FED limiter designs

Journal

29X

PUMPING AND EROSION

of Nuclear

Materials 1 II & I I? (IYXL) 2YX~304 North-Holland Publishing Companv

RATES FOR THE TFTR AND INTOR/FED

D. HEIFETZ, D. POST, M. ULRICKSON

LIMITER

DESIGNS

and J. SCHMIDT

Plasma Phvsics Laboratotyv, Princeton University. Princeton, New Jersey 08544, USA

Wall power deposition and erosion rates due to charge-exchanging neutrals, along with pumping rates. are computed for various TFTR and INTOR/FED limiter and divertor schemes. It was found that only -7% of charge exchange power was deposited on the first wall in a proposed INTOR/FED “T” limiter. Thus, the first wall will not have to be replaced as often as the limiter.

1. Introduction Limiters will be used on TFTR to provide impurity and particle control [ 11. Limiters are also being designed to provide impurity control and to meet the pumping requirements for large scale reactor experiments such as FED and INTOR [2,3]. Some of the key questions involved in the design of limiter and pumping systems are the erosion of the limiter itself, the erosion of the limiter support structure and adjacent vacuum vessel wall or liner, and the transport of recycling neutral atoms and molecules through pumping structures. These questions have been examined using a two-dimensional Monte-Carlo neutral gas transport calculation [4] for a variety of fixed plasma conditions and geometries. The erosion rates of the limiters due to physical sputtering by ions and the erosion rates of the limiters and vacuum vessels due to physical sputtering by neutral hydrogen atoms have been calculated. An estimate of the pumping efficiencies of a proposed INTOR/FED pump limiter designs has also been calculated.

rough walls, resulting in a cosine distribution in reflected polar angle. Physical sputtering of limiter and first wall materials is computed as a function of incident energy and polar angle using the code DSPUT [7].

3. TFTR neutral particle wall loads TFTR will have both a toroidally symmetric “bumper” limiter on the inner wall, and a movable limiter at one toroidal location. Our model of the bumper geometry is shown in fig. 1. A beam heated 2.2 s dis-

2. Physical models Typical discharges in TFTR and INTOR/FED were modeled using the one-dimensional transport code BALDUR (51. The scrape-off model in BALDUR [6] is used to model conditions in the limiter regions. The two-dimensional Monte-Carlo calculation described in ref. 4 computes neutral transport using plasma profiles from the BALDUR results. This neutral transport code models a wide range of neutral/plasma reations. Neutral/wall interaction is modeled assuming

0022-3115/82/0000-0000/$02.75

0

20 (ml

Fig. 1. Poloidal cross section of TFTR, plasma discharge and the bumper limiter.

0 1982 North-Holland

showing

a circular

299

D. Heifeir et al. / Pumping and erosion rates Table 1 Assumptions

for BALDUR

simulation

of a base case TFTR

discharge Major radius = 265 cm Minor radius= 85 cm Scrape-off region from 85 to 95 cm Toroidal field on axis= 52 kG Ion recycling rate = 90% 32 MW tangential injection D beams. for 1.5 s starting at 0.2 s (0.5 s is the current design, 1.5 s is an extended performance design) Full beam energy= 120 keV 70% T, 30% D. initially In main discharge (0~ r < 85 cm): x = 1 X neoclassical + 7 X lO”/n e x f = 1 X neoclassical D,=lXneoclassical+5X103+5X103(r/a)3 + 5 X 1016/n, In scrape-off region (85 < r =S95 cm): x = 1 Xneoclassical+7X lO”/n e XI= 1 X neoclassical D, =2X lo4

charge, representing an extended performance design for TFTR, was calculated as a base case using BALDUR, under the assumptions in table 1. Plasma properties at the times 0.2, 0.7, 1.7, and 2.2 s are given in table 2. Effects of neutral recycling of neutrals born at the limiter are given in fig. 2-4. Density of Do at t = 0.7 s, after 0.5 s of beam injection which is the current TFTR design performance parameter, is shown in fig. 2. Since the mean free path length of neutrals born near the center of the limiter is typically less than 10 cm, the neutral density is localized near the limiter. This holds true during the entire discharge. Power deposition of the charge-exchanging neutrals born at the limiter is shown in fig. 3 for the times 0.2,

