Bernert, M.

Bobkov, V.

Carralero, D.

Cavedon, M.

Conway, G.

Dux, R.

Guimarais, L.

Happel, T.

Herrmann, A.

Kocan, M.

Leitenstern, P.

Müller, H.

Nikolaeva, V.

Noterdaeme, J.

Ochoukov, R.

Puetterich, T.

Simon, P.

Viezzer, E.

Wolfrum, E.

English

MPG.PuRe

Experimental study of a possible role of the scrape off layer radial electric field in determining H-mode confinement properties on ASDEX Upgrade   R. Ochoukov1, V. Bobkov1, M. Bernert1, D. Carralero1, M. Cavedon1, G.D. Conway1,  R. Dux1, L. Guimarais1,2, T. Happel1, A. Herrmann1, M.Kocan3, P. Leitenstern1,  H.W. Mueller1, V. Nikolaeva1,2, J.-M. Noterdaeme1,4, Th. Puetterich1, P. Simon1,  E. Viezzer1, E. Wolfrum1, and the ASDEX Upgrade Team  1Max-Planck-Institut für Plasmaphysik, Boltzmannstr. 2, D-85748 Garching, Germany 2Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal 3ITER Organization, Route de Vinon sur Verdon, 13115 St Paul Lez Durance, France 4UGent, Applied Physics Department, 9000 Gent, Belgium  1. Introduction The high confinement (H-) mode is an attractive regime in which to operate fusion-grade tokamak plasmas. The H-mode formation is associated with the establishment of steep electron and ion pressure gradients over a radially narrow region in the plasma edge (the pedestal, 0.96 < pol < 1.00, where pol is the normalized poloidal flux coordinate), as well, as the development of a deep radial electric field (Er) well across the same region. The goals of this study are: 1) to develop a viable technique that can modify (or bias) the Er in the scrape-off layer (SOL, pol > 1.00) using Ion Cyclotron Range-of Frequencies (ICRF) power operated in non-standard phasing and, if successful, 2) to assess the impact of the SOL Er on the H-mode performance. The first results show that a significant level of ICRF power (PICRF ≥ 1 MW) operated in non-standard phasing (π/2 compared to standard π phasing) leads to a change in the edge localized mode (ELM) behaviour from predominantly large amplitude ELMs to small ELMs. At the same time, the change in the ELM behaviour is accompanied by an increase in tungsten (W) sources from the active ICRF antenna limiters, an increase in the core W contents and the associated increase in the core radiated power, a decrease in the non-radiated power crossing the separatrix (Psep), an increase in the normalized electron collisionality at the pedestal top (e* at pol ~ 0.96), and little to no change in the Er well profile in the pedestal region. At this time it remains unclear if ICRF power operated in non-standard phasing was able to modify (or bias) the SOL Er profile. 2. Experimental Method In order to significantly modify (or bias) the Er value (|ΔEr| ≥ 10 kV/m) across the SOL region (~5 cm wide region at the outer midplane for a standard H-mode discharge on ASDEX Upgrade), it is necessary to raise the SOL plasma potential (p) by several hundred volts (p ≥ 500 V for the case of a standard H-mode on ASDEX Upgrade). ICRF power is known to 41st EPS Conference on Plasma Physics P2.012increase the SOL p in the vicinity of the ICRF limiter to values >100 V [1-3]. Note, that SOL biasing with ICRF power is expected to generate plasma  potentials that peak radially near the ICRF limiters and, hence, apply negative Er’s (Er = -p) across the SOL region radially between the separatrix and the limiter, see Fig. 1. The discharge parameters for this study were: the toroidal magnetic field BT = -2.5 T and the plasma current Ip = 0.8 MA. The edge safety factor q95 = 5.4. The working gas was deuterium. The plasma shape was a lower single null. The H-mode was maintained by a combination of 4.9 MW of neutral beam power (PNBI) and 1.4 MW of electron cyclotron heating power (PECRH). The operating frequency of the ICRF antennas was 36.5 MHz with antenna strap phasing varying between standard ( and non-standard (/2). 