Lengyel, L.

Neuhauser, J.

English

MPG.PuRe

- 82 - A P Ho 017 Ignition and Fuelling Scenarios Based on the Injection of Pellets in Thermonuclear plasmas . L.Lengyel and J. Neuhauser Max-Planck- Institut fur Plasmaphysik, EURATOM Association, 0-8046 Garch:ng, Fed. Rep. of Germany Some of the potential of pellet injection for futur;e tusion machines is demonstrated by means of reactor scenario calcula-tions performed with the help of the BALDUR /1/ l-D tokamak transport code, which incorporates an extensive Monte Carlo treat-ment of the neutral particles and a divertor model. The code is supplemented by a pellet ablation routine that is based on the so-called neutral gas shielding model /2/ . The parameters of the recipient plasma are assumed to be identical with the standard INTOR parameters /3/ . The present calculations are part of an extensive assessment performed for NET (Next European Torus) /4/. Density Ramp- up and Ignition by Pellet Injectio~ Early analyses of ignition scenarios in NB-heated tokamaks (see , for example, Watkins et al. /5/, Holmes et al /6/ , and Houlberg et al. /7/) have shown that optimum path to ignition in the n , T space always starts with a low-density high-temperature plasma state . The calculations of Watkins et al. show that , ir-respective of the scaling laws assumed by them (Alcator-Intor , Copp i-Mazzucato, Ohkawa, and a radially dependent version of Alcator-Intor scaling) , the minimum B for ignition is always associated with low densities (ni S 10 20 m- 3). The calculations of Houlberg et al. /7/ indicate the importance of the NB pene-tration depth, i.e . heating profile , on the optimum ignition power characteristics. The purpose of the pr esent ignition scenario cal culations is to assess the potentia l of density r amp-up by pellet injection in planned fusion machines such as NET, based on the INTOR design and plasma parameters. Special attention has been paid to the possibility of reducing the beam energy envisaged for INTOR ~1 75 keV) py simultaneously increasing the envisaged beam power (from 75 MW to 100 to 110 H\v). Three beam energy levels were con-sidered: 120 keV, lOO kgV, and 80 keV, respective ly). The respec-tive power fractions associated with the 1/1 , 1/2 , and 1/3 energy components were assumed to be: 0 . 59 : 0.22 : 0.19 (120 keV) , 0.62 : 0.21 : 0. 17 (100 keV) , and 0.64 : 0 . 20 : 0 . 16 (BO keV) /B/. In al l cases , -the scenario started with plasma in the so- called "hot ion mode " (in low- density NB-heated plasmas the energy is preferentially coupled into the plasma ions and thus the ion tem-perature may substantially exceed the electron temperature in the heat deposition zone) . As soon as the minimum B value necessary for ignition has been reached , a pellet of preselected size was injec-ted into it , thus adiabatically ramping the plasma density up to. - 83 -the value necessary for ignition (i.e. for a self-sustained alpha reaction rate). The values of Bign and nign were previous-ly determined from standard INTOR start-up phase calculations (Ebi : 175 keV, Pbi : 75 MW) with marginal ignition ( bi : 4 s) and were found to be: Bign = 2.5 and n ign 9 . 5 x 1019 m- 3 , respectively. Details of these calculations can be found in /9 / . The pellet velocity assumed (approx. 2400 m/s after correction for the ellipticity of the plasma cross-section) corresponds to values not very far from the presently obtainable ones . The ab-lation rates yielded by the neutral gas shielding theory /2/ were used without corrections for possible magnetic shielding effects. Thus the actual pellet penetration depths may be larger under reactor conditions than those calculated here /4/ . (This may again further relax the requirements on the NB and pellet injec-tor systems.) Since the intense deuterium beams make the dis-charge rather tritum-!ean in the reactor zone, only tritium pellets were used for ramp-up purposes. It should be noted that the results of reactor scenario cal-cu lations such as those described here strongly depend on , the scaling laws (transport models) used. The present calculations are limited to Alcator-Intor scaling, on which the standard INTOR and NET calculations are based. The NB heat pulse data and pellet sizes required for ignition by means of density ramp- up via pellet injection can be summa-rized as follo~s: Particle Total beam Beam pulse Pellet Pellet injection energy current duration 'radius time 120 keY 1. 25 kA 2.16 s 0.37 cm 2.15 s 100 keY 1. 40 kA 2.68 s 0.34 cm 2.67 s 80 keY 1. 