Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session UO07: MFE: Heating, Current Drive, and ScenariosOn Demand
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Chair: Robert Pinsker, General Atomics - San Diego Room: Rooms 315-316 |
Thursday, November 11, 2021 2:00PM - 2:12PM |
UO07.00001: The Physics of Electron Cyclotron Assisted Initiation of Fusion Plasmas Joyeeta Sinha, Peter C de Vries, Michael L Walker, Devon J Battaglia, Francesca Turco, Alan Hyatt, Hyun-Tae Kim, Joerg Stober, Ryota Yoneda, Yuri Gribov, Shaun R Haskey, Igor Bykov, Eric M Hollmann Electron Cyclotron (EC) wave injection can assist plasma initiation by providing additional heating during the burn-through phase, and also by pre-ionizing the neutral gas (pre-plasma) prior to the application of loop voltage (Vloop). The first mechanism is well understood and can be modelled and extrapolated to ITER, but that is not the case for the pre-ionization mechanism. New experiments have been conducted on DIII-D to improve the physics understanding of the EC plasma initiation process by clearly separating the effect of these two mechanisms. This has been achieved by applying an EC pulse before the Vloop is applied. For this study, pre-plasma formation was characterized in terms of the maximum density and temperature attained prior to Vloop application. Parametric scans were performed to study the influence of the EC injected power, EC injection angle, and pre-fill gas pressure on the pre-plasma creation process. The experiments showed that the pre-plasma assists the subsequent Vloop induced plasma initiation process by providing a high degree of ionization before Vloop is applied. It increases the likelihood of successful burn-through at relatively low values of toroidal electric field (~0.4 V/m). This particular aspect of the data is being modelled with the DYON code. |
Thursday, November 11, 2021 2:12PM - 2:24PM |
UO07.00002: Modeling very high electron heating by radio frequency waves on EAST Yueheng Huang, Nong Xiang, Jiale Chen, Longhao Ma, Erzhong Li, Liqing Xu, Xianzu Gong, Jinping Qian, Haiqing Liu, Zong Xu In recent EAST experimental campaign, a very high central electron heating, fully-non-inductive discharge with the core electron temperature over 9keV has been achieved. Such high central electron heating was realized by injecting radio frequency waves, including 1.8MW lower hybrid wave (LHW) and 0.8MW electron-cyclotron waves (ECW). In this work, integrated modeling is performed to investigate the physical mechanisms of such high electron heating. The modeling confirms that the core electron temperature can be sustained around Te≈5.5keV by LHW alone which is consistent with the experimental measurement. The electron temperature increases rapidly after the ECW is injected which is due to the interaction between the ECW and the electrons. With the increase of the electron temperature, the electron flux induced by the trapped electron modes (TEMs) and the electron temperature gradient driven modes (ETGs) is enhanced in the core region. The electron temperature increases slowly in a longer time scale. It is found that the slow increase is mainly due to the flattening of the density profile. Such flattening of the density profile can decrease the electron thermal diffusivity mainly induced by the TEMs, leading to a higher core electron temperature for a given heating source. |
Thursday, November 11, 2021 2:24PM - 2:48PM |
UO07.00003: Mitigation of RF sheaths via an ICRF antenna with electrically insulating sidewalls Gurleen Bal, Bart V Compernolle, Mike Martin, Patrick Pribyl, Troy A Carter A single strap, high-power (~150kW), RF (2.4MHz) antenna was used to study RF sheaths in a magnetized helium plasma with plasma parameters ne ~ 1018 – 1019 m-3, Te ~ 1 – 10 eV, and B0 ~ 0.1 T. The experiments were conducted on the Large Plasma Device (LAPD). Three experiments were carried out on the Large Plasma Device (LAPD) using different plasma-facing materials on a single-strap ICRF antenna. These experiments demonstrated that electrically isolating the antenna walls from the bulk plasma can significantly reduce near-field rectification. The three different enclosure sidewall materials included copper, MACOR (electrically isolating), and MACOR over copper (MACOR-copper). In the case of the MACOR-copper sidewalls, the non-conductive MACOR material was exposed to the bulk plasma but a layer of copper was added below to allow for image currents to flow. All three of the experiments had similar plasma density, temperature, and background magnetic field. In the case of the copper enclosure, RF rectified potentials and associated formation of convective cells were observed and reported1. In the experiments with MACOR and MACOR-copper enclosures, RF rectification was significantly reduced. Additionally, these experiments showed no evidence of convective cell formation. Although the results from the MACOR experiment are reminiscent of the results obtained in ASDEX-U with the 3-strap antenna optimized to reduce image currents on the antenna limiters2, the MACOR-copper experiment seems to suggest that insulating plasma-facing materials have a stronger impact on potential rectification than a lack of image currents at the antenna boundary. |
