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Boronization of COMPASS
Journal of Nuclear North-Holland
Materials
journal of nuclear materials
186 (1992) 217-226
Boronization
of COMPASS
H.G. Esser ‘, S.J. Fielding ‘, S.D. Hanks ‘, P.C. Johnson ‘, A. Kislyakov ’ and J. Winter b aAEA Fusion, Culham Laboratory, Euratom / AEA Fusion Association, Abingdon, Oxon OX14 3DB, United Kingdom ’ Institut fiir Plasmaphysik, Kemforschungsanlage Jiilich GmbH, Association Euratom / KFA, D-51 70 Jiilich, Germany ‘A. F. Ioffe Physico-Technical Institute, Academy of Sciences of the USSR, Saint Petersburg, USSR Received
2 August
1991; accepted
31 August
1991
Boronization of COMPASS has been carried out using trimethyl boron, a less hazardous alternative to diborane used to boronize other machines. Two separate boronizations, using different gas and glow discharge parameters, have been udertaken. Both have resulted in excellent long-term impurity control, albeit with a significant change to the hydrogen recycling characteristics
1. Introduction Boronization, the plasma-chemical depostion of thin boron/carbon films onto the inner wall of a tokamak, has been developed as a surface conditioning technique to reduce impurity fluxes into the plasma. This procedure was first used on TEXTOR [l], which had previously been conditioned by carbonisation (deposition of thin hydrogenated carbon films [2,3]). The boronized layer was deposited at a wall temperature of 300-350 “C using an rf-assisted glow discharge in a throughflow of 0.8He + O.lB,H, + O.lCH,. The resulting film, of average thickness 50 k 20 nm, was found to have a nonstochiometric B to C ratio c 1. A strong reduction in oxygen influx during tokamak discharges, by a factor - 4, and carbon by a factor _ 2 was observed compared with good carbonised conditions, and good hydrogen recycling control was achieved. These beneficial conditons in TEXTOR lasted for several hundered discharges, including numerous shots with ICRH at power levels greater than 2MW. Following the favourable results from TEXTOR, boronization has been used on ASDEX [4], TCA [5,6] and TFTR [7]. In ASDEX and TCA boronization was carried out at room temperature, with similar results to those observed on TEXTOR in terms of film constituency and plasma improvement. In all the above cases diborane (B,H,) was used as the boron-containing gas. Diborane is very toxic and 0022-3115/92/$05.00
0 1992 - Elsevier
Science
Publishers
highly explosive and requires special handling techniques, involving considerable and costly modifications to standard gas and vacuum systems [8]. Because of this, a programme of work was carried out at KFA Jiilich to identify a less hazardous gas for boronization which would allow more convenient usage. Trimethyl boron, B(CH,),, a gas which is pyrophoric rather than explosive, was found to be the most promising and was used in TEXTOR with apparently similar results to diborane [9]. However boronization by diborane gives rise to a very durable surface film, which lasts for hundreds of tokamak discharges, and hence results from the use of trimethyl boron on TEXTOR could well have been influenced by the residues from previous boronizations. In this paper we report the use of trimethyl boron for the boronization of COMPASS, a tokamak not previously boronized, or carbonised. We describe the layer deposition procedure and the resulting improvements to the tokamak performance. We also describe a subsequent boronization using a different gas mixture. The experimental physics programme on COMPASS, involving second harmonic ECRH, requires clean, low density, low loop voltage plasmas with minimal runaway/ slideaway content. These conditions had nto been obtainable by conventional wall-conditioning techniques. Boronization with trimethyl boron provided a relatively easily implemented means of attaining low impurity plasmas.
B.V. All rights reserved
H.G. Esser et al. / Boronization of COMPASS
218
2. Boronization
H,
system
Prior to boronization, COMPASS wall conditioning was by means of rf-assisted golw discharge cleaning (GDC) in hydrogen or helium at room temperature. Moderate vessel baking to 120 o C was used only after a major vacuum opening and simultaneous baking and GDC was not possible. The COMPASS GDC system consists of a single stainless steel rf/dc anode mounted on a horizontal probe, capable of being driven in to the minor axis of the torus. Up to 40 W of rf at 13.6 MHz, and 650 V dc can be applied to the water-cooled electrode. Some modification was made to the gas handling and exhaust systems to accommodate the use of trimethyl boron. The Roots and rotary pumps in the turbopump backing lines were operated wtih dry nitrogen gas ballast. Exhaust gas was vented to atmosphere via a pipe fitted with a flame arrester, through the roof of the COMPASS experimental building. Test boronization in a small planar rf diode plasma system had been previously studied at Julich [9]. In these Jiilich experiments samples were negatively biased to 200-350 V. The sample temperature was varied between 100 and 230” C, with a trimethyl boron pressure of 2.7 X lo-’ mbar. No dilution by other gases was used. The B/C ratio of the resulting film was measured to be 0.6 (compared with 0.33 in the gas). For boronization in TEXTOR an admixture of 85% helium was used, to a total pressure of 3 X lop3 mbar, with the vessel liner and limiters heated to 350 o C. The helium was added in an attempt to reduce the hydrogen content of the films but resulted in a of lower B/C ratio of - 0.4. For initial boronization COMPASS it was decided to use trimethyl boron alone, in order to maximise the boron content of the deposited films. Fig. 1 is a schematic plan view of the COMPASS vacuum vessel showing the positions of the gas inlet, GDC electrode and pumping port. Also shown is the location of a removable probe containing samples of silicon, stainless steel and graphite positioned at the wall radius, for film analysis. Residual gas analysis was by means of a Spectra-Metrics Multi-Quad quadrupole mass spectrometer, coupled to the torus by means of a 100 to 1 pressure reducing aperture, enabling RGA measurements to be made at high torus pressures. The GDC gas feed system consisted of a manually operated needle valve, with pressure monitoring by means of a Leybold VM211 spinning rotor gauge programmed to molecular mass 56. For standard GDC operation on COMPASS in hydrogen or helium the
Jmpurity
Spectroscopy
Pumps
Fig.
