I ndian Journal of Pure & Appl ied Physics Yol. ]1.). January-February 200 1 . pp. 4- 1 4

Folded tandem ion accelerator facility at BARe P Si ngh

Nuclear Physics Division, Bhabha Atomic Research Centre. (BARC). M umbai 400 0RS

Received 7 November 2000

The design. fabrication and instal lation of the Folded Tandem Ion Accelerator (FOTI A), with a terminal voltage of 6 M Y . has been completed and the commissioning is in progress. After achieving a voltage of 3.4 MY on the terminal with nitrogen and carbon dioxide mixture as the i nsulating gas beam trials were started and the accelerated oxygen beam of about 1 2 nA. at terminal voltage of I .R MY, has been obtained. I n order to i ncrease the terminal voltage further. where transmission is expected to improve substantial ly, the accelerator tank is being charged with SF(, gas. In th is tal k. some o f

the sal ient features of FOTIA and i t s present status are d iscussed.

1 Introduction

The FOTIA project I i nvo lves designi ng, fabrication, instal lation and commissioni ng of a Folded Tandem Ion Accelerator (Fig. I ) . It would del iver a variety of heavy and l ight Ion beams for bas ic and appl ied research .

In FOTlN, ion beams from the SNICS-IJ negati ve ion source ( l ocated outside the pressure vessel and inside a h igh voltage deck) are pre-

180" BENDING CORONA STABILISING

SYSTEM EL. QUAD. DOUBLET

FC-5

accelerated up to 1 50 ke V before injecting i nto the low energy accelerat ing tube through a combination of a 70" -magnet and a 20"-electrostatic deflector. Three electrostatic steerers have been i ncorporated at suitable p laces, to shift the beam to the desired d irection . An e lectrostatic quadrupole tri plet i s used to focus the low energy beams. In the termi nal , the ion beams are focussed to a spot of about 3-4 mm diameter on the stripper" where e lectrons of the negative ions are stripped and posit ive ions with

- TERIlINAL SHELL __ ���--�--�FC-4

__ ��.s�1j--1r----F�OIL STRIPPER

GAS STlUPPER

H.E. TUBE ---I----F.I �----..- L.E. TUBE

MAGNETIC QUADRUPOLE --It---llrlil TRIPLET

SCATTERING SWITCHING FC-7 CHAMBER MAGNET i I MQT-2

BPM-7 I

BPM-6 ___ _ FC-6---,--ICl

Fig. I - Schematic d iagram of the folded tandem ion accelerator

SINGH: FOLDED TANDEM ION ACCELERATOR :;

h igh charge states are produced. The positive ion beams are then bent by the 1 80"-magnet4 into the high energy tube. At the exit of the 1 80" magnet the beam is d ivergi ng and therefore a lens i s required to reduce the divergence of the beams before they enter the high energy accelerating tube. An electrostatic quadrupole doublet has been designed, fabricated and instal led . It is located after the 1 80" magnet i nside the h igh voltage terminal . In view of the necessity of gas stripper for experiments at low energies and to create enough space for beam handl ing components l i ke electrostatic quadrupole doublet and Faraday cup etc . inside the high voltage dome the number of modules were reduced from 7 to 6. A jacketed cyl indrical shel l was designed and fabricated at the Central Workshop, B ARC, to compensate for one module d imensions. The shel l is a doubled wal led she l l and is also used as water reservoir for cool ing the 1 80"-bending magnet.

The high energy beams are focussed using a magnetic quadrupole triplet before being analyzed by the 90"-magnet . For sh ifting the beam ax is, if required, magnetic steerers5 have been incorporated in the high energy beam l ine. In Table I, final beam energies, ME/q� values (rounded) and relative intensities correspondi ng to different charge states produced at the foi l stripper in the terminal are l i sted . Beam profi le monitors (BPM), precision double s l i ts and pneumatical ly operated Faraday cups (FC), developed indigenously, have been provided at strategic points for monitoring and measurement of the beam intensities. A pel let charging system was designed, fabricated, instal led and used in the present system and its performance

has been satisfactory. The pel let charging system gives better voltage stabi l ity and lower ripple. The new SF(, gas handl i ng o i l free system!" which consists of an oil free compressor, a centrifugal blower, a heat exchanger, dust filters, dryers and a vacuum pump, was designed, i nstal l ed ancl commissioned. In FOTIA, whi le the high voltage column section remains stationary the pressure vessel weighing around 1 2 tons is l ifted and kept on the base plate. For this purpose, a crane was procured and insta l led . The old crane, which was in continuous service i n the Van de Graaff accelerator since 1 962, became obsolete and unrel iable. The work regarding design, instal lation and commissioning of the above crane was coordi nated by TSD .

The i nsta l lation, testing and i ntegration o f a l l the components ( injection l ine, h i gh voltage column section, charging assembly, sg, gas handl ing and computer control systems, magnets, electrostatic and magnetic lenses, steerers, etc .) has been completed and commissioning is in progress7•X• III this paper, some of the sal ient features of FOTJA and its present status are discussed.

