Buechl, K.

Kaufmann, M.

Lang, P.

Lang, R.

Lorenz, A.

Maraschek, M.

Mertens, V.

Mueller, H.

Neuhauser, J.

Salzmann, H.

English

MPG.PuRe

CONTROLLED HIGH DENSITY OPERATIONBEYOND THE GREENWALD LIMIT ON AUGBY INBOARD PELLET INJECTIONP.T. Lang, K. Büchl, M. Kaufmann, R.S. Lang, A. Lorenz, V. Mertens, M. Maraschek,H.W. Müller, J. Neuhauser, H. Salzmann and ASDEX Upgrade TeamMax-Planck-Institut f. Plasmaphysik, EURATOM-IPP Ass.,Boltzmannstr. 2, 85748 Garching1. IntroductionThe injection of frozen deuterium pellets from the magnetic high field side (HFS) of atokamak has the potential to become an efficient particle refueling source in next generationfusion devices [1]. Despite being technically more demanding HIS-injection proved to besuperior to injection from the torus outside due to the nature of the toroidal curvature whichtends to expel ablating pellet clouds injected from the magnetic low field side (LFS) [2]. Thesame effect is very beneficial for HFS injection where the refueling flux penetratessignificantly deeper into the core enabling rapid bulk density enhancement and efficient bulkdensity control. At the same time, conservation of plasma energy and energy confinementtime are crucial to sustain a lasting refueling procedure. In a series of experiments the pelletinjection hardware was adapted to operate at very high repetition rates to study and if possibleoptimize the operation space for controlling densities beyond the empirical density limit [3] asenvisaged in ITER scalings [4].2. Experimental setupThe refueling studies were performed on ASDEX Upgrade (Ro = 1.65 m, plasma radiusao= 0.5 m, Vplasma = 13 m3, b/a =1.6) with lower single null configuration, Ip = 0.8 MA,Bt ≅ 2 T, q95 ≅ 4, PNI = 5 - 10 MW. The set up of the pellet injection system is shown in Fig. 1with two injectors available, a centrifuge and a blower gun. The latter was used only in proof-of-principle experiments [l] where pellets were injected horizontally at a velocity of 130 ms-1and repetition rates of up to 17 Hz.The centrifuge injector [5], able to deliver pellets of variable velocity and mass, oper-ated in the mode of vpel= 240 ms-1 and mpel = 3.8 ⋅ 1020 particles. The velocity corresponds toa centrifuge revolution frequency of 60 Hz, yielding so far pellet repetition rates of up to 30Hz. In order to cover the large refueling particle flux required the system was optimized to al-low for repetition rates of 60 Hz. On leaving the centrifuge pellets are guided via a funnel anda 5 m long teflon tube toward the magnetic high field side of the torus. They are injected intothe plasma under an angle of 44o to the horizontal plane in order to maximize the radius of theguiding tube to minimize the centrifugal forces on the pellet.Pellets were video monitored on their entrance into plasma. The plasma densityenhancement was measured by the DCN-interferometer, the Thomson scattering system andthe edge lithium beam diagnostic.3. Experimental resultsThe measured increase of the number of particles in the target plasma divided by the numberof particles in the pellet leaving the cryostat is defined as the fueling efficiency εf. The graphin Fig. 2 compares HFS- and LFS-refueling efficiencies for both injectors. HFS-fuelingefficiency is dominated by pellet mass losses of at least 60% occurring in the guiding tube.However, losses in the plasma approach zero and the fueling efficiency does not degrade withincreasing heating power. In contrast, LFS-refueling does not suffer from tube losses, butexperiences increasing plasma losses with rising heating power. Expectably, the efficiency ofblower gun HFS-refueling is about 15% higher than that of the centrifuge be-cause of thedifferent injection planes and the additional interaction with the funnel experienced by thecentrifuge pellets.Fig. 1.  Experimental arrange-ment for pellet injection fromthe magnetic low- (grey) andhigh field side (black) centri-fuge and blower gun  injectors.Fig. 2.  Maximum fuelingefficiencies achieved forpellet injection from themagnetic low- (gray) andhigh field sides (black),curves of centrifuge(solid) and blower gun(dashed). 