1. Schematic
plan
and diagnostic
view
positions
of COMPASS relevant
showing
vacuum
to boronization.
glow is ignited by raising the gas pressure to - I X IO- ’ mbar. After ignition the gas flow rate is reduced to give the required operating pressure, - 2 X 10m3 mbar, while at the same time the applied dc voltage is increased (typically to 350-400 V) to set the current to the level of - 1 A. It was intended to follow this procedure for the boronization, however it was found that even close to the ignition pressure, - 5 X lo-* mbar, the glow discharge was very unstable, with the current fluctuating on a time scale of - 1 s between 15 mA and 1 A, and the cathode voltage varying between 400 and 200 V. Any reduction in the gas pressure led to immediate termination of the glow. This behaviour of the glow was similar to that observed for Hz, He GDC on COMPASS after a vacuum opening, where a few minutes of high pressure operation (- 5 x 10-z mbar) was usually adequate to condition the walls and enable low pressures to be accessed. Accordingly the boronization glow was operated for several minutes at high pressure, and each time attempts were made to reduce the pressure the glow extinguished. After - 15 min of this high pressure glow the available 10 g of B(CH,), was exhausted and the boronization was terminated. Fig. 2 shows RGA measurements taken during the boronization, for the same flow rate of BfCH,), (4 mbar/s), with GDC on and off. Winter et al. [9] have made a tentative identification of the cracking pattern of B(CH,), and present an assessment of the plasma and wall chemistry involved in the boronization process. The COMPASS RGA data show qualitatively similar effects. When the GDC is turned on, disintegration of B(CH,), occurs to a large degree and there
H.G. Esser et al. / Boronization of COMPASS B(CHJ)J
219
off
GDC
100
SHOT
2823
Q Q
10
20
30
40
50
60
Mass/emu
GDCon
NCH,),
0
I,10
Time
0 0
10
20
30
40
50
60
Mass/amu
Fig. 2. Residual gas analysis measurements in B(CH,), and during glow discharge boronization.
before
is copious evolution of hydrogen, with methane and C, hydrocarbons around m/q = 28 also produced. After the boronization procedure was completed the samples were removed from the torus. Depth probe measurements at Culham using a Sloan Dektak surface profile measuring system gave a layer thickness of 40-70 nm. Data from later analysis of the film composition by Electron Probe Microanalysis technique is given in table 1, together with the data from similar
21m
(ms)
Fig. 3. Plasma parameters in standard hydrogen discharges (I= 100 kA, B, = 1.1 T, <= 1.8X 10’” mm’) before and after the first boronization.
measurements taken during trimethyl boronization of TEXTOR. It can be seen that the B/C ratio in the COMPASS films is much lower (- factor 2) than in TEXTOR. In addition the estimated film thicknesses given in table 1 are considerably less than those actually measured.
3. Plasma parameters 3.1. Before boronization
Table 1 A posteriori
analysis
of films - first boronization
Sample
Mass thickness
type
(kg/cm’)
Silicon Stainless steel TEXTOR Si sample a Assumes
Boron
Carbon
B/C
Calculated a layer thickness (nm)
0.32 0.45
2.6 2.1
0.13 0.24
24.3 21.3
8.6
0.38
96.3
2.95
film density
of 1.2 g/cm”
Atom ratios
The COMPASS vacuum vessel (0.57 m major radius, 0.22 m minor radius, 316 stainless steel) has a single poloidal ring graphite limiter, radius 0.196 m and 64 small graphite tiles (each with _ 0.02 m2 plasma-facing surface area) at 0.21 m. A series of standard test discharges in hydrogen was run, prior to boronization, at 1.1 T B,, 100 kA I, and 1.5 to 2.0 X 10’” mm3 average line-of-sight density. Typical waveforms are shown in fig. 3a. Detailed measurements were made with all available dianostics and
H. G. Esser et al. / Boronization of COMPASS
220 Table 2 COMPASS standard discharges Wall Gas Shot no. B,(T) Current &A) Loop volts (V) T, (eV)/(SiLi) Ti (eV) (10” m-7) Wall CII (a.u.) Wall 011 (a.u.) Wall Cr I (a.u.) Wall H y (a.u.) BIV PrBd(kWs) E,
Prad
/Pohm~c
X-ray anom. (SBD array) Z,ff
UB
Bl
H2
H2
2823 + 1.1 100 2.4 730 * 20 190 +10 1.7 580 270 150 300
2875 + 1.1 105 1.3 690 *30 250 * 1.5 1.6 120 <4 <4 650 3000 40 0.3 2.1 1.1
180 0.75 75.6 2.9
Bl He 3225 + 1.1 100 1.75 646 &23 210 k 25 1.6 300 40 90 40 2900 87 0.5 n/a 1.5
Bl
Bl
B2
B2
H?
