Setup for potential bias experiments on the Saha Institute of Nuclear Physics tokamakJ. Ghosh, R. Pal, and P. K. Chattopadhyay Citation: Review of Scientific Instruments 70, 4557 (1999); doi: 10.1063/1.1150112 View online: http://dx.doi.org/10.1063/1.1150112 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/70/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Suppression of electric and magnetic fluctuations and improvement of confinement due to current profilemodification by biased electrode in Saha Institute of Nuclear Physics tokamak Phys. Plasmas 19, 072510 (2012); 10.1063/1.4739074 Dynamics of turbulent transport in the scrape-off layer of the CASTOR tokamak Phys. Plasmas 13, 102505 (2006); 10.1063/1.2359721 Multiscale coherent structures in tokamak plasma turbulence Phys. Plasmas 13, 102509 (2006); 10.1063/1.2357045 Effect of the vacuum vessel on magnetic measurements in TCABR tokamak Rev. Sci. Instrum. 75, 5038 (2004); 10.1063/1.1807001 Dependence of up–down asymmetry on the edge safety factor in the Saha Institute of Nuclear Physics–Tokamak(SINP–Tokamak) Phys. Plasmas 11, 1453 (2004); 10.1063/1.1650847

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Setup for potential bias experiments on the Saha Institute of NuclearPhysics tokamak

J. Ghosh,a) R. Pal, and P. K. Chattopadhyayb)

Plasma Physics Group, Saha Institute of Nuclear Physics, Calcutta 700064, India

~Received 13 July 1999; accepted for publication 31 August 1999!

An experimental setup for studying the influence of the radial electric field on very lowqa plasmaon the Saha Institute of Nuclear Physics tokamak is presented. A high current, high voltage pulsedpower supply, using a semiconductor controlled rectifier~SCR! as a dc switch is developed and usedto bias a tungsten electrode inserted inside the plasma. The electrode’s exposed length and itsposition inside the plasma are controlled by a double bellows assembly to optimize theelectrode-exposed length. We show that using the force commutation method to turn the SCR off toget the power pulse desired has good potential for carrying out similar kinds of studies, especiallyin a low budget small tokamak. ©1999 American Institute of Physics.@S0034-6748~99!02412-0#

I. INTRODUCTION

Since the discovery of high confinement(H-mode! dis-charges in the ASDEX tokamak1 by neutral beam injection,this low to high confinement mode transition(L–H transi-tion! phenomenon has drawn a lot of interest, both theoreti-cally and experimentally, in the fusion community. TheHmode has since been observed in many tokamaks worldwidethat employ different heating techniques with divertors aswell as with limiter configurations. Since the radial electricfield was thought to be one of the major causes behind theL–H transition, Tayloret al.2 were able to induceH-mode-like ohmic discharges by generating a radial electric field inthe edge region of the tokamak plasma by introducing anegatively biased electrode a few centimeters inside theplasma. Since then, in many tokamaks2–4 a similar kind oftransition introducing a biased electrode into the plasma hasbeen observed.

Depending upon the size and the plasma parameters ofdifferent tokamak plasmas, different size electrodes wereused and different radial currents were reported for achievingH-mode plasmas. For example, in the CCT tokamak2 a biasvoltage of 1.5 kV was necessary to draw a radial current of20 A on an electrode surface area of approximately 50 cm2 toenable the L–H transition, whereas in the TUMAN-3tokamak3 only 150 V was sufficient to draw over 100 A ofradial current on an electrode exposed area of only 3 cm2 forachieving the same transition. However, no systematic studyhas been made along these lines.

We felt it was important to undertake a parametric studyon the effect of the bias voltage and the dimension of theexposed electrode to optimize theL–H transition in any to-kamak. With this in mind we constructed a setup involving adouble bellows mechanical assembly and a pulsed powersupply using a semiconductor controlled rectifier~SCR! as adc switch. Novel features of this setup are~1! the double

bellows assembly was made so that it allows freedom inchanging the electrode exposed length as well as its positioninside the plasma and~2! we used a low cost SCR to switchthe bias voltage on fast; however, by applying the force com-mutation method with another SCR we could switch the firstSCR off quickly and get the pulsed power desired for biasingthe electrode with a plasma load. This is the first time we arereporting this method of switching off the bias voltage insimilar kinds of tokamak experiments. Normally, gate turnoff ~GTO! thyristors or integrated gate bipolar transistors~IGBTs! are used for such applications, but they are muchmore expensive compared to the method used by us involv-ing the low-cost SCRs.

The setup has been tested on the Saha Institute ofNuclear Physics~SINP! tokamak5 and the preliminary bias-ing results obtained by this method are presented in this ar-ticle. This article is outlined as follows: in Sec. II the designand the mechanical construction of the double bellows me-chanical assembly is described, and in Sec. III details of thedesign considerations in fabricating the pulsed power supplyare explained. Section IV is devoted to the experimental re-sults achieved with the systems on the SINP tokamak andfinally, in Sec. V, we discuss our conclusions.

