Asok, Saha

Kumar, Ganguly Ashim

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Indian J. Physica. 50, 083-689 (1976)Operational characteristics of a pressurised Van de Graaff generatorAshim Kumar Ganotjly and Asok Saha*Saha Institute of Nuclear Physics, Calcutta 700 009 {Received 10 July 1975, revised 17 January 1976)Bofcailo^ l juvostigatioiLS on ilxo cporational ohaT-actoriaticjs of a j)i‘o,s- Hiirifjod Van. do G^aaff GoiLorato^ ' wo^ ’o undortakon Tho input ajxd transkiT oha»*aota^ ‘istios, di^ 5t.ril)ution of cnrronts, and tJxo t(!>rminal voltago and ita lojLd cJiaractx^ n.^ tios wou> Htudiod at differt«nt nitrogon gas pvossuroR iipto 751h/iji® g1. IntroductionVan de Graaff gonoratorB are expeuBivcly used for prcM-.iaion experiments ijuiuclear pjiysics Some operational el\aT‘acteri,sties of such genoralors midei atmosplieric conditions iiLrci available in the lilcc’atui'o tlu'orgh the pioneering uorka ol R. J. Van de Graaff and l;is associates (Van Atta el al 1036) and of J I ’. Sinoo (Smee 1944) However, no such detailed study under pressnnsed condition has been »’ep(uiul inl.lv lilfiraturi) (Ke^b el al 1937, IVump & VandeG’’aaff 1939, Vande G'‘aaff c/ al 1946, Craggs <fc Meek 1954, Hub 1959, Living‘^ ton <!t Bknvett 1962, 'rj'ump 1964) Wo, tlie’’(»fo''‘o, uivlertook inbeusive sti'.dies on the operational cUa’'acie‘"istu>s of a pAusiirised Vau. dr- G^ 'aatf geiifU’ato’’ in the one million volt range, which has been designed and const.'’uctod utilizing indigenously available iTiatoi’ials and lot;al t'ngiiv*o'ing falu’ication facilities (Chattcujoc el al 1973), fo*‘ a b('tt(U' imdiU'stauding of its opc’'ation Some ol the results obtained from iJieSi^  experiments liave boon bvJ(^ fly "■epo-''tod elsewb.eie (Gangitly & Saba 1973)2, Experimental Setup and ProceduresThe Van do G^ a^aff generator (CHratUu jeo el al 1973) used in our experiment is a liorizontal machine housed inside a pressure vessel having an ovei'all dim<ui>sioii of about 3 ft dia x5 ft long Perspox was used as tlxo insulating support sl 'uc- ture material tor the hjgli voltage terminal made of spun aluminium Tho ter­minal itself Avas a 22" dia cylinder (nxding as a hemisphere of radius 11" at tho top terminal end, its total length, b(»ng 22". The gap between the two c oaxial cylinders, viz., the high voltage tormmal and the ground-^ Kl p’ ossiu'o vessel was about 5J" on the sides. The p,^ ossiu-e vesKid Avas fi^ 'st evaoi'ati^d to hotter tlian lO-’^ mm of Hg to remove traces of moisture before filling it with coinmoreial•Present address : Nuclear Physics Laboratory, Bose Institute, Calcutta-700 009.683684 Ashim Kumar Ganguly and Asok Sahaquality nitT-ogon gas (water vapour coixtont ~  600 ppm) obtairuHl in oylindors from M/s, Indian Oxygen Ltd, TJu^  mc^ an room temperatuKi dining the experi- mentH was ~  2S'’C.A 18 cm wide X mm tjijrk endless nylon-woven (;liargo tranKport belt mounted on two 3" dia solid mild stool roller,s, one ea(;]i at ground and l\igli. voltage ends, was drn^ui by a 3 please 2 H P. motor at a linear spo(d of 1300 metros/min. Corona eoinbiS for tho spraying and eollection of charge,s were made fiom J" long gramophone nowlles fixed on a V  dia brass rod, th(^  distance between two ad­jacent needle,s being J". The relative pei’formancos ol diffeient comb oonfigiu'o- tions and their gi^ omet'uos for transport of charges to tl\e high voltage terminal Jiave been studied oa?'lior under atmo,spheric conditions (Ganguly & Saha 1975). Por sijudios under pre,SHu,ri,s(d niti'ogen gas weliavc used the arrangement of c’ombs isliown in figuio 1 ivliich was Found to b(^  the most convenient one under atmos- ph.eric conditions (Configuration IV 2a of Ganguly & Saha 1975)The terminal voltage wa.s moasiu'od by a giuierating volt-metier Its response was found to be linear above 15 kV and was practically independent of prc!,ssuro and tile type ol‘ t]>.e filling ga,s The load (‘iirront Icn 1^ mn the bjgli voltage, termi­nal was drawn with t3\e lu>lj) <>1 an adjustable coi'ona needle as.