H2
H2
D2
4338 + 1.1 95
4874 + 1.1 100 1.5 n/a _
4215 - 1.1 100 1.7 600 +20 110 + 10 1.6 150 50 12 270 900 n/a n/a 14.1 1.3
1.75 600 *50 130 & IO 1.4 205 54 21 430 n/a n/a n/a 26.6 1.4
1.75 80 5 <4 400 3000 _
5757 + 1.1 100
1.45 750 *50 155 +15 1.8 80 IO <4 180 3000 _
_ 1.6
UB: unboronized; Bl, B2: first and second boronizations.
values of characterising parameters are given in table 2. These parameter values correspond to a well-conditioned torus, in that the vacuum vessel had been baked, many hours ( > 100) of hydrogen GDC had been imple-
mented and several hundred tokamak shots had been run since the last vacuum opening. However table 2 shows that the resulting plasmas were relatively poor, with the high Z,, value (calculated from the measured
Fig. 4. Comparison of visible line emission, centred at 432.5 nm, in standard hydrogen discharges (1, = 100 kA, B, = 1.1 T, <= 1.8~ 10” mm”) before and after the first boronization.
221
H.G. Esser et al. / Boronization of COMPASS
loop voltage and central electron temperature (SiLi), assuming a parabolic-squared temperature profile, Spitzer resistivity plus neo-classical corrections and Z,, independent of radius) indicating a high impurity content. Visible line emission spectroscopy was used to separately view the limiter and wall regions of the tokamak. Measurements were make using a calibrated medium resolution 0.6 m SPEX monochromator fitted with a diode array, collecting line emission data over a 12.5 nm spectral region centred at 432.5 nm. This instrument provided time resolved (17 ms) chord integrated line emissions from 011 (435.1 nm), CII (426.7 nm), Fe I (427.2 nm), Cr I (425.4 nm) and H y (434.0 nm), see fig. 4a. Photon efficiencies (number of ionisations per photon) are available for all these lines [lo], but apart from hydrogen are strongly temperature dependent. In this phase of COMPASS operation no measurements of boundary region plasma parameters were made and hence the measured photon fluxes could not be converted to absolute impurity influxes with any degree of accuracy. However the rate of change of photon efficiency with temperature is quite similar for all the impurity lines (apart from thydrogen) and hence relative influxes can be derived. It should be noted that the singly ionised or neutral states observed may well be located within the scrape-off layer, with a speciesdependent probability of penetrating to the main body of the plasma. Hence the relative influxes calculated from the chord integrated line emissions may not necessarily reflect the content of the impurity influx into the main plasma. With this caveat the line emission data of table 2 yield influx ratios of approximately 1: 3 : 0.05 for C : 0 : Cr, respectively, indicating that oxygen was probably the dominant impurity in these preboronized plasmas. The high loop voltage and impurity content resulted in substantial hard X-ray emission (energy > 250 keV) from these plasmas, fig. 3. 3.2. After boronization 3.2.1.
Density control
and recycling
behaviour
Initial attempts to run tokamaks immediately after boronization resulted in short pulses (< 10 ms) with a very high density not amenable to control by reducing the prefill, indicating very high recycling and wall retention. GDC in He was then used to remove hydrogen from the walls. Fig. 5 shows the time history of the RGA mass 2 signal (H,) during the 15 min glow in 3 X 10m3 mbar He, at 1 A, 400 V. It can be seen that the hydrogen signal rapidly falls with an e-folding time of approximately 2.2 min.
0 0 0
I
I” 10-6.
0
5 Time
(mins)
Fig. 5. Time histow of H, partial pressure during He GDC (3 X 10m3 mbar, 400 V, 1 A).