Thursday, November 11, 2021 2:48PM - 3:00PM |
UO07.00004: Electron Bernstein Wave heating and current drive for MAST Upgrade Simon Freethy, Alf Koehn-Seeman, Bengt Eliasson, Luca Garzotti, Helen Webster, Sam Gibson Microwave-based current drive brings significant technological advantages in a reactor environment. However, the cyclotron harmonics in many spherical tokamak designs are covered by the plasma cut-off, prohibiting the use of ECCD. Electron Bernstein Waves (EBWs) offer a promising alternative. EBW-CD is predicted to be highly efficient, however EBW-CD has never been demonstrated on the spherical tokamak. |
Thursday, November 11, 2021 3:00PM - 3:12PM |
UO07.00005: New ICRF antenna Characterization and Performance in EAST Xinjun Zhang, Chengming Qin, Shuai Yuan, Hua Yang, Yongsheng Wang, Yuzhou Mao, Lunan Liu, Lei Wang, Yanping Zhao, Xianzu Gong, Yan Cheng, Xu Deng, Kai Zhang, Songqing Ju, Jianggang Li, Baonian Wan, Yuntao Song, Wei Zhang, Lin Ai, Qichao Liang, Guanghui Zhu Coupling and Heating associated with ion cyclotron range of frequency (ICRF) heating was a major challenge to ICRF utilization in the past on EAST. In order to increase the antenna loading and thereby increase the heating, a lower k parallel spectrum ICRF antenna has been designed and installed for EAST 2021 experimental campaign. In order to decrease the maximum voltage on the transmission line, we developed an impedance transformer near the antenna port. The new antenna with 3D curved Faraday screens and current strap is also expected to improve further the coupling. Using the old B-port antenna as a reference, the coupling and heating efficiency are significantly higher for the new antenna than the old B-port antenna. The coupling loading and heating efficiency of the new ICRF antenna is ∼3-7 times greater than the old B-port antenna. The plasma stored energy is increased by 30 kJ/MW in the high βp H-mode discharges . The improved performance is consistent with simulations indicating that the new design of ICRF antenna has greatly improved the coupling relative to the old antennas. In addition, the long pulse operations of 1.1MW/60s, 1.5MW/40s and 1.8MW/22s are achieved for the new ICRF antenna in the high βp H-mode discharges. Here, we will report results on the coupling characterization of new ICRF antenna and an overview of the main results from new ICRF antenna in the EAST 2021 experimental campaign. |
Thursday, November 11, 2021 3:12PM - 3:24PM |
UO07.00006: Commissioning and first results of the DIII-D helicon system Bart G Van Compernolle, Michael W Brookman, Robert I Pinsker, Charles Moeller, Jared P Squire, Andrea M. M Garofalo, A. Nagy, Antonio Torrezan, Dan Ponce, Carl J Pawley, Satyajit Chowdury, Neal A Crocker, Genevieve H DeGrandchamp, Edward T Hinson, John Lohr, Alessandro Marinoni, Elijah H Martin, Craig C Petty, Miklos Porkolab, Chris C Rost, Oliver Schmitz, Kathreen E Thome, Huiqian Wang, Jonathan G Watkins, Kurt Zeller A 1 MW-level helicon launcher system at DIII-D is being commissioned to demonstrate efficient off-axis current drive in high- plasmas, where full single-pass absorption is expected. A 30-module traveling wave antenna of the comb-line type can be fed from either side to allow co- or counter- current drive. A klystron at 476 MHz has been commissioned into a test load at 1 MW forward power. During antenna conditioning ~0.3 MW of helicon power was coupled successfully to a highly reproducible L-mode target plasma, in quasi-steady RF pulses as long as 0.5 s. Clear conditioning progress was observed in repeated discharges. Several dedicated helicon diagnostics obtained data for the first time: Langmuir probes adjacent to the antenna observed changes to the SOL density; High-Frequency Ion Cyclotron Emissions (HICE) and Doppler Back Scattering (DBS) observed magnetic and density fluctuations near 476 MHz and nearby sidebands. Additional diagnostics to measure wave propagation and antenna-region density profiles will be installed this year. |
Thursday, November 11, 2021 3:24PM - 3:36PM |
UO07.00007: Initial assessment of Parametric Decay Instabilities during high power helicon wave injection into DIII-D* Miklos Porkolab, Robert I Pinsker, Genevieve H DeGrandchamp, Seung Gyou Baek, Michael W Brookman, Bart V Compernolle, Severin Denk, Andrea M Garofalo, Craig C Petty, Kathreen E Thome High power helicon waves (whistler or very high harmonic fast waves) at a frequency of 0.476 GHz have been launched on DIII-D with the goal of demonstrating efficient off-axis current generation in DIII-D AT plasmas. We have shown in recent work that under typical experimental conditions strong parametric decay instability (PDI) is expected in the pedestal region at injected RF power levels of 0.1-1.0 MW with corresponding electric fields of 10-30 kV/m [1]. The dominant driver of the PDI is the ExB and polarization drift velocity which can drive ion cyclotron quasi-modes and lower hybrid sideband waves unstable. Initial experimental results have been obtained with powers up to 0.3 MW showing evidence of strong PDI measured with high-frequency one-turn magnetic probes at frequencies set by the usual selection rules and the toroidal field strength at the outboard edge of the plasma; weak PDI has been observed at power levels as low as 10 kW. Here we present analytic and numerical assessment of growth rates, frequencies and convective thresholds for the PDIs, and compare them with experimental observations. |
