Edwards, A.

Gill, R.

Pasini, D.

Weller, A.

Zasche, D.

English

MPG.PuRe

Simulation of soft x-ray emissivity during pellet-injection and h-mode in JET

SIMULATION OF SOFT X-RAY EMISSIVITYDURING PELLET—INJECTION AND I-l-MODE IN JETD. Pasini . A. lidwards .R, Gill . A, Weller”) and D. Zitsclie“H.llf'l‘Joi/u (flit/winking. .-lhiugt/mt, ONO/1.. (LY/43154. United [vinyl/(mt“" Perniuneut tall/rests .‘ IIJI’. Gare/tiny, WU" UW'H‘W.“Introduction . Spatial profiles of soft Xeray intensity (E 113' I .75 keV ) are measured on JETllSlllg one vertical and one horizontal X ray diode array (ltlll detectors total). Usingtomographic reconstruction techniques spatial distributions of local X—ray emissivity are thenderived. Remarkable differences in the Xeray emissivity distributions have been observeddepending on the type ol‘ plasma discharge. Xeray distributions measured during limiterdischarges usually have a gaussian shape. In deep contrast. extremely peaked profiles ofX—ray emissivity. with most ol’ ol~ the emission coming from a narrow central region. havebeen observed alter pellet injection and flat or even hollow emissivity profiles have beenobserved during the l’lemode (,ifX17t'riiit discharges. In part I of this paper simulations whichreproduce well both the shape and absolute emissivity of the measured profiles arepresented They show that peaked or hollow emissivity profiles rellcct similar changes in thedensity distributions of the dil'l'ercnt species.The vertical Xeray camera which directly views the trajectory of pellets injected into theplasma shows very intense ln‘etusstrahlung emission from the interactions of plasma electronswith pellet particles. In part 2 ol' this paper a model which accounts for the main aspects ol‘the observations will be discussed1. Simulation of emissivity profile .Peaked profile— Fig. I shows the X—rav distribution before (t:4.44 s) and after (t=7.52 s) theinjection ol~ two D pellets (4 mm pellet at 4.5 s and 2.7 mm pellet at 5.5 s) into an ohmicallyheated plasma (74’ l3572.l=2.5 MA. [3:].HT). 'l'he first pellet. coolingr the plasma. allowsthe second pellet to penetrate deep into the plasma. leading to a dramatic increase of thecentral emissivity and consequently to a Strong peaking ol' the X—ray distribution. Verysimilar observations made on other experiments have already been reported 1‘1./_ \ Mm 939 WW 3'. Max 733i; Wit“/ l ti. j t' ‘ after t»: vétauwtinnDolor? mllet in echon t" t‘‘ l l t l 1:152 st:4.445 I-' ' ‘ 4 i il: ai titan tugszrt“‘\;‘-s44 . .M Ilt tr radiusFig. I — X ray emissivity distribution belore and aiter pellet injection.Fig _shoVVs the 1adi111 emissiVitV profile ot the peakeddistribution (t:7 53 5) together withtwo ditlerent simulations usinu the radiation code lONEU" 'l'aking in account thet1ansmissior1 of the filter the code caeulates for each radiating species a radial profile ofemissivity and these are then summed up to yield 11 total profile. The radial charge statedistribution for each species is calculated assuming coronal equilibrium and using theimpurity concentrations derived from PllA spectra and the profiles of T1 and 11!, measuredwith the LIDAR 'l'homson scattering diagnostic. 'l'he rrtcasured profile of II“ is also verypeaked In the first simulation (nine 1) which reproduces well the shape and absoluteemissiVitV ot the measured profile the densitV distributions L11 the dilleient plasma species areassumed to be as n“. The calculation shoVVs that the observed emission is mainly due tofree—free radiation from fully ionized C and O. In the second simulation (curve It the radialdensity distribution of D is taken as 11‘, but 11 much broader pre—pellet distribution is used forthe C O. ('1 and Ni impurities. A peaked l) Llcnsitv profile is to be expected as the secondpellet deposits 111 this case. a large part of its particles 111 the pl 1sr111t center l’he peaking ol'the impurities is moreLsarprisine 1111L1111Llicates an increasL ott'the 111w111Ll convection velocityThis result can be understood according to the neoclassical theory which predicts that thelarge density gradient of the 1) ions should drive the impurities inward However since theneoclassical theory has not been adequate to describe the majority of experiments thisinterpretation should be looked 111V\ith caution and more work is needed to confirm it.Hollow profile — Hollow X—ray profiles are observed 111 certain Xerpoint discharges correlatedwith hollow electron density profiles. Fig. 3 shows such Xiray emissivity profile measuredduring an llemode together with two simulations. '1'l1e simulation which reproduces the111easured profile well (curve 1) was obtained using the hollow electron density profilemeasured by interferometry and assuming tor the other plasma species the same shape ofdensity profile. A second simulation (curve 1). assuming 1'1111 density profiles over the centralregion, was run to verily that the 1111111111111ess111111e Xeeray profile was not due to line radiationfrom N1 111-("l btrt 11 rather 11 direct consetlence of the hollow densities,Measurement Calculationwttht hollowEttatprolrle 01 nE incentral regroneSOiimBL1C -2 ‘119V1U.Calculation‘1 peaked 