Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session VI3: Tokamak Operations, Control, Scenarios, and RF Heating |
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Chair: Steve Scott, Princeton Plasma Physics Laboratory Room: Ballroom BC |
Thursday, November 1, 2012 3:00PM - 3:30PM |
VI3.00001: Advances in tokamak control: from multi-actuator MHD control to model-based current profile tailoring Invited Speaker: Federico Felici Recent experiments on TCV have demonstrated integrated control of the sawtooth and Neoclassical Tearing Mode (NTM) instabilities in a combined preemption-suppression strategy. This strategy is enabled by new sawtooth control methods (sawtooth pacing) in which modulation of sawtooth-stabilizing electron cyclotron power during the sawtooth cycle stimulates the advent of the crash. Rather than controlling the average sawtooth period, the precise timing of each individual crash can now be prescribed. Using this knowledge, efficient preemptive stabilization of NTMs becomes possible by applying power on the rational surface only at the instant of the crash-generating seed island. TCV experiments demonstrate that this approach, reinforced by NTM stabilization as a backup strategy, is effectively failsafe. This opens the road to inductive H-mode scenarios with long sawteeth providing longer inter-crash periods of high density and temperature. Also Edge Localized Modes are susceptible to EC modulation and it is shown that individual ELM events can be controlled using similar techniques. For advanced tokamak scenarios, MHD control is to be combined with optimization and control of the plasma kinetic and magnetic profile evolution in time. Real-time simulation of a physical model (RAPTOR) of current transport, including bootstrap current, neoclassical conductivity and auxiliary current drive, yields complete knowledge of the relevant profiles at any given time. The pilot implementation on TCV shows that these calculations can indeed be done in real-time and the resulting profiles have been included in feedback control schemes. Integration of this model with time-varying equilibria and internal current profile diagnostics provides a new framework for real-time interpretation of diagnostic data for plasma prediction, scenario monitoring, disruption prevention and feedback control. [Preview Abstract] |
Thursday, November 1, 2012 3:30PM - 4:00PM |
VI3.00002: Progress Toward Fully Noninductive Discharge Operation in DIII-D Using Off-axis Neutral Beam Injection Invited Speaker: J.R. Ferron The new 5 MW off-axis neutral beam capability on DIII-D results in discharges with improved access to fully noninductive plasma regimes. Off-axis beam injection broadens the current density profile by changing the distribution of beam-driven current; broadens the pressure profile; and extends the tearing mode stable duration. In cases with the most off axis current, plasmas can now be sustained without $q=2$ resonances. The measured $T_e$ is lower on axis, and the calculated fast ion pressure profile is less peaked, resulting in the broadened pressure profile. This plus the additional current near the conducting vacuum vessel wall results in an increase in the calculated ideal $n=1$ $\beta_N$ limit to above 4, close to the $\beta_N$ that must be achieved for fully noninductive operation in a reactor. With no $q=2$ surface, neither the most deleterious, low-order (2/1) tearing modes nor off-axis fishbone modes are observed; instead, if present, tearing modes are higher order (5/2 or 3/1). A decrease in the normalized confinement results from the off-axis injection, with a further decrease when the minimum in $q$ is above 2. With more peaked current profiles so that the minimum in $q$ is around 1.4, stability is also improved with off-axis injection, with pulse duration at $\beta_N =3.5$ increased to 2 current relaxation times, 3 s, in a discharge that projects to $Q=5$ in ITER. Modeling indicates that in DIII-D, solutions for fully noninductive operation have $\beta_N$ above 4, making the increased ideal stability limit that results from off-axis beam injection a requirement, and the minimum in $q$ is above 2 with about 50\% of the input power from off-axis injection. This new approach to modification of the current profile in DIII-D, then, is providing crucial insight into the development of a fusion steady-state scenario. [Preview Abstract] |
