Abstract. After completion of the tungsten coating of all plasma facing components, ASDEX Upgrade has been operated without boronization for 1 1/2 experimental campaigns. This has allowed the study of fuel retention under conditions of relatively low D co-deposition with low-Z impurities as well as the operational space of a full-tungsten device for the unfavourable condition of a relatively high intrinsic impurity level. Restrictions in operation were caused by central accumulation of tungsten in combination with density peaking, resulting in H-L backtransitions induced by too low separatrix power flux. Most important control parameters have been found to be the central heating power, as delivered predominantly by ECRH, and the ELM frequency, most easily controlled by gas puffing. Generally, ELMs exhibit a positive impact, with the effect of impurity flushing out of the pedestal region overbalancing the ELM induced W source. The restrictions of plasma operation in the unboronized W machine occured predominantly under low or medium power conditions. Under medium-high power conditions, stable operation with virtually no difference between boronized and unboronized discharges was achieved. Due to the reduced intrinsic radiation with boronization and the limited power handling capability of VPS coated divertor tiles ( 10 MW/m 2 ), boronized operation at high heating powers was possible only with radiative cooling. To enable this, a previously developed feedback system using (thermo-)electric current measurements as approximate sensor for the divertor power flux was introduced into the standard AUG operation. To avoid the problems with reduced ELM frequency due to core plasma radiation, nitrogen was selected as radiating species since its radiative characteristic peaks at lower electron temperatures in comparison to Ne and Ar, favouring SOL and divertor radiative losses. Nitrogen seeding resulted not only in the desired divertor power load reduction, but also in improved energy confinement, as well as in smaller ELMs.
The conditions for a magnetically con fined plasma to ignite are a plasma tem perature above 100 Million degree (10 keV) and a product of density ne and energy confinement time τE in excess of 2 x 1020 m-3 s. (Technical require ments such as low plasma contamina tion by impurity ions have additionally to be fulfilled.) Under steady state condi tions, the energy confinement time de termines the energy content of the dis charge at given heating power or descri bes its decay when the heating power is switched-off. On the large tokamak at Princeton, the PLT, an ion temperature of 7.5 keV was achieved in 1979 with natu ral injection (Nl) auxiliary heating 1) and on Alcator C, another tokamak device in the USA, using resistive heating by the plasma current (the same process which heats a normal conductor), the less critical n τE breakeven limit was reached in 1984, albeit with a low tem perature of 1.5 keV. Thus the road to an ignited tokamak plasma seemed to be clear. However there turned out to be an un expected road block: the good confine ment properties of resistively heated plasmas could not be maintained at high plasma temperatures. With increasing auxiliary heating power, the plasma con finement was found to degrade severely and ignition conditions were in effect not approached. This degradation in confinement was a worldwide obser ved phenomenon which was found to be independent of the heating technique and which seemed to threaten the ulti mate goal of the fusion programme. On ASDEX, the AxiSymmetric Diver tor tokamak Experiment at Garching, the same problem was encountered: τE decreased from 70 ms, established in a resistively heated discharge with a typi cal Ohmic input of 0.4 MW, to 20 ms during a Nl pulse of 3 MW. The particle confinement time was also observed to decrease. However, on ASDEX a solu tion of this problem was found by sur rounding the plasma with a thermal insulation layer. This solution to the heat leak problem is in itself not unique-every house owner applies it in an effort to increase the room temperatures (im prove the energy confinement time) without having to uppgrade the furnace. Fig. 1-Poloidal cross-section of the dou ble-null divertor configuration of ASDEX. We should not feel inclined to report on it if the thermal resistivity of the insulation layer surrounding the ASDEX plasma were not to surpass that of a product such as Styrofoam by three orders of magnitude, and if degraded confine ment as a limitation of the present expe riments were not thereby mitigated. Design Features of ASDEX The toroidal plasma in ASDEX has major and minor radii of 1.65 m and 0.4 m; maximum plasma current is 0.5 MA; stabilization is provided by a toroidal ma gnetic field of up to 2.8 T. The Nl system of ASDEX delivers a beam of hydrogen atoms with 40 keV energy and with a maximum power of 4.4 MW. The main research goal of ASDEX is to demon strate the efficiency of the magnetic di-vertor concept in providing ultraclean hydrogen discharges by minimizing the interaction of the h...
Steady-state discharges with improved core confinement and H-mode edge with edge localized modes (ELMs) are investigated. In plasmas with an upper triangularity δ top close to zero an H -factor of H ITER89-P = 2.7 and β N = 2.2 could be maintained for 1 s and H ITER89-P = 2.4 and β 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 × 10 19 m −3 the central temperatures reach values of T i = 10 keV and T e = 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, β is limited by neoclassical tearing modes. Raising the density by edge gas fuelling and the simultaneous increase of the neutral beam power, H ITER89-P remained unchanged up to n e = 5.5 × 10 19 m −3 , accompanied by a substantial reduction of Z eff . Increasing δ top to 0.2, both confinement and β-limit improved reaching values of H ITER89-P = 3.0 and β N = 2.4 at densities above n e = 5 × 10 19 m −3 . This resulted in the highest fusion product of n D,0 T i,0 τ E = 0.9 × 10 20 keV s m −3 so far observed in ASDEX Upgrade.
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