2 Low Energy Line

The low energy beam l ine'! of FOTIA consists of a negative ion source (Fig. 2), pre-accelerating tube, 70"-magnet, double sl it, Faraday cups, beam profi le monitors, e lectrostatic 20"-deflector, steerers, quadrupole triplet, an einzel lens and a low energy accelerating tube . The entire l ine has been instal led. The 70"-magnet was tested for its absol ute magnetic fie ld and uniformity . A magnetic fie ld of 1 4 .5 kG

Table I - Final beam energies at a terminal voltage of 6 M Y Ion Z 4+ Relative% Er<MeY) M E/q

2 Ion Z q+ Relative% EdMeY) M E/q� I H I 1 00 1 2 1 2

2xSi 1 4 5 1 6 3 6 40

'He 2 2 1 00 1 8 1 8 6 34 42 33 1 2C () 3 1 2 24 32 7 3 1 411 27

4 5 2 3D 23 8 1 3 54 24

5 33 36 1 7 .�2S 1 6 6 2 8 42 37

1 ('0 X 4 24 30 30 7 34 411 3 1

5 47 36 23 8 1 9 54 27

6 23 42 1 9 37C1 1 7 6 29 42 43 1-tMg 1 2 4 30 45 7 33 48 3 1

5 24 3(1 35 II I II 54 27

6 39 42 32 41 1Ca 20 7 3 1 48 39

7 25 48 24 8 26 54 34

R 6 54 20 9 1 2 60 3()

INDIAN J PURE & APPL PHYS, VOL 39, JANUARY-FEBRUARY 200 1

was real i sed at a current of 1 80 Amp (Fig. 3) . The field un iformity of (0. I %) was also found to be as per speci ficat ions I/). In FOTIA, negati ve ions extractecl from the SNICS-Il source are pre­accelerated up to 1 50 keY. They are then i njected into the low energy accelerat ing tube through the 70"-magnet and 20"-e lectrostatic deflector. The electrostatic triplet and the e inze l lens focus and match the beam parameters to the acceptance of the low energy tube.

Fi&! . :2 --- lo l l sou rce asscll1 h l y with ext rac!or and ci nzel

Tahie' :2 - Anal yscd he. l n l CU ITC l l tS I'ro ll1 l 1l': i o n sourcc w i t h d i llncnt c'll hodc samples

Req u i rcd Sarn p i c Current Currcnt h;:;Ull lIsed i!:!' I-C i ( p A ) (�) FC2 ( p A l ' I I ri l l :25 4 5 " L i L i F ''V' 0 3 " C Gr;lr i l i t l' 3( 1 5 .0 i I'C)' Fc,O, 3� 24 ' )o.: { , . .) 1 S i ( Nat u ra l ) ::! 5 1 3 " ( ' 1 N;I( , I .� ) I I

The beam l i ne wa.� made operat ional with appropriate power suppl ies and cool ing

arrangements. An u ltra h i gh vacuum of I x l O x Torr i s ach ieved" , up to the entry of the low energy accelerat ing tube, using a turbomolecular and ion pumps.

c.J � "0 -(I)

� u . ", ---' (I) C OD co

:2:

� <.'J � "0 v r""

u ...., :v � tll) ro

?

1 6 1 4 [a ] ---

1 2 1 0

(l -

6 4 - / 2 // 1 0

0

1 2

1 0 -

H

0

r 4

') '-l ,'i

, , I ,

[b]

I , , I 50 1 00 1 50

Current (Amp) I I , ,

r----.-._� .... �\ / \ I \

\ J \ I / \

, I

/ C Ul'J'el l l. = 1 00 \ A \ , / \

I , , •• _1_1-__ _ L. I 1 _ 1 _ _ .--& I -· 1 U C, ( ) ;'i 1 0

200

1 -j

� i :I I i �

. 1 . , I I !)

C r o s s s e c t i o n a l p o s i t i o n ( e rn )

Fig. 3 - Magnetic !ield and i ts d istr ihu t i on I'm t ile 7 ( )" magnet

Severa l beams were extracted and then analysed using the 70"-magnet. An a lysed bealll c u rrents of several microamperes were measu red (Table 2) Oil

the Faraday cup located after the 70"-tnagnet and t rans ported to the beam profi le moni tor ( R PM) l ocated at the entry of the l ow energy acce lera t i n g

lube. In these meas u re llle nts , t he cathOlk vo l t age was fi xed at 2 kY . S i nce the iun beams are ext racted

us ing pos i t ive voltage, the c u rrent at Faraday cup

(FC I ) , mounted be fore the 7( )"- magnet, conta i n s electrons and other negat i v e i on s produced frolll the

corresponding samples used at the ion �nurce . A typical mass spectrum obtained w i t h Fe l), cathode sample is �hown in Fig. 4 . For 1 00 ke V pre­accelerated beam, the transmission through the 7 ( )" magnet was found to be more than <)O%> . The contro l l i ng and moni tori ng of the vari ous

parameters of the ion source located at the h i gh

SINGH: FOLDED TANDEM ION ACCELERATOR 7

1 0 2

1 0 1

....---.. < :::i.

