On cryopumps for selective pumping of “globus” tokamak

ICEC 14 Proceedings

ON ORYOPUMPS FOR SELEOTIVE PUMPING OF "GLOBUS" TOKAMAK

V.B.Yuferov ~ S.F.Grishin z V.O.Ilyicheva ~ N.A.Kosik ~ Yu.V.Kholod z L.G.Sorokovoj * G.M.Vorobjev ~* A.N.Novokhatskij "z Kharkov Institute of Physics & Technology, Ukraine 310108, Kharkov, Akademicheskaja,

I

Ioffe Institute of Physics & Technology, Russia, 194021,St.-Petersburg, Politekhnicheskaya, 26

Some

problems

pumping

of

of impurity

cryopumps gases

in

for selective

"Globus"

tokamak

are considered. The pumping speed of the system determined in

dependence

was

on geometry in molecular

regime and when taking into account the influence of unpumped

hydrogen.

cryopumping

The

values of heat load

system are estimated

rating conditions. are proposed.

Some

in different

types of cooling

in ope-

system

INTRODUCTION The impurity

removal

from

the

volume

~

is the long-term process

due to the high value of ~/~: ~/~ >I0 for the most of existing fusion devices

(~ is the pumping speed). In principle, it is possible to ob-

tain W/~
impurity pumping.

But

this results

pumping and leads to

the operating gas

significant for

deuterium

should

the

be advantageous t o

use

and the

in increasing of losses that

system

is used

is particularly it

selective function pumps that CO, C02, CmHn, etc.

Due to this requirement the whole pumping complicated and includes several pumping systems. first

hydrogen

tritium case. In this sense

do not pump the hydrogen but effectively remove

The

and to accele-

for pumping f r o m

system

becomes

the atmospheric

pressure to the high vacuum, providing the vacuum leak test and preconditioning procedure provides the selective and conditioning of the

of the vacuum system. The second system impurity pumping when operating with plasma vacuum

chamber

by low-power plasma dis-

charge. The third system is the "guard" pump for vacuum maintenance between plasma experiments. Cryogenics 1992 Vol 32 ICEC Supplement

155

ICEC 14 Proceedings

The Fusion Device and Requirements "Globus"

tokamak

with

low

aspect

ratio (R/a ~ 1.5)

and

plasma

chamber volume about 3 m 3 is considered . Total volume of the vacuum vessel is 5 m 3. Plasma

chamber

wall

surface area ~ 12 m 2 and total

surface ~25 m 2. Plasma pulse duration is 0.3 - 0.5 s. Plasma density 3.10130m -3.

In

the

2OO,6OO mm

rectangular

connection

with

of vacuum

chamber.

equatorial

after

Torr.1/s.cm 2 pulses in

section

The

whole

The

impurity

discharge

cycles

at

gases).

The

steady

helium condensation

The

selectivity

the

adsorption

can

be

a

performed

isotherms

(80 K)

A temperature

NH 3

)

20

of the

plasma being

K

is

the

pressure the

condensing partial

upon

at

surface

pressure of

values

to the oryooondensation oryoadsorption

pumping

of

vapor

pumping at a at

60-80

K.

region of interest the absorption dynamic

value of

absorbability from 5.10 -5 to 1.10 -4 of oryopumps

the volume of fusion device. Moreover,

is

comparable with

one should take into account

deterioriation of impurity gas

hydrogen pressure of

condensation

( 02, N2, 00, 002, CmHn,

dependence

1.Torr/g. It means that the volume the appreciable

between

atmosphere

and

the values of

in

The alternative

give

5.10 -11

by the oryopumping methods:

NO,

the

to

pumping (i.e. titanium sorp-

At,

However, in

of

time

hydrogen

typical for fusion devices

of

be heated up

level

dwell

impurity gases

temperature

the

with a pressure about 1.10 -9 Tort.

should be below 21K according to

temperature.

used for

pumps). The selective pumping of

at low temperatures

even lower temperatures.

N20 ,

of

chamber at a level of 1.10 -4 Tort eliminates the

impurity gases has to be realized

H20 ,

12 ports

ports are

system will

use of high capacity pumps for hydrogen tion pumps and

are

made of stainless steel with outgassing rate

conditioning

(for

vacuum

is about 10 min. The

the

there

diagnostic equipment for plasma heating and pumping

20oO0. Vacuum vessel is expected

plane

isotherms under the

1,10 -4 Tort.

