Session GO4: C-Mod, ADX, Lockheed Martin CFR

Overview of Alcator C-Mod Research

Research on C-Mod supports next-step-devices: RF heating, current and flow drive, divertor/PMI physics, non-ELMing regimes with enhanced confinement, and disruption mitigation/runaway dynamics. Disruption mitigation experiments in MHD-unstable plasmas show MGI works equally well with and without locked modes. The L-I-mode threshold is found to be independent of magnetic field, opening an expanded operating range at high field. The toroidal and radial structure of power deposition of RF waves into the edge plasma has been systematically quantified, through the use of a unique set of fast time resolution edge diagnostics. Progress in understanding multi-channel core transport has been significant. Full-physics, ITG/TEM/ETG gyrokinetic simulations show that nonlinear cross-scale coupling enhances both ion and electron heat flux to match experiments, explaining the origin of electron heat flux and stiffness. Dynamic, passive measurements of the core rotation velocity profiles with X-ray imaging crystal spectroscopy show the direction of intrinsic rotation reversals depends on central safety factor, not on the magnetic shear. Design studies for ADX and SPARC are establishing the engineering, economics and physics for a fusion energy development path leveraging new superconducting magnet technologies.   

ADX: A high Power Density, Advanced RF-Driven Divertor Test Tokamak for PMI studies

The MIT PSFC and collaborators are proposing an advanced divertor experiment, ADX; a divertor test tokamak dedicated to address critical gaps in plasma-material interactions (PMI) science, and the world fusion research program, on the pathway to FNSF/DEMO. Basic ADX design features are motivated and discussed. In order to assess the widest range of advanced divertor concepts, a large fraction (\textgreater 50{\%}) of the toroidal field volume is purpose-built with innovative magnetic topology control and flexibility for assessing different surfaces, including liquids. ADX features high B-field (\textgreater 6 Tesla) and high global power density (P/S $\sim$ 1.5 MW/m$^{2}$) in order to access the full range of parallel heat flux and divertor plasma pressures foreseen for reactors, while simultaneously assessing the effect of highly dissipative divertors on core plasma/pedestal. Various options for efficiently achieving high field are being assessed including the use of Alcator technology (cryogenic cooled copper) and high-temperature superconductors. The experimental platform would also explore advanced lower hybrid current drive and ion-cyclotron range of frequency actuators located at the high-field side; a location which is predicted to greatly reduce the PMI effects on the launcher while minimally perturbing the core plasma. The synergistic effects of high-field launchers with high total B on current and flow drive can thus be studied in reactor-relevant boundary plasmas.   

Modeling of Edge Plasma in X-Point Target Divertor Configuration

Innovative divertor configurations with secondary X-points in the divertor volume or close to the target plate have been actively studied in the tokamak edge plasma community in recent years. Such configurations include the cusp divertor [1], snowflake-like divertor [2], X-divertor [3], and X-point target divertor [4]. The presence of a secondary X-point in the divertor has hindered the application of most established tokamak edge plasma transport codes. To meet the demands for comprehensive modeling tools for such innovative configurations, the UEDGE code has been recently advanced to allow for inclusion of a secondary X-point in the divertor region. The present physics application is focused on the X-point target divertor configuration that has been speculated to allow for stable, highly radiating detached plasma regimes in the ADX tokamak concept [4]. The results of application of the upgraded UEDGE to an X-point target divertor configuration will be presented and physics implications will be discussed.\\[4pt] [1] H. Takase, J. Phys. Soc. Japan, 70, 609, 2001;\\[0pt] [2] D.D. Ryutov. Phys. Plasmas, 14, 064502, 2007;\\[0pt] [3] M. Kotschenreuther et al., 2004 IAEA FEC, paper IC/P6-43;\\[0pt] [4] B. LaBombard et al., Nucl. Fusion 55, 053020, 2015.   

Overview of Alcator C-Mod Contribution to the 2015 JRT on Off Axis Current Drive

The goal of the US Department of Energy FES 2015 Joint Research Target (JRT) is to conduct experiments and analysis to quantify the impact of broadened current and pressure profiles on tokamak plasma confinement and stability. Broadened current profiles are achieved on Alcator C-Mod through the use of Lower Hybrid Current Drive (LHCD). C-Mod experiments from the recent run campaign have focused on several areas, such as MHD stability of discharges with broad current profiles, energy transport barriers and high-Z impurity transport in non-inductive discharges, momentum transport in discharges with altered safety factor profiles, and validation of LHCD actuator models.   

