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Development of distributive pumping Ambuj Tripathi. L Senapati and D K Avasthi, Nuclear Science Centre, PB No 70502, JNU Campus, New Delhi- I 10067, India

received for publication 29 March 1993

The development of distributive pumping was carried out with the aim of using it as a cheap and effective pumping system in the beam lines of the existing 15 UD Pelletron and the planned booster. Non-evaporable getter (NEG) strip St 707 was laid in a test beam line with a modified, simpler and more economical design as compared to that of earlier works. An ultimate vacuum of 4 x IO-” torr was achieved using the distributive pumping of NEG strip in addition to ion pump and titanium sublimation pump. The activation of NEG strip even at 30 A reduced the rate of rise of pressure in the power off period by a factor of 0.004 in the system pumped by an ion pump. The rate of pressure rise was further reduced by a factor of - 0.06, to a value of 7.2 x 70m9 torr h-‘, with the incorporation of a titanium sublimation pump and activation of NEG at 60 A. The partial pressure measurement of various gases was also carried out.

1. Introduction

The beam lines at NSC Pelletron ’ facility have the ion pump and titanium sublimation pump (TSP) as pumping devices, providing a vacuum of -5 x 1O-9 torr. We aim at better vacuum with less pressure gradient in the beam lines to be provided by distributive pumping along the beam line for the planned superconducting

LINAC facility. The objective of the present work, therefore, has been to develop an economic distributive pumping device to achieve (1) uniform and better ultimate pressure in the beam line, (2) to reduce the load on the main pumping station and (3) to maintain high vacuum during long power shut down periods by reducing the rate of rise of pressure.

Non-evaporable getter (NEG) strips2r3, possessing the prop- erty of pumping out the gases after activation by heating in vacuum, suit our requirement. The performance characteristics of the strip have been studied recently by Sang Ryul In et a14,

Benvenuti and Francis’ and Halama et af6. Mechanically, also, it suits the requirement as it can be laid inside the beam pipe, without hindering the path of accelerated ion beam. Out of the two materials of NEG strips, e.g. St 101 and St 707, the latter is chosen because of its activation at lower temperature. The NEG strips have been shown to be useful in accelerator beam lines by Ferrario et al 3 and Hseuh et al ‘. A simpler design requiring less insulator material as compared to earlier works’,’ is successfully attempted in this work. The characteristics of TSP have also been studied.

2. Experimental set-up

A 4 in. internal diameter stainless steel (SS) pipe of 1 m length is taken as a test beam line. It is connected to a cross fitted with a TSP, a starcell ion pump, a BA type ion gauge head and a residual gas analyser head. The test beam line is isolated from the cross by an all metal straight through valve. A schematic diagram of the test set-up is shown in Figure 1. Two 1.33 in. conflat flanges with necks are argon arc welded to the extreme ends of the beam pipe. Two high current feedthroughs are mounted on these

flanges to pass high current for heating the strip. Two machinable ceramic pieces are fabricated to accommodate the conductors of high current feedthroughs, a quartz rod and the NEG strip. Two annular rings of machinable ceramic are provided on the quartz rod, so that the strip does not touch the beam pipe even if it sags due to thermal expansion during its activation. It is estimated from the given linear expansion coefficient, that the increase in length of the strip is approximately 7 mm at the activation tem- perature. The design ensures that the strip does not touch the beam pipe in case of thermal expansion of a similar order. The arrangement for mounting the strip is shown in Figure 2. We feel that the present design is easier to implement as compared to that of Hseuh et aZ’ and it reduces the material involved and machining costs. A thermocouple feedthrough is mounted on the NEC flange of the beam pipe to monitor the temperature of the strip during its activation. A K-type thermocouple is tightly pressed between the strip and a SS annular ring for good contact. The strip is 32 mm away from the centre of the beam pipe and the conductor of feedthrough is 25 mm away from the centre of beam pipe, which is considered enough for the passage of an ion beam. The whole arrangement was helium leak tested using a mass spectrometer leak detector, before performing the tests.

NALYSER HEAD

ELECTRICAL POWER FEEDTHROUGH

ALL MET&L PNNMATK STRAIGHTTnRWCH “ACVE TARCELL ION PUMP

Figure 1. Schematic diagram of the experimental set-up.

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Ambuj Tripath et al: Distributive pumping

NEG St 707 STRIP

Figure 2. Mechanical arrangement for laying the NEG strip in the beam line.

3. Observations

The system was roughed with a rotary pump provided with a liquid nitrogen trap. The ion pump was switched on when press- ure was better than 1 x lo-’ torr after baking the whole set-up at 150°C for 24 h. The vacuum achieved after 23 h of pumping was 4 x lo-’ torr. It was observed that ultimate pressure remains the same in a long run of several days. At this stage the ion-pump was switched off to observe the rate of rise of pressure with time. This is plotted as shown in curve A of Figure 3. The large rate of rise could be due to a minor leak, or due to imperfect cleaning of the surface, as all the items used except the beam line were not electroetched. After plotting the rate of rise, the ion pump was switched on to reach the previous ultimate pressure of 4 x lo- * torr. The NEG strip was then activated, only with 30 A current in the beginning, for 75 min. This heating, which raised the temperature to about 210°C was then stopped and the pressure improved to 1.0 x lo-’ torr. The ion pump was again switched off. The pressure rise with time is shown as curve B of Figure 3.