Following the He GDC the resulting tokamak discharges were of long duration but with very low density (< 1 x 10” mP3) and loop voltage I 0.2 V. Nevertheless the complete absence of hard X-rays indicated a slide-away rather than runaway regime. The gas prefill and gas puff rates were then increased shot-by-shot for a sequence of six discharges, each shot showing only a slight increase in density until standard shot conditions were reached. The subsequent shot however, with the same gas valve program, featured rapidly rising density. All further discharges in this series had densities rising to disruption, despite reducing the gas puff to zero. After 5 min GDC in He, strongly pumping walls were again produced and the same discharge behaviour cycle was repeated, with one discharge of the required density produced per cycle of _ six shots. This recycling behaviour (strongly pumping walls after He GDC followed by strong wall outgassing after a series of discharges) is similar to the behaviour observed on DIII-D after carbonisation [II]. In DIII-D density control was only possible by means a wall conditioning procedure involving accurately timed GDC sequences in hydrogen and helium between each tokamak discharge. The large dimensions of the DIII-D vacuum vessel enabled stable low pressure GDC plasmas to be initiated without application of RF by means of an electrode positioned at the wall radius and there
222
H.G. Esser et al. / Boronization of COMPASS
was little decrease in the shot repetition frequency. On COMPASS the GDC system was not sufficiently well automated to enable this procedure to be used. Hydrogen discharges with reasonable density control could only be obtained by He GDC for 5 min followed by several low density tokamak shots. The next 2 or 3 discharges would have reasonable recycling conditions enabling the required density to be achieved by preprogramming the gas valve voltage waveform and then the cycle would have to be repeated. After using this procedure for approximately 100 tokamak discharges with no indication of any change in the recycling behaviour it was decided to abandon operation in hydrogen and change the working gas to helium. Good density control was immediately obtained, with no requirement for GDC. 3.2.2. Impurity behaciour Standard discharges in hydrogen after boronization showed a dramatic improvement in impurity content. Fig. 4b shows a comparison of line emission from standard discharges before and after boronization. After boronization there is a virtual absence of oxygen, chromium and iron influxes and a considerable reduction in carbon (X i). The BIV triplet at 282.4 nm was used for boron influx monitoring. The large reductions in intrinsic impurity emission are quantified in table 2 together with other derived and measured parameters. As a result of the reduced impurity influx Zeff falls from 2.9 (preboronized) to 1.1 (postboronized). The loop voltage decreases from 2.4 to 1.3 V (see fig. 3b) with a corresponding reduction in ohmic power input, but with little change in electron temperature, corresponding to - 2 X increase in global electron energy confinement time. The total impurity radiation falls from 75% of the ohmic input to 30%. Impurity radiation in the soft X-ray region is dramatically reduced, the soft X-ray diode array system (energy range l-20 keV), suffering a drop in signal intensity by a factor - 40. It can be see from table 2 that after boronization there is an appreciable increase in ion temperature, but no simple explanation is available for this observation. If the protons and impurities are considered as a single ionic fluid then the equipartition power input would be expected to decrease with Zen, leading to a decrease in Ti for unchanged xi. The observed increase in Ti suggests an improvement in ion energy confinement. In contrast to the impurity behaviour the hydrogen influx is observed to increase after boronization. Several H, monitors are installed on COMPASS and all
indicated an approximate doubling in signal. The particle influx from recycling is considerably greater than the gas puff rate (> 10 x , corresponding to a coefficient of recycling of > 0.9) for these standard shot conditions and hence the increase in the H, remote from the gas feed position corresponds to a proportionate increase in the total hydrogen ionisation source term. Since the density is unchanged this would seem to suggest a decrease in particle confinement time by a factor 2. However the high Z,, value prior to boronization indicates that the impurities were making a substantial contribution to the electron density. If we assume for the sake of simplicity that the dominant impurity both before and after boronization is carbon, in the fully-stripped ionisation state, then the measured Z,, values give proton to electron density ratios of 0.56 and 0.9, respectively. The measured increase in hydrogen recycling influx is therefore consistent with this increase in discharge proton content, within the approximations of the simple analysis and lack of detailed experimental data, without invoking a decrease in particle confinement. 3.2.3. Helium discharges Helium had not been used as the working gas in COMPASS prior to boronization and hence comparison data are not available. Table 2 shows data from a set of standard helium discharges in the boronized torus following - 1.50 hydrogen and - 210 helium discharges (of typical duration - 200 ms). A considerable increase in impurity influx can be seen when compared with hydrogen discharges, almost certainly due to increased sputtering yields, but the impurity influxes are substantially lower than in preboronized hydrogen discharges. The BIV signal initially increased (X 1.7) after the change-over to helium indicating an enhancement in the erosion rate of the layer. The Z,, value of 1.5 indicates considerable contamination of the discharge by hydrogen, although it should be noted that the calculated Zeff is dependent on the particular temperature profile assumed and Zen values closer to 2 can be obtained by choosing different profiles. There is however a substantial reduction in H y, corresponding to the hydrogen recycling flux being considerably reduced, and good density control is obtained in all shots. During this series of helium discharges the torus was let up to atmosphere because of a diagnostic window failure. After 6 h of GDC in He good operating conditions were recovered. This behaviour is to be compared with the experience of preboronization vacuum openings where tens of hours of GDC followed by
223
H.G. Esser et al. / Boronization of COMPASS
Table 3 Boronization glow parameters
Ignites
Extinguishes
I I
I I
Volts
First boronization Gas Pressure Current
Voltage Duration
Second boronization 5% B(CH,),, 95% He
B(CH,L * 10-l mbar 1-15 mA 200-400 v _ 15 min
9x lo-” mbar
400 mA 375 v 115 min
225v
I I 0.4A
\-----7;
impurity fluxes. A significant reduction in B IV signal is also seen corresponding to film erosion/ removal.