II. DOUBLE BELLOWS ASSEMBLY

The electrode used for the edge biasing experiment inthe SINP tokamak was made of a high purity tungsten rod orstainless steel. In Fig. 1 the construction of the double bel-lows system holding the electrode is depicted schematically.The whole assembly can be divided into two major parts, oneis the stainless steel body, which holds the electrode andcontrols its movement and the second is the ceramic sleevewhich moves on the end of the electrode to change its lengththat is exposed inside the plasma. The bellows are connectedto two stainless steel pipes of different diameters. The pipewith the smaller diameter holds the electrode and that withthe larger diameter holds the ceramic sleeve. The larger pipeslides over the smaller one, and the smaller one can movethrough the larger one independently. Movement of the bel-

a!Electronic mail: joy@plasma.saha.ernet.in.b!Present address: Department of Physics, University of Wisconsin, 1150

University Avenue, Madison, WI 53706.

REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 70, NUMBER 12 DECEMBER 1999

45570034-6748/99/70(12)/4557/5/$15.00 © 1999 American Institute of Physics

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lows, which maintains accurate linearity, is achieved by get-ting them to slide over four fixed guide rods. Therefore, onebellows controls the movement of the electrode whereas theother bellows controls the movement of the ceramic sleeve,which in turn adjusts the exposed length of the electrode asthe sleeve moves over it. The sleeve is made of machinableceramic~shown in Fig. 1 inset!. It consists of a thin coppercylinder sandwiched between two ceramic cylinders. Thecopper cylinder acts as a shield to the magnetic field gener-ated by the electrode current due to the unexposed part of theelectrode. This magnetic field must be shielded out, other-wise, it can lead to unwanted local magnetic field perturba-tion. The maximum variation of the electrode-exposed lengthis over 3 cm whereas its position can be changed up to 3 cminside the plasma, with an accuracy of less than 1 mm.

III. PULSED POWER SUPPLY

To supply voltage to the electrode there exist a numberof requirements for the power supply, which must be takeninto account during its design. The requirements can be de-rived from the characteristics of the SINP tokamak plasmaand from the results obtained in similar biasing experimentsperformed on other tokamaks. The requirements can be sum-marized as follows.

~1! The duration of very lowqa~VLQ! discharges in theSINP tokamak is 2–4 ms; accordingly we need to havethe flexibility of adjusting the pulse length of the powersupply to suit this time scale.

~2! During discharge it is required that the plasma be biasedat some particular instant which is dependent on the shotconditions. So a switching mechanism is needed to de-liver power at the desired time which must also be ad-justable. In addition, the switch must have fast turn onand turn off times which are required by our experiment.

~3! As is evident from previous experiments in other toka-maks, the radial current drawn by the electrode signifi-cantly exceeds the ion saturation current and these highcurrent densities are inconsistent with those predicted byneoclassical theory.3 It is also evident from previous ex-periments that application of the bias voltage to the elec-trode inserted inside the plasma leads to charging of themagnetic surface that is intercepted by the electrode tip.Hence, in this case the current drawn to the electrodedoes not depend on the electrode area, rather, it dependson the area of the average magnetic surface and the per-pendicular conductivity. The plasma potential at the lo-

cation of the electrode now can be given byVp5ErLandI e5 j rA, whereEr is the radial electric field,L is thedistance between the intercepted magnetic surface andthe last closed magnetic surface,j r is the perpendicularcurrent density, andA is the area of the average magneticsurface in the electric layer. The perpendicular currentdensity j r can be calculated from the radial momentumequations in terms of radial electric fieldEr and the ef-fective collision frequencyn6,7 and results in8

jr5@~nimin!/BT2#3Er .

Calculating the value ofj r using SINP tokamak param-eters and putting into the equationI e5 j rA, with an elec-tric field of the order of 50 V/cm and the electrode tippositioned 2 cm inside the limiter,I e is estimated to beof the order of hundreds of amperes. So the power sup-ply to be used should be designed in such a way that itcan deliver such high current drawn by the electrode, andthe switch should also have such higher power ratings.

A further restriction on the supply, which may be moreimportant, is set by the very low budget of our experiment.

With these considerations in mind we designed ourpower supply based on a capacitor bank consisting of elec-trolyte capacitors. The advantage of using an electrolyte ca-pacitor is that it made the system very economical and easyto handle. A total of 16 capacitors~made by RESCON, In-dia! each having a capacitance of 10 mF and maximum volt-age rating of 250 V was used. The total capacitance of thebank is 40 mF and it can deliver a maximum of about 1000A at 500 V for, say, 1 ms with a drop of only 25 V. A 3 kVAtransformer with a regulator circuit, shown in Fig. 2~a!, wasused to charge the capacitor bank.

