THE LIFE TIME CHARACTERISTICS OF NEG PUMP
Kenji Itoga, Kazunori Ikegami, Itsuo Kodera
Third Group, Advanced Electrotechnology Department,
Central Reseach Laboratory Mitsubish Electric Corporation
8-1-1 Tsukaguchi-hon machi, Amagasaki, 661 Japan

2Abstract
We measured the equilibrium pressure and outgassing rate inorder to examine the life time characteristics of NEG pump forseveral times air exposure cycles. The degrading rate of NEG
pump for nine times air exposures was small.

1Introductio nNon-evaporable getters(NEG) pumps have been applied to theaccelerator and the nuclear fusion experimental devices to getthe ultra high vacuum. The NEG pumps are higher in pumping
speed par unit volume and robuster for the fluctuation of themagnetic field than ion pumps.
The NEG pumps are planned to apply the large accelerator in
next step i.e. ESRF (1] and APS (2]. Since the accelerator is not
yet industrial products but the experimental device, the NEG. pumps built in the accelerator have experience of air exposurein many times for maintenance. Consequently we must pay
attention to contamination of the NEG pumps.
Our final goal is to find the way of procedure for the accelerator maintenance to protect against the contamination of theNEG pump. In the first step, we experimentally examined how
the pumping speed performance degrades by air exposure cycle.

Experimen tal setup

The experimental setup is schematically shown in Fig.l. NEG
pump was installed in the main vacuum test chamber made from
SUS-304 which was evacuated by the turbo molecular pump(TMP) through the orifice and the narrow duct. The total vacuum pressure was measured by two ionization gauges ( nudeB-A type ) attached to the vacuum test chamber and to theTMP head. The partial residual gas pressure was measured by
the mass spectrometer attached to the vacuum test chamber.
The pumping speed of the vacuum test chamber was calculated from the conductance of the orifice and the narrow duct.
The diameter of the orifice was equal to 12 mm. The inner diameter and the length of narrow duct were equal to 38 mm and300 mm, respectively. The pumping speed was equal to 3.1 1/s
in calculation. The inner diameter and the length of ~uum test
chamber were equal to 148mm and 300mm, respectively.
The NEG pump was St707 GP100W cartridge type suppliedwith Sa:es Getters Corporation. The material of St 707 is Zr-VFe alloy and can sorb the gases after the activation at 300°C5000C. The nominal pumping speed is 2001/s for H2 and 751/sfor CO.

NEG PUMP

TMP
MASS SPECTROMETER

Fig.l Experimental setup~ 146-

3Experimen tal procedure

In order to evaluate the net pumping speed of NEG pump, thebackground outgassing rate was measured without NEG pump.
The outgassing rate was obtained by the following equation,plotted only odd times. This figure indicated that the pressure
was higher than that obtained by previous operation. This result
suggested that since the equilibrium pressure was obtained after
about seven hours, the evacuating time was sufficient enough toevaluate the outgassing rate.

( 1)
where Q is the outgassing rate in unit of torr· l/s, S is thepumping. speed which was equal to 3.1 l/s calculated by theconductance, P 1 and P2 are the pressure of the vacuum test
chamber in which NEG pump was installed and the pressureof the TMP head respectively. Then the outgassing rates werel.lx1o- 7 at 93°C and 1.4x1o-s at 51°C.
The air exposure operation was carried out according to following procedures:
1) Expose the NEG pump to the air for an hour,2) Evacuate the air and bake the chamber at 180°C for fourhours with ribbon heater typically,3) Activate the NEG pump at 360°C for two hours, or at420°C for an hour. In both cases the activation rate reaches
100%.
These air exposure operations were repeated many times toexamine the life time characteristics. The operational parameterfor each air exposure operation is shown in table 1. The humidity when the NEG pump was exposed to the air was 40%-50

1st Operatlcn--o- 3rd Operation--6-- 5th Opera~on-+--7th~--x-- 9th Operation
t:.!!

2%.

436

57TIME( hours I

Fig.2 Change of total pressure

Table 1 The list of the air exposure cycle.
ACTIVATIONOPERATIONEXPOSURE

NUMBER

TIME

TIME

TEMP,TIME

TEMP.

