Recent experiments at ASDEX Upgrade have achieved advanced scenarios with high β N (>3) and confinement enhancement over ITER98(y, 2) scaling, H H98y2 = 1.1-1.5, in steady state. These discharges have been obtained in a modified divertor configuration for ASDEX Upgrade, allowing operation at higher triangularity, and with a changed neutral beam injection (NBI) system, for a more tangential, off-axis beam deposition. The figure of merit, β N H ITER89-P , reaches up to 7.5 for several seconds in plasmas approaching stationary conditions. These advanced tokamak discharges have low magnetic shear in the centre, with q on-axis near 1, and edge safety factor, q 95 in the range 3.3-4.5. This q-profile is sustained by the bootstrap current, NBI-driven current and fishbone activity in the core. The off-axis heating leads to a strong peaking of the density profile and impurity accumulation in the core. This can be avoided by adding some central heating from ion cyclotron resonance heating or electron cyclotron resonance heating, since the temperature profiles are stiff in this advanced scenario (no internal transport barrier). Using a combination of NBI and gas fuelling line, average densities up to 80-90% of the Greenwald density are achieved, maintaining good confinement. The best integrated results in terms of confinement, stability and ability to operate at high density are obtained in highly shaped configurations, near double null, with δ = 0.43. At the highest densities, a strong reduction of the edge localized mode activity similar to type II activity is observed, providing a steady power load on the divertor, in the range of 6 MW m −2 , despite the high input power used (>10 MW).

ASDEX Upgrade has recently finished its transition towards an all-W divertor tokamak, by the exchange of the last remaining graphite tiles to W-coated ones. The plasma start-up was performed without prior boronization. It was found that the large He content in the plasma, resulting from DC glow discharges for conditioning, leads to a confinement reduction. After the change to D glow for inter-shot conditioning, the He content quickly dropped and, in parallel, the usual H-Mode confinement with H factors close to one was achieved. After the initial conditioning phase, oxygen concentrations similar to that in previous campaigns with boronizations could be achieved. Despite the removal of all macroscopic carbon sources, no strong change in C influxes and C content could be observed so far. The W concentrations are similar to the ones measured previously in discharges with old boronization and only partial coverage of the surfaces with W. Concomitantly it is found that although the W erosion flux in the divertor is larger than the W sources in the main chamber in most of the scenarios, it plays only a minor role for the W content in the main plasma. For large antenna distances and strong gas puffing, ICRH power coupling could be optimized to reduce the W influxes. This allowed a similar increase of stored energy as yielded with comparable beam power. However, a strong increase of radiated power and a loss of H-Mode was observed for conditions with high temperature edge plasma close to the antennas. The use of ECRH allowed keeping the central peaking of the W concentration low and even phases of improved H-modes have already been achieved.

Extension of the ECRH operational space in ASDEX Upgrade 2 Abstract. ASDEX Upgrade is operated with tungsten-coated plasma-facing components since several years. H-mode operation with good confinement has been demonstrated. Nevertheless purely NBI-heated H-modes with reduced gas puff, moderate heating power or/and increased triangularity tend to accumulate tungsten, followed by a radiative collapse. Under these conditions, central electron heating with ECRH, usually in X2 polarisation, changes the impurity transport in the plasma centre, reducing the central tungsten concentration and, in many cases, stabilizing the plasma. In order to extend the applicability of central ECRH to a wider range of magnetic field and plasma current additional ECRH schemes with reduced single pass absorption have been implemented: X3 heating allows to reduce the magnetic field by 30 %, such that the first H-modes with an ITER-like value of the safety factor of q 95 = 3 could be run in the tungsten-coated device. O2 heating increases the cutoff density by a factor of 2 allowing to address higher currents and triangularities. For both schemes scenarios have been developed to cope with the associated reduced absorption. In case of central X3 heating, the X2 resonance lies close to the pedestal top at the high-field side of the plasma, serving as a beam dump. For O2, holographic mirrors have been developed which guarantee a second pass through the plasma centre. The beam position on these reflectors is controlled by fast thermocouples. Stray-radiation protection has been implemented using sniffer-probes.

Currently, a new multi-frequency ECRH system is under construction at the ASDEX Upgrade tokamak experiment. This system employs, for the first time in a fusion device, multi-frequency gyrotrons, step-tunable in the range 105–140 GHz. The first two gyrotrons, working at 105 and 140 GHz, were installed and tested. The matching optics unit includes a set of phase correcting mirrors for each frequency as well as a pair of broadband polarizer mirrors. The transmission line consists of non-evacuated corrugated HE11 waveguides with an inner diameter (ID) of 87 mm and has a total length of about 70 m. Transmission losses were deducted from calorimetric measurements both at the beginning and at the end of the transmission line at both frequencies and are in reasonable agreement with theory. Two transmission lines are completed so far and first plasma experiments with the new system have started. The first gyrotron Odissey-1 is currently being equipped with a broadband chemical vapour deposition (CVD) diamond Brewster output window and will become a step-tunable gyrotron with the additional frequencies 117 and 127 GHz. A tunable double-disc CVD-diamond window will be mounted at the torus. The system includes fast steerable launchers at the front end that will allow very localized feedback controlled power deposition in the plasma.

Ahstraet-Enpcrimental rcsults on thc coupling or a 2 x 24 waveguide grill are compared with theory.Giaphitc tiles attached dircctly to the grill and intcnded to protect the grill against particle bombardment h a w been found to degrade the coupling subslanlially if they protrude only ii icw millimrtcrs beyond the actual grill surfice. Measurements in chis case are compatible with calculations where a vacuum gap is introduced bctween thc grill and the plasma. Low reflection is obtained whcn the tiles do not protrude. Coupling can be maintained whcn the r.f. pulse is started whilc thc plasma is near l o thc grill and then moved to a larger grill-separatrix distancc where othcrwiie total relirction would occur. I . I N T R O D U C T I O N THE LOWER HYBRID GRILL used in ASDEX consists of two arrays of 24 waveguides each, arranged one on top of the other. The inncr dimensions of the guides are 10 x 109 mm with 4 mm walls in between. The vertical separation between the centers of the two grills is 160 min. With a phasing of d, = Onon,, . in successive waveguides we generate a symmetric spectrum centcred around NI, = c/uPh = *4.4, with a width AN,, = 0.8. The phase can be set arbitrarily with a corresponding decreasc in NI, of the main peak, leading to asymmetric spectra used for current drive, like A 4 = f90" where it is centered around NI, = +2.2. A symmetric spectrum with the same low N!! = +2.2 can be launched with a grill phasing of d, = OOnnOOnn.. . . The experiments have been performed in ASDEX, a Tokamak with a poloidal divertor (KEILHACKER et U/., 1981). The major radius R, was varied between 162 and 168 cm with a corresponding shift of the separatrix position R, from 202 to 208 cm. *This paper is an cnpandcd version ormateri:d which originally was a contributed prcscntstion st thc 17th EPS Plasma Physics Division Conicrence. Amsterdam. The Netherlands. June 1990.