3. Experimental Results and Discussion Changing ICRF strap phasing to a /2 configuration is expected to increase unabsorbed ICRF power in the SOL through a combination of higher near- and far-ICRF fields. The increase in the ICRF field strength, in turn, leads to stronger RF rectification [4] and, hence, plasma potentials in the SOL. Fig. 2 shows the plasma response to a change of ICRF antenna phasing on ASDEX Upgrade, the time periods of interest are highlighted. The key observations are: 1) an increase in the W sources from the active ICRF antenna limiter surface (WLim, Fig. 2 (e)). The WLim increase is present both during the ELM and between the ELM cycles, ruling out the change in the ion temperature within the ELM cycle as the cause of the WLim increase. Figure 2: The time histories of the key plasma parameters in the presence of ICRF power in standard () and non-standard (/2) phasing. The quantities shown are (a) the ICRF power PICRF, (b) the radiated power Prad, (c) the stored plasma energy Wmhd, (d) the non-radiated power crossing the separatrix Psep, (e) the tungsten source from the limiter surface WLim, (f) the tungsten source from the divertor surface WDiv, and (g) the core tungsten concentration cW. Figure 1: SOL Er biasing with ICRF power. 1.01.5340.40.54548022 3 4122 3 4  (MW)30629PICRF30631  Prad(MW)(g)(f)(e)(d)(c)(b)/2 phasing(MJ) Wmhd phasing(a)  Psep (MW)  (au)WLim  (au)WDiv(au)Time (s)cWTime (s)41st EPS Conference on Plasma Physics P2.012The observed WLim increase is consistent with increased p (or biasing) at the limiter surface, as significant sputtering of W surfaces by D+ ions requires incident ion energies in excess of 200 eV [5]. Such high ion energies have been measured previously on ASDEX Upgrade in ICRF-heated discharges in the vicinity of the limiter surfaces [6]. 2) Simultaneously, we also observe an increase in the core W contents cW (Fig. 2 (g)), a corresponding increase in the radiated power Prad (Fig. 2 (b)), and a drop in the non-radiated power crossing the separatrix Psep (Fig. 2 (d)). The drop in Psep (Psep = Pheat – Prad – dWmhd/dt, where Pheat is the total heating power and Wmhd is the plasma kinetic energy) is dominated by the increase in the radiated power as the total heating power and the stored energy is little changed over the highlighted time periods (Fig. 2 (c)). 3) The response of the W sources from the divertor (WDiv) is the opposite of the limiter response (Fig. 2 (f)): WDiv drops with the application ICRF power in non-standard phasing. The drop in WDiv is consistent with the change in the ELM behaviour, which will be the subject of a later paragraph. One of the key features of an H-mode plasma is the appearance of an electron and ion pressure pedestal. The response of the plasma density ne and the electron temperature Te across the pedestal region to ICRF power in non-standard phasing is shown in Fig. 3: the ne profile shows a radial inward shift (Fig. 3 (a)), while Te at the top of the pedestal (pol ~ 0.96) shows a small decrease (Fig. 3 (b)). The net effect of the Te and ne changes is the increase in the pedestal electron collisionality e* (e* ~ ne/Te2) from 2.0 to 2.6 (where the effective charge Zeff is assumed ~1). Within the experimental uncertainties the Er profiles across the pedestal (as measured with charge exchange recombination spectroscopy [7]) show little to no variation before and after the application of ICRF power in non-standard phasing (Fig. 3 (c)). Note that Er measurements in the SOL region on ASDEX Upgrade are typically carried out using Doppler reflectometry [8]. However, due to a combination of high stray ICRF fields and low amplitude of the poloidally propagating turbulence in the SOL, the Doppler reflectometer diagnostic was unable to resolve the SOL Er profiles for these discharges. Figure 3: The radial profiles of (a) the plasma density ne, (b) the electron temperature Te, and (c) the radial electric field Er across the pedestal region in discharges heated with ICRF power in standard and non-standard phasing. 