1 0 ° -------+-->

� Q> h h :=1 1 0 -- 1 c..>

S ,

Cd ::c: (1)

1 0 - 2 CO

1 0 - 3

I C)

c:.o

, ::x:: C> � !::: N c::>

:g <U

""""

C a t h o d e s a m p l e F e 2 0 3

o 2 5 5 0 M a g n e t

7 5 1 0 0 c u r r e n t

1 2 5 ( Am p )

1 5 0 1 7 5

Fig. 4 - Mass spectrum for Fe20, cathode sample

voltage, was done using the fiber optic data telemetry system'2 developed for FOTIA. The performance of the fiber-optic data telemetry system and the local console for the i njection system, has been satisfactory .

The low energy accelerating tube consisting of 6 modules has been instal led . Each module was leak tested using a he l ium leak detector and al igned with laser beam to an accuracy of less than 250 micron . Design, fabrication and instal l ation of chambers for foi l and gas strippers (60 cm long) was completed. They were tested for a vacuum of 2 x 1 0'x Torr before instal l ing i n the system. The foi l stripper was procured from Mis NEe and 1 20 foi ls could be mounted on i t .

The 1 80"-magnet 1 3 (ME/q2 = 1 0, R=30.5 cm) has been tested for its fie ld uniformity which was found to be ± 0. 1 5 %. A magnetic fie ld of 1 0.2 kG was measured at 1 00 A(Fig.5) . The l 00A, 30V power supply presently used for the 1 80" bending magnet, mounted inside the terminal , was modified to suit the power frequency of 330 Hz avai lable from the 5 kY A alternator i nsta l led inside the FOTIA termina l . This invol ved modifications in the i nput l ine fi l ters and power factor correction circuits.

12

1 0 / (.!) .!:<: "C 8 -Q)

.... Q 6

:;J Q) t:: t>D 4

/ ctl � 2

0 0 20 40 60 80 100 120 Current (Amp)

Fig. 5 - Magnetic field versus current for the I RO° magnet

The two coi l s of the magnet made of 75 turns each, are wound from copper stri p conductors of size 0.78 mm x 5 1 m m (Table 3 ) . The c lose loop cool ing system, for the ma!!net, consist incr of cool ino-� b b jackets, a reservoir (jacketed dome), a circu lating pump, valves etc, was i nsta l led and tested in the

INDIAN J PURE & APPL PHYS, VOL 39, JANUARY-FEBRUARY 200 1

Tahle :1 - Parameters of the magnets

Parameter 70il I XOO 90il

( A) Core deta i. l s A i r gap(mm) 40 1 5 40

Gap width ( m m ) 1 1 0 42 1 1 0 Bendi ng radius( m m ) 400 305 750 Gap field( KG) 1 4 1 4 1 4 Fielcl uniformity(%) 0. 1 0 0. 1 5 0. 1 5 Pole geometry A N A C NOR M A L A N A C

( B ) Coi l deta i l s

M aterial Copper Copper Copper Conductor Stri p Stri p Hol low copper

tuhes S ize(mm x m m ) . O.X()x75 O.7Xx50 1 2x l 2 No. or coi l s 2 2 2 No. or turns per coi l I SO 75 56 Resistance per coi I X5 Ill!l 75 mQ 40 mQ Power suppl y current 200 amp 1 20 amp 500 amp I n ter-turn i nsulation M yl ar Nomex Fiber glass Max. coi l temp 70" 50" 70"

actual operating condit ion . However, it was ohserved that the temperature of the coi ls ri ses to ahove IOO"C with the exist ing water cool ing system. This problem was studied l� , in detai l , in col laboration with RSD. In order to bri ng down the temperature of the magnet coi l s to an acceptable val l i e of about 5()"C it w i l l be necessary to blow about 1000 l itre/min of SF(, inside the magnet coi ls and to i ts surrounding areas. An addit ional l ine has heen erected through the centre of the column section for this purpose. Appropriate modifications for this purpose are being made in the exist ing gas hand l ing system. To monitor the temperature of thl:: 1 80"-magnet coi l s and the surrounding areas, inside Ihe terminal where control e lectronics is mounted , temperature sensors us ing IC LM335 were designed, fabricated and insta l led . The sensor c ircuit , used conti nuously, is a �round referred thermometer and gi yes vol tage output of 1 0 m V /nc. With these sensors. t he temperatures can be moni tored remote ly from the main control console. The work regard ing 1 80°-magnet i s done in coll aboration with APPD, BARC. An electrostatic (]uadrupole doublet (aperture = 40 mm) was des igned, fabricated and instal led in the beam l i ne. I t uses four ± 1 5 kV power suppl ies which are also mounted inside the termi na l .