111111111111; protrlesB prorpetlet impurity Drolfles#8 11') F1711?»1-71‘11111yEm5311111‘-‘-tt.111‘Emrssrwty:103Wcmat- 1"11 .1Radms (cm)Fig. 2 7 Profile ot X—ray emissiVity alter pellet 112.1- Measureat and simulation ol hollowinjection and two simulations assuming peaked \ 11IV emissiVitV profile during an 11 modeor broad density profiles tor the impurities.2. X-ray ablation emission . Detailed observations on D pellets injected into .I ET hare beenmade with the vertical soft X—ray camera which directly views the pellet trajectory Strongemission (Fig-l) is seen from the pellet-plasma interaction region due to bremsstrahlungradiation from collisions between the plasma electrons and the ablated pellet particles theobservations hayc been used extensively to determine the pellet velocity and depth ofpenetration. The time dependence ol‘ a single channel has shown that the emission originatesfrom within a region with a diameter 2 r‘ = 7 cm in the major radial direction and I}, is takenas the critical radius at which the ablatant flows along the magnetic field litres. The length ol’the hoze of ablatant along the field lines was determined from toroidally spaced X—raydetectors as much less than 3 m . Measurements with dillcrent Be lilters showed that thecl'l'ectiye temperature of the electrons in the region ot‘ the pellet was close to that ol‘ theplasma belore pellet injection.X'lj‘, ti'tErtStW Pelletalflala‘miMaitnradiusFig. 4 — The ablation ol a pellet as seen by the \vertical X' ray cameral’hc Pellet ablation model of Parks and 'l'urnbulll has been used to calculate the absoluteintensity of the X rays. For the shots considered here the model also gaye an accurateprediction ol' the pellet range although a more complicated model5 is generally required topredict the pellet range iii .l ET. In this model the incident plasma electrons interact with theablated particles which expand spherically with high density and low temperature near thepellet surface. In a later rcl'inement to the tltctii'y"“ the ablated particles flow along themagnetic field lines at I" = 2,5 rp . but we prefer to keep I“ as a free parameter as themeasurements show a much larger (solution of PT but outside we have found the corresponding solution for cylindricalgeometry. The temperature of the incident electrons dillers significantly from I; close to thepellet surface only. As the X7‘use the plasma \‘alue ot’ I“ eycrywlicre ’l'he ablatant density isl ‘ t 1lalizp, Ht?)' . rw I" : p:/t —,a1X) yalue Inside I“ we use the analytic asymptotic1y emission comes l'rom mttcli larger radii it is reasonable towhere n l " is a constant near unity. r and p. are the radius and the density at the sonicsurface and : a \‘ariablc along the field lines. As the velocity of the ablated particles is muchless titan the plasma electrons the X ray emission may be calculated from the hydl'ogenicbremsstrahlung formulaP : rm to ‘3 1’ no TuHi ‘\' ‘’7 ll (Wicl)! 7' )Where [1‘ is the energy cutol'l. Integrating over the field of View ll~ig5l giyes. 1 v. 7r." .m " ‘ J ’ipii i ”u\/7u ‘7 ‘ Ii (H i'This expression (accurate to s-ZtI‘M) depends only weakly on r], confirming that there is little., J.13‘ i 7.9 .v, to“ ’—emission from the region near to the pellet. I" has been compared with experiment (Fig.6)for the ease of a 3.0 mm D pellet injected into a D plasma withI : 3 MA. I3 = 3.8 I'. H“ = 1.35 X. llt'” m“ and It : 4.3 Irel'. The calculated emissiongenerally exceeds the ohsened value in the outer part of the plasma. hut decreases to wellbelow the observed value at the end ot‘ the pellet trajectory. The ohsenation that I" alwaysincreases sharply at the end oi the trajectory is not adequately explained h) this model.Z k lUZIGJPellet . , IZFC ;' O J 1 >1 1 3 7l >31_ 1 1'lll _, {7 _Plasma hose 2g, Spherical o , 7 Calculationexpanston FD : 72.6 cm ,l : 6.9 cm 7 ( .Q to‘ 4 (I 17m} 18Fig. 5 — Schematic of the ahlatineV plasma withinthe field of \‘ie\\ ot an X—ray detector (D is It.For r .- r_ the particles How along the t'ield linesFig. 6 ~ Comparison between measured (eachdata point corresponds to a particular detector)and Calculated soil X-t'a)‘ power versus radial(/. direction). position.In conclusion : ( l) the Xera)‘ measurements support the concept ot' the formation of a plasmahose. but its radius is larger than predicted and it is quite short ("3 3m). (3) the Pl modelprovides an approximate estimate ol' the ohserred emission except towards the end of thepellet’s range where the calculated value is much too small. (3) the X—ray energy spectrum isapprt'mimately as expected.ReferencesI R. l’ett'assoela|..Ph)s,Re\‘.l.ett.57(1‘)t\'h)]|)(l7.3 M, Cireenwald ct al.. ll’i’ lnt. ('onl'. Plasma l’hys. and (iotur. Nuel. l—us. Res. Kyoto. Woo.l/\Ei\-(‘N—~l7»“Ailll I.A. Weller et al. JET internal report. .lE'l‘-ll{(t\’7)llt. IUH7.'i l’. B. Parks and R. .l. Turnhull. l’hysl'luids lli 1"7H)1735-5 M. L. \Vatkins. FPS (‘onll(Schliet‘see) \'o|. [UV ( liloollSh.I‘ l’. B. l’arks. Nuclear Fusion. lt l‘NlBl l.

http://hdl.handle.net/21.11116/0000-0009-8A47-C

Simulation of soft x-ray emissivity during pellet-injection and h-mode in JET

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