Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session TI2: Pedestal, LH Transition, Non-Inductive Startup |
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Chair: Jerry Hughes, Massachusetts Institute of Technology Room: OCC Oregon Ballroom 203 |
Thursday, November 8, 2018 9:30AM - 10:00AM |
TI2.00001: H-mode grade confinement with L-mode edge in negative triangularity plasmas on DIII-D Invited Speaker: Alessandro Marinoni Plasmas with Negative Triangularity (NT) shape on the DIII-D tokamak sustain H-mode level confinement and high normalized beta (H98,y2 = 1.3, βN = 2.6) for several energy confinement times, despite featuring edge pressure profiles typical of an L-mode plasma without Edge Localized Modes. |
Thursday, November 8, 2018 10:00AM - 10:30AM |
TI2.00002: Self-consistent pedestal prediction for JET Invited Speaker: Samuli Saarelma In H-mode plasmas the energy confinement is strongly linked to pressure pedestal height (pped). At the same time, the core pressure has a stabilising effect on the peeling-ballooning modes (PBM) that limit pped. Therefore, in a truly predictive model the core and pedestal have to be resolved self-consistently. The most widely used model for pedestal predictions, EPED requires advance knowledge of the density pedestal (nped) and the global beta which are not available when predicting future experiments. We have further developed the EPED model to incorporate For the prediction of nped, we have used a model based on neutral penetration. While this model lacks the important contribution of inter-ELM pedestal transport, it is able to predict nped in a large JET-ILW database with an average of 22% error. For the self-consistent core and pedestal prediction we use heating power as input and couple pedestal prediction with BohmgyroBohm transport model. For experimental JET plasmas found at the PBM stability limit this method accurately predicts both the pedestal and core as long as fast particle pressure is included in the prediction. When the experimental pedestals are not limited by PBMs, the pedestal height is overpredicted. We also investigated the effect of the mass of the main ion isotope (mi) on the pedestal prediction. The isotope affects directly the diamagnetic stabilisation of the PBMs. This alone does not explain the higher pedestals of deuterium plasmas compared to hydrogen. However, the experimental density pedestal increases with mi (both nsep and nped increase from H to D). With these changes
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Thursday, November 8, 2018 10:30AM - 11:00AM |
TI2.00003: Long-lived predator-prey dynamics in the pedestal of near-zero torque high performance DIII-D plasmas Invited Speaker: Kshitish Barada Novel turbulence dynamics are found to regulate ELM-free pedestal gradients of near-zero applied torque, wide-pedestal quiescent H-mode plasmas. ELM-free, low torque operation of future burning plasmas, such as in ITER, make this an important research area. In these plasmas, a long-lived predator-prey type limit cycle oscillation (LCO) regime is sustained for 3-12 energy confinement times immediately following suppression of edge harmonic oscillations (EHOs). This LCO regime is characterized by coupled time dependent dynamics in E×B velocity shear (V'), electron temperature gradient (∇ Te), and density fluctuation amplitude (ñ). A pulse-like radially inward propagation of perturbations in E×B velocity is found to be critical for the existence of this unique system. The temporal dynamics can be described as follows: ñ modifies transport, transport modifies gradients, gradients modify E×B velocity. The spatiotemporally varying E×B velocity due to the above mentioned pulse like propagation modifies E×B shear which modifies ñ, and the cycle repeats. These quasi-stationary E×B velocity perturbations are observed to have toroidal and poloidal symmetry but are found inconsistent with turbulence driven zonal flows. LCO modulated transport, as evidenced by increased levels of divertor ion saturation current and divertor Dα intensity on LCO timescales, can serve to partially replace missing EHO related particle transport and allow continued ELM-free operation. This LCO transport is hypothesized to prevent the pressure gradient increases necessary to trigger ELMs. The unique LCO dynamics discussed here is a potentially important mechanism to control and regulate pedestal structure in future fusion plasma devices. |
Thursday, November 8, 2018 11:00AM - 11:30AM |