3 High Energy Beam Line

In FOTIA, the beams accelerated 111 the h i gh energy tube are focussed using a magnet ic quadrupole triplet (MQT). For th is purpose, a quadrupole triplet has been designed, fabricated and i nsta l led . For shift ing the beam ax is, if required, magnetic steerers have been fabricated and installed in the l i ne . The high energy beam i s analysed by the 90"-d ipole magnet designed for a magnetic field of 1 4 kG and ME/q2 = 50 with a radius of curvature of 75 cm (Table 3) . The fabricat ion of the magnet was done i n the CWS, BARC. The coi ls for th is magnet were made at CAT, Indore. Each of the two coi l s. made of 56 turns of 1 2 mm x 1 2 mm hol low (hole d ia. 8 mm) copper conductor has a resistance of 40 mQ and can carry a max imum current of 500 Amp. The magnet has been insta l led after i ts cal ibration.

Fig . 6 - High voltage co lumn sect ion of the FOT I A

4 High Voltage Column Section

The h igh voltage column sect ion I.' consists of s ix modu les (Fig . 6); each designed for one mi l l ion volt and separated by th in stai n less steel plates. These plates were fabricated in the Central Workshops

SINGH: FOLDED TANDEM ION ACCELERATOR

with stringent specifications. Assembly of the h igh voltage column section with ceramic insulators was completed . Each modu le has 4 ceramic insulat ing posts which can withstanrl h igh compress ive stresses and have exce l lent characteristics from h igh voltage point of view. The insulat ing posts are ceramic to metal bonded with 1 8 corona gaps which are connected by equipotential hoops. To ensure that each i nsulat ing column post was under compressive stress a screw jack assembly was designed, tested and instal led. The screw jack assembly consists of a conical seat, spherical washer, round nut and a bolt (Fig. 7) .

INSULATOR SUPPrlRT

TVPICoAl f'LoATE

F"ELXJBLE PLAT[

SEAT

Fig. 7 - Screw jnck assembly for insu l at ing columns

Before erecting the high voltage column section with actual ceramic column posts, a number of tests were carried out. Stress in i ndi vidual column posts, coupl ing among the column members and v ibration tests were carried out on column assembly of 2 modules consist ing of 8 dummy column posts made of mi ld stee l . Strai n gauges were instal led on al l the eight column posts and strains were measured for different loads. On an average, column posts show smal l tens i le stress. Coupl ing between two column posts was found to be al most negl igible. A systematic study of bending load was also carried out. These studies were carried out in col laboration with APPD, CWS, RSD and RED.

The electrical power, required in the terminal , for 1 80°-magnet, ion pump, foi l and gas strippers, quadrupole doublet. Faniday cup and other electronic components. is generated by the 5 KV A, 3 phase, 440 volts al ternator (brush less, permanent magnet type) . The alternator, connected by a long perspex shaft, is dri ven by a 1 0 HP motor rotating at

1 450 rpm. Desi gn of the motor mounting assembly was modified to flange type in order to reduce the vibration levels . The motor is mounted, vertical ly, on the bottom plate of the column section. It was also observed that long rotat ing perspex shaft (75 mm d ia. and more than 1 500 mm long), used in the beginn ing, lead to vibrations which were unacceptable. Again the design of the shah assembly was modified and the shafts of approximately 99cm long were fabricated and machined to an accuracy of 1 0011. The natural frequency of the shaft assembly was found to be far off from the rotational frequency of the shaft (25 cycles/sec) . The v ibration measurements were made with s ix modules constructed using actual ceramic column posts with two separator plates combined together to form a smal l dead section. This dead section accommodates a special l y designed flexible coupler. The max i mum vibration ampl i tude ' " with both perspex shaft and pel let chain running was found to be less than 5011 (Fig. 8 ) . Based on the experience at 1 4 UD B ARC-TIFR pel letron at Mumbai , it was decided to accept these vibration levels and proceed with the instal l ation ancl commission ing of the accelerator. The velocity and acceleration were also found to be within the safe l i mi t (Table 4).

/ 1\ I

6 I \ 5

, i I i i I i

I i I i

6

5 4

tt- 0 Z

I ! 3 '" 3 i[

r--' f " ! � I

I i I i 2 ., r-' i I i I

L li I

o } o

i ci i z

I ! w J ,� >-

� ll"ll1 :5 0..