The System of Selective Pumping For the

pressure

of impurity

with expected values of gas

gases

about

release and leaks

I ,I0 -9 Tort be achieved the

pumping speed of

system of selective pumping has to be ~1.3.104 1/s. The system of selective pumping mounted in formed in

consists

of

10 - 12

cryopumps - one-type

modules

diagnostic chambers (Fig.1). Selective pumping is perthe medium of unpumped gas. As a result, pumping speed

decreases and at 10 -4 Tort according our culculations its decrease is 20 % [I ,2]. The condensing surface for nonscreened oryopumps has to ~ I .8,1 03 ~ ..2 ... and for those ','~÷~ n ~ t r^~~n screon ~°~ to be ~5.103cm 2 . l,.l~

156

~

Cryogenics 1992 Vol 32 ICEC Supplement

vv.L

u A ,11,

,~

ICEC 14 Proceedings

The

Selective Cryopump Thermophysios

Several processes determine the heat load and heat yield in the cryopump system,so the corresponding values can be presented as :

where load

~rad

is

the

heat emission of the surrounding walls; ~g-heat

relative to residual gas and due to condensation; ~s- heat load

through the supports; ~p- all kinds of ~c-heat yield

due

as oryoagent); values

of

radiation produced by plasma;

to ortho-para conversion ( mainly in the hydrogen

~mf-heat

~rad

of

yield

due

to eddy currents.

nonscreened

pumping

surface

The

oulculated

and

of

the

nitrogen screen are ~ 4.10 -3 W/cm 2 and ~ 3,10 -2 W/cm 2 respectively; ~g is about 2.10 -3 small as a

W/cm 2. The value of ~mf is

size of pumps (current circuits) and constructive material.

negligible

using

due

to

stainless steel

The increase of a pulse number from 1 to 10 pulses/hour results in rather great

influence of plasma pulse upon thermal characteristics

of the cryopump, even if the pulse duration is small. As the expected radiation flows are =0.5, an average solve

20 - 30 W/cm 2, pulse duration T = 0,3 s,

heat load is

the cryopump out of not

within the

the problem,

1,10-3W/cm 2. The

installation

of

field of vision, i.e. "in the shadow", does because

the absorption ability of the cryo-

panel is much higher than that of the

walls. This situation

improved by use of special light traps. Without these

can

be

traps the de-

sorption of the cryodeposit will take place at the moment of plasma pulse, which is undesirable as the subsequent long cleaning of the chamber surfaces will be required. Hence, for a period of pulse duration

(

I s )

it

the chamber volume

seems by

means

valves is also necessary for

reasonable

to separate

oryopumps from

of valves. The use of the separating the independent regeneration

of oryo-

pumps, as well as for simplification of their repair. Cryomaintenanoe The v a r i a n t s o f

System oryomaintenanoe systems for selective pumps were con-

sidered in [2]. In present paper we p~gpose for pump feeding a system of hydrogen-neon J-T expansion-valve liquefier supplying hydrogen to collecting volume with the subsequent distributing it among individual modular oryopumps. The combination of hydrogen-neon liquefier seems to be rather profitable, since the hydrogen liquefying coefficient can be obtained within the range of 0.8- 0.95, which simplifies the hydrogen equipment design. Heat load to cryopumps being about 4- 15 W for nonscreened and screened type of oryopumps will Cryogenics 1992 Vol 32 ICEC Supplement

157

ICEC 14 Proceedings

result in two- or threefold increase

of the total energy losses at a

temperature of 20 K.

CONCLUSION The design of cryopumps for selective "Globus" tokamak is load

at

nitrogen

a

discussed, Without the

temperature of

screen

the

pumping

20 K is

of

impurity gases in

nitrogen screen the heat

0.3 - 1,10 -3 w.i-l,s and with

heat load can be at the same temperature level

(2OK) ~ I - 3.10 -4 W.l-l.s and

~ 2.5.10 -2 W.l-l.s at the temperature

80 K( within the pressure range considered 1.10-6-1.10 -4 T0rr).

REFERENCES

1

Grishin,

S.F.

et al,

Selective

Devices: Preprint KFTI 91-53 2

Pumping

of

Impurities

(1991)

Yuferov, V.B. et al, The Proposals on Design and Selective Pumping

of "Globus" Tokamak Plasma Devices and Operations

Z

Figure 1 Schematic selective pumping system

158

in Fusion

Cryogenics 1992 Vol 32 ICEC Supplement

(be published)