^ enibly wh.ich wa,s grouudeil. tlv(>ugj>. a liurient mea,SpTing meleJ* 'I’lie gap b(.t\^ 'e(‘n Ihe corona needle tip aud the dome wa,s varii t^ to c,(>nl''(>l tho t ivniiiial voltagi* Wlicn thi» ((n'OT»a ncodlf# toiuO',( ,s tb,e dome, tlie i.ii'riait flowii'.g t]e‘ovf,l. li. is tlic sho '^f cir- cuil Cl maltTJ'.e relation bi t^wecn difioroat ciuTonts under sliovt uiaaui condition. ,shown in figure I, arc as followsU  -- I + + I lc and Is=^J^~\I-P,rom tjioae equations wo can write( 1 )and/+  -  T s~ I .J l c  —  “  / c + / _ —(3)(4 )Thu,s, aaid Jlc estimated I'rom t)io inea,siu'od vahu^ s of Jq, I s aiid / „  under slioj't circuit condition Also. t]u> r<^ Ku.ltant inpui, curiort at the ground endJin =  Jc~VJ~and hfwie can be calculated from tho measured values of Jc and /_.(5)Van de Oraaff generator 685Fig. 1. Schomatic diagram shoAving the distribution of currents mider short ciromt condition (Configuration IV 2a of Ganguly & Saha 1975).C'l* Ca : ground end corona combs; Vc : ground end spray voltage; I c  : gi'oimd end spray current: / ,  : current duo to positive idiarges going up with the bell; / „  ; current due to negatjA'^ o chaiges coming down with the belt and collected by the comb C ;^ Tf,c : loakage current at the ground end; G^ , (7^  : corona combs inside the dome; Is  ■ short circuit currentDistance of the gromid ond roller oentrelino from the ground end baso plate djt =  2 y , distances of the combs Ci, from the ground end roller centrolino dcBi ~  dcJiB 2^  respectively; distance of tlie dome ond loller centreline from the domebase plate dj^  =  2^ ^^ ; distance of the comb C][ from the dome baso plate =  H", static gaps between the belt and needle tips of the combs C^ , 0 ,^ L' Cg : g\ =  g>2. = =  3/8^3. BESTHiTS AND DISCUSSIONSThe plot of ground end spray current Ic  against spray voltage Vc is an iuput charactoridtic of tJto genoT’ato?-. A typical plot of 7q vs Vc along Avith the corres­ponding ground ond spraying oflicioncjy /.,7/c at a nitrogen gaH proSHuio ol 3 atmospHeros absolute aj'e shown m figure 2(a) Tlxe spray current increases tvith the spray voltage Tlie initial slow rJ>so is duo to leakages The abrupt rise in Ic  from Fc'^19kF coinoidos with the threshold of spiaying efficiency marking686 Ashim Kumar Ganguly and Asok SahatUo oxisiot of cho’ go spraying through corona. From hero the increase in the spray current and the spraying oificiency is due to tlio increase in charge density on the belt TJi,e spraying oflioiency roaches a peak at Vc 25kV indicating a saturation in t)ie i5)i,ai’ge density Subsequent increase in Ic  alojig with the decrease iji spraying offiiuoney indicates the onset of leakage of charges in excess of saturation' 04■#i" j"I1(a) 0»)rig. 2(ii) Variation of giound end spray cuiiont Ic  and the ground end .siDraying offiojienoy 1 , /7c’With ihe sipray voltage Fe at a nitrogen gaa presauro of 3 atmoapheros ahsohito.-Fig, 2(b) De)ieiidoiKo of short circuit current Is  and over all charge transfer efficiency Isjlin on tho roaiiltani. mpui current at the ground end /*„ at a nitrogen ga.s pressure of 2 atmospheres absolute.from tbe surface of the bolt. Input eharactei’istic curves of similai' nature have been found fo’’ pi'essuies upio G atmospheres nitrogen The thrc>s]xo]d fo  ^ corona was found to increase linearly with gas pT-oSsure—a characto^istii  ^ feature of the gas bioakdown pb.onomejia. The peak ground end spraying efficiency was found to remain constant to a value ~ 7 0 %  irrespective of tho pressure of nitrogen gas upto 0 atmospheres absolute.The dependence of ilie f h^ort circuit current Is  on the resultant input current i^n represents a U'anslV cJiaractt^Istic of tlxe Van do Graaff generator. Tlie variation of short circuit current Is  and the overall chaTge trajisfer ( f^flcionoy Islhn  ^he resultant input cu.went / / „  wore investigatf^i at different nitrogen gas p’^ esMiies upto G atmospheres absolute. A typical plot o f Is  and Islhn  li^ for 2 atmospheres nitrogen are shown in figure 2(b). Tho chai’acteristios aj’e similar for different preseu»'os Tlxe maximum overall charge transfer effi­ciency remains well above 80% “•nd is practically pressi^re indeiKindent