Generally, for switching on and off purposes, we needcontrollable switches like power bipolar junction transistors~BJTs!, metal–oxide–semiconductor field effect transistors~MOSFETs!, GTO thyristors, or SCRs with force commuta-tion. The MOSFETs have lower voltage ratings whereas asufficiently large base current~dependent on the collectorcurrent! is needed for the BJTs to be on fully. GTOs may bethe ideal solution for this kind of high power requirement.However, they are quite expensive. Instead, we chose to usea much cheaper SCR from GE~GE c180 PB! with a powerrating of 1200 V and 180 A in our circuit. Its characteristicturn on time is around 30–40ms and it has a similar turn offtime. We found in our tests that it can deliver a current of

FIG. 1. Layout of the double bellowssystem that holds the electrode. Theinset shows the front part of the sys-tem consisting of the movable insu-lated sleeve that controls the exposedlength of the electrode.

4558 Rev. Sci. Instrum., Vol. 70, No. 12, December 1999 Ghosh, Pal, and Chattopadhyay

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nearly 10 times its rated dc value for a limited time of theorder of milliseconds. Figure 2~a! shows all of the circuitryof the power supply. The most difficult part of using thecircuit lies in turning off the main SCR (S1) while conduct-ing at a high current. We used the force commutation tech-nique to turn off this SCR carrying the load current. To dothat we made use of a capacitor (C2) and a second SCR (S2)with a much lower rating of 600 V and 70 A~made byBHEL, India!. A separate charging circuit was used tocharge this capacitor@shown in Fig. 2~a!#. Both SCRs weretriggered into the on state by applying a pulse of positivegate current for a short duration in its forward blocking state.Figure 2~b! shows the electronic circuitry of the trigger cir-cuit. We used monoshot integrated circuits~ICs! ~DM74121!to generate the trigger pulse for turning on the SCRs at thetime desired. The master pulse was taken from the tokamakpre-ionization signal, which was fed into the monoshots. Bychanging the resistanceR we can vary the pulse width andhence control the time of trigger for the SCRs. The output ofthe monoshots is then amplified and put into the gate of theSCRs through a pulse transformer, which is essential forelectrical isolation of the trigger circuit. When SCRS1 istriggered into conduction the voltage of the main capacitorbank (C1) is applied to the load. Depending upon the re-quirement of the pulse width, after some time SCRS2 istriggered using another set of trigger circuits. This enablescapacitorC2 to drive a momentary reverse voltage onS1 toturn it off. We chose a capacitorC2 of value of 100mF andchanged the voltage on it to get the desired turn off time toturn the main SCR (S1) off. This voltage can be calculatedfrom9

tc5C2Vc2 /I D ,

wheretc is the turn off time,Vc2 is the voltage on capacitorC2 , andI D is the load current. Whentc becomes more thanthe rated turn off value of SCRS1, it stops conducting. Fig-ure 3 demonstrates turning off of the main SCR with theplasma load. It shows a momentary transient voltage on theload during the turn off interval. This does not disturb theaim of our experiment because we want to see the effect ofbias for sufficiently longer times~@the turn off time of theSCR!.

IV. RESULTS

The present experiment of edge biasing was done inVLQ discharges of the SINP tokamak, that has a major ra-

FIG. 2. ~a! Schematic of the power supply circuitry for biasing the electrode.~b! Schematic of the trigger circuit for triggering the SCR.

FIG. 3. Demonstration of turning off the main SCR with the plasma load.

4559Rev. Sci. Instrum., Vol. 70, No. 12, December 1999 Potential bias

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dius (R) of 30 cm and a minor radius(a) of 7.5 cm. In VLQdischargesqa lies between 1 and 2, whereqa is the safetyfactor at the edge, defined asqa5aBF /RBq(BF5toroidalmagnetic field,Bq5poloidal magnetic field!. The typicalplasma parameters are plasma current (I p)525– 28 kA, tor-oidal magnetic fieldBF50.45 T, average electron densityne'231019 m23, and peak electron temperatureTe'50eV. The plasma has a circular poloidal cross section and theelectrode assembly is mounted on the top port of the ma-chine in order to insert the electrode vertically. Figure 4shows a schematic diagram of the whole setup. It is evidentthat before the bias circuit is turned on the electrode will actas a floating probe.