Analysis of Weakly Coherent mode on C-Mod with the BOUT++ code

Edge turbulence in I-mode is characterized by a strong reduction of mid-frequency turbulence and the appearance of a higher-frequency fluctuation, dubbed the ``weakly-coherent mode'' (WCM). WCM has been well characterized by BOUT++ code. In the simulation, the magnetic equilibrium is generated using the kinetic EFIT with measured pressure profile and the calculated bootstrap current from the Sauter model. The linear simulations are carried out using fits to measured plasma density, electron temperature and electron field profiles, assuming quasi-neutral. WCM in I-mode is driven by Drift Alfven wave (DAW) mode and Ballooning mode, and resistivity plays an important role. In the linear simulation, strong DAM instability has been found at n>=20, and both the mode structures and the electron diamagnetic direction are consistent with the DAW mode. In the nonlinear simulation, the frequency spectrum of the mode at n=20 is similar to reflectometry measurements at the same location. Particle diffusivity is larger than thermal, normalized density fluctuation is larger than the electron temperature fluctuation, and simulated Xe and Xi are close to experimental Xeff. Some predictions about how to drive WCM will also be presented.   

Analysis of Runaway Electron Synchrotron Radiation in Alcator C-Mod

In Alcator C-Mod, runaway electron (RE) beams can reach energies of $\sim$30 MeV or higher, emitting synchrotron radiation in the visible wavelength range. Two spectrometers, with spectral ranges of 350-1020 nm and resolutions of $\sim$4 nm, have been installed and absolutely calibrated on C-Mod to observe this emission. Recent theoretical work [1,2] predicts that the RE distribution tends toward a mono-energetic bump as the synchrotron radiation and collisional friction balance the electric force. Our analysis of RE synchrotron spectra shows that it is possible to distinguish the emission of a mono-energetic and mono-pitch beam from that of a distribution of energies and pitch angles (as calculated in [3]). Preliminary results indicate that the mono-energetic bump is formed as predicted, instead of a broader distribution.\\[4pt] [1] A. Stahl, et al. Phys. Rev. Lett. 114, 115002 (2015).\\[0pt] [2] P. Aleynikov, et al. Phys. Rev. Lett. 114, 155001 (2015).\\[0pt] [3] A. Stahl, et al. Phys. Plasmas 20, 093302 (2013).   

Perturbative Thermal Transport Studies on Alcator C-Mod and ASDEX Upgrade

Perturbative thermal diffusivity has been measured on Alcator C-Mod and ASDEX Upgrade via the extended-time-to-peak method with heat pulses generated by partial sawtooth crashes. Heat pulses generated by sawtooth crashes have been used extensively in the past to study perturbative diffusivity [Lopes Cardozo, N.J., PPCF 37, 799 (1995)], but the details of the sawtooth event lead to non-diffusive ``ballistic'' transport, invalidating their use for measuring perturbative diffusivity [Fredrickson, E.D. et al., PoP 7, 5051 (2000)]. Partial sawteeth generate a heat pulse without the ballistic transport of full sawteeth [Fredrickson 2000]. Partial sawtooth analysis was applied to over 50 C-Mod shots containing both L- and I-Mode, as well as ASDEX Upgrade plasmas, though partial sawteeth were less common on AUG. Results indicate correlations between perturbative diffusivity and confinement regime (L- vs. I-mode), as well as correlations with local temperature, density, the associated gradients, and gradient scale lengths (a/LTe and a/Ln). Finally, diffusivities calculated from partial sawteeth were compared to perturbative diffusivities calculated with the GYRO gyrokinetic code, leading to quantitative agreement with multi-scale GYRO simulations.   

