Cripwell, P.

Duperrex, P.

Edwards, A.

Gill, R.

Granetz, R.

Hubbard, A.

Malacarne, M.

Matthews, G.

Simonet, F.

Snipes, J.

Weller, A.

MPG.PuRe

93 l"LUC'l'UATIONS AND CONf'INl::Ml':NT IN Jl::T P A Duperrex• , P Cripwell" , A W Edwar·ds, R D Gill , ll S Grc1netz, A Hubbard", M Malacarne , G Matthews•, F Simonet 1, J Snipes , A Weller• Jl::T Joint Undertaking , Abingdon, Oxfordshire , OX14 3EA , JK On attacnmenL from: + CRPP , Lausanne , Swi tzerland , Al:!:>Tl1ACT • Imperial College , London, UK • Cul ham Laboratory , AIJi ng<.Jon , Oxfordshire , UK 1 CEA , Cadarache , France 0 !PP, Garch i ng , l"RG FlucLuat1on measurements in Jl::T are pres~1Led and their correlat i on wiLh the confinement is d iscussed for limiter and X-point discharges . INTRODUCTION Mea~;urement of turbulence in Lokamak is motivated by Lhe fact that anomalous tr·ansporL i s o fl.en attrilJuLed to m1cro-insta1Jilitles . The results presented here conct:rn fluctuations analys is in JET using dir!'erent diagnostlcs : edge located pick-up coils for polo1dal magnetic fluctudtio11s (b0 ), diodes for fluctuations or sofL X- r·ay an<.! visible light cm1ss1on , a reflecLomel.t:r for density nucl.uaLions and Langmuir probes for elec~rostati c fluctuati ons . 11 LIMITER DISCHAHGl::S Magn<::tic pick-up coils in Jin detect 1,11e usu.il Mir·nov <.1ctiv1ty (f' - 0 . 3 ~ "f kllz) 11 J, events linked wiLh lhe 1nte1·nal d1s1·uption (the so-called gong [2 ) and fiuctuaLions [3] . The signal i s eiLl1er r·ecorded wi th r elallvcly fasL ADC (sampling rre4uency: 110 kHz) 01· 111011itort:<.l by tl l1d1·dwa 1·e band-pass f i l tcr·s (from 5 to 56 kllz) . Tile dlllpll tude decr·eases wiLll frequency: boc' f'-l . 5 ±_ 0 . 5 above 2 kHz . The Lotal 11ormali sed ampl1lude is typically: o0!1:l0 - 10-· - 10->. Cross cor r elaL10 11 techniques have been used to determ111e the spatial characteristics of these fluctuations. Two different types of act ivity can be observed in Lhe frequency range 0 to 20 kllz . The rirst , domi11anL at frequencies up to 10 kHz , 1s obsc r·ved 1,0 propagal.e in L11e electron <.liagmagnetic drift veloc iLy d1recL1on (a l ao w1Lll Neutral Ueam lnjectio11) . On the conLrary , ti1e second Lypc (dominant from 10 kllz) appears lo be sLationar·y cverr durirrg NUI with 11=\ and m equal to ll1c ouLermosL irrteg r a l q~1 v.rluc . 1:!0L l1 typea are :;Lrcngly eorTelated <.llong a d11·ecL1011 par·allel l.o Lile e4uilibnum 11\dgneLi c field (U0 ) (!'1g . I) . The co1·1·esp0mllllg co1·1·elatio11 le11gt11 is lar·ger· l.lian L11c 10:.jo1· r ad ius (H) . Tile pllase :;t11 !'L beLween 2 lllJ(ilh.!ttc pr~obe:J dlong t111:.; d1rectiu11 i s vcr·y clo!.:.u to Zt!r1..> for· dll frcqueneie:; indicat1ng k ~. - O for Lile broadban<.J spectrum . 0.4 0.2 o.o -0.2 12.30 12.31 12.32 94 12.33 12.34 Time (s) F . 3 "spikd'activity at the edge during the L phaoe ?rior to an H lg . : mode transition . n 10850 a.) PROBE 5 (T/ s) 12 10 6 -· -6 - 8 - 10 ------~ - 12 1354 13S8 1J62 1366 1310 TIME (s) Fig . ~= Magne t i c signals at the H-L a) probe 5 near the X-point f or which edge fluct uations s ignal domi nates . b) PROBE 1 (T /s) 121---- - - 200ms --- - --! 10 - 8 L-mode -1o•L------- · .. ----- 1l lJ 54 1356 1362 1J66 1370 TIME (s) transiti on. 0 10789 b) probe 1 behind the limiters , dominated by coherent oscil la tions resonant deeper i nside t he plasma. 95 The 50 level aepends strongly on t he plasma-probe distance and increases when additional heating is applied . This enhancement is observed for all pick-up coils and therefore is not due to a simple shift o f the plasma . It also increaEes wi t h Ip and decreases witl1 BT though it is not poss ible to determine if i t is a genuine enhancement or is due to the closer location of the resonant layer at_lower qljl(a) . However, taking these effects into account by multiplying b0 by qljl(a) (or By/BP) the normalised l evel is shown in Fig . 