Stationary Zonal Flows during the Formation of the Edge Transport Barrier in the JET Tokamak

Abstract

High spatial resolution Doppler backscattering measurements in JET have enabled new insights into the development of the edge Er. We observe fine-scale spatial structures in the edge Er well with a wave number krρi≈0.4-0.8, consistent with stationary zonal flows, the characteristics of which vary with density. The zonal flow amplitude and wavelength both decrease with local collisionality, such that the zonal flow E×B shear increases. Above the minimum of the L-H transition power threshold dependence on density, the zonal flows are present during L mode and disappear following the H-mode transition, while below the minimum they are reduced below measurable amplitude during L mode, before the L-H transition.

Poloidal asymmetric flow and current relaxation of ballooned transport during I-phase in ASDEX Upgrade

“…Linearizing Eqs. (14) and (12) in the electrostatic limit yields iω JP p e cos s = J sin s and iμω JP J sin s = −(1 + τ i ) p e cos s . This exhibits an eigenfrequency of ω JP = (1 + τ i )/μ (renormalization gives f JP = m i /m e (c si /(2πqR))) in the few hundred kHz range for ASDEX Upgrade parameters.…”

“…(14) and up-down asymmetric poloidal magnetic field fluctuations corresponding to up-down asymmetric parallel current fluctuations Eq. (12). All other terms are neglected.…”

“…Replacing the parallel electron Eq. (12) by that of the ions (13) flow, which is the Stringer spin-up [32,33]. Linearizing both equations gives iω SSU ũ sin s = (1+τi) ǫ p e cos s and iω SSU p e cos s = − ũ sin s corresponding to an eigenfrequency of ω SSU = (1 + τ i )/ǫ (renormalization gives f SSU = c si /(2πqR)).…”

“…The turbulent generated flows appear as lowfrequency zonal flows [2][3][4][5][6][7][8][9]12] or geodesic acoustic oscillations [1]. Whereas the basic mechanism of turbulence suppression by zonal flows [13][14][15][16], the existence of zonal flows during the L-H transition [12] and the predatorprey dynamics in general [17] have been experimentally demonstrated, turbulence suppression by flow generation appears strong enough to trigger a transition into the Hmode [1,2,4,5,9,18] but some experiments show only weak or insufficient zonal flow activity at the L-H transition [6,7,19,20]. Recently also magnetic fluctuations are observed during these limit cycle oscillations [7,8,[20][21][22] even though the predator-prey model [10,11] is electrostatic in nature.…”

The role of turbulence–flow interactions in L- to H-mode transition dynamics: recent progress

“…Recent DBS results from the JET tokamak provide strong evidence of mesoscale zonal flows on the edge of ohmically heated (OH) and L-mode plasmas [81]. JET exhibits a pronounced well structure in the radial electric field E r already during the OH phase.…”