indicating that oiu’ generator is a low current loss system.Van de Oraaff generator 68 ^Tho variation of the short circuit current 1 $, current duo to positive cha>-gos going up cun'ent duo to negative charges coming down 7.., and tlie loakao<^  current at tho ground end Ii,c as a function of spray current I c  v^ oro studiwl at different nitrogen gas pressures under sliort circuit condition A typical plot for 3 atm. Ng is shown in figure 3(a). The currents Is, 1+ and all shoAv satiu a-Fig. 3(a) Variation of different currents with tho ground end spray current 7c at a nitrogen gas pressure of 3 atmpshercs absolute.Fig. 3(b) Dependence of current due to negative charges coming down 7_ on the current due to positive charges going up /,  at a nitrogen gas pressure of 6 atmospheres absolute.tion with Ic  The leakage curront I lc increaso,s rapidly after saturation in /+ is attained. Tho linoai' incri'.aMo in tho saturation values of Is, I   ^ and 7_ witli iiitrogon gas pressure is sliown in figure 4 Th(^  systematic difforcmcc in tlio saturation values of I^ _ itud I. is inforrod to lie duo to diffortmt sticking prohabi- litios of different typos of ions onto the chajge ti anspovt boltThe dopondonco of tho current duo to negative charges coining dow^ n 7_on tlu^  curront due to positive cha,rgos going up was studic^d at different nitrogon gas pressures A typical curve for 5 atm. Ng is shown in figure 3(b) Thc>re is a tlireshold value of I^ from whe.ro /_  stai ts, l^urthor, I_  ultimately slrows satu­ration with due to saturation in the negative charge density on tho bolt At a particular prassure, this thershold value of /+ is the minimum amount of up- going positive charge curront that is necessary to produce the required roller to dome voltage for negative charge spraying to commence The thresliold value of I .^ was fouird to increase linearly with nitrogen gas pressure.The maximum steady tei*minal voltages attained in our Van do Graaff generator at different nitrogen gas pressures are shown in figixre 4 A maximum terminal voltage of 1.1 million volt was achieved at 76 Ib/in* g nitrogongasprossure688 Ashim Kumar Ganguly and Asok Sahawitjx a Hprajr voltago of 38 kV. The load characteristics of the gonorator at dif- iei’(int nitrogen gas prossiu’os are s]iown in figiu'e 5. The curves reveal the essen­tially constant current feature of the geiuirator.Fig. 4. Dependence of terminal voltage and the saturation values of the ctuients Is^ I + and J_ on nitrogen gas pressureFig. 6. Load charaoteriaticB of the Van de Graaff generator at different gas pressures. ACKNOWIiBUGMENTWe are grateful to Professor I) N. Kundu, Director of this Institute, for his kind intoroHt and support. We are thankful to Dr A Ohatterjee for timely aflvioe and to Dr. N. K. Majuindar and Dr. S. K. Ghosh for helpful discussions.We also wisJi to thank Mr. A. N. Sinha, Mi\ H K. Das and Mr. K. C Pal for assistance and technical helpK b feben oicsChafcterjee A., Ganguly A., Ghosh S. K., Majumdar N. K„ Mukhorjee S., Baha A & Barker R. X973 Scie,fkc.e. and Culture, 39, 136.Graggs J. D. & Meek J. M. 1964 Ilitjh Voltage Laboratory Techniquea, Butiorworths Scientific Publications, Ijondon.Ganguly A. K. & Saha A. 1973 Nuclear Phyazca and Solid State Phyaica (India) 16B, 204. Ganguly A. K. & Saha A 1976 Ind. J. Phya. 49. 287.Herb R. G. 1959 Handhuch dcr Phyatk 44, 64,Herb R. G.. Parkinson D. B. & Korst D. W. 1937 Phya. Rev. 51, 75.Livingston M. S. & BlewoU J P. 1962 Particle Accelerators, McGraw-Hill.Smoo J. P. 1944 J, Inat. Med. Engra. Pt. 1 91, 422.Trump J. G. 1964 Nucl. Inatr, & Meth. 28, 10.Trump ,T. G. & Van do Graaff R, J. 1939 Phya. Rev. 55, 1160.Van Atta L. C., Northrup D L., Van Atta G. M. & Van do Graaff R. J, 1936 Phya, Rev. 49, 761 Van de Graaff R. J., Trump J. Q. & Buechnor W. W. 1940-47 Rep. Prog. Ph/ya. 11, 1.Van de Qraaff gmtraiot 68d

Operational Characteristics of a Pressurised Van De Graaf Generator

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Operational Characteristics of a Pressurised Van De Graaf Generator

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