70 kA 3.00 s 2xO.29 cm 1 s 2.99 s The pellet injection times shown are counted from the start of the beam pulse. In the first two cases considered, the starting plasma density was 4 x 10 19 m-3 , while in the third case it was reduced to 1 x 10 19 m- 9 . Because of the rather large beam current associated with the 80 keV case and the corresponding rather tritium- lean discharge, it was found necessary to inject a tri-tium pellet already in the early discharge phase , thus ensuring an acceptable O-T mixture in the reaction zone to the end of the NB heating pulse. In conclusion, the potential of density ramp-up in NB- heated plasmas by pellet b,jection may be' sununarized as follows: • - 84 -al The injection scenario can be started with plasma densities sufficiently low to ensure deep beam penetration, central particle deposition and thus peaked temperature and density profiles. Advantage may be taken of the hot ion mode in this case . bl While in the case of density ramp-up by gas puffing the ramp-up time is finite and the beam may be blocked off at the plasma periphery before the heating pulse is off, density ramp-up by pellet injection is practically instantaneous and unimpaired beam penetration may be granted during the entire heating phase. cl Pellet injection transports fresh fuel to the reaction zone on a time scale that is much shorter than the diffusion time characterizing the method with gas puffing. Hence the timing of the moment of density ramp- up with respect to the heat pulse is less complicated in the case of pe llet injection than in the second case. d) Transition from NB - driven f usion to self - sustained alpha par-ticle production may be possible at lower beam energies and shorter beam pulses (with increased beam currents) than in the case of gas puffing. Continuous Fuelling by Pellet In jection In a series of calculations performed for NET, the optimum combinations of pellet sizes , pellet velocities , and pe llet fre -quencies applicable to reactor fuelling were investigated by in-jecting a s':ring of 10 pellets ("continuous" fuelling) into an already ignited standard INTOR plasma. Before and after pellet injection , the density was kept at a constant level by gas puf -fing . The same transport scal ing laws were assumed here as in the case of the igniti on scenarios (Alcator-Intor ) with an adden-dum : a "soft beta limit " was applied t o obtain steady- state burning conditions. Details of these calculations can be found in /9/ as wel l . The results obtained have shown that the response of the rec i -pient discharge to the injection ' of a continuous string of pel-lets , i . e . the change of the temperature and density distribu-tions , the onset of favourable or unfavourable profiles and the resulting changes of the particle and ene rgy confinement times are functions of the pellet mass - velocity-frequency combination s applied. Hence, for determining the optimum injection characteri -stics , systematic scanning of these parameters was performed on the basis of the following considerations : First, deep pel l et penetration pr oduces favourable density profiles , improves the confinement time , and increases the alpha production rate . Hence for a given velocity the pellet mass shoul d b~ large enough to ensure deep penetration and yet small enough not to cause large -ampl i tude density fluctuations. Second , the pellet frequency re -quired for continuous fuelling depends on the particle confine -- 85 -ment time associated with the modulated density and temperature profiles developing during continuous pellet fuelling. Care should be taken that the frequencies chosen and the respective injection periods are different from the characteristic response time associated with burn control (~1 5). Third, the particle exhaust flux should be large enough to ensure the rate required for helium pump- off. In conclusion, reactor fuelling scenario calculation~ have shown, that fuelling by pellet injection at technically feasible pellet velocities is superior to fuelling by gas puffing alone: a) It produces favourable density and temperature profiles, thus increasing the particle and energy confinement times; b ) it in-creases the central and average BT values and may cause a sub-stantial increase of the thermal alpha power production; c) the particle exhaust flux corresponding to pellet fuelling is some-what reduced compared with edge fuelling but is still more than sufficient for effective helium pump-off. This study is part of an analysis performed for NET on the basis of the EURATOM contract No . 138 /83 -1 1/NET . Useful dis-cussions with K. Borrass of the NET Team are gratefully acknow-ledged. References /1/ D.E.Post et al., Rept. No. ' 33, PPPL, TFTR Physics Group, 1981. /2/ P.B.Parks and R.J.Turnbull, Phys. Fluids 21 (1978) 1735. /3/ INTOR , phase One, IAEA, Vienna 1982. /4i L.L.Lengyel, Assessment of Pellet Injection for NET, IPP Repts. 1/231 to 1/233 (1984), and 1/236 (1985 ) . /5/ M. L.Watkins, T.E.Stringer, A.Gibson, et al., pI. Phys. and Contr . Nucl. Fus. Res. (Proc . 8th Int. Conf. Brussels, 1980), IAEA , Vol. 1 (1981), 639 . /6/ J.A.Holmes, J.A.Rome, W. A. Houlberg, et al . , Nucl. Fus. 20, (1980) , 59 . /7/ W.A.Houlberg , S.E . Attenberger, and L.M . Hively, Nucl-. Fus . 22 (1982), 935 . /8/ E.Speth , IPP Garching, privat communications . /9/ Paper submitted to Nuclear Fusion; see also IPP Rept. 1/236 (1 985) . 

Ignition and Fuelling Scenarios Based on the Injection of Pellets in Thermonuclear Plasmas

http://hdl.handle.net/21.11116/0000-0008-FB99-1

Ignition and Fuelling Scenarios Based on the Injection of Pellets in Thermonuclear Plasmas

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