Fig. II - V i bration amp l i tudes across the column st l'uctU I't'

1 0 INDIAN J PURE & APPL PHYS, VOL 39, JANUARY-FEBRUARY 200 1

Tahle 4 - V ibration levels in the FOTI A column sect ion Platc No. Bottom # 1 #2

Ampl i tude in !l m 23 25 1 9

Velociry( cm/sec ) O I l) o. I 8 0. 1 4

Acceleration (cm/sec2) 0.25 0. 1 0 0.32

In FOTIA, pel let chain charging system, made of metal l ic pel lets and nylon l inks, is used for generating the voltage on the terminal . The charging takes place by induction method. Metall i c pel l ets pass through a lumin ium inductors, which are appl ied negative h igh vol tage. The inductors i nduce posit ive charge on the metal pel lets. When these pel lets enter the termi nal , the charge of the pe l lets is transferred to the terminal and gradual ly terminal attai ns h igh vol tage. On i ts way back pel lets are negat ively charged thus double-charging takes place.

The pel let chain i s dri ven by a 7.5 HP, 600 rpm motor which is mounted on a see-saw mechanism. The tension on the chain can be adjusted by varying the counterweight. Normal ly , 80 pounds of tension is required to transfer the fu l l charging current at rated vol tage . The charging chain was tested in open air. I t was observed that up going charge and the down coming charge were not equal . After carefu l testing it was found that the loss of charge was taking place at the idler pul leys and it was required to min imise transverse osc i l lation in the chain . By adj usting the d iameter of idler wheel th is problem was rectified . The entire system has been instal led and tested .

5 High Voltage Tests

The h igh voltage system of FOTIA consists of different components l ike chargi ng chain mechanism, h igh vol tage measurement and control system. ]n the begi nn ing, i t was noticed that part of the chargi ng current was lost at the gu iding pul ley arrangement in the fi rst dead section. The problem was sol ved by adj ust ing the gaps between guiding pu l leys and the moving pel let chain . The h igh vol tage measurement system of FOTIA uses a generating voltmeter (GYM) mounted on the inside surface of the tank, in front of the h igh voltage terminal . The GVM was cali brated by mounting at its actual posi tion inside the tank . The high voltage control system uses a corona probe mounted ins ide the tank opposi te to the GYM. This corona probe

#3 #4 #5 #6

25 1 8 20 1 6

0. 1 8 0. 1 1 O.<J7 0. 1 1

0. 1 3 0.08 0.02 0.03

can be moved back and forth . The corona probe was tested for pressure and vacuu m tightness and i ts free mechanical movement over a d istance of about 40 cm.

All the components associated with the h igh voltage generation, measurement and control were tested i nd ivi dual l y i n air. The gap between the charging inductor and chain pel l et is around 6 mm

. and we cou ld ach ieve only 1 2 !lA of charging current as voltage of the charging i nductor could not be raised beyond - I OkY in air. After this , h igh voltage tests were carried out us ing N2+C02 mi xture as an i nsul at ing gas. At a tank pressure of 98 psig, the i nductor voltage cou ld be raised to -25 kV which gave charging current of 35 !lA. With these parameters, a sustained voltage of 3.4 MY was ach ieved l7 • At th is terminal voltage, column and probe currents were 35 !lA and 5 !lA respecti vely .

6 SF (i Gas Handling System

In the old 5 . 5 MV Van-de-Graaff accelerator, a mixture of 80% ni trogen and 20% Carbon dioxide was used as insulat ing gas, at a pressure of 225 psig. However, i n FOTIA, the insu lat ing gas w i l l be SF(" which has a dielectric strength of approxi mately 2 .2 t imes that of the N2+C02 mixture at 98 psig. S ince hydrocarbon free environment is required i nside the accelerator tank, it is necessary to use o i l free equipments. The new gas hand l i ng system(" which cons ists of an oil free compressor, a centrifugal blower, a heat exchanger, dust fi l ters, dryers and a vacuum pump etc, was designed, instal l ed and commissioned in col laboration with RSMD. The gas hand l ing system, is used mai n ly : a) To transfer SE, gas from storage tank to accelerator tank and vice versa. b) To evacuate the accelerator tank to a pressure -0.5 Torr. Thi s maintains purity of gas hy mini mizing the contamination of SF(, by residual air. c) To remove moisture and breakdown products by recircu lat ing gas in a close loop system contai n ing acti vated alumina dryer, heat exchanger, blower and fi l ters, d) To mai ntain the temperature, inside the accelerator tank, to its designed val ue. During the

SINGH: FOLDED TANDEM ION ACCELERATOR I I

testing of the column section it was found necessary to incorporate an addit ional dust fi lter at the entry of the accelerator tank . A fi l ter assembly, capable of stopping dust particles rcyond 0. 1 � s ize, has been instal led in the gas l ine. The layout of the equipments in the gas hand l ing system i s shown Fig. 9 .

7 Vacuum System

In FOTIA. i on are injected and then accelerated

through the accelerat ing tubes and final ly transported to the experimental beam l i ne (Fig. 1 0) covering a long path of about 35 m. S ince the charge exchange cross sections, for heavy ions, are very large i t i s necessary to min imize the residual gas pressure and maintain u ltra h igh vacuum in the accelerator tubes and rest of the beam trans pori system of such accelerators. This is also required to reduce the loss of intensity and spread in energ:- of the ion beams. A l so, the accelerating tubes are subjected to very h igh voltage gradient, 20.4 kV/cm.