I I
I 6x10
-’
IO -’
3x10-2
Pressure
(mb)
Fig. 6. Operating diagram for second boronization glow discharge, 0.5 X 10e3 mbar B(CH,), +variable He partial pressure.
several tokamak conditioning shots would be required. This resilience of the boronized layer to air exposure and its reactivation by plasma contact have been reported for all the boronized tokamaks ([1,4,5,7]). 3.2.4. Further hydrogen discharges Following more than 400 discharges in helium, hydrogen was again used as working gas. Good density control was now possible for tens of consecutive shots, with - 5 min of He GDC leading to recovery after eventual loss of density control. Low Z,, and loop voltage were observed, only slightly higher than in discharges immediately following boronization. A gradual deterioration in discharge quality with shot number was observed and it can be seen from table 2 that after a total of 1500 discharges (I, - 100-200 kA, n = (l-5) X 10” rnm3, T, - 700 eV, Ti - 150 eV) following boronization significant increases were apparent in all
4. Second boronization 4.1. Procedure Re-boronization of COMPASS was implemented after a substantial vacuum break for diagnostic installation. The vacuum vessel was baked to - 100 ’ C for 8 h followed by 50 h of GDC in H, and 30 min in He. Following the lack of control over the glow discharge parameters during the first boronization, an admixture of B(CH,), and He was used for the second boronization. Prior to running the boronization discharge the B(CH,), flow rate was set to give a partial pressure of 0.5 x lo-” mbar. The He flow rate was adjusted to give the ignition pressure of - 10-l mbar, and then reduced to as low a value as possible compatible with the required current to sustain the discharge. Fig. 6 shows the observed operating regime in discharge current/voltage versus pressure space. The transition from the high-current low-voltage high-pressure regime to the low-current high-voltage low-pressure mode as the He pressure was reduced was very sudden and usually resulted in the complete extinguishing of the glow, as the resistance of the discharge sharply increased. The glow could only be sustained by a rapid increase in
Table 4 A posteriori analysis of films - second boronization Mass thickness (pg/Cm*)
Sample
type Stainless Stainless Stainless stainless
steel steel steel steel
1b 2b 1’ 2’
Boron
Carbon
10.12 10.22 9.68 9.82
24.23 24.41 23.58 24.05
a Assumes film density of 1.2 g/cm3. ‘L Correspond to different measurement
positions on same sample.
Atom ratios
Calculated a layer thickness (nm)
0.46 0.46 0.46 0.46
286 288 271 282
H.G. Esser et ~11./ Boronizationof COMPASS
224 50 r
Le.07
4687
Time
imsl
Time
lmsl
Fig. 7. Electron density (n,) and gas puff programme (@) versus time in a sequence of hydrogen discharges following the second boronization. The plasma current is constant at 100 kA from 50-250 ms.
supply voltage simultaneous with the discharge mode transition. Table 3 summarises the glow parameters for the second boronization and compares them with the previous parameters. Probe-introduced samples were again used for film analysis. These showed a measured thickness of 170 nm and a B/C ratio of 0.45, see table 4. 4.2. Plasma performance 4.2. I. Density control, recycling and isotope exchange The initial performance of the tokamak after this boronization closely duplicated that after the first boronization. Helium GDC (15 min) was used to remove excess hydrogen from the walls. For tokamak operation in hydrogen this resulted in strongly pumping walls with Iow density slide-away discharges despite strong gas puffing. Repeating the same puff led to higher density shot by shot, with the density showing rapid pump-out after termination of the gas puff, even at high densities (fig. 7). After a number of discharges (- 10) the density would rise uncontrollably with prefill only. Discharges with helium prefill produced high densities even with no gas puff. Neutral particle analyser (NPA) and spectroscopic diagnostic data showed substantial hydrogen recycling in these plasmas. Dis-
charges with hydrogen prefill and helium gas puff resulted in more recycling than the use of hydrogen alone and it was noticeable that when the helium gas puff was turned off there was no signi~cant density pumpout. The observed density behaviour points to the presence of a large reservoir of hydrogen in the deposited films. Glow cleaning in helium results in a depletion of the near-surface portion of this reservoir, the released hydrogen being pumped away. The wall surface can then absorb hydrogen until the depleted layer is refilled, either by tokamak operation in hydrogen or else by diffusion from the deeper regions of the films. Tokamak operation in helium also evolves hydrogen from the walls but the released gas is absorbed into the plasma and leads to an unwanted density increase. The helium is not pumped by the walls and is completeiy recycled. This behaviour is consistent with the concept of a large reservoir of hydrogen in the boronized layer not accessible to depletion by GDC and constantly replenishing the near-surface layer by diffusion. It is possible to get a very approximate figure for the replenishment rate by monitoring the hydrogen partial pressure during He GDC. As described above this shows a rapid decrease over a time scale - minutes, corresponding to the removal of - IO”’ H2 molecules or - 2 monolayers, to a constant level (see fig. 7). We assume that it is held at this value, - 5 X 10” mol/s, by an equilibrium between the rate at which hydrogen is removed from the layer (equal to the rate at which it diffuses into the near-surface region) and the rate of removal from the vacuum vessel by the pumps. The time-scale for replenishment of the near-surface region, after He GDC conditioning, is - 3 h, which is consistent with the observation that when He GDC is carried out the end of an operational week the first tokamaks of the following week exhibit high recycling. It is consistent also with the time period in an operating day over which low recycling persists after He GDC (although this ignores the effect of tokamak discharges on the wall loading). During this period of time a closed loop density feedback system, operating from the 2 mm microwave interferometer, was installed, leading to much better control over plasma behaviour. Short duration He GDC (- 5 min) was still required to recondition the walls prior to commencing a day’s tokamak programme, and also 1 or 2 times during the day. After operation for some 150 shots in hydrogen, with occasional discharges in helium, the working gas was changed to deuterium. Mass resolved NPA measurements showed that for several tens of discharges
H.G. Esser et al. / Boronization of COMPASS
D,/H, agreed with the NPA nD/nH ratio to within 20%. On TCA changeover of the working gas isotope species was investigated after boronization [6]. They report that only a few discharges were required to reduce the concentration of hydrogen to < 5% (measured spectroscopically)
1.6
1.4
1.2
=
ee
1
225
4.2.2. Impurity behauiour The effect of the second boronization on impurity behaviour was similar to that observed previously. The oxygen, chromium and iron influxes were reduced to below the experimental measurement threshold. Carbon was substantially reduced and the hydrogen influx approximately doubled (see table 2). Low voltage, Z,, and total impurity radiation resulted. When the working gas was changed to deuterium only a slight increase in impurity influxes was observed.