Bumper Limiter

(bl

-

,

Fig. 2. (a) Density, noo, of the Do originating at the bumper limiter at r =0.7 s in the TFTR base case, showing an edge at the limiter to - 107.‘/cm3 density fall off from - 10”/cm3 opposite the limiter. The density of To behaves similarly. (b) Contour plot of log(n,o) from fig. 2a, showing a drop in the density of Do radially in the plasma from 107.5-10’o at the edge to - IO5 at the magnetic center.

Table 2 TFTR plasma parameters (Here a =85, n,(r) is the ion density average ion density, and T the volume average ion temperature)

at radius

r, Ti(r)

the ion temperature

at radius

(eV)

Time

n SO)

T,(O)

ni(a)

T,(a)

(s)

(parts/s)

(W

(parts/cc)

(ev)

;darts/cc)

0.20 0.71 1.70 2.21

5.17x 5.44x 5.66X 4.79x

1231 16460 29450 17280

1.96X 1.33 x 1.14x 1.96X

63 111 129 95

3.62~ 340x 3.40x 3.54x

10’3 10’3 10” 10’3

1Ol3 10’3 10’3 lOI

T,

lOI 10’3 10’3 10’3

667 6495 11800 5342

r, iii the volume

300

D. Heifetz et al. / Pumping and erosion rates

I = 0.2 set

1 =a7

Tolol=6.98~l0’Watts

set

Total:723xlO’Watts

1.38~16

3.85d

~~~~~~~~~~~4,~~~~~~~~~

*,,2x,0-z t : 1.7 5ec Total

:l.69x106

___

----

----

-----t z2.2

Watts

Totol:3.66x105

-----

set Watts

Fig. 3. Power deposition, in W/cm’, of charge-exchanging neutrals born at the TFTR bumper limiter. at times 0.2, 0.7. 1.7. and 2.2 s, in the TFTR base case. Note that at each time the power drops off by a factor of - 10d3 from the limiter center to a point directly opposite the limiter.

Fig. 4. Physical erosion, in cm/y. by charge-exchanging neutrals born at the TFTR bumper limiter, at times 0.2. 0.7, 1.7. and 2.2 s. in the TFTR base case. Results assume a 100% duty cycle. The first wall is stainless steel, and the limiter is assumed to be carbon.

301

D. Heifetz et al. / Pumping and erosion rates

0.7. 1.7, and 2.2s. Assuming up/down symmetry, only half the poloidal cross section is drawn. Note that though the total power on the first wall varies greatly with time, from 6.98 X lo4 W at 0.2 s to 1.69 X lo6 W at 1.7 s, the dropoff in power intensity from the limiter center to the point poloidally opposite is approximately 10e3 at each time. Physical erosion by charge-exchanging neutrals is shown in fig.4. Here we assume a carbon limiter (for erosion rate purpose), and a stainless steel wall. No self-sputtering or chemical erosion is included. A 100% duty cycle is assumed. Peak erosion is at the limiter center, varying from 0.64 cm/y at 0.2 s, to 1.7 1 cm/y at 1.7 s. The sputtering there due to ion impact varied from 3.04 cm/y at 0.2 s to 8.12 cm/y at 1.7 s, a rate approximately 4.75 times larger than the charge exchange rate.

4. INTOR

neutral particle wall loads and pumping rates

A number of design proposals have been made for INTOR/FED poloidal divertors and pump limiters [3]. One such poloidal divertor design has been previously modeled by our algorithm, and the results are described in ref. 4. We describe here our modeling of the proposed toroidally symmetric pump limiter shown in fig. 5. Since heavy erosion of the limiter itself is expected, the limiter segments would sit in drawers between poloidal field

