Overview of ASDEX Upgrade results

Abstract

ASDEX Upgrade was operated with fully W-covered wall in 2007 and 2008. Stationary H-modes at the ITER target values and improved H-modes with H up to 1.2 were run without any boronisation. The boundary conditions set by the full W-wall (high enough ELM frequency, high enough central heating and low enough power density arriving at the target plates) require significant scenario development, but will apply to ITER as well. D retention has been reduced and stationary operation with saturated wall conditions has been found. Concerning confinement, impurity ion transport across the pedestal is neoclassical, explaining the strong inward pinch of high-Z impurities in between ELMs. In improved H-mode, the width of the temperature pedestal increases with heating power, consistent with a β pol,ped 1/2 scaling. In the area of MHD instabilities, disruption mitigation experiments using massive Ne injection reach volume averaged values of the total electron density close to those required for Runaway suppression in ITER. ECRH at the q=2 surface was successfully applied to delay density limit disruptions. The characterisation of fast particle losses due to MHD has shown the importance of different loss mechanisms for NTMs, TAEs and BAEs. Specific studies addressing the first ITER operational phase show that O1 ECRH at the HFS assists reliable low-voltage breakdown. During ramp-up, additional heating can be used to vary l i to fit within the ITER range. Confinement and power threshold in He OV/2-3 are more favourable than in H, suggesting that He operation could allow to assess H-mode operation in the nonnuclear phase of ITER operation.

Modelling of the JET current ramp-up experiments and projection to ITER

“…The current ramp up phase in tokamaks has recently attracted the attention of experimentalists [1][2][3] and modellers [1, 4 -6] in connection with future experiments on ITER. The important objectives of the current ramp up phase on ITER are maintenance of a current density profile compatible with vertical stability of the plasma column [7] and the formation of the target q-profile favourable for subsequent burn phase (with q 0 above one for an extended sawtooth-free H-mode and Hybrid scenario or reversed q profile for the advanced operation with an internal transport barrier (ITB)) [8].…”

“…The important objectives of the current ramp up phase on ITER are maintenance of a current density profile compatible with vertical stability of the plasma column [7] and the formation of the target q-profile favourable for subsequent burn phase (with q 0 above one for an extended sawtooth-free H-mode and Hybrid scenario or reversed q profile for the advanced operation with an internal transport barrier (ITB)) [8]. Keeping in mind these objectives the effects of various parameters, such as the current ramp rate [3], plasma density [3], additional heating [2,3] and variation of the plasma shape and volume [1] on the optimisation of the current ramp up phase have been investigated. The current profile diffusion is a key issue of this study.…”

Two-fluid tokamak equilibria with reversed magnetic shear and sheared flow

“…(2)(3)(4)(5)(6)(7)(8) are advantageous in two respects: (i) the momentum equation includes the electric field and therefore the pressure gradient contribution to E can be calculated from this equation; this contribution is missing in the frame of MHD because E is calculated by Ohm's law, E + v × B = 0, and (ii) the current density J is related self-consistently to the fluid species velocities [Eq. (8)].…”

“…Because of symmetry any equilibrium quantity depends solely on the radial distance r; therefore Eqs. (2), (4)[and (9)], (6), and (7) are identically satisfied. Also the flow for both fluid species is incompressible (∇ · v α = 0).…”

“…Long quasi-steady or steady ITB states have been obtained in different tokamaks, e.g ASDEX Upgrate [1,2], JT-60U [3], Tore-Supra [4], and JET [5] where ITBs were maintained for up to 11 s. The ITBs usually are associated with reversed magnetic shear profiles [6], [7] and their main characteristics are steep pressure profiles in the barrier region [8] and radial electric fields associated with sheared flows [9,10]. The mechanism responsible for the formation of ITBs and the underlying physics is not completely understood.…”