H. Meister

Confinement physics of the advanced scenario with ELMy H-mode edge in ASDEX upgrade

The confinement physics of the improved H-mode is analysed in detail. It is shown that this scenario can be largely unified with that of the standard H-mode. Transport barriers exist in the early phase of the discharge just after the switch on of the heating. In steady state, however, no clear internal transport barriers can be indentified in the ion temperature profiles. The profiles are then found to be stiff and the improved H-mode follows the same ion temperature scaling as the standard H-mode. The improvement of the confinement against the H-mode scaling is partly due to the dependence of this scaling on the line averaged density, and is partly obtained through density peaking appearing at low densities. It is shown that pre-heating is not essential in reaching the good confinement.

Characterization of ion heat conduction in JET and ASDEX Upgrade plasmas with and without internal transport barriers

In ASDEX Upgrade and JET, the ion temperature profiles can be described by R/LTi which exhibits only little variations, both locally, when comparing different discharges, and radially over a wide range of the poloidal cross-section. Considering a change of the local ion heat flux of more than a factor of two, this behaviour indicates some degree of profile stiffness. In JET, covering a large ion temperature range from 1 to 25 keV, the normalized ion temperature gradient, R/LTi, shows a dependence on the electron to ion temperature ratio or toroidal rotational shear. In particular, in hot ion plasmas, produced predominantly by neutral beam heating at low densities, in which large Ti/Te is coupled to strong toroidal rotation, the effect of the two quantities cannot be distinguished. Both in ASDEX Upgrade and JET, plasmas with internal transport barriers (ITBs), including the PEP mode in JET, are characterized by a significant increase of R/LTi above the value of L- and H-mode plasmas. In agreement with previous ASDEX Upgrade results, no increase of the ion heat transport in reversed magnetic shear ITB plasmas is found in JET when raising the electron heating. Evidence is presented that magnetic shear directly influences R/LTi, namely decreasing the ion heat transport when going from weakly positive to negative magnetic shear.

Response of Internal Transport Barriers to Central Electron Heating and Current Drive on ASDEX Upgrade

Internal transport barriers with the central electron temperature as large as the central ion temperature both in excess of 10 keV have been achieved in the Axi-symmetric divertor experiment (ASDEX Upgrade) [H. Vernickel et al., J. Nucl. Mater. 128, 71 (1984)]. By applying central electron cyclotron heating and current drive to negative central shear discharges, established by neutral beam heating in the current ramp, the core electron temperatures could be raised by more than a factor of 2. Despite the fivefold increase of the central electron heat flux, the ion and electron energy and also angular momentum transport did not deteriorate. For neutral beam injection alone and also with additional central electron cyclotron counter-current drive, a double tearing mode and the associated detrimental effect on the plasma confinement is destabilized only transiently, when the minimum of the safety factor (qmin) passes through 2. For co-current drive, however, the confinement does not recover after qmin has dro...

Confinement and transport studies of conventional scenarios in ASDEX Upgrade

Confinement studies of conventional scenarios, i.e. L and H modes, in ASDEX Upgrade indicate that the ion and electron temperature profiles are generally limited by a critical value of ?T/T. When this is the case the profiles are stiff: core temperatures are proportional to pedestal temperatures. Transport simulations based on turbulence driven by an ion temperature gradient show good agreement with the ion experimental data for H?modes. Studies specifically dedicated to electron transport using electron cyclotron heating with steady state and modulated powers indicate that the electron temperature profiles are also stiff. Candidates for turbulence having a threshold in ?Te/Te may be trapped electron modes and electron temperature gradient driven instabilities. The critical threshold (?Te/Te)c and the increase of the stiffness factor with temperature are found experimentally. In contrast, the density profiles are not stiff, but the variation in shape remains moderate in these conventional scenarios. As a consequence of this profile behaviour, the plasma energy is proportional to the pedestal pressure. The global confinement time increases with triangularity and can be good at densities close to the Greenwald limit at high triangularity. In this operational corner and at q95 around 4, the replacement of large type?I ELMs by small ELMs of type?II provides good confinement with very reduced peak power load on the divertor plates. This regime is believed to be adequate for a fusion reactor.

Stationary advanced scenarios with internal transport barrier on ASDEX Upgrade

Steady-state discharges with improved core confinement and H-mode edge with edge localized modes (ELMs) are investigated. In plasmas with an upper triangularity top close to zero an H-factor of HITER89-P = 2.7 and N = 2.2 could be maintained for 1 s and HITER89-P = 2.4 and N = 2.0 for 6 s, the latter corresponding to 40 confinement times or 2 1/2 resistive time scales for current redistribution, only limited by the duration of the possible discharge length. At a line averaged density of 4 × 1019 m-3 the central temperatures reach values of Ti = 10 keV and Te = 6.5 keV. The stationarity of the current profile is explained by magnetic reconnection driven by strong (m = 1, n = 1) fishbones, which, in the absence of sawteeth, also expel energy and impurities. Further increasing the pressure, is limited by neoclassical tearing modes. Raising the density by edge gas fuelling and the simultaneous increase of the neutral beam power, HITER89-P remained unchanged up to ne = 5.5 × 1019 m-3, accompanied by a substantial reduction of Zeff. Increasing top to 0.2, both confinement and -limit improved reaching values of HITER89-P = 3.0 and N = 2.4 at densities above ne = 5 × 1019 m-3. This resulted in the highest fusion product of nD,0Ti,0E = 0.9 × 1020 keV s m-3 so far observed in ASDEX Upgrade.

Turbulence Reduction in Internal Transport Barriers on ASDEX Upgrade

A reduction in turbulent density fluctuations is observed in internal transport barrier (ITB) discharges in the ASDEX Upgrade tokamak with an L-mode edge and reversed magnetic shear. The turbulence reduction is localized spatially to around the ITB gradient and the ITB foot-point region, coincident with strong shearing in the toroidal rotation velocity. The radial extent and degree of reduction is related to the strength of the ITB. Strong (narrow and steep) ITBs are well localized and display greater turbulence reduction. Weaker ITBs are less localized with less reduction. The formation of a subsequent H-mode edge barrier moves the velocity shearing region to the edge and weakens the ITB to the point of extinction. During the ITB the turbulence is reduced across a broad frequency range, however a residual level of low-frequency fluctuations (f<10 kHz) remains. Comparison of linear growth and shearing rates suggest that E×B shear may be a factor in the turbulence reduction.

Formation criteria and positioning of internal transport barriers in ASDEX Upgrade

Advanced tokamak discharges on ASDEX Upgrade can generate an internal transport barrier (ITB) during the current ramp-up phase early in the discharge. Formation of the ITB has become more reliable with the discovery that a low density is necessary for it to form. These ITBs form in very low or negative central shear regions. There is no clear evidence of integer q magnetic surfaces triggering the ITB phase. The appearance of an integer q surface (usually q = 2) often leads to a second ITB (i.e. another steepening in the ion temperature gradient) inside the first ITB.Investigation of possible turbulence suppression mechanisms suggests that something is missing from the generally accepted model of sufficient E × B sheared flow suppressing the turbulence since the E × B shearing rate was not greater than, or comparable with, the maximum linear growth rate in about half of the ITB discharges analysed. Thus, it does not explain the reduced radial transport observed in these discharges.