Recent Experimental Results in ADITYA Tokamak

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Sylvia Dorsey
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1 Recet Experimetal Results i ADITYA Tokamak R. Jha ad the ADITYA Team Istitute for Plasma Research, Bhat, Gadhiagar , INDIA rjha@ipr.res.i Abstract. Recet studies o measuremets of edge turbulece i ADITYA are reported. The measured particle flux i the edge plasma shows bursty ature. The Lévy aalysis of plasma fluctuatios ad the fluctuatio iduced trasport shows self-similarity over a time scale 3 times the auto-correlatio time. I the short time scalig rage, the Lévy scale idex, α = ad α=2 i the log-time scalig rage. The results idicate that the trasport due to small-scale fluctuatios is covective whereas that due to large scale is diffusive. The observed bursty trasport ca be explaied i terms of Lévy flights. I additio, several ew diagostics have become operatioal which have allowed experimets o low- impurity ijectio, gas-puff ad limiter biasig with a commo goal of improvig eergy cofiemet time. 1. Itroductio The ADITYA tokamak has bee i operatio for about a decade. Several ew diagostics have become operatioal i the last year, which allow us to carry out studies of plasma cofiemet. The ew diagostics which have become operatioal are measuremet of electro temperature (T e ) profile usig ECE (electro cyclotro emissio) ad soft X-ray, sigle poit measuremet of core T e usig Thomso scatterig, impurity radiatio power usig bolometry, edge desity ad temperature usig Lithium blow-off ad hydroge ioizatio rate usig H α itesity ad desity fluctuatio measuremet usig µw reflectometry at 22.4 GHz. I additio, a MHz, 200 kw Io Cyclotro Resoace Heatig (ICRH) system has bee itegrated with ADITYA ad experimets to measure couplig efficiecy have bee carried out. Similarly a 28 GHz, 200 kw gyrotro based Electro Cyclotro Resoace Heatig (ECRH) system has bee successfully commissioed ad experimets to iitiate plasma discharge have bee carried out. I ohmically heated ADITYA discharges, we have carried out ew measuremets o the ature of plasma turbulece i the edge. It is observed that floatig potetial ad desity (io saturatio curret) fluctuatios exhibit self-similarity of short time scales (up to 3 times the auto-correlatio time) ad simultaeous absece of log-rage correlatio. The observatio implies the existece of self-similar coheret structures i the edge plasma. The measured particle fluxes exhibit bursty ature ad self-similarity of small scales. I the followig we summarize these activities. 2. Turbulece studies The floatig potetial ad io saturatio curret are measured usig a poloidal array of Lagmuir probes made up of molybdeum wires (legth = 3.5 mm, diameter = 1 mm,

2 separatio= 3 mm). The probes o a movable shaft are mouted o the top port, toroidally 72 O away from the limiter i the electro side. The 24 kb data are acquired at a samplig rate of 1 MHz i the time widow (25-49 ms or ms) where the mea plasma parameters are early costat. I the preset experimet the probe array is located at 3 mm behid the limiter. The istataeous particle flux is determied from the measuremets of io-saturatio curret ad floatig potetial at alterate probes i the probe array. The odd umber probes measure the floatig potetial, whereas the eve umber probes measure the io saturatio curret. Graular structures propagatig i the poloidal directio ca be see i both measuremets. FIG 1: Results for floatig potetial. The probability of retur to the origi p( =0) vs. graph shows two distict scalig rages: =1-40 ad >40. The Lévy scale idices i the two scalig rages are: 1.2 ad 2 respectively. The re-scaled PDFs for the two scalig rages are show i the iset (larger width for the log time scalig rage). The istataeous value of fluctuatio iduced particle flux = δ e δv r, where the radial velocity, δv r is obtaied from the poloidal electric field, δe θ ad the plasma desity, δ e is assumed to be proportioal to the io saturatio curret. The δ sig idicates fluctuatio i differet quatities. The poloidal electric field at the locatio of a eve umber probe is determied from the differece of floatig potetials at the adjacet probes. The fluctuatio iduced particle flux shows bursty ature. The bursty ehacemet over the mea flux (=0.25) is by a factor of The floatig potetial, io saturatio curret ad the particle flux show o-gaussia behaviour as idicated i excess kurtosis. We have carried out Lévy aalysis of these data with a objective to search for self-similarity of o-gaussia stochastic process. It has bee show by Lévy that the sum of idepedet stochastic variables, with a probability distributio havig power-law wigs, coverges to a stable process characterized by the Lévy distributio [1]. The process is stable because the PDF of the sum, = X i, of the stochastic variables {X}, has the same fuctioal form as the PDF of X i. The PDF of a symmetrical Lévy stable process is give by [2]: p( ) 0 α ) = (1/ π ) exp( γq )cos( q dq where α (0 <α 2) ad γ(>0) are the scale idex ad scale factor respectively. If the cetral regio of the distributio is well described by a Lévy stable process, the the probability of retur to the origi is give / by: p( 0) (1/ α) /[ πα( γ ) 1 α = = Γ ], where Γ is a Gamma fuctio. The Lévy stable 1

symmetrical distributios rescale uder the followig trasformatios: = 1/α s / ad 1/ α ps ( s ) = p( ) Although Lévy stable process is characterized by ifiite variace, the existece of fiite variace i