.o
0.8
0.6
0.4
5290
5260
5300
5. Summary Shot
Number
Fig. 8. Isotope ratio nr,/nH for a sequence of shots in D, (I, = 100 kA, B, = 1.1 T, K=(1.5-3.2)~ lOI m-‘) following 15 min He GDC, after the second boronization.
after the changeover hydrogen dominated the discharge (11,,/n~ < 0.5). The D/H ratio increased after He GDC and then decreased shot-by-shot. Fig. 8 shows this behaviour in a sequence of D, discharges immediately after He GDC, some 160 discharges after the working gas change-over, and it can be seen that no/n” is still low, u 1. This behaviour is again consistent with a large amount of hydrogen deep in the layer and a continually replenished near-surface region. The D/H ratio was eventually increased to 2-3 by 16 h of GDC in D, followed by 15 min GDC in He but after 100 shots had again fallen to < 2, where it remained for the rest of this operational period of COMPASS, involving some 1400 shots after the changeover to D,. It should be mentioned that the isotope ratio of the recycling influx is monitored by D,/H, spectroscopy both close to the limiter and 180” away toroidally. Both sets of observations yield D/H ratios considerably greater than the NPA no/n” value (by a factor _ 3). This is not understood at present, especially since during a short series of shots just prior to the second boronization deuterium puffing was used in hydrogen discharges and the measured
Trimethyl boron, a much less hazardous material than diborane, has been used to boronize COMPASS, a tokamak not previously conditioned by any film deposition technique. First boronization of COMPASS used a glow discharge in pure B(CH,),. The glow discharge was unstable and incapable of being operated at low pressure. The resulting deposited film was of low B/C ratio. A second boronization used an admixture of 5% B(CH,), and 95% He at a total pressure of 9 x 10-j mbar. The stable low pressure glow resulted in the deposition of a much thicker film with higher B/C ratio. A dramatic reduction in impurity influxes was observed following each boronization. The oxygen, chromium and iron line emissions were reduced to below the experimental measurement threshold, and carbon was reduced by a factor 5. As a result the Z eff, loop voltage and impurity radiation fraction were much lower than in similar preboronization plasmas, and the electron energy confinement time was approximately doubled. The observed hydrogen influx increase was consistent with the increased proton content of the plasmas, with no change in particle confinement. Impurity control lasts for several hundred discharges. Even after 1500 shots the plasmas had not deteriorated to the preboronized state. Rapid recov-
226
H.G. Esser et al. / Boronization of’ COMPASS
ery to a well-conditioned torus was observed after vacuum openings. After each boronization, density control in hydrogen tokamak discharges was difficult to achieve, the torus walls acting either as a strong pump or a gas source, depending on the number of tokamak discharges since the last He GDC session. This was avoided after the first boronization by operating in He for several hundred shots. Subsequent discharges in hydrogen exhibited good density control and low impurity levels. The second boronization resulted in a much larger reservoir of hydrogen in the walls with only the near-surface region being depleted by He GDC. Tokamak operation in He led to density increase and lack of control. Density control was made possible by implementation of a closed-loop feedback system in conjunction with occasional He GDC. The plasma isotope mixture could not be changed arbitrarily. Due to the high hydrogen recycling flux, deuterium discharges were still heavily polluted with hydrogen several hundred shots after changing the working gas. It is planned to use dcuterated trimethyl boron in the next boronization of COMPASS (after installation of the D-vessel) when significantly isotopically-purer deuterium plasmas are expected to result, but ideally a solution to avoid the high gas retention in the films is required.
Acknowledgements We are grateful to D.C. Robinson and T.N. Todd for advice and encouragement during the period of this work, to the COMPASS Physics Team for diagnostic measurements and to the Technical and Engineering Sections for machine operations. Analysis of the boronized samples was performed by P. Karduck at GfE, Technical University Aachen.
References [l] J. Winter et [2] J. Winter et [3] J. Winter et [4] U. Schneider
al., J. Nucl. Mater. 162-164 (1989) 713. al., J. Nucl. Mater. 122 & 123 (1984) 1187. al., Nucl. Instr. and Meth. B23 (1987) 538. et al., J. Nucl. Mater. 176 & 177 (1991) 350.