2 (for f llO kHz in tl1is example) to i ncrease with 1~1 {1E •energy conf inement time). Fluctuations f rom visible light emission were measured by 2 arrays of soft X-ray diodes withou t foi l s . Tomographic reconstruction confirms that the measured emiss i on is essentially located at the pl asma edge. Cross-corr elation analysis shows the presence of high m wave numbers . In the frequency range 15- 20 kHz the measured m numbers are about 25 - 35 and for the frequency range 40-60 kHz the corresponding m numbers are m -70- 100. Density fluctuations have been measured with a reflectometer . Coherent oscillations are sometimes seen which correlate well with the magnetic signals . However no correlation between broadband densi ty a nd magnetic f luctuations has yet been found . Langmuir probes in the scrape-off layer can also detect fluctuations in the range 0 to 20 kHz. Preliminary results indicate that the correlation length is shorter than the distance ( 10 cm) between the 2 probes. No correlation has been obtained either with the magnetic or the reflectometer data as far as the broadband spectrum is concerned . llI FLUCTUAT!OtlS DUR I NG X-POINT DISCHARGES During X-point di scharges, vi s i bl e ligh t f luctuati ons are seen to correlate with the first type of magnetic activity (the propagating type) measured with a coil which is near the X-point and very close to the plasma for thi s configuration , whereas the n• l low m magnetic standing fluctuations do not have any visible counterpart. A new type of edge activity in JET has also been observed. Perturbations appear t o dominate in the phase preced ing the transition fr~m a low confinement regime (L-mode) to a high confi nemen t regime (H-mode) (fig . 3) : regular spi kes are observed on the magnetic, reflectometry (when probing the edge density) and visible light emission signals. They are correlated with a sudden flattening of the soft X-ray emission at the edge. The re~etition fr equency of these spikes is slowed dow~_by sawtooth disruptions , one of which often marks the final transition to the H-mode . In contrast H-.L transitions (which are preceded by a large increase of edge radiat i on) are characterised by a sudden burst of broadband {turbulent) activity wi t h m ~ 6-8 and n ~ 1, (frg 4a) . Coherent osci llati ons resonant on surfaces deeper inside the p: asma are apparently unaffected by the transitions (fig 4b) . The X-ray emission drops at the edge and Haradiation from the single X-point region shows a 96 lar·ge i ncr·ca3e ; very often spikes reappear after U1is initial burst . During tile II-mode , edge tur bulent fluctu<.ttions are still present , tliougll dt a much lower level . lV DlSCUSSlON AND CONCLUSION Turbul encti up to 50 kHz has be en observed with oif f erent diagnost i cs . Magnetic fluctua tions l1a ve a very long corr e l at i on l e ng th (~ R) . T11is could conf i rm tha t t he magnetic connec t i on lengtl1 i s o f the oroer of -qH . No corr ~ l ati~1 ha~ yet been found between the broad-band fluctua tions o f tile di fferen t oiagnos t ics for limi t e r discharges . During X-point di sc11arge~ t he light emiss i o n exhibits some c orrela t io11 with a .oiagnetic co il near the X-po int for whi ch edge fluctua t ions dominate the signal. The enhance ment of t\J du ring add!ti o na l heati ng propor tionally t o 1E;' points out a poss i ble l i nk between b0 and the a nomal ous t r ansport . Edge perturbat i ons appear t o play an impor t a n t r ole in the phy s i cs of the hi gh confi nement r egime ( II-mode) . HEFERENCES l l J l 2 Snipes J et al : EPS Schlier see 1986 , Vol I , p152 . Duperrex P A, Keller R, Malacar·ne M, Pochelon A: EPS Budapes t 1985 , Vol 1 , p 126 . l3J Maldca rne M, Duperrex P A: JET Report JET-P (8b)3 1 . 0o 10 20 frequency (kHz) - 40kHz b0.q1/l(a) 3 2 " NB o n 0 .__ __ _.___ _ _,_ __ _,__ _ _ __, 0 2 3 4 7E1 (s - 1) rig . 1: Coherence spect rum fig . 2 : Normalised l e vel i).a ( a t 40 between 2 ma gne tic coi ls kHz) versus 1~ 1 • Ip• 2 , 3 , 4 MA , A) along~ • • B) across ~o · BT: 2 . 5 , 2 . 8 , 3 .4 T . 

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Fluctuations and confinement in JET

Fluctuations and confinement in JET

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MPG.PuRe

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