Thursday, November 11, 2021 3:36PM - 3:48PM |
UO07.00008: Reduced threshold parametric instabilities during X2 heating in ASDEX Upgrade Mads G Senstius, Asger S Jacobsen, Søren K Hansen, Stefan K Nielsen, Joerg Stober Second harmonic X-mode (X2) heating is popular in several fusion devices such as the ASDEX Upgrade (AUG) tokamak. X2 heating benefits from the availability of efficient microwave sources, favorable beam accesibility and strong absorption. Furthermore, X2 heating has generally been considered free of undesired nonlinear wave effects known as parametric decay instabilities (PDIs). However, new observations from AUG show scattering due to PDIs during X2 heating at a greatly reduced gyrotron power threshold which is correlated with the passing of a rotating island. The generated scattering presents a danger to microwave sensitive equipment and the instability may result in substantial power absorption into unintented regions of the plasma. We present a theoretical model on why the island reduces the PDI power threshold. The island can act as a cavity, trapping PDI daughter waves and giving rise to an absolute- instead of a convective instability. The scattering is not visble when the gyrotron beam intersects the O- or X-point of the island but rich spectra emmerge between these points. This is because only an intermediate density in the decay region can excite two trapped PDI daughter waves necessary to overcome the power threshold. Dedicated particle-in-cell simulations confirm this. |
Thursday, November 11, 2021 3:48PM - 4:00PM |
UO07.00009: Impact of rotated perpendicular wavevector of lower hybrid waves to lower hybrid current drive profiles on EAST and Alcator C-Mod Seung Gyou Baek, Bodhi Biswas, Gregory M Wallace, Paul T Bonoli, Bo Jiang Ding, Miaohui Li, Youngchun Li, Yunfei Wang, Mao Wang, Chenbin Wu, Guanghou Yan, Jiale Chen, Xumei Zhai, Andrea M. M Garofalo, Wilkie Choi, Francesca M Poli, Syun'ichi Shiraiwa Standard lower hybrid current drive model generally predicts a broad power deposition profile with off-axis peaking, while experiments show on-axis peaking. In the present modeling study, spectral modification is introduced in an ad-hoc manner by modifying the initial orientation of the perpendicular wavevector, which may arise from wave scattering by turbulence in front of the launcher. The ray-tracing/Fokker-Planck solver GENRAY/CQL3D is utilized within the python-based πScope framework. The study finds that rotating the perpendicular wavevector in such a way as to increase the initial poloidal component of k^ is effective in reproducing the centrally peaked current profile observed in monotonic shear plasmas on both EAST and C-Mod with an inclusion of radial transport of fast electrons. These waves can readily be absorbed to the central plasma, which reduces the sensitivity of the power deposition profile to a slight change of the plasma condition. Off-axis power deposition is reproduced in reverse shear plasmas. The results presented here suggest that spectral modification arising from edge density fluctuations in a tokamak may need to be considered in understanding wave propagation and absorption. |
Thursday, November 11, 2021 4:00PM - 4:12PM |
UO07.00010: Additively Manufactured RF Launchers: Advantages, Challenges, and Future Prospects Andrew Seltzman, Stephen J Wukitch A system for high field side launch of lower hybrid waves at 4.6 GHz is planned for DIII-D at n||=-2.7 and is expected to access improved efficiency current drive of 140 kA/MW at r/a=0.6-0.8. A novel multijunction launcher reduces circulating power to lower electric field below the 9.3 kV/cm multipactor threshold over a wide range of edge densities (ne=1017 to 1018 m-3) with a traveling wave power divider and by matching aperture impedance to the plasma edge. |
Thursday, November 11, 2021 4:12PM - 4:24PM |