Thursday, November 1, 2012 4:00PM - 4:30PM |
VI3.00003: Heating and current drive requirements for ideal MHD stability and ITB sustainment in ITER steady state scenarios Invited Speaker: Francesca Poli Steady state scenarios envisaged for ITER aim at optimizing the bootstrap current, while maintaining sufficient confinement and stability to provide the necessary fusion yield. Non-inductive scenarios will need to operate with Internal Transport Barriers (ITBs) in order to reach adequate fusion gain at typical currents of 9 MA. However, the large pressure gradients associated with ITBs in regions of weak or negative magnetic shear can be conducive to ideal MHD instabilities in a wide range of $\beta_N$, reducing the no-wall limit. Scenarios are established as relaxed flattop states with time-dependent transport simulations with TSC [1]. Fully non-inductive configurations with current in the range of 7-10 MA and various heating mixes (NB, EC, IC and LH) have been studied against variations of the pressure profile peaking and of the Greenwald fraction. It is found that stable equilibria have $q_{min} > 2$ and moderate ITBs at 2/3 of the minor radius [2]. The $\mathbf{E}\times \mathbf{B}$ flow shear from toroidal plasma rotation is expected to be low in ITER, with a major role in the ITB dynamics being played by magnetic geometry. Combinations of H\&CD sources that maintain reverse or weak magnetic shear profiles throughout the discharge and $\rho(q_{min})\ge0.5$ are the focus of this work. The ITER EC upper launcher, designed for NTM control, can provide enough current drive off-axis to sustain moderate ITBs at mid-radius and maintain a non-inductive current of 8-9MA and $H_{98}\ge1.5$ with the day one heating mix. LH heating and current drive is effective in modifying the current profile off-axis, facilitating the formation of stronger ITBs in the rampup phase, their sustainment at larger radii and larger bootstrap fraction. The implications for steady state operation and fusion performance are discussed.\\[4pt] [1] Jardin S.C. \textit{et al}, J. Comput. Phys. \textbf{66} (1986) 481\\[0pt] [2] Poli F.M. \textit{et al}, Nucl. Fusion \textbf{52} (2012) 063027. [Preview Abstract] |
Thursday, November 1, 2012 4:30PM - 5:00PM |
VI3.00004: Field-Aligned ICRF Antenna Characterization and Performance in Alcator C-Mod Invited Speaker: Stephen Wukitch Impurity contamination associated with ion cyclotron range of frequency (ICRF) heating remains a major challenge to ICRF utilization in magnetic confinement devices, particularly with metallic plasma facing components. Here, we report results on an experimental investigation of a high power, magnetic field-aligned (FA) antenna, designed to reduce parallel electric (E$\vert \vert )$ field through symmetry and thereby reduce RF related impurity contamination. Using the standard non-field aligned antennas (ST) as a reference, the impurity contamination and sources on the antenna are significantly lower for the FA-antenna than the ST antennas. In addition, the radiated power is reduced for given injected power for the FA-antenna compared to the ST-antennas in L and H-mode discharges. The improved performance is consistent with simulations indicating that the FA-antenna has reduced integrated E$\vert \vert $ relative to the non-aligned antennas. However, the simulation also predicts that so-called monopole phasing, where antenna strap current has [0,0,0,0] phase, should have the lowest integrated E$\vert \vert $. The initial results suggest that monopole phasing has a stronger impact on the plasma potential and higher core impurity contamination and sources at the antenna. Utilizing gas puff imaging, the radial electric field profile in the scrape-off-layer (SOL) is readily measured. For the ST and FA-antennas, fine structure (variations of order $\sim $0.5 cm) in the radial electric field is observed and radial penetration of the rectified potential structures is $\sim $10 times greater than the skin depth. This anomalous penetration appears to be consistent with including cross-field RF polarization currents in the sheath model. Further comparisons of the FA- and ST-antennas are being carried out with an extensive array of boundary plasma diagnostics to characterize the impurity behavior and impact on the SOL transport and SOL density profiles; the latest results will be presented. [Preview Abstract] |
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