15% losscommon to all types ofinjection is already takeninto account.Taking into account the losses the particle flux supplied by the centrifuge to the torusinside reduced from 2.40 ⋅1022 particles/s to 1022 particles/s. In spite of this, an ice reservoirof up to 96 pellets enabled density control over 3 s bearing in mind that permanent operationat 60 Hz was not required due to the very fast density ramp up at this frequency(trise < 100 ms). Quasi-steady-state operation was attained by feedback controlling twoparameters, the line averaged electron density and the divertor neutral gas pressure, no,div, as ameasure of the separatrix density. An exemplary discharge is displayed in Fig. 3. After therapid density ramp-up steady state H-mode operation is retained around the Greenwald limitfor a 2 s fueling phase controlled by bursts of 60 Hz pellet trains. The particle flux issupported by very little gas puffing such that the divertor pressure could be kept at low value,one of the conditions for good energy confinement [6].The plasma energy falls by about 20% during the refueling phase despite the low levelcontrol of no,div. In addition to the neutral pressure dependence confinement degrades withMHD-mode activity [7]. A first series of discharges aiming to improve energy confinementduring HFS-refueling was performed (one discharge was LFS-refueled). The divertor gaspressure was controlled by additional cryopumping. The switching on of the neutral injectionheating power, PNI was adjusted so as to delay the onset of neoclassical tearing modes, andthe influence of the radiated power, Prad was studied by the injection of impurity gases such asneon and CD4.The results are summarized in Fig. 4 (note suppressed zero). The plasma energy isplotted against line averaged density, and each discharge is represented by the temporalevolution of its energy confinement. Apparently, there is a clear cut limit in energyconfinement independent of auxiliary heating and pumping scenario. Although at mediumFig. 3.  Quasi steady-statepellet fueling at a feedbackcontrolled density of1.2⋅1020 m-3 (dashed line,ne,GW ~ 1.07⋅1020 m-3) byHFS pellet injection.densities plasma energies beyond values expected by the ITER H92Ptot(y) scaling law areobtained, densities close to and beyond the Greenwald limit are only achieved with increasinglosses in energy confinement. Possible indications for transport enhancement during theserefueling phases are discussed in a paper by V. Mertens et al. [8]. Expectably, a gas-pufffueled discharge trace plotted for comparison seems to subscribe to the density limit.4. SummaryHFS pellet injection using a centrifuge confirms that the fueling efficiency does not decreasewith increasing heating power. The system has been upgraded to enable controlled highdensity operation during refueling phases as long as 3 s. A series of discharges aimed at betterenergy confinement by trying to delay MHD-mode activity and adjusting divertor gaspressure and impurity puffing revealed a preliminary confinement limit. The underlyingscenarios have to be understood, but scaling laws such as ITERH92Ptot(y) should be usedwith caution when extrapolating to density regions close to and beyond the Greenwald limit.References[1] P.T. Lang et al.: Phys. Rev. Lett. 79, 1487 (1997)[2] P.T. Lang et al.: Proc. 24th Eur. Conf. Berchtesgaden, 1997, Part IV, EPS, Geneva (1997)[3] N. Greenwald et al.: Nucl. Fusion 28,2199 (1988)[4] T. Takizuka et al.: 16th IAEA Fusion Energy Conf., IAEA-CN-64/FP20, Montreal (1996)[5] C. Andelfinger et al.: Rev. Scient. Instr. 64(4), 983 (1993)[6] V. Mertens et al.: Nucl. Fusion 37, 1607 (1997)[7] M. Maraschek et al.: subm. to Plas. Phys. Control. Fusion (1998)[8] V. Mertens et al.: this conference.Fig. 4.  MHD plasma energyvs. ne in pellet refueleddischarges; note suppressedzero; all discharges have thesame magnetic configurat-ion.Legend: achieved plasmaenergies at different PNIlevels (10 MW - black; 7.5MW –gray, 5 MW - lightgray) are compared to thevalues calculated using theITERH92Ptot(y) scaling.The dotted line marking theconfinement limit is only fororientation. 5                 _     10                       15ne (1019 m-3)P = 10 MWP = 7.5 MWGass puffingP=7.5 MWP = 5 MWTurbopumpsCryopumpLFS (Cryopump)0.70.50.3PLASMA ENERGY (MJ)   ITERH92Ptot(y)ne,Gw

Controlled High Density Operation beyond the Greenwald Limit on ASDEX Upgrade by Innboard Pellet Injection

http://hdl.handle.net/21.11116/0000-000A-5FAB-C

Controlled High Density Operation beyond the Greenwald Limit on ASDEX Upgrade by Innboard Pellet Injection

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