The ion pump was switched on again. The TSP was then activated in auto mode in a cycle of on-time for 2 mm and optimized off-time of 15 min. Before the activation of NEG strip, it was observed that the TSP and ion pump combination give an ultimate pressure of 5 x lo- 9 torr, which matches with our experience on accelerator beam lines. The rate of rise, after switching off both the pumps, is shown as curve C of Figure 3.

The NEG strip was activated to its optimum temperature of about 480°C for nearly 70 min by passing 60 A current. When

lo8 I , ,,,, I ,,,,, I , ,,, 10 10’ lo2 103 104

T I mnutes,

Figure 3. Rate of rise of pressure in power off period for different cases ; (A) ion pump only ; (B) ion pump and NEG activation at 30 A ; (C) ion pump, titanium sublimation pump and NEG activation at 30 A ; (D) ion pump, titanium sublimation pump and NEG activation at 60 A.

the heating was stopped, the pressure started improving and went down to 8 x lO-‘O torr in 6 h. The pressure kept on decreasing gradually with the combined effect of ion pump and NEG pump- ing. The pressure as observed after two and half days was 5.6 x 10~” torr and 4.0 x lo--” torr after two weeks. The rate of pressure rise was 7.2 x lOmy torr h- ’ after switching off all the pumps. The pressure rose to only 7.0 x lo- 7 torr, even after four days. The rise of pressure vs time is plotted as curve D of Figure 3. When only the ion pump was switched on, pressures in the range of lo-” torr was attained within 10 min of operation, giving very small load to the ion pump.

The present study establishes the procedure to achieve a press- ure of - lo- lo torr by the use of distributive pumping of NEG strip along with TSP and ion pump. The rate of rise of pressure in the power off period is also reduced drastically and it has been shown that the ultimate pressure is restored within 10 min, even after a power off period of four days. Although the pressure at the other end of the beam line is not measured, it is logical to assume that the pressure at both the ends will be of the same order, due to the distributive pumping of NEG strip.

4. Partial pressure measurements

During the study, the residual gas spectra were taken with a

Hiden HAL quadrupole gas analyser. The changes in com-

Table 1. Partial pressure of different gases during the operation of (A) ion pump; (B) ion pump and titanium sublimation pump; (C) ion pump, titanium sublimation pump and NEG strip activated at 60 A

Partial pressure (torr)

Mass (amu)

2 12 16 18 28 44

(A) Ion 1x10~” 2x 10-g 3 x 10-v 6x 1O-9 3 x lo-’ 4x lo-’ pump only (B) Ion pump 3 x 1om9 2x lo-“’ 9x lo-‘” 1x10-’ 2 x lO_‘O 2x lo-” and TSP only (C) Ion pump, TSP and NEG 3 x lo- “1 3 x lo-” 7x lo-” 2 x lo- ‘(1 4x lo-” 7 x lo-” activated at 60 A

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Ambuj Tripathi et al: Distributive pumping

position of residual gases were recorded after switching on every pump. As the pumps were switched on in a sequence, the pumping effect of each pump for different gases could be observed. The ob- served partial pressures for different gases are shown in Table 1.

The TSP seemed to be more effective at masses 12,28 and 44. The NEG showed good pumping for masses 2, 16 and 18. TSP and NEG strip together seem to be an appropriate choice for a heavy ion accelerator like ours, as both of these reduce the partial pressure of the heavier masses quite effectively.

5. Conclusion

The distributive pumping with an optimum, economical and simple design using NEG strip St 707 is successfully carried out in a 1 m test beam line of 4 in. internal diameter. To conclude : for ultimate pressure in the present study for the test set-up, the Ion pump gives a pressure of about -4 x lo-* torr, which is Improved to - 5 x 10e9 torr by TSP. The NEG strip further improves the ultimate vacuum to 8 x lo-” torr after partial activation at 30 A and the vacuum is further improved to 4 x lo- lo torr after full activation of strip at 60 A. Encouraged

by these results, we now plan to lay the NEG strips in some segments of the future beam lines.

Acknowledgements

We are thankful to Prof G K Mehta for his advice to take up the development work on distributive pumping. The help of T Madhusudan and S Mandal is gratefully acknowledged.

References

’ G K Mehta and A P Patro, Nucl Instrum Meth, A268, 337 (1988). ‘T A Giorgi, B Ferrario and B Storey, J Vat Sci Technol, A3,417 (1985). ’ B Ferrario, F Doni and L Rosai, Mini-symp on uhv system in connection with Particle Accelerator, Stockholm, Sweden, 9-11 October 1984. 4 Sang Ryul In, T Maruyama, S Yokouchi and S H Be, J Vat Sot Japan, 34,28 (1991). 5C Benvenuti and F Francis, J Vat Sci Technol, AS, 3864 (1990). 6H J Halama and Yaohua Gao, J Vuc Sci Technol, A9,20?0 (1991). ‘H C Hseub. I Feieenbaum. M Manni. P Stattel and R Skelton. Particle Accelerator konf, iancouvkr, Canada; 13-16 May 1985. (BNL 36453) Brookhaven, USA.

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