3 symmetrical distributios rescale uder the followig trasformatios: = 1/α s / ad 1/ α ps ( s ) = p( ) Although Lévy stable process is characterized by ifiite variace, the existece of fiite variace i physical systems is treated by usig a trucated Lévy distributio (TLD) where a expoetial tail is imposed o the values of the stochastic variable. The TLD is a Lévy stable process. Further details ca be foud i [2]. FIG. 2 : Results for io saturatio curret. Other details are same as i FIG. (1). FIG. 1 shows the probability of retur to the origi, p( =0) as a fuctio of the umber of covolutio () for the floatig potetial fluctuatios. There are two scalig rages = 1-32 ad 64 with well defied slopes: -0.9 ad -0.5 respectively. The correspodig Lévy idices are 1.11 ad 0.5 respectively. I the iset, we have plotted the re-scaled PDFs, p s ( s ) ad it is observed that the PDFs collapse to distict PDFs i the two scalig rages. The differece of log[p s ( s =0)] values i the first ad the secod scalig regimes provides a measure of the `distace', betwee the two scalig regimes. The 0.6 for the results show i Fig.(1) idicates that the 'distace' is sigificatly large. FIG. 2 shows the probability of retur to the origi, p( =0) as a fuctio of for io saturatio curret. The self-similarity is preset ad the Lévy scale parameters (α) i the short ad the log scalig rages are 1.2 ad 2.0 respectively. The short scales are skewed ad o-gaussia whereas the log scales are Gaussia. The two scales are well separated with 0.6. FIG. 3: Results for particle flux. Other details are as i Fig. (1). The re-scaled PDFs of oly scales 1-32 µs are show i the iset. For higher time scales, the PDFs do ot collapse to a sigle PDF as show i the isets of FIG. (1) ad FIG. (2). FIG. 3 shows peak of p( ) vs. for the particle flux. For the first six scales, the peak is close to p( =0). For larger scales, the peaks move towards the positive values, ad hece

4 1/ α p( = ) relatio is ot applicable. The re-scaled PDFs with α =1.33 for the first six scales are show i the iset. 3. Cofiemet characteristics The discharge parameters i the preset experimets are: B φ = 0.75 T, R= 0.75 m, a=0.25 m, I p =50-80 ka, < e >= (8-16) x } m -3 ad discharge duratio of 50 to 100 ms. The plasma curret is early costat durig ms. The eergy cofiemet times, τ E determied from the measured Ohmic power ( ) kw, the radiated power (20-50 kw) ad the heat cotet ( J) are i the rage ms. The estimate of the total umber of electros i the discharge [(5-11)x10 18 ] is obtaied from the µwiterferometry ad that of the ioizatio rate [(2-6)x10 21 s -1 ] is obtaied from the H α itesity. The particle cofiemet time, τ p determied from these estimates is typically 2 times the eergy cofiemet time (τ E ). Several other experimets have bee carried out towards a commo goal of improvig plasma cofiemet. These are limiter biasig, gas puff ad low- impurity ijectio. FIG. 4 shows diagostic sigals whe Li-C impurity has bee ijected at 46 ms ito the ADITYA discharge. Icrease i plasma desity ad impurity radiatio ad Soft X-ray emissio ca be observed simultaeously with decrease i H α itesity for a duratio of 4-5 ms. It is ot yet cofirmed that the icrease i desity is due to improvemet i the cofiemet time. Similarly we have carried out limiter biasig experimet i which there is a clear reductio of fluctuatios i floatig potetial measured by Lagmuir probes. Detailed studies are beig carried out ad will be reported separately. FIG. 4: Diagostic sigals i a experimet with Li-C impurity ijectio at 46 ms. The < e > uit is m Prelimiary RF Experimet Io Cyclotro Resoace Heatig system at MHz ad 200 KW has bee istalled o ADITYA. Iitial experimets relate to optimizatio of matchig coditios at differet frequecies. Atea system is matched with RF geerator usig matchig uit comprisig stubs ad phase shifters. Voltage probes (20 os.) are placed i betwee atea system ad matchig uit where VSWR is quite high. RF power coupled to the atea has bee optimized at 25.5 MHz. This is doe off-lie based o the sigal

5 obtaied from the directioal coupler positioed at load ed ad the sigals from the twety probes to measure the VSWR. Atea curret is measured usig a calibrated curret loop placed at the back plate of the atea box. A probe assembly cosistig of electric dipoles, magetic loops, Lagmuir probes ad capacitive probe is placed iside ADITYA vacuum chamber through radial port. This assembly ca be moved by 50 mm to sca maily SOL regio. RF electric ad magetic field compoets are measured usig dipoles ad magetic loops. The same heatig system is beig used i experimets of plasma breakdow ad for preioizatio assistig ohmic breakdow. FIG. 5 shows successful plasma breakdow with 100 kw ECRH system istalled o ADITYA. The diagostic sigatures from optical light ad Lagmuir probe measuremets show a plasma desity of 5x10 11 cm Cocludig remarks I coclusio, we have reported results of some recet experimets i ADITYA tokamak. The direct measuremet of particle flux shows bursty ature. It is show that edge fluctuatios ad fluctuatio iduced particle flux exhibit differet scalig behaviors at short- ad log-time scalig rages. I the short time scalig rages, the Lévy scale idex, α= This idicates the domiace of coheret structures at these scales which ca lead to covective trasport. At loger scales, the value of α is close to 2 idicatig that at those scales, fluctuatio lead to diffusive trasport. I additio, some ew diagostics have become operatioal which allow cofiemet studies. The measured τ E is i the rage 1-3 ms ad τ p is 2 times loger. Several experimets with limiter biasig, gas-puff ad low- impurity ijectio is beig carried out.the iitial RF experimets demostrates the lauchig of RF ito ADITYA ad feasibility of usig it for iitiatio of discharge which may be of importace i our ext geeratio tokamak (SST-1). Refereces FIG. 5: Plasma breakdow i ADITYA usig ECRH. [1] P. Lévy, "Théorie de ladditio des Variables Aléatoires", Gauthier-Villars, Paris, (1937). [2] R. Jha et al., "Evidece of Lévy stable process i tokamak edge turbulece", Submitted to Phys. Plasmas.

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