Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session GI2: MFE: Pedestal & ImpuritiesInvited
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Chair: Howard Wilson, University of York Room: 210 CDGH |
Tuesday, November 1, 2016 9:30AM - 10:00AM |
GI2.00001: Pedestal shape, stability and inter-ELM evolution for different main ion species in ASDEX Upgrade Invited Speaker: Florian M. Laggner In tokamak plasmas with different main ion species as hydrogen isotopes or helium, a change of confinement occurs, known as isotope effect. To identify the processes defining the pedestal structure and evolution, experiments comparing hydrogen (H), deuterium (D) and helium (He) plasmas have been performed. Their goal was to match the pedestal top electron density and temperatures and compare the pedestal shape and stability. A factor of almost $10$ higher gas puff as well as a factor of $2$ higher heating power were required in H to achieve the same pedestal top values as in the D reference. While the pedestal electron temperature profiles do not differ, the density profile in H has shallower gradients. These can be explained by a lower particle confinement in H, if the ionization source profile is assumed to be similar. In He plasmas owing to the larger effective charge, the stored energy at similar pedestal top electron density is roughly a factor of $1.5$ smaller than in the references, leading to the absence of ELMs. In summary the experimental results suggest different particle and energy confinement for different main ion species, however, peeling-ballooning theory can sufficiently describe the pedestal stability and ELM behavior. [Preview Abstract] |
Tuesday, November 1, 2016 10:00AM - 10:30AM |
GI2.00002: Observation and Calculation of the ECRH Effect on the Tracer Impurity Accumulation in LHD Invited Speaker: Naoki Tamura In a magnetic confinement fusion reactor various Z impurities will exist inside the plasma. When the amount of the impurities exceeds the acceptable level by an accumulation, this will lead to impermissible plasma performance degradation due to the radiation losses and plasma dilution. Therefore, it is crucially important to develop effective schemes for controlling the impurities in the core plasma and to understand the underlying physical mechanisms of such schemes. Recent LHD experiments show the ability of ECRH to control the impurity accumulation. Experiments on the LHD have used a tracer-encapsulated solid pellet (TESPEL), which is embedded with vanadium, to introduce the extrinsic and non-recycling impurity directly inside the last-closed flux surface (LCFS) region. Therefore, the confinement time of the vanadium impurity can be directly evaluated from the time history of highly ionized vanadium ion. In cases where the collisionality between the impurity ions and the bulk ion is in the Banana-Plateau regime (but close to the collisional Pfirsch-Schl\"{u}ter (PS) regime), the impurities in the LHD plasma are strongly accumulated into the core plasma. When the 1.5 MW 154 GHz ECRH is applied for such plasma just after the TESPEL injection, the accumulation of the vanadium ions was almost completely suppressed. This result indicates that applying ECRH changes the direction of the radial vanadium particle flux from the inward to the outward. Although the neoclassical ambipolar radial electric field in stellarators has a stronger impact on the transport, particularly on the impurity transport, than in tokamaks, there is no conclusive data regarding a radial electric field measured with a charge exchange spectroscopy diagnostic to support the view that the change in the radial electric field would be attributed to the outward flow of the vanadium ions in the LHD plasma. In this contribution, the results of ongoing evaluations of the neoclassical (e.g., PENTA/DKES that includes the momentum conservation) and turbulent (e.g., GKV-X) transport will be presented to elucidate their respective roles in 3D toroidal plasmas. [Preview Abstract] |
Tuesday, November 1, 2016 10:30AM - 11:00AM |
GI2.00003: On the recovery of pedestal temperature of JET-ILW plasmas with injection of low-Z impurities Invited Speaker: Carine Giroud The pedestal confinement has significantly decreased in JET with its metallic ITER-like wall with reference to the carbon wall phase of JET (JET-C). A reduction in pedestal temperature is observed in all scenarios regardless of the level of D-gas injection or value of $\beta _{\mathrm{N}}$. In particular, the JET-ILW 2.5MA/2.7T high-$\delta $ ($\delta =$0.4) plasmas at n$_{\mathrm{ped}}$/n$_{\mathrm{GW}}\ge $0.7, discharges most comparable with JET-C, the pedestal pressure has reduced by 40{\%} with a decrease in pedestal temperature from 0.9keV to 0.5keV with the change of wall. The pedestal stability has been modified with the new wall: the reference JET-C plasmas pedestals had an operational point in the corner of the Peeling-Ballooning (PB) diagram, with pressure limited by intermediate n-numbers (n$=$5-20), whereas the JET-ILW unseeded plasmas have a lower pressure gradient limited by high n-numbers $\ge $70 (ballooning modes). Seeding N, a low-Z impurity, almost recovers the thermal stored energy, pedestal pressure and pedestal temperature to JET-C levels and with an operation point in the corner of the PB diagram. The mechanisms linked to the pedestal recovery with N are likely related to the mechanisms leading to a decrease in pedestal temperature in the absence of C in the plasma composition. The improved pedestal stability with N is not solely linked to the ideal linear PB stability since N-seeded plasmas in JET-ILW can be in type-III ELM regime and have a higher pedestal pressure than unseeded type-I ELMy H-mode. An increased pedestal pressure via an inward movement of the pedestal pressure from the separatrix is not observed with N seeding. However, we have identified two mechanisms responsible. A first initial mechanism linked to the change in ELM energy losses which raises modestly the average global beta by 10{\%} but allows in return a second mechanism to take place. The considered high-$\delta $ plasmas can then benefit, if in type-I ELM regime, from the virtuous cycle (2nd mechanisms) of an increased Shafranov shift, higher pedestal pressure allowing increased core pressure. The operational point can climb towards the corner of the PB diagram. The 1$^{\mathrm{st\thinspace }}$mechanism which reduces the average ELM energy losses has to be identified but seems to be linked to the SOL/separatrix conditions. The effects of ion diamagnetic drift and plasma rotation on the stability of high-n ballooning modes are being investigated. [Preview Abstract] |