!KIND :

i8l- IWl. v.u.n: iXI- GLOB VAtn

�'ftm -Y1- BtlTTDrLT v.u.n

! UHJ: -t>K1- RDDU: VAtn: "

Ii I I I I OIl!nCl � a.ann IIa.DP v.u.n:

i 0 I'IIII:SSUU caUG!

0) TIIIDIOCOUPa i! P-1.P-2 D08T nLTm

'-3 OIL rD.'ID D-1'])-1 DRYD K IlUftR

Fig. I) -- Layollt o r the SF" ga� hand l i ng system

Fig. I () - Experi mental heam l i ne i ncluding ,catlering chamher

1 2 INDIAN J PURE & APPL PHYS, VOL 39, JANUARY-FEBRUARY 20() 1

which requires a hydrocarbon free and clean vacuum for smooth operation of the accelerator. The FOTIA, vacuu m system l l comprises of about 35 m long, 1 00mm diameter beam l i nes incl uding various diagnostic devices, two accelerating tubes, and three narrow vacuu m chambers. The cross sections of the vacuum chambers are 14 mm x 24 mm for the 1 80" magnet and 38 mm x 60 mm for' the 70"- & 90"­bending magnets. A l l the beam l i ne components are UHV compatible, fabricated from stain less steel 304L grade material fitted with metal gaskets. This enables degassing of the beam l i nes at e levated temperature.

A distributed pumping system having seven pumping stations has been des igned to maintain UHV for the total volume of about 3 x 1 05 cm' and surface area of 1 .5 x I 05 cm� for the whole accelerator inc luding an experimental beam l i ne and a 80 cm d iameter scattering chamber. The spacing among the pumps is chosen such that the p ipe conductance equals the speed of the . individual pumping stations, which gives un iform pumping throughout the beam transport system. The type of the pumps i nstal led in a particu lar section is based on the gas load i n that section. The ion source sect ion has a large gas load, which increases substantial l y whenever samples are changed in the ion source. A turbo-molecular pump, with the speed of 1 600 l itres/sec, maintains ultra h igh vacuum in this region. The other sections, which are thoroughly degassed and have very sma l l degassing loads from metal surfaces, are pumped by a combination of ti tan ium subl i mation and sputter ion pumps or only by sputter ion pumps. The FOTIA vacuum system has been div ided into seven sections. The deta i ls of the individual sections with mai n components, vacuum and surface areas, gas loads and the capacity of the pumps lIsed, are gi ven in Table 5 .

Control System and Electronics

For contro l l i ng and mon i toring the parameters of the FOTIA a PC based control system has been designed, fabricated and installed 12 . This system has been desi gned as a network of PCs with a front-end interface using CAMAC instrumentation.

There are large number of devices ( l i ke extractor, e inzel lens, cathode, fi lament etc .) located inside the h igh vol tage ion source deck which is raised to -ISO kV. The control and moni toring of

these devices is done from ground leve l . A ti me mult iplexed optical fiber, for transmitting and receiving signals, has been designed, fabricated and instal led . The special feature of the system is that it has only one fiber for sending the signals and another one for recei ving al l the channe l s as compared to mu ltiple fiber system which uses one fiber for each channel .

For control l i ng the devices (foi l stripper, ion pump, Faraday cup, fold ing magnet and temperature sensors etc .) located ins ide the 6 MV terminal , a fiber optic system was obtained from NEC. This has 2 1 channels for contro l l ing and moni toring above devices. These devices were i nterfaced with the fiber optic system. The h igh vol tage terminal of FOTIA has a very l i mited space for instal l i ng the above mentioned devices and they also require EMIIRFI shie lding from the transients, an appropriate double shielding box was fabricated and instal led ins ide the terminal .

The control lers, for interfacing e lectrostatic and magnetic steerers, quadrupole lenses, ±SO kV charging power suppl ies, Faraday cups and the 70 kV einzel lens power supply, were designed, fabricated and instal led . In order to control and mon itor the magnet power supplies thi'ough CAMAC, Electronics Di vision has added software to FOTIA Control and Information System (FOTIA­CIS) developed by them. The injection and analys ing magnet power supplies (developed by TPPED) have been i nterfaced to PC via CAMAC RS-232 module . This a l lows precise sett ing of the magnet current. A dedicated window, through existing FOTIA-CIS, permits the operator to control and moni tor the magnet currents.

In accelerators, due to h igh voltage breakdowns, large amount of transients are generated which are harmfu l for a l l electronics devices. To minimize the effect of these trans ients to an acceptable l i mit spark protection boards were desi gned, fabricated and instal led in the input channels of CAMAC system.

