low density, n$_{e} <$ 0.5x10$^{20}$ [m$^{-3}$] the ratio falls below unity indicating a reversal. For n$_{e}$ $>$ 0.5x10$^{20}$ [m$^{-3}$] the ratio is shown to scale linearly with n$_{e}$/I$_{p}$, qualitatively consistent with theory.   

Experimental and gyrokinetic investigation of core impurity transport in Alcator C-mod

A new multiple pulse laser blow-off system coupled with an upgraded high resolution x-ray spectrometer with spatial resolution allow for the most detailed studies of impurity transport on Alcator C-mod to date. Trace impurity injections created by the laser blow-off technique were introduced into plasmas with a wide range of parameters and time evolving profiles of He-like calcium were measured. The unique measurement of a single charge state profile and line integrated emission measurements from spectroscopic diagnostics were compared with the simulated emission from the impurity transport code STRAHL. A nonlinear least squares fitting routine was coupled with STRAHL, allowing for core impurity transport coefficients with errors to be determined. With this method, experimental data from trace calcium injections were analyzed and radially dependent, core values ($<$ r/a $\sim $.6) of the diffusive and convective components of the impurity flux were obtained. The STRAHL results are compared with linear and global, nonlinear simulations from the gyrokinetic code GYRO. Results of this comparison and an investigation of the underlying physics associated with turbulent impurity transport will be presented.   

Tungsten migration in Alcator C-Mod: sputtering and melting

A row of bulk tungsten (W) tiles were installed near the typical outer strike-point location in the Alcator C-Mod divertor in 2007. In the 2009/2010 campaign, one of the W tiles mechanically failed resulting in significant W melting at that location. Post-campaign PIXE surface analysis has been used to observe tungsten (W) deposition and migration patterns in the divertor for the typical operations (sputtering only) and operation with melted components. For sputtering conditions, W deposition of up to 20 nm equivalent thickness is observed at various divertor surfaces indicating prompt re-deposition at the outer divertor, neutral and ion transport through the private-flux region and ion transport in the scrape off layer. For melting conditions, W deposition of up to 400 nm equivalent thickness is observed at some locations at the outer divertor. However, the toroidal distribution of W on the outer divertor is strongly non-uniform. There is no W deposition measured on the inner wall limiter. These results indicate that impurity migration is affected by the erosion mechanism and source, with the migration from melting being less predictable and uniform than from the sputtering case. Supported by USDoE award DE-SC00-02060.   

Recent Measurement and Interpretation of Stable and Unstable Alfv{\'e}n Eigenmodes (AEs) in the Presence of Fast Ions in Alcator C-Mod

Stable AEs of various toroidal mode number are excited by a wide-toroidal-spectrum antenna and detected by a fully resolved toroidal array of probes. Stable GAEs with $n = 0$ and damping rates around $\gamma/\omega_0 = 1\%$, and stable TAEs with $n = 1$ and damping rates around $\gamma/\omega_0 = 1.5\%$ are observed. In the same discharges, TAEs of higher toroidal mode numbers including $n = -4$ and $n = 6$ are more centrally localized and do not couple to the diagnostic until driven unstable by fast ions. Such non-linear behavior via mode-particle interaction is intrinsic to the complete picture of AE instability. A composite spectrum ($|n| = 0-10$) is rendered from the collection of stable and unstable mode observations, and from NOVA-K calculations; peak instability is reached around $|n| = 5$, in rough agreement with the theoretical scaling of fast ion drive. Within this broad spectrum, however, stability between adjacent modes is not smooth - it varies strongly with mode number and plasma shape. These local islands of stability could be exploited through subtle changes in equilibrium parameters to stabilize AEs in burning plasmas.   

Investigating Highly Energetic Ions in the Alcator C-Mod Tokamak

Alcator C-Mod uses ICRF power for the bulk auxiliary heating and relies primarily on hydrogen minority heating scenarios. Measuring the hydrogen ion distribution provides an opportunity to validate ICRF simulations that include a full-wave field solver with a non-Maxwellian ion response (AORSA), and a zero-orbit width Fokker Planck code (CQL3D). The Compact Neutral Particle Analyzer (CNPA) is a diagnostic that measures this fast ion distribution function in C-Mod. The diagnostic has been upgraded to allow extended measurements in both energy and space. Results show a significant presence of highly energetic ($>$ 500 keV) ions. In this talk, results from the diagnostic will be presented. The fast ion tail temperature varies inversely with plasma density, exhibits depressed high energy tails at low plasma current likely due to orbit loss, and has no clear dependence on ICRF power. The measured distributions allow for an assessment of integrated simulations. Comparisons of measurements with predictions of a synthetic diagnostic will also be shown.   

Measurement of ICRF mode conversion and minority ion tail influence on wave absorption

Waves at ion cyclotron range of frequencies (ICRF) are measured with phase contrast imaging (PCI) and compared to RF simulation codes. Self-consistent wave electric fields and ion distribution functions are simulated by the full-wave code AORSA [1] and the Fokker-Planck code CQL3D [2]. Simulation agrees well with measurements for D(H) minority heating scenarios ($n_H/n_e = 0.1$) when the minority ion distribution function is allowed to evolve as it absorbs the wave, which indicates the impact of the ion tail back on wave absorption. However, the fluctuation intensity of the waves measured by PCI in mode conversion heating scenarios of D-H ($n_H/n_e = 0.2$) and D-$^3$He ($n_{^3He}/n_e = 0.14$) is smaller than simulation by a factor of 2-5. RF coupling efficiency as well as the accuracy of the simulated toroidal wave field structure is being investigated as possible causes for this discrepancy. \\[4pt] [1] E. F. Jaeger et. al., Phys. Plasmas, 8, 1573\\[0pt] [2] Proc. IAEA TCM on Advances in Sim. and Modeling of Thermonuclear Plasmas, Montreal, 1992, p. 527   

Study of Density Fluctuation via Microwave Imaging Reflectometry on KSTAR

The first prototype Microwave Imaging Reflectometry (MIR) system [H. Park et al., RSI \textbf{74}, 4239 (2004)] clearly demonstrated the shortcomings of the conventional reflectometry when the probe beam encountered a large amplitude and/or high fluctuation wavenumber at the reflection layer in laboratory tests However, the distinctive advantages shown in these tests were not fully realized in plasma operation. The discrepancies between the laboratory test and plasma application have been thoroughly investigated at POSTECH and possible causes of the MIR performance degradation on TEXTOR will be presented together with a conceptual multi-frequency MIR system design that will be developed for the KSTAR 2012 campaigns.   

KSTAR ECE Imaging for Temperature and Edge Density Measurements

ECE imaging (ECEI) system is becoming a powerful diagnostic for studying MHD instabilities in tokamaks. A prominent example is the 2D visualization of sawtooth reconnection physics in the TEXTOR, which revealed the high field side crash and collective heat transport [Park, et al., PRL 96, 195003 (2006)] forbidden in the Ballooning mode model. An improved ECEI system with considerably wider spatial coverage (spanning both high and low field sides) and $\sim\mu$sec time resolution has been installed on the KSTAR. The first KSTAR ECEI images showing the 2D spatial structures of the MHD dynamics of inner/outer regions (e.g. NTMs and ELMs) and Alfv\'en eigenmodes/waves will be presented. The feasibility of using ECEI data from the optically thin edge region to recover 2D electron density changes during L/H mode transitions is also discussed.