Sensitivity of Alcator C-Mod Dissipative Divertor Operation to Toroidal Peaking of Extrinsic Low-Z Impurity Seeding

In most present experiments, mitigation of heat and particle fluxes to plasma facing components is not necessary to avoid engineering limits, while dissipative divertor operation will be a requirement for reactor-scale facilities. ITER will use distributed sub-divertor impurity injection and seeks to explore the impact of non-axisymmetric divertor seeding, a possible result of injector failure. Results are presented from Alcator C-Mod experiments exploring the sensitivity of pedestal temperature and energy confinement degradation to the toroidal distribution of extrinsic low-Z seeding. At moderate N$_{2}$ fueling levels, outer divertor power loading could be strongly reduced, reaching P$_{\mathrm{ODIV}}$/P$_{\mathrm{NET}}$ \textless 10{\%}. In these case high confinement, H$_{98}$ $\sim$ 1, is sustained and plasmas are insensitive to the toroidal localization of the impurity seeding. Experiments with elevated N$_{2}$ that access a pronounced or fully detached regime demonstrate a transition to a reduced confinement H-mode, H$_{98}$ $\sim$ 0.7, which is sensitive to the toroidal peaking of the N$_{2}$ fueling. When utilizing all injection locations, minimizing non-axisymmetric effects, high confinement was sustained at total N$_{2}$ fueling rates that were at least 30-40{\%} higher than if impurities were introduced at a single location.   

High-field side scrape-off layer investigation: scaling of the power e-folding width and impurity screening behavior in near-double null configurations

Fluctuation-induced transport measured on the C-Mod high-field side (HFS) scrape-off layer (SOL) is extremely low; n, T profiles there become very sharp in near-double null configurations and, unlike on the low-field side (LFS), no far SOL ``shoulders'' are seen. In single-null discharges, this transport asymmetry drives near-sonic parallel flows. A strong impurity screening behavior is also evident -- 6x higher N puff rate on the HFS compared to LFS produces the same core N content. It has been proposed that future tokamaks should exploit this remarkable behavior [1] -- locate all RF actuators and close-fitting wall structures on the HFS and employ near-double-null topologies, for example. C-Mod is presently investigating this physics more fully: (1) How does the HFS power e-folding width scale with plasma current, $\sim$ 1/Ip as seen for the LFS? (2) Does the favorable screening behavior extend to balanced-double null behavior where the HFS SOL flows become stagnant, or must some unbalance be required? Latest experimental results will be presented.\\[4pt] [1] \textit{ADX: a high field, high power density, advanced divertor and RF tokamak}, Nucl. Fusion \textbf{55} (2015) 053020.   

Investigations of Impurity Penetration and Transport with the Application of ICRF Power

High power ICRF heating has typically been accompanied by elevated core impurity concentrations. The causes of this have been variously attributed to increased impurity sources due to RF-enhanced sputtering, to increased impurity penetration due to ICRF-induced potentials and convention cells in the far-SOL, or to a combination of these effects. In Alcator C-Mod ICRF-induced potentials are observed in the far-SOL both locally (i.e. along flux tubes that intersect or pass in front of an energized ICRF antenna) and globally (i.e. in regions that are not magnetically connected to an energized antenna). We have performed experiments to investigate these effects by puffing known amounts of N impurity from four locations that have different mappings to an ICRF antenna energized to stepped power levels. We find no significant change in the penetration efficiency for the locally-puffed impurity with the ICRF power level, even though the local ICRF-induced potentials are present at the puff location, and we take this as evidence that these potentials do not directly enhance impurity penetration. Nonetheless, we observe that the core concentrations of non-puffed impurities are well correlated with the ICRF power. We interpret this to be a result of source enhancement or penetration enhancement by the ICRF at locations other than the four tested puff locations.   

First Measurements of Edge Transport Driven by the Shoelace Antenna on Alcator C-Mod

The Shoelace antenna is a unique device designed to couple to the Quasi-Coherent Mode (QCM, $k_{\perp}\sim1.5$~cm$^{-1}$, $50 < f < 200$~kHz) and Weakly-Coherent Mode (WCM, $k_{\perp}\sim1.5$ ~cm$^{-1}$, $200 < f < 500$~kHz), continuous edge fluctuations that sustain high-performance confinement regimes by exhausting impurities. The antenna is used to explore whether modes like the QCM and WCM may be exploited to actively regulate edge transport. In initial experiments, the antenna excited a resonance at the QCM frequency and phase velocity, but transport measurements were unavailable. A subsequent redesign of the winding pitch allows the antenna to be field-aligned while mapping magnetically to the Mirror Langmuir Probe (MLP) on the last-closed flux surface. This has enabled the first measurements of edge transport induced by the antenna-driven fluctuation, which has been further enhanced by quadrupling the antenna source power.   