In order to have stable ion beam through FOTIA, the terminal voltage requires good stab i l i zation. Generat ing voltmeter and control s l i t s ignals are u sed for control l ing the terminal voltage. The corona stab i l ization system provides regulated discharge from the terminal through corona drain system to ground. A proper circuit us ing 402 1 vacuum tube was designed for stabi l iz ing the

SINGH: FOLDED TANDEM ION ACCELERATOR

Table 5 - Detai l s of the pumping systems Section Name of the section · M ain components Yolume(cm·�) & Gas load Pumping No. I Ion source Ion source. Ace. Tube.

E.S. Steerer

2 I njection l ine BPM . FC, 70" M agnet chamber, 20" Detlector. E.S. Steerer

3 Low energy Einzel lens, Ace. Tube. accelerating tube E.S. Steerer, valve

4 Terminal Stripper and 1 80" Magnet Chambers, Pumping unit

5 H igh energy Ace. Tube, Mag. Quad. Triplet, accelerating tube Pumping unit

6 Analysing magnet 90" Magnet chamber, BPM, FC. Sl its, Pu mping unit

7 Experimental beam Scatt. Chamber, S/W Magnet. l ine Mag. Steerer. MQT.

terminal voltage using GVM output as a feedback signal for the corona control system. This unit was fabricated in house and has been instal led .

Remote ON/OFF control of the charging chain motor, mounted inside the acceler<ltor tank, was done from the main control console. In order to monitor the parameters such as chain current, column current, GVM picku'p etc a local control unit was fabricated ( with a signal-conditioning card integrated inside the unit. Provision was also made for remote monitoring of these parameters through CAMAC system.

Digital panel meters for measurement of the high voltage, terminal temperature, tank pressure, chi l led water temperature etc were instal led and integrated to the mai n control console.

The fol lowing features have also been added recently to the FOTIA control system:

• A separate dedicated window is provided for magnetic operation invoked from FOTIA CIS window.

• Graphical meter is provided for current read back.

• Edit box is provided for current control with a fac i l ity to change the step size.

surface area(cm2) . Ctorr- l i t/sec) speed ( l i t/sec ) 477 1 3 1 .44 x 1 0'; · 1 600

1 0822 5 1 6 1 6 6.3 x 1 0-1> H2O

1 9287 22407 1 .08 x 1 0-1> 1 20

1 7 1 94 9628 9.34 x 1 0-7 1 20

4473 3 1 455 7.5 x I O-r. ' 1 20

208 1 2 40834 1 .0 x I O-{' I (AO

1 8393 · 1 06306 9.5 x lO·r. ; 3000

50 1 98

• System can be configured Via a database . window.

• Different magnetic systems can be selected . .

• Window provides Faraday Cup selection and contro l .

• Incorporates the feature of interlock checking and displays the message if any interlock fai lure during the operation :

• Dragging of s l ider during control session for parameter now can be selectively disabled from database to prevent sudden change of any parameter value to be control led .

• Upper l imit can be set for the s l ider dragging to l imit the range of a parameter to be control led .

• Continuous operation for increment/decrement buttons

The entire control system was used extensively during high voltage tests and beam trials and it has worked satisfactori ly .

9 Present Status

The column structure of the FOTIA has been tested for high voltage with N2+C02 mixture fi l led in the accelerator tank at 98 psig. A voltage of 3 .4

1 4 INDIAN J PURE & APPL PHYS, VOL 39, JANUARY-FEBRUARY 200 1

MV was achieved on the terminal 17. The terminal voltage is expected to exceed 6 MV with SF(" as the electric breakdown strength of SF(, insulating gas, at 98 psig, is about 2.2 times that of the N2+C02 mixture. In order to test the functioning of the integrated assembly, the beam trials were started

6

7

(PATPAA-96).Calcutta. 1 996.

Gupta S K, Patkar R V, S ingh P, Shallom E, Agarwal A. Santra S , Rajesh Kumar,Sarode S P. RaUL P J , Jain B K. Gogte S V . Patankar R R, Andhansare M G. Tikku A C & Khadi lkar M G, Proc DA E Symp on Nucl Phys B, 42 ( 1 999) 438.

8 with nitrogen and carbon dioxide mixture as the insulating' gas and the accelerated oxygen beam (,604+) of about 1 2 nA, at terminal voltage of 1 .8 ' 9 MV, has been obtained . To increase the terminal voltage further, where transmission is expected to improve substantially, the accelerator tank is being charged with SFr, gas.

Singh P, Proc DAE Symp on Nucl Phys A . 39 ( 1 996) 1 1 3 .

S ingh P, Workshop on Heavy Ion Accelerators (TIFR, Mumbai), Dec. 29-30, 1 998.