[5] C. Hollenstein et al., J. Nucl. Mater. 176 & 177 (1991) 343. [6] T. Dudok de Wit, B.P. Duval, C. Hollenstein and B. Joyce, Proc. 17th EPS Conference on Controlled Fusion and Plasma Heating, Amsterdam, 1990, p. 1476. [7] H.F. Dylla, J. Nucl. Mater. 176 & 177 (1991) 337. [8] H.G. Esser, H.B. Reimer, J. Winter and D. Ringer, Fusion Technol. 1 (1988) 791. [9] J. Winter and H.G. Esser, J. Nucl. Mater. 176 & 177 (1991) 481. [lOI K.H. Behringer, J. Nucl. Mater. 1455147 (1987) 145. [II] G. Jackson et al., J. Nucl. Mater. 176 & 177 (1991) 311.
Materials
journal of nuclear materials
186 (1992) 217-226
Boronization
of COMPASS
H.G. Esser ‘, S.J. Fielding ‘, S.D. Hanks ‘, P.C. Johnson ‘, A. Kislyakov ’ and J. Winter b aAEA Fusion, Culham Laboratory, Euratom / AEA Fusion Association, Abingdon, Oxon OX14 3DB, United Kingdom ’ Institut fiir Plasmaphysik, Kemforschungsanlage Jiilich GmbH, Association Euratom / KFA, D-51 70 Jiilich, Germany ‘A. F. Ioffe Physico-Technical Institute, Academy of Sciences of the USSR, Saint Petersburg, USSR Received
2 August
1991; accepted
31 August
1991
Boronization of COMPASS has been carried out using trimethyl boron, a less hazardous alternative to diborane used to boronize other machines. Two separate boronizations, using different gas and glow discharge parameters, have been udertaken. Both have resulted in excellent long-term impurity control, albeit with a significant change to the hydrogen recycling characteristics
1. Introduction Boronization, the plasma-chemical depostion of thin boron/carbon films onto the inner wall of a tokamak, has been developed as a surface conditioning technique to reduce impurity fluxes into the plasma. This procedure was first used on TEXTOR [l], which had previously been conditioned by carbonisation (deposition of thin hydrogenated carbon films [2,3]). The boronized layer was deposited at a wall temperature of 300-350 “C using an rf-assisted glow discharge in a throughflow of 0.8He + O.lB,H, + O.lCH,. The resulting film, of average thickness 50 k 20 nm, was found to have a nonstochiometric B to C ratio c 1. A strong reduction in oxygen influx during tokamak discharges, by a factor - 4, and carbon by a factor _ 2 was observed compared with good carbonised conditions, and good hydrogen recycling control was achieved. These beneficial conditons in TEXTOR lasted for several hundered discharges, including numerous shots with ICRH at power levels greater than 2MW. Following the favourable results from TEXTOR, boronization has been used on ASDEX [4], TCA [5,6] and TFTR [7]. In ASDEX and TCA boronization was carried out at room temperature, with similar results to those observed on TEXTOR in terms of film constituency and plasma improvement. In all the above cases diborane (B,H,) was used as the boron-containing gas. Diborane is very toxic and 0022-3115/92/$05.00
0 1992 - Elsevier
Science
Publishers
highly explosive and requires special handling techniques, involving considerable and costly modifications to standard gas and vacuum systems [8]. Because of this, a programme of work was carried out at KFA Jiilich to identify a less hazardous gas for boronization which would allow more convenient usage. Trimethyl boron, B(CH,),, a gas which is pyrophoric rather than explosive, was found to be the most promising and was used in TEXTOR with apparently similar results to diborane [9]. However boronization by diborane gives rise to a very durable surface film, which lasts for hundreds of tokamak discharges, and hence results from the use of trimethyl boron on TEXTOR could well have been influenced by the residues from previous boronizations. In this paper we report the use of trimethyl boron for the boronization of COMPASS, a tokamak not previously boronized, or carbonised. We describe the layer deposition procedure and the resulting improvements to the tokamak performance. We also describe a subsequent boronization using a different gas mixture. The experimental physics programme on COMPASS, involving second harmonic ECRH, requires clean, low density, low loop voltage plasmas with minimal runaway/ slideaway content. These conditions had nto been obtainable by conventional wall-conditioning techniques. Boronization with trimethyl boron provided a relatively easily implemented means of attaining low impurity plasmas.