UO07.00011: Application of a newly developed radial directional electron probe to the edge unidirectional electron current measurement in EAST Shaocheng Liu, Yunfeng Liang, Ning Yan, Liang Liao, Wenyin Wei, Lingyi Meng, Liang Chen, Shuai Xu, Nan Zhao, Ran Chen, Guanghai Hu, Yongliang Li, Xiaoju Liu, Tingfeng Ming, Youwen Sun, Jinping Qian, Long Zeng, Guoqiang Li, Liang Wang, Xianzu Gong, Xiang Gao A newly developed radial directional electron probe (DEP) has been applied to the unidirectional electron current measurement on EAST tokamak. The DEP consists of two radial arrays of channels which have opposite directions and align along the local magnetic field line. Each radial array has 6 holes with a radial interval of 5 mm. Every channel has a hole with 0.5 mm radial width, 3 mm depth and 15º poloidal opening angle. The graphite collector embedded inside the hole is biased to positive potential to repel low energy ions, and high energy ions are blocked by the hole surface because their Larmor radii are larger than the radial width of hole. A particle orbit simulation is performed based on the Boris algorithm, which demonstrates the validity and rationality of the DEP. According to the simulation, the ion collected probability is quite small if compared with the electron collected probability, consequently the collected current is mainly contributed by electrons. The collected currents of thermal electrons under Maxwell-Boltzmann distribution from both channels are almost equal, and the net current is driven by non-thermal electrons. The ion current collected by the DEP collector can be ignored in contrast with the electron current, as demonstrated by the I-V characteristics in a DEP commissioning experiment. The difference of collected current between two opposite channels signifies the unidirectional electron current in the flux tube. In a lower hybrid wave (LHW) modulation experiment, the amplitude and radial structure of non-thermal electron current induced by LHW is measured directly by this radial DEP array, and the LHW filament current covers over 20 mm radial region with a maximum of 20 A/cm2. |
Thursday, November 11, 2021 4:24PM - 4:36PM |
UO07.00012: Model based optimization of Advanced Tokamak plasma start-up Raphael Schramm, Alexander Bock, Maximilian Reisner, Emiliano Fable, Joerg Stober, Marc Maraschek, Hartmut Zohm AT scenarios offer improved stability, confinement and pulse length compared to standard scenarios due to an increase of the plasma’s bootstrap current jbs∝q∇p. They are accessed by externally manipulating the safety factor profile q, which can be applied during the current ramp-up, or after an equilibrium is reached. The former allows for a longer discharge and more unconventional current distributions, but due to the volatility of the early plasma, creating such a scenario experimentally, with feed-forward control usually takes a lot of trial and error. |
Thursday, November 11, 2021 4:36PM - 4:48PM |
UO07.00013: Can global simulations of magnetically confined plasmas be done with the implicit particle in cell method ECSim? Giovanni Lapenta, Joost Croonen, Yong-Seok Hwang, Choongki Sung Implicit PIC is an alternative to other reduced models that step over smaller scales. While reduced models rely on analytical derivations to eliminate the scales deemed not of interest, the implicit method uses numerical techniques that automatically step over and damp the unresolved scales. The approach has had significant success in space applications [1]. But can the method also be used for magnetically confined plasmas? A new recent development is the ECsim method that conserves energy exactly and allows more flexibility on the choice of resolved scales [2-4]. To apply the new code to fusion devices we will address a number of questions arising. For example the presence of boundary layers, typically non present in space, but source of many important processes directly affecting confinement [5]. We will present our recent results on the application of the method to several alternative fusion concepts such as field reversed configurations, poly well, spherical torus etc. In particular, we will consider the opportunities for ECsim in spherical tokamaks. |
Thursday, November 11, 2021 4:48PM - 5:00PM |
UO07.00014: Elevating zero-dimensional predictions of tokamak plasmas to self-consistent theory-based simulations Tim Slendebroek, Joseph Mcclenaghan, Orso M Meneghini, Brendan C Lyons, Sterling P Smith, Jeff Candy A new workflow in the OMFIT integrated modelling framework (STEP-0D) has been developed to make theory-based prediction of tokamak scenarios starting from zero-dimensional (0D) quantities. The workflow starts with the PRO-create (profiles creator) module, which generates physically plausible plasma profiles and a consistent equilibrium from the same 0D quantities as the H98Y2 confinement scaling. These results form the starting point for the STEP (Stability, Transport, Equilibrium, and Pedestal) module, which then iterates between state-of-the-art theory-based physics models for the equilibrium, sources, core transport, and pedestal to obtain a self-consistent solution. A systematic validation against the H98Y2 database and the global confinement database (DB5) demonstrated that on average STEP-0D is capable of predicting the energy confinement time with a mean relative error (MRE) <18%. A neural network-based energy confinement predictor, TaueNN (EPED-NN, TGLF-NN), was found to be faster but less accurate with an MRE of 23%. Lastly, STEP-0D was used to predict the performance of negative triangularity DIII-D experiments, and derive a theory-based confinement time scaling law for negative triangularity plasmas. |
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