coils for easy withdrawal and replacement. However, heavy erosion of the first wall, which is difficult to replace, may make the design impractical. Thus, it is important to calculate the mass and energy transport by charge-exchanging neutrals born at the limiter. Assumptions in our INTOR base case calculation are given in table3. At 6.6s into the discharge, one second after the end of neutral beam injection, central ion density is 1.31 X 10’4/cc, central ion temperature is 35.8 keV, and ion current on to the limiter is 3.16 X 1O23/s. The effects of neutral recycling by the limiter at this time are shown in figs. 6-8. Power depositions by charge-exchange neutrals on the limiter and first wall are shown in fig. 6. A total of 25.60 X lo6 W falls on the limiter, and 1.77 X lo6 W on the first wall. Thus, only 6.5% of the charge exchange power went to the first wall. Since the mean free path lengths of the neutrals born at the top of the limiter are only 4-10 cm, the neutral density falls quickly away from the limiter (fig. 7). Thus, these neutrals deposit their energy on the first wall mainly in two peaks just beyond the limiter edges (fig. 6b). Note that the peak at the right limiter edge is higher and narrower than the one on the left. This may be because the field lines are more compressed on the right side (fig. 5). There is also a third peak directly across from the neutralizer plate due to the current on the plate, which is 9% of the total current on the limiter. Physical erosion of carbon by charge-exchanged neutrals in units of cm/y is shown in fig. 8 assuming a

Mid-olane

Table 3 Assumptions INTOR/FED

Limiter

Fig. 5. Poloidal cross section of a proposed INTOR/FED pump limiter [3], showing a “T” limiter located at the bottom of the vacuum

vessel.

for BALDUR discharge

simulation

of

Major radius = 520 cm Minor radius = 160 cm Scrape-off region from 160 to 170 cm Toroidal field on axis = 55 kG Ion recycling rate = 95% 75 MW D neutral beam injected for 4.5 s starting Full beam energy= 175 keV 90% T, 10% D, initially In main discharge (0 4 r =Z160 cm): ~~=lXneoclassical+5XlO”/n, xi = 3 X neoclassical + ripple trapping D,=3Xneoclassical+l.25X10”/n, In scrape-off region (1606 r =c 170 cm): xe = 1 Xneoclassical+SX lO”/n, xi = 3 X neoclassical + ripple trapping D,=2X lo4

a

base

case

at t =0.5 s

D. Heifetr et al

302

Pumping and erosion rates

ial

I 084’

2.10’ / ;Tkl.67kl.59 5.99’ ‘,2.7 I

(bl

Fig. 6. Power desposition. in W/cm2, by charge-exchanging neutrals born on the limiter. at 6.6 s into the INTOR/FED base case discharge. Fig. 6a shows the deposition on the limiter. and fig. 6b shows the deposition on the first wall. Note that of the total power of 27.4 MW, 25.6 MW falls on the limiter.

100% duty cycle. Again no self-sputtering or chemical sputtering is included. Total charge-exchange erosion on the limiter top is comparable to that by the plasma ions (6.72 X lo6 g/y versus 6.05 X lo6 g/y). This charge-exchange erosion on the limiter top results in a loss of - 7-8 cm/y carbon fairly uniformly across the limiter top. Again three peaks appear on the first wall, behind the limiter edges at - 1 cm/y, and across from the Thus, the limiter neutralizer plate, at - 1.75 cm/y. lifetime would seem far shorter than the first wall lifetime. However, a study of limiter and wall material redeposition will be necessary before conclusions can be drawn about limiter lifetimes. Raising the edge temperature and lowering the edge density should increase neutral transport, thus increasing the relative load on the first wall. Also, the presence of hydrogenic molecules affects the loads on the limiter. In the above calculation, all D/T absorbed by the wall is assumed to desorb as atoms. A recomputation, assuming the same plasma conditions but now assuming that

Limiter

Fig. 7. Contour plot of the log of the density of the D” originating at the INTOR/FED limiter at 6.6 s into the INTOR/FED base case discharge , showing the Do population concentrated in the limiter region.