0.8 0.9 1.0 1.10.02.0x10194.0x10196.0x10198.0x10190.8 0.9 1.0 1.1020040060080010000.92 0.94 0.96 0.98 1.00-50-40-30-20-10010  phasing, PICRF = 1.6 MW /2 phasing, PICRF = 1.1 MW(c)(b)30629, 30631 ne (m-3)pol(a)ICRF Limiter  Te (eV)pol30629, 3063130629, 30631  phasing, PICRF = 1.6 MW /2 phasing, PICRF = 1.1 MW  Er (kV/m)pol41st EPS Conference on Plasma Physics P2.012Therefore, it remains unknown if ICRF power in non-standard phasing affects the Er profiles in the SOL region. ELMs are another key feature of a steady-state H-mode on ASDEX Upgrade. The evolution of the ELM behaviour (amplitude and frequency spectrum) in the presence of ICRF power in /2 phasing is shown in Fig. 4: the ELMs change from predominantly large amplitude, low frequency ELMs (Fig. 4 (a)) to predominantly small ELMs with a broad frequency spectrum (Fig. 4 (b)). The change in the ELM behaviour is consistent with the change in Psep, which suggests that the ELMs change from predominantly Type-I to predominantly Type-III [9]. However, an unambiguous identification of the dominant ELM type requires additional analysis of the pedestal stability limit and the magnetic ELM precursor signatures. 4. Conclusion Operating ICRF antennas in /2 phasing on ASDEX Upgrade has a strong effect on the H-mode plasmas: WLim increases (Fig. 2 (e)), cW contents and Prad increase (Figs. 2 (g) and (b)), while Psep decreases (Fig. 2 (d)). Simultaneously, the ELM behaviour changes from predominantly large amplitude to predominantly small amplitude ELMs with a broad frequency spectrum (Fig. 4). While the increase in WLim is consistent with increased biasing of the SOL region, the radial extent of biasing remains unresolved. Future experiments will focus on decoupling the effect of tungsten impurities either by using boronization or an externally applied W source. 5. Acknowledgements This project has received funding from the EURATOM research and training programme 2014-2018. 6. References [1] V. Bobkov et al., Nucl. Fusion 50 (2010) 035004. [2] I. Cziegler et al., Plasma Phys. Control. Fusion 54 (2012) 105019. [3] R. Ochoukov et al., Plasma Phys. Control. Fusion 56 (2014) 015004. [4] H.S. Butler and G.S. Kino. Physics of Fluids, 6:1346–1355, 1963. [5] R. Behrisch, W. Eckstein, Sputtering by Particle Bombardment, Springer, 2007. [6] L. Colas et al., 20th International Conference on Radio Frequency Power to Plasmas, Sorrento, Italy, 2013. [7] E. Viezzer et al, Rev. Sci. Instrum. 83, 103501 (2012). [8] G.D. Conway et al., 8th International Reflectometry Workshop - IRW8 (St.Petersburg) 2-4th May 2007. [9] H. Zohm, Plasma Phys. Control. Fusion 38 (1996) 105–128. Figure 4: The evolution of the ELM amplitude and frequency spectra in the presence of ICRF power operated in (a) standard and (b) non-standard ICRF phasing, the relevant time windows are highlighted (3.1<t<4.1 s). The response of the ELM frequency spectra is shown in (c). 2.5 3.0 3.5 4.0 4.50.05.0x1031.0x1041.5x1042.5 3.0 3.5 4.0 4.5 101102103102103104105106  30629ELM Amplitude (A)Time (s)(a)30631Time (s)(b)/2 phasing/2 phasingPRF = 1.1 MW  Power Spectrum (au)ELM Frequency (Hz) phasingPRF = 1.6 MW30629, 30631, 3.5 < t < 4.0 s(c) phasing41st EPS Conference on Plasma Physics P2.012

Experimental study of a possible role of the scrape off layer radial electric field in determining H-mode confinement properties on ASDEX Upgrade

http://hdl.handle.net/21.11116/0000-0008-8737-2

Experimental study of a possible role of the scrape off layer radial electric field in determining H-mode confinement properties on ASDEX Upgrade

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