¥:

(hours)(hours)(" C)(hours)

(" C)

~

1LONG

30150

2360

243144·150

2360

31201802360~

4120180

2360

I \5I

4180

1420614180I420

7I

4180

1420

\~\T.,..______ _814180

1420

1 'ti'.._9I

4180

1420

BAKING-r

TOTAL PRESSURE

I+--+x---x

II

PRESSURE OF Hz•
PRESSURE OF Hzo•
PRESSURE OF co•o---ow.,n1!<---ll.

,.,

111

PRESSURE OF c.o•PRESSUR OF Clio+~~I'lb I

n .,

~:''I

x....:...X---x...----- ----+--+•--+---+

4 '·<Q_.............__'-x---x---)E-.--.-M--~-- ... K- .... -x--o---o-!"..:O:.."!"~--- ............. __ ~+--o.__ ..,__....,__~-~--6---A----ll,.

--6-------•--54Results

The change of total pressure after the activation is shown infigure 2. The pressure was measured since the time when heater
for activation of NEG pump was off. The measured pressure wasFig.3 Typical change of partial pressure. This example is refered to 8th operation. Outgassing rate of H 2 was increased with
the NEG material temperature, and other gases were reduced
as time passes.

-147-

Main residual gases were observed to be H 2, CH4 , H20, CO,
C02 by the mass spectrometer installed in the vacuum testchamber. The typical changes of partial pressure were shownin figure 3. The partial pressure was measured since the time
when NEG heater was on. The partial pressure of H 2 was dominant in the residual gases during the activation, and other gases
were reduced as time passes.

The degrading rate was not more than 5% for 3rd-9th operations. We neglected 1st and 2nd operations, because the condition of air exposure cycle was different from other operations.

Table 2 The pumping speed of NEG pump for each operation
obtained in eq.(3)This is the evidence the sorption mechanism of H 2 is different from that of other gases. Since H 2 gas is adsorbed on thesurface of NEG material, its outgassing rate is increased withthe material temperature. Other gases are absorbed and diffuse
into the NEG material. Therefore, once the gases are absorbed
in the bulk of the NEG material, they don't get out from it[3]. Whenever the NEG material is heated, its surface becomes
active to get in the gases.

5OPERATION

p 1

P2TEMP.

outgasSNEGNUMBER(torr)(torr)• C)( 1/s )

(1/s)

21.0

Discussion

The degrade of the pumping speed will be discussed considering the temperature dependence of the outgassing rate. Theoutgassing rate Q depends on the temperature T as followingequation, [4],Q = Aexp(K/T).

(2)

Substituting Q and T obtained experimentally in section 3
into the equation (2), the coefficients were solved to be A =2.59xlo- 1 and K = 5.37x103.
The pumping speed SNEG of the NEG pump can be obtainedby the following equation,

(3)

61

8.5E-10

5. 2E-928.04. 6E-92

1. 1E-9

3. 7E-9

27.0

4. 3E-9

11. 23

7. 8E-10

3, 8E-9

24.3

3. 7E-9

16.54

7. 8E-10

3. 8E-9

20.0

2. 8E-915.4

5

8.9E-10

4. 9E-9

20.0

2. 8E-9

16.96

1. OE-9

4. 4E-925.8

4, OE-914.4

7

9. 7E-IO

4. 4E-9

20.8

3. OE-9

13.98

1. IE-9

5. 7E-9

23.1

3. 4E-9

15. 99I. 2E-9

5. 7E-929.1

4. 9E-9

15.5

Conclusion

We observed the degrading r.ate of NEG pump for several times
air exposure cycle.The operating pressure was the order ofw- 10torr. The performance of NEG pump didn't deteriotate
so much for nine times air exposures.
The pumping speed of NEG pump was measured for variousgases. The difference of evacuating process between H2 andother residual gases was recognized experimentally.

References[1] J;'.Aigrain et a!., ESRF-FPR, pl88-p200, Feb.(1987).

The pumping speed of the NEG pump for each operation is
obtained in equation (3) as the pressure of test vacuum chamber
becomes equilibrium state after seven hours. Calculated results
were summarized in table 2.

[2] A.Schriesheim et a!., ANL-87-15, April(1987)[3] K.Watanabe, INS-T(Inst Nucl Study Univ Tokyo) N0.486,
p8-20, (1988).
[4] H.Kumagai G.Tominaga, Physics and application of vacuum, pl39, Shokabo,(1977).

The degrading rate R is presented in following eq.( 4),(4)where n is the number of times of air exposure and S 0 is theinitial pumping speed of NEG pump.

~ 148-