We induced the radial electric field in the edge region ofVLQ discharges of the SINP tokamak.10 Initially, to test thecurrent handling capability and the performance of the bias-ing power supply we used a stainless steel cylindrical rodwith a length of 10 mm and a diameter of 15 mm as theelectrode. Its tip was placed 2.1 cm inside the limiter. Aftergetting stable and reproducible discharges we inserted theelectrode inside the plasma. Figure 5 shows the plasma cur-rent, bias voltage, and bias current in a typical dischargewhere a bias voltage of 80 V was applied to the electrode. Aradial current as high as 800 A was seen to be drawn by theelectrode without any appreciable drop in the capacitor volt-age. The electrode voltage, shown in Fig. 5, depicts the localfloating potential before the bias is turned on; as soon as thebias was on, it jumped to the 80 V level in less than 50ms.In this shot we were able to turn off the main SCR, therebydisconnecting the power supply in less than 100ms. As soonas the SCR is turned off, the electrode is seen to come backto the floating potential level.

However, if the voltage on capacitorC2 was not suffi-ciently high as mentioned in Sec. III, then we could not turnoff the power supply. In Fig. 6 such a shot is shown forcomparison. Plotted in Fig. 6 are electrode current, electrodevoltage, and floating potential from one Langmuir probe toshow the effect of the bias voltage on the plasma potential.Three different plasma discharges~shot Nos. 30595, 30597,

and 30598! with same plasma parameters are presented. Inshot No. 30597 no bias voltage was applied to the electrode.In shot Nos. 30595 and 30598 a positive voltage of 80 V wasapplied to the electrode. However, in one of these discharges~shot No. 30595! we could not put off the main SCR due toinsufficient voltage on capacitorC2 while in the other~shotNo. 30598! we could. As can be seen in Fig. 6~c! the voltageon the electrode remained throughout the discharge in theformer case. In Fig. 6~b! we plotted the floating potential ofa Langmuir probe placed at a different toroidal location. Itclearly shows that when we biased the electrode the floatingpotential changed with respect to that of the unbiased case.In shot No. 30598, when the main SCR was turned off aftera certain time interval, the floating potential returned back tothe value of that of the unbiased case, whereas in shot No.30595, the main SCR was not turned off and the change inthe floating potential remained throughout the discharge asdid the bias voltage, which remained until the end.

Using a tungsten rod 6 mm in diameter in the doublebellows assembly we varied the exposed length of the elec-trode and checked the disturbances created by it. Up to anexposed length of 2 cm, the electrode had no significant ef-fect on the plasma parameters without the bias. Figure 7shows variations of the radial current with different voltagesapplied to the electrode for two exposed lengths, 3 and 10mm. The electrode tip was placed 1.8 cm inside the limiter.The electrode current did not seem to increase much as theexposed length was increased which in a way indicates the

FIG. 4. Schematic of the experimental setup for biasing the electrode fromthe top port of the SINP tokamak. The Rogowski coil in the biasing circuitmeasures the electrode current (I e).

FIG. 5. Plasma current, electrode current, and electrode voltage in a typicalvery low qa discharge in the SINP tokamak with application of a biasvoltage of 80 V.

4560 Rev. Sci. Instrum., Vol. 70, No. 12, December 1999 Ghosh, Pal, and Chattopadhyay

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charging of the magnetic flux surface as we mentioned inSec. III.

V. DISCUSSION

To study the transition toH mode by electrode biasing ofthe SINP tokamak we fabricated a power supply that is verymuch more cost effective and its performance was demon-strated here. By using low-cost electrolyte capacitors andSCRs to switch on the bias it has been possible to quitereliably draw the large amount of current that is normallynecessary to trigger theH-mode transition. To switch off thebias at a desired time we have successfully employed anotherSCR with the force commutation technique for the first timein this kind of tokamak experiment. A sophisticated double

bellows assembly was also constructed to hold the electrodeand it has the much needed flexibility to change the elec-trode’s position and exposed length to enable a reasonablyversatile parametric study of the biasing experiment.

ACKNOWLEDGMENTS

One of the authors~J.G.! would like to thank ProfessorG. Van Oost for helpful discussions. The authors would liketo thank N. Adhikari and the members of SINP workshop forfabricating the mechanical assembly. Further thanks are dueto P. S. Bhattacharya, S. Basu, S. S. Sil, and Dipankar Dasfor their help in designing and fabricating the power supply.Support during the experiments from the members of thePlasma Physics group at SINP is also appreciated.

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FIG. 6. ~a! Electrode current,~b! floating potential, and~c! electrode voltagefor three different VLQ discharges~shot Nos. 30595, 30597, and 30598!with same plasma parameters except shot No. 30597 was with no bias volt-age ~long dashed line!, shot No. 30595 was without turning off the biasvoltage~solid line!, and shot No. 39598 was with turning off the bias volt-age~short dashed line!. The dotted vertical line shows the time of applica-tion of the bias voltage.

FIG. 7. Variation of the electrode current with the electrode voltage fordifferent exposed lengths of the electrode.

4561Rev. Sci. Instrum., Vol. 70, No. 12, December 1999 Potential bias

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