TI2.00004: Safety Factor and Turbulence Dynamics Dependence of the L-H Power Threshold on DIII-D Invited Speaker: Zheng Yan The L-H transition power threshold (PLH) is found to have a significant but previously unidentified dependence on the plasma current, or q95, at mid-density (ne~3.2e19 m-3) on DIII-D. Comprehensive 2D turbulence and flow measurements in the plasma edge reveal the co-existence of two frequency bands of broadband modes across the L-H transition with higher flow shear at lower current, which can help explain the linear decrease in PLH as the plasma current is reduced from 1.4 MA to 1 MA. At lower density, ne~1.5e19 m-3, there is little dependence of PLH on the plasma current. Density fluctuation measurements by beam emission spectroscopy (BES) show that the lower frequency band (<20 kHz) of the broadband modes propagates in the ion diamagnetic direction at low plasma current, whereas the higher frequency band (> 20 kHz) propagates in the electron diamagnetic direction (in the lab frame). This bimodal turbulence structure has been previously observed in plasmas with lower PLH, such as those with ion grad-B drift towards X-point and in the isotope dependence (D lower than H). Nonlinear simulations with BOUT++ shows similar dual-mode with experiments, with the most unstable modes being resistive ballooning modes. Additionally, the amplitude of long wavelength (KperpRhoI<1) density fluctuations is higher at lower currents, implying a higher drive for a Reynolds Stress driven zonal flow. This is consistent with the higher (turbulence driven) flow shear observed at low current, and is qualitatively consistent with the lower zonal flow collisional damping at higher q (lower current). Understanding the physics behind the PLH dependence on multiple parameters and accurately predicting PLH is important to ensuring that future burning plasma experiments such as ITER can access high confinement modes of operation. |
Thursday, November 8, 2018 11:30AM - 12:00PM |
TI2.00005: Reconciling L-H threshold dependencies on JET-ILW with the ErxB shear mechanism Invited Speaker: Ephrem Delabie H-mode access with external heating only is a critical issue for ITER. Experiments on JET-ILW highlight strong dependencies of the L-H threshold power, such as a factor 2 variation in threshold depending on strike point positions, that cannot be extrapolated without understanding of the underlying physics principles. We will show that most of the observed trends in the JET L-H database can be reconciled with the paradigm of turbulence suppression by sheared ErxB flows but all terms contributing to the Er shear as well as parametric dependencies of the turbulence drive are required to explain all the observed trends. An explanation of the divertor configuration effect will be presented based on a near-SOL contribution to the shear in the outer part of the Er well. Observations of a stronger peaked sheath voltage at the outer strike point in the pulses with lowest threshold are in qualitative agreement with EDGE2D/EIRENE modelling, pointing to the effect of the recycling pattern on broadening the target temperature profiles. The influence of the near-SOL on the transition could also explain some of the dynamics of dithering transitions as the inner divertor plasma attaches and detaches during each cycle, pointing to a breakdown of the sheath and the associated poloidal ErxB flow in the SOL at the transition into H-mode. An interpretation of the isotope effect in the context of ErxB shear stabilization remains elusive. Both the stored energy as well as the edge kinetic profiles before the transition are well matched between hydrogen and deuterium. This implies a similar Er well structure as there are no explicit mass dependencies in the radial force balance equation. The requirement of a similar critical ErxB stabilization would imply a similar turbulence drive, which is at odds with the observation of higher transport in hydrogen. |
Thursday, November 8, 2018 12:00PM - 12:30PM |
TI2.00006: Application of transient CHI plasma start-up to future ST and AT devices Invited Speaker: Kenneth Hammond Employment of non-inductive plasma start-up techniques would considerably simplify the design of a spherical tokamak fusion reactor. Transient coaxial helicity injection (CHI) is a promising method, and is expected to scale favorably to next-step reactors. However, the implications of reactor-relevant parameters on the initial breakdown phase for CHI have not yet been considered. In particular, successful handover to steady-state heating and current-drive sources requires that the transient CHI discharge reach a certain minimum temperature, which in turn puts a stringent limit on the allowable gas pre-fill. On the other hand, insufficient pre-fill could prevent breakdown at the beginning of the CHI phase. In order to meet these potentially conflicting requirements, we evaluate CHI breakdown using an extension of the Townsend avalanche theory often applied to analyze Ohmic startup. The model predicts the duration and location of breakdown based initial vacuum conditions: field line connection length, gas pressure, electrode bias, and, if applicable, loop voltage. The predictions of this model have shown good agreement with observations of breakdown in NSTX for both Ohmic and CHI experiments [Hammond et al., Nucl. Fusion 58, 016013 (2018)]. In addition, the model provided an explanation for a CHI failure mode which was previously not well understood, in which breakdown occurred in an unexpected and undesired part of the vessel. We now apply this model to concepts for next-step reactors, determining the feasibility of breakdown and analyzing the risks of parasitic arcing in scenarios for solenoid-free startup. Scenarios for both spherical tokamak and advanced tokamak configurations are studied, with supporting simulations using the TSC code, to compare breakdown and current ramp-up conditions for different aspect ratios. |
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