The physics of electron internal transport barriers in the TCV tokamak

Abstract

Electron internal transport barriers (eITBs) are generated in the TCV tokamak with strong electron cyclotron resonance heating in a variety of conditions, ranging from steady-state fully noninductive scenarios to stationary discharges with a finite inductive component and finally to transient current ramps without current drive. The confinement improvement over L-mode ranges from 3 to 6; the bootstrap current fraction is invariably large and is above 70% in the highest confinement cases, with good current profile alignment permitting the attainment of steady state. Barriers are observed both in the electron temperature and density profiles, with a strong correlation both in location and in steepness. The dominant role of the current profile in the formation and properties of eITBs has been conclusively proven in a TCV experiment exploiting the large current drive efficiency of the Ohmic transformer: small current perturbations accompanied by negligible energy transfer dramatically alter the confinement. The crucial element in the formation of the barrier is the appearance of a central region of negative magnetic shear, with the barrier strength improving with increasingly steep shear. This connection has also been corroborated by transport modelling assisted by gyrofluid simulations. Rational safety-factor (q) values do not appear to play a role in the barrier formation, at least in the q range 1.3-2.3, as evidenced by the smooth dependence of the confinement enhancement on the loop voltage over a broad eITB database. MHD mode activity is however influenced by rational q values and results in a complex, sometimes cyclic, dynamic evolution.

Overview of the RFX fusion science program

“…This coincides with a region of null magnetic shear, where the density of rational surfaces and thus the probability of mode coupling are lowered. This is similar to what is described in [7] for tokamaks, where in the presence of dominant electron heating a reversed magnetic shear and especially a shear null represent a sufficient condition for a strong electron barrier to develop [8,9]. In RFX-mod, the SHAx states are accompanied by significant E × B sheared poloidal flow.…”

Understanding the core density profile in TCV H-mode plasmas

“…The density peaking in the C-Mod ITBs was thought to result from the Ware pinch, with the ITB density gradient during electron heating determined by the balance of Ware pinch and TEM turbulent fluxes, thus acquiring a strong inverse dependence on temperature. In TCV electron ITBs, where transport barriers are seen mainly on the electron temperature profile [5], the density peaking results from a strong thermodiffusive pinch [6] component which can be explained [3] using the quasi-linear model that shall be used in this paper .…”

Controlling the internal transport barrier oscillations in high-performance tokamak plasmas with a dominant fraction of bootstrap current

“…While tokamaks remain the front runners for the first generation of thermonuclear fusion power plants, the need to provide continuous electrical power makes steady state (SS) operation of future tokamak reactors more attractive than the pulsed mode associated with purely Ohmic (OH) inductive drive which, on top of the cyclic thermal and mechanical stresses it implies, must rely on some energy storage system to feed the electrical grid during transformer recharge [1]. Internal transport barriers (ITBs) have hence become a milestone in the route to SS economically viable fusion reactors: besides improving core confinement, their steep pressure gradient induces very large fractions of the selfgenerated bootstrap (BS) current, enabling the plasma current to be fully driven non-inductively with only a small amount of power recirculated to the external current drive (CD) sources, in so-called advanced tokamak (AT) scenarios which have attracted increasing interest [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. According to present-day theoretical understanding and experimental evidence, two key ingredients for ITB formation and sustainment are the shears in the magnetic field and in the E × B drift velocity, s and ω E×B , respectively, the emerging picture being that ITB dynamics is mainly governed by some synergistic combination of the stabilizing effects due to reversed, non-positive s and high ω E×B , the latter shearing rate being compared with the linear growth rate of ion temperature gradient (ITG) modes or of some other type of drift-wave plasma instabilities [2][3][4][14][15][16][17][18].…”