Santra S, Gupta A K, Rajesh Kumar, Gupta S K. Kansara M J , Mittal H R, Shallom E, Agarwal A. Vaze M Y. Bhagwat P V, Patkar R V, Sridevi N. Sarode S P, RaUl P J . Fernandes N, Ojha S C. Singh P & Ja in B K, Proc Nation.al Symp on Vacuum Science & Tech and Power Beams (lVNSNS-97), (BARC, Mumbai). 1 997.

Acknowledgement

We thank Dr S S Kapoor and Dr B K Jain for their encouragement and continuous support to the project. We also thank Dr Anil Kakodkar for his valuable suggestions regarding cooling of the folding magnet and keen interest in the project. Our sincere thanks are due to all members of the FOTIA working group and several Divisions of BARC, for their excellent cooperation and contributions to the project.

References

I Singh P, Proc Inl Conf on Vacuum Science & Technology & SRS Vacuum Systems, (CAT, Indore), Vol . I , 1 995, p. 1 47- 1 54; Singh P et at., Proc Int NucL- Phys Symp (INPS-95), (BARC, Mumbai) , 1 995, p. 13; Singh P, 4th National Symp on Phys & Tech of Part Accelerators & their Applications (PATPAA-96),Calcutta, 1 996.

2 Singh P, Indian J Pure & Appl Phy:;, 35 ( 1 997) 1 72; Singh P, IPA Seminar on Current Perspectives in Nuclear Physics Research & Accelerators, (BARC, Mumbai), 1 998.

3 Hattangadi V A. Singh p, Thakur N M , Kerekatte S S, Surendran P & Eswaran M A. Proc 3rd National Symp on Phys & Tech of Part Ace & their Applications (PATPAA-93), Calcutta, 1 993, p. 76; Sanlra S & Singh P, Proc DAE Symp on Nucl Phys B, 42, ( 1 999) 404 .

4 Singh P, Desai S V, Sanlra S & Kerekatte S S, Proc lilt Nucl Phy:; Symp (INPS-95), (BARC, Mumbai). 1 995, p. 14: Desai S V, M ajumder R & Ron P H , Proc 3'" National Symp on Phys & Tech of Part Ace & their Applications (PATPAA-93), Calcutta, 1 993, p. 82.

5 Santra S, Singh P & Gupta S K, 4'11 National Symp on Phy:; & Tech of Part Accelerators & their Applications

1 0 Sanlra S, Singh P. M ittal H R. Shal lom E, Kansanl M J , Raut P J , Patkar R V , Sarode S P.Fernandes N. Rajesh Kumar, Vaze M Y, Gupta S K, Agarwal A & Jain B K. Proc DAE Symp on Nucl Phys, (India). B40 ( 1 997) 472 .

I I Gupta S K , Santra S, Raut P J. Sarode S P, Patkar R V . Mittal H R. Shallom E, Rajesh Kumar & Singh P. Proc National Symp on Vac Science & Tech and Power Beams (lVNSNS-97), (BARC, Mumbai). 1 997: Gupta S K. Santra S, Agarwal A, Rajesh Kumar. Shallom E. Sarode S P. Patkar R V. Sparrow H. Ekambaram M, Raut P J. Singh P & Jain B K. Proc National Symp on Vacuum Science & Technology UVSNS-99), (CAT, I ndore). 1 999.

1 2 Kansara M J , Vaze M Y, Sridevi N & Singh P. Indian J Pure & Appl Phys, 35 ( 1 997) 2 1 2; Kansara M J , Sridevi N, Vaze M Y & Singh P, 4th National Symp on Phy.I' & Tech of Part Accelerator & their Applications (PATPAA -96), Calcutta, 1 996,

1 3 S ingh P; Thakur N M & Hattangadi V A. Proc 3rd National Symp on Phys & Tech of Pari Accelerators & their Applications (PATPAA-93), Calcutta. 1 993. p. 354 .

1 4 Bhasin V, Vaze K K & Kakodkar A. (private communication ).

1 5 Bhagwat P V . Vaze M Y. Gupta S K. Agarwal A. Rajesh Kumar, Kerekatte S S. Singh P. Majumder R. Mukesh Kumar & Desai S V, Proc Int Nucl Phys Symp, ( INPS-95). (BARC, Mumbai), 1 995, p. 124.

1 6 Bhagwat P V, Suthar R L, Singh P. Majumder R, Ojha S C. Agarwal A, Rajesh Ku mar, Rama Rao A. Jose P G. Jain B K, Ron P H & Kapoor S S. Proc DAE Symp on NlIcl Phys B, 42 ( 1 999) 448.

1 7 Singh P, Bhagwat P V , Gupta S K, Vaze M Y, Agarwal A. Rajesh Kumar, Santra S, Kansara M J, Shal lom E, Patkar R V. Sapna P, Sarode S P, Subrahmanyam N B V , RaUL P J. Ojha S C. Jain B K & Kapoor S S. Proc DAE SYIIl{J on Nucl Phys B. 42 ( 1 999) 366.