all desorbed D/T is molecular, reduced the power load on the limiter by a factor of greater than 2, to 12.0 X 10h W. This may be due to the following reasons. The total density of molecular hydrogen at the limiter face was 1-2 times the total density of atomic D/T there. Since the predominant molecular dissociation reaction is e + H; - Ho + HC +2e, there are half as many neutrals striking the plate than in the previous computation. hence the lower loads. However, a self-consistent plasma transport calculation including the cold ions produced from this reaction is necessary before final conclusions can be made. A calculation of limiter pumping was done using the same physical assumptions as in the wall power and erosion calculation above (with no molecules). The plasma conditions across the pump opening, shown in fig. 9a and computed using the model in [8], were assumed constant along magnetic field lines. The pump geometry studied, representing half of a 2-sided pump

303

D. Heifetz et al. / Puniping and erosion rates

Limiter Side

4.5BJ ‘-2.06

L-4

Mid-Plane

I,(D/T)z

-----7

Lim’iter Side

Will Side

Wdll Side

1.43~10~~

I,(He)=7.16x1020

5cm+

Limiter

Is,cK(D/T)=9.8x1020

-80

cm-

Wall

IsAcK (He):3.9xlO”

IpuUp(D/T)=1.9x102’

I puvp (He1 = 7.6 x IO” Fig. 8. Physical erosion, in cm/y, by charge-exchanging neutrals born at the limiter, at 6.6 s into the INTOR/FED base case. A 100% duty cycle is assumed. Both the limiter (fig. 8a),

and the first wall (fig. 8b) are carbon.

limiter, is shown in fig. 9b. A pumping speed of lo5 l/s for room temperature helium is assumed. Approximately 4.5% of the particle flux current to the limiter flows onto each side of the neutralizer plate. Thus assuming 5% He, 7.16 X 10” parts/s of He flow into each pump channel. We compute that 7.6 X lOI parts/s, or 10.6% of the He flows out the pump, and 5.48 flows back out the throat. With two channels, a total of 1.52 X 102’ parts/s of He is then pumped, which is 75% of the required 2 X 10” parts/s rate. However, since - 80% of the neutrals produced at the plate reionize near the plate, a self-consistent calculation may show a plasma density increase and temperature decrease, effecting the pumping rate [9]. We propose our result as a conservative lower bound.

Fig. 9. (a) Electron density, ne, and temperature, 7”, across the INTOR/FED pump opening, at 6.6 s into the INTOR/FED base case. (b) Conductances in the INTOR/FED pump, assuming the constant conditions given in fig. 9a. Only half the pump is shown. A total of 1.52X 10zo/s He atoms are pumped, 75% of the required rate. However, since 80% of the neutrals born at the plate are reionized, this calculation is not self-consistent.

Acknowledgment This work was supported by the US Department Energy Contract No. DE-AC02-76-CHO-3073.

of

References [1] J. Cecchi, J. Nucl. Mater. 93-94 (1980) 28. (21 C.A. Flanagan et al., ORNL/TM-7777, Union Carbide Corporation (June 1981). [3] INTOR, Phase One IAEA, Vienna (1982). [4] D. Heifetz, D. Post, M. Petravic, J. Weisheit and G. Bateman, J. Comp. Phys. 46 (1982) 309-327

304

D. Heifetz et al. / Pumping and erosion rates

[5] D. Post et al., TFTR Physics Group Report No. 33, Princeton University, Plasma Physics Laboratory, Princeton, N.J. (1981) unpublished. (61 J. Ogden et al., IEEE Trans. Plasma Sci. PS-9 (1981) 294. [7] D. Smith et al., Ninth Symp. Engineering Problems of Fusion Research, Chicago, IL 26-29 October 198 1.

[8] M. Ulrickson, PPPL-1901, Princeton University 1982). [9] M. Petravic et al.. Phys. Rev. Lett. 48 (1982) 326.

(May.