I ndian Journal of Pure & Applied Physics

Vol . 39. January-February 2001 . pp. 8 1 -83

New shorting mechanism for pelletron accelerator

Q N Ansari, C D 'Costa*, S K Sarkar*, P V Bhagw!lt

Bhabha Atomic Research Centre. Trombay. Mumbai 400 085

"Tat a I nstitute of Fundamental Research. Homi B habha Road. Mumbai 400 085

Received 7 November. 2000

A new mechanism to electrica l ly short Pel letron Accelerator modules has been developed to replace the present manual ly operated system that contains some l i mitations. Its salient features over the existing system are that the operation wi l l be fu l l y automatic. any module can be independently shorted from control console and its status read back wi l l be avai lable on the control console. Since the new system is not external to the main Accelerator Tank. chance of S F" leakage is el iminated.

1 Introduction

The 1 4UO Pel letron Accelerator is an electrostatic tandem accelerator. The potential grad ient or the electrostatic fie ld is establ ished by al lowing current to pass through corona discharge or resistance path . When lower terminal voltage is needed, i t i s general ly customary to electrical ly short-c ircuit a few modules of the accelerator column . It is also some t imes necessary to short­circuit a set of modu les for isolating them for diagnostic purpose or for systematic conditioning of the accelerator for withstanding h igher voltages. Thus, module short-ci rcuiting mechanism IS an important part of these accelerators.

2 Problem Definition

The exist ing system of short-circuiting modules of this accelerator is achieved by means of a string of rods. The high vol tage column i s housed in a large pressure vessel which is ti l l ed with SF" gas at 5-6 atmospheric pressure. A motorized driver from outside the tank drives the string of steel rod through a smal l port. The string is general ly formed with nylon and steel rods in a particular combination depending on thc position of the column to be shorted .

The maj or problem encountered with this systcm is that some l imes the string breaks inside the tank. In such situations, the operation of the accelerator hecomes impossible . The entire SF" gas inside the accelerator tank is required to be transferrcd to the external reservoirs . It amounts to

i ncreased down t ime of the accelerator. The fai lure statistics of th is shorti ng rod system was examined and it is found that quite a large break-down t ime of accelerator can be attributed to the fai lure/breakage of this shorting system alone. Over and above, the system of rods and drivers, etc . needs careful handl ing to avoid compl ications and it requires a

large maintenance time.

3 The New Mechanism

The design of new mechanism consists of one­eighth inch braided copper wire used for shorting: the one module, a nylon string coupled with the copper wire via a steel coupler, a spri ng arrangement for posit ive contact and two micro­switches to know the status. As shown in Fig. I , the nylon and copper wires are wound on threaded pu l leys of 70 mm diameter, which wi l l he housed inside the casting at either end of the module. The pul ley wi l l be rotated by stepper motor. The braided copper wire w i l l be inside the casting when moduk is l ive, i .e . only the ny lon string wi l l be exposed to the h igh voltage. When module is shorted nylon wire wil l go inside the cast ing and braided copper wire wi l l short the module . S pecial care has been taken from high voltage design poi nt of view whi le selecting and fabricating the components of this ncw mechanism. A scheme of shorting fur second module is shown in Fig. 2. 4 Stepper Motor Drive and Controller

The control ler for the stepper motor I S

developed around a micro-control ler for rel iable operation. Tht: whole system can be d i vided into

K2 INDIAN J PURE & APPL PHYS, VOL 39, JANUARY-FEBRUARY 200 1

Tahle I - Comrarat ive advantages of the new system

I .

2 .

4 .

6 .

Proposed silort i n !! mechanism

The oreratioll i s fu l l y automatic.

Any module or mod u les can i ndependcntly he shorted from control console i t sel f.

S tallis read-hack of the module w i l l he avai l ahle on the cont rol console mon i tor.

System is not extei'nal 1 0 t he main tank . hence no chance of S ri, gas le'l k .

Easy to short a n y mod u l e as each l1lodule h a s i ts own system for short ing .

No expert i se i s nceded to short any module .

two parts, viz. Control Electronics and Stepper Motor Dri ver.

8· __

Idler Pullcy Spring arrangement For positive contact

.. ---.----�

Braided Correr Wire ' �

Micro-switch

Self fixing r-�-"F;r-'Ro., Arrangement

Nylon Thread

-- Micro-switch

Motor Pulley -------- - - - - - - - - -----

Fig. I - S hort i ng arrangement for one module

I .

2.

3 .

4.

5 .

6.

Exist in!! short i n !! mechanism

The oreration is manual .

Comhinations of stecl and nylon rods are req u i rcd t o short a part icu l ar module o r mod u les from tank tor and/or tank bottom.

No status read-hack i s avai lahle on the control console. A separate loghook has 10 he maintaincd.

System is external to t he main wnk. I n case o f fai lure. t here i s a chance o f S F(, gas leakage. and rhysical i n jury.

Rods are i n serted sequent ia l ly . so changing conligurat ipn is not easy.

Experience and exrertise is necessary. part icu larly to handle Nylon rods.

In Control Electronics, Intel -89C5 1 micro­control ler has been used . This micro-control ler has 4 kbyte of program memory, 256 byte of data memory and 4 I/O ports. The micro-contro l ler lIses a 1 1 .059 MHz clock for its operation . The bus port

Idter Pulley -

Casting Opening

Braided Copper \Vire

Spring arrangement ft'or positive contact

.Micro-switch

'Setf lixi l lg Arr::1I1getllt:ll!

(iuiding tnbc

Micro-switch

-- Motor Pulley

Fig. 2 - Shorting arrangement for second !nodule

ANSARI et (II. : S HORTING M ECHANISM IN PELLETRON ACCELER ATOR R3

PO is used for driv ing the stepper motor e lectronics. The port P2 is used for user input/user control signal . Rest of the ports is unused. The micro­control ler, upon receivins user input for forward or reverse rotation of the stepper motor, sends signals to driver electronics for the stepper motor.

The design of stepper motor driver electronics consists of power dri vers, which receive control s ignal patterns in appropriate sequences as decided by the micro-control ler.

Thus, the entire circuit has been designed in a compact manner and selection of components as such to improve the rel iabi l ity. The embedded software has been developed in low-level assembly language . However, no back-up power supply is provided for this system. Therefore, position sensing upon power fai lure and restoration is achieved by

means of two micro-switches.

5 Test Results

Comparati ve advantages of the new system are presented in Table I . The system has been tested thoroughly outside the Accelerator tank on the test bench . One such un it has been i nstal led inside the 281h module of the Accelerator, which is working satisfactori ly s ince January, 200n.

References

Alston. High v{}lt(l�e technology.

2 Livingston & Blewett. Particle accelerator.

3 In tel. 8-bit embedded colltroller halld/wok.

4 Douglas V . Hal l . Microprocessors alld intn/cwing. 5 Sax & Herb. Stepper mntor drit'ing.