Tuesday, November 1, 2016 11:00AM - 11:30AM |
GI2.00004: Theoretical explanation for strong poloidal impurity asymmetry in tokamak pedestals Invited Speaker: Silvia Espinosa Stronger impurity density in-out poloidal asymmetries than predicted by the most comprehensive neoclassical models have been measured in H-mode tokamak pedestals during the last decade. However, these pioneering theories\footnote{P. Helander, Phys. Plasmas 5, 3999 (1998)} neglect the impurity diamagnetic drift, while recent measurements indicate that it can be of the same order as the ExB drift that is retained\footnote{C.Theiler et al.,Nucl Fusion 54,083017 (2014)}. In order to keep both drifts self-consistently, stronger radial gradients of the impurity density must be allowed. As a result, radial impurity flow effects need to be included for the first time. These effects substantially alter the parallel impurity flow. The resulting modification in the impurity friction with the banana regime background ions then allows stronger poloidal variation of the impurity density, temperature and potential. Even the six-fold high field side accumulation of boron density measured on Alcator C-Mod\footnote{R.M.Churchill et al.,Phys Plasmas 22,056104 (2015)} can be explained without invoking anomalous transport. Moreover, the potential can no longer be assumed to be a flux function since the impurity density variation gives a poloidally varying potential that results in strong poloidal variation of the radial electric field. The fact that the magnitude of the negative radial electric field and the impurity temperature are both larger on the low field side is also correctly predicted. Finally, this pedestal neoclassical model with radial flows may provide insight on how to control impurity accumulation in JET. [Preview Abstract] |
Tuesday, November 1, 2016 11:30AM - 12:00PM |
GI2.00005: Experimental pathways to understand and avoid high-Z impurity contamination from ICRF heating in tokamaks Invited Speaker: Matthew Reinke Recent results from Alcator C-Mod and JET demonstrate progress in understanding and mitigating core high-Z impurity contamination linked to ICRF heating in tokamaks with high-Z PFCs. Theory has identified two likely mechanisms: impurity sources due to sputtering enhanced by RF-rectified sheaths and greater cross-field SOL transport due to ExB convective cells. New experiments on Alcator C-Mod and JET demonstrate convective cell transport is likely a sub-dominant effect, despite directly observing ExB flows from rectified RF fields on C-Mod. Trace N$_{\mathrm{2}}$ introduced in the far SOL on field lines connected to and well away from an active ICRF antenna result in similar levels of core nitrogen, indicating local RF-driven transport is weak. This suggests the core high-Z density, n$_{\mathrm{Z,core}}$, is determined by sheath-induced sputtering and RF-independent SOL transport, allowing further reductions through antenna design. ICRF heating on C-Mod uses a unique, field aligned (FAA) and a pair of conventional, toroidally aligned (TAA) antennas. The FAA is designed to reduce rectified voltages relative to the TAA, and the impact of sheath-induced sputtering is explored by observing n$_{\mathrm{Z,core}}$ while varying the TAA/FAA heating mix. A reduction of approximately 50{\%} in core high-Z content is seen in L-modes when using the FAA and high-Z sources at the antenna limiter are effectively eliminated, indicating the remaining RF-driven source is away from the limiter. A drop in n$_{\mathrm{Z,core}}$ may also be realized by locating the RF antenna on the inboard side where SOL transport aids impurity screening. New C-Mod experiments demonstrate up to a factor of 5 reduction in core nitrogen when N$_{\mathrm{2}}$ is injected on the high-field side as compared to low-field side impurity fueling. Varying the magnetic topology helps to elucidate the SOL transport physics responsible, laying a physics basis for inboard RF antenna placement. [Preview Abstract] |
Tuesday, November 1, 2016 12:00PM - 12:30PM |
GI2.00006: MHD modeling of DIII-D QH-mode discharges and comparison to observations Invited Speaker: Jacob King MHD modeling of DIII-D QH-mode discharges and comparison to observations Nonlinear NIMROD simulations, initialized from a reconstruction of a DIII-D QH-mode discharge with broadband MHD, saturate into a turbulent state, but do not saturate when flow is not included. This is consistent with the experimental results of the quiescent regime observed on DIII-D with broadband MHD activity [Garofalo et al, PoP (2015) and refs. within]. These ELM-free discharges have the normalized pedestal-plasma confinement necessary for burning-plasma operation on ITER. Relative to QH-mode operation with more coherent MHD activity, operation with broadband MHD tends to occur at higher densities and lower rotation and thus may be more relevant to ITER. The nonlinear NIMROD simulations require highly accurate equilibrium reconstructions. Our equilibrium reconstructions include the scrape-off-layer profiles and the measured toroidal and poloidal rotation profiles. The simulation develops into a saturated turbulent state and the n=1 and 2 modes become dominant through an inverse cascade. Each toroidal mode in the range of n=1-5 is dominant at a different time. The perturbations are advected and sheared apart in the counter-clockwise direction consistent with the direction of the poloidal flow inside the LCFS. Work towards validation through comparison to magnetic coil and Doppler reflectometry measurements is presented. Consistent with experimental observations during QH-mode, the simulated state leads to large particle transport relative to the thermal transport. Analysis shows that the phase of the density and temperature perturbations differ resulting in greater convective particle transport relative to the convective thermal transport. [Preview Abstract] |
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