Is turbulence indeed reduced in the tokamak edge pedestal? Mysteries of PEDESTAL poloidal ASYMMETRIES revealed

It has been suggested that the sudden transition between states of low and high confinement involves the reduction of turbulence by sheared radial electric fields. For H-mode pedestals, the amount of turbulence may be only large enough to affect high order phenomena, such as heat transport. Neoclassical collisional theory may thus be expected to properly treat low order phenomena, such as flows. However, H-mode edge pedestals on Alcator C-Mod exhibit significantly stronger poloidal asymmetry than predicted by the most comprehensive neoclassical models developed to date. We propose a novel self-consistent neoclassical theoretical model that allows us to explain these poloidal asymmetries. First, impurity temperature asymmetries can be driven by inertial effects, which are significant when impurities are allowed to reach sonic speeds. Second, a much stronger impurity density in-out asymmetry than given by just the magnetic field can be introduced by the poloidally varying impurity diamagnetic drift. This is achieved by allowing the diamagnetic drift contribution to be comparable to the poloidal and toroidal flows used to measure the radial electric field. We thus provide a more realistic predictive model for pedestal observations without the need of invoking anomalous transport.   

Imaging of X-point turbulence in Alcator C-Mod

A nearly tangential view of the lower X-point region of Alcator C-Mod has been coupled to a high-speed camera filtered for D-alpha line emission. Recording at $\sim$400,000 frames per second, the system detects filaments propagating in the private flux region that are approximately aligned with the local magnetic field. This behavior appears similar to what has recently been observed in the MAST tokamak [1]. Turbulence and transport into the private flux region is potentially important. It may be a mechanism to spread heat across field lines and reduce peak heat fluxes on divertor targets. It may also explain how transport-driven flows seen in the high-field side scrape-off layer [2] are accommodated, being otherwise too large compared to the particle flux arriving at the inner divertor target plates. The dynamics of these filaments are analyzed, as is the rate at which they are generated. Correlation analysis is used to determine the speed and trajectories of the filaments. Radial speeds of $\sim$1 km/s are found. Clear changes are observed in the X-point-region fluctuations at the L-to-H-mode transition.\\[4pt] [1] J.R. Harrison et al., J. Nucl. Mater. 463 (2015)\newline [2] N. Smick et al., Nucl. Fusion 53 (2013)   

Path to Efficient Lower Hybrid Current Drive at High Density

Recovery of lower hybrid current drive (LHCD) efficiency at high density was demonstrated on Alcator C-Mod by modifying the scrape-off layer (SOL) plasma. RF probe measurements around the C-Mod tokamak indicate that the LH wave amplitude at the high field side wall significantly attenuates with plasma density. This is interpreted as enhanced collisional loss due to the increase in the SOL density and width. By taking advantage of the narrower SOL width by doubling plasma current to 1.1 MA, it is found that the LH wave amplitude maintains its strength, and an effective current drive is extended to above 1x10e20 m-3. An order of magnitude increase in non-thermal Bremsstrahlung emission is consistent with ray-tracing results which take into account the change of SOL profiles with current. In the coming campaign, a further investigation on the role of the ~SOL plasma is planned by raising plasma current above 1.1 MA. This will be aided with newly developed RF magnetic loop antennas mounted on a radially movable probe head. This system is expected to intercept the LH resonance cone on the first pass, allowing us to measure radial profiles of both the wave amplitude and dominant parallel wavenumber in the SOL for the first time. These data will be compared with the GENRAY ray-tracing code.   

Overview of the Lockheed Martin Compact Fusion Reactor (CFR) Program

The Lockheed Martin Compact Fusion Reactor (CFR) Program endeavors to quickly develop a compact fusion power plant with favorable commercial economics and military utility. An overview of the concept and its diamagnetic, high beta magnetically encapsulated linear ring cusp confinement scheme will be given. The analytical model of the major loss mechanisms and predicted performance will be discussed, along with the major physics challenges. Key features of an operational CFR reactor will be highlighted. The proposed developmental path following the current experimental efforts will be presented. \copyright 2015 Lockheed Martin Corporation. All Rights Reserved.