B.V. All rights reserved
H.G. Esser et al. / Boronization of COMPASS
218
2. Boronization
H,
system
Prior to boronization, COMPASS wall conditioning was by means of rf-assisted golw discharge cleaning (GDC) in hydrogen or helium at room temperature. Moderate vessel baking to 120 o C was used only after a major vacuum opening and simultaneous baking and GDC was not possible. The COMPASS GDC system consists of a single stainless steel rf/dc anode mounted on a horizontal probe, capable of being driven in to the minor axis of the torus. Up to 40 W of rf at 13.6 MHz, and 650 V dc can be applied to the water-cooled electrode. Some modification was made to the gas handling and exhaust systems to accommodate the use of trimethyl boron. The Roots and rotary pumps in the turbopump backing lines were operated wtih dry nitrogen gas ballast. Exhaust gas was vented to atmosphere via a pipe fitted with a flame arrester, through the roof of the COMPASS experimental building. Test boronization in a small planar rf diode plasma system had been previously studied at Julich [9]. In these Jiilich experiments samples were negatively biased to 200-350 V. The sample temperature was varied between 100 and 230” C, with a trimethyl boron pressure of 2.7 X lo-’ mbar. No dilution by other gases was used. The B/C ratio of the resulting film was measured to be 0.6 (compared with 0.33 in the gas). For boronization in TEXTOR an admixture of 85% helium was used, to a total pressure of 3 X lop3 mbar, with the vessel liner and limiters heated to 350 o C. The helium was added in an attempt to reduce the hydrogen content of the films but resulted in a of lower B/C ratio of - 0.4. For initial boronization COMPASS it was decided to use trimethyl boron alone, in order to maximise the boron content of the deposited films. Fig. 1 is a schematic plan view of the COMPASS vacuum vessel showing the positions of the gas inlet, GDC electrode and pumping port. Also shown is the location of a removable probe containing samples of silicon, stainless steel and graphite positioned at the wall radius, for film analysis. Residual gas analysis was by means of a Spectra-Metrics Multi-Quad quadrupole mass spectrometer, coupled to the torus by means of a 100 to 1 pressure reducing aperture, enabling RGA measurements to be made at high torus pressures. The GDC gas feed system consisted of a manually operated needle valve, with pressure monitoring by means of a Leybold VM211 spinning rotor gauge programmed to molecular mass 56. For standard GDC operation on COMPASS in hydrogen or helium the
Jmpurity
Spectroscopy
Pumps
Fig.
1. Schematic
plan
and diagnostic
view
positions
of COMPASS relevant
showing
vacuum
to boronization.
glow is ignited by raising the gas pressure to - I X IO- ’ mbar. After ignition the gas flow rate is reduced to give the required operating pressure, - 2 X 10m3 mbar, while at the same time the applied dc voltage is increased (typically to 350-400 V) to set the current to the level of - 1 A. It was intended to follow this procedure for the boronization, however it was found that even close to the ignition pressure, - 5 X lo-* mbar, the glow discharge was very unstable, with the current fluctuating on a time scale of - 1 s between 15 mA and 1 A, and the cathode voltage varying between 400 and 200 V. Any reduction in the gas pressure led to immediate termination of the glow. This behaviour of the glow was similar to that observed for Hz, He GDC on COMPASS after a vacuum opening, where a few minutes of high pressure operation (- 5 x 10-z mbar) was usually adequate to condition the walls and enable low pressures to be accessed. Accordingly the boronization glow was operated for several minutes at high pressure, and each time attempts were made to reduce the pressure the glow extinguished. After - 15 min of this high pressure glow the available 10 g of B(CH,), was exhausted and the boronization was terminated. Fig. 2 shows RGA measurements taken during the boronization, for the same flow rate of BfCH,), (4 mbar/s), with GDC on and off. Winter et al. [9] have made a tentative identification of the cracking pattern of B(CH,), and present an assessment of the plasma and wall chemistry involved in the boronization process. The COMPASS RGA data show qualitatively similar effects. When the GDC is turned on, disintegration of B(CH,), occurs to a large degree and there
H.G. Esser et al. / Boronization of COMPASS B(CHJ)J
219
off
GDC
100
SHOT
2823
Q Q
10
20
30
40
50
60
Mass/emu
GDCon
NCH,),
0
I,10
Time
0 0
10
20
30
40
50
60
Mass/amu
Fig. 2. Residual gas analysis measurements in B(CH,), and during glow discharge boronization.
before
is copious evolution of hydrogen, with methane and C, hydrocarbons around m/q = 28 also produced. After the boronization procedure was completed the samples were removed from the torus. Depth probe measurements at Culham using a Sloan Dektak surface profile measuring system gave a layer thickness of 40-70 nm. Data from later analysis of the film composition by Electron Probe Microanalysis technique is given in table 1, together with the data from similar
21m
(ms)
Fig. 3. Plasma parameters in standard hydrogen discharges (I= 100 kA, B, = 1.1 T, <= 1.8X 10’” mm’) before and after the first boronization.
measurements taken during trimethyl boronization of TEXTOR. It can be seen that the B/C ratio in the COMPASS films is much lower (- factor 2) than in TEXTOR. In addition the estimated film thicknesses given in table 1 are considerably less than those actually measured.
3. Plasma parameters 3.1. Before boronization
Table 1 A posteriori
analysis
of films - first boronization
Sample
Mass thickness
type
(kg/cm’)
Silicon Stainless steel TEXTOR Si sample a Assumes
Boron
Carbon
B/C
Calculated a layer thickness (nm)
0.32 0.45
2.6 2.1
0.13 0.24
24.3 21.3
8.6
0.38
96.3
2.95
film density
of 1.2 g/cm”
Atom ratios
The COMPASS vacuum vessel (0.57 m major radius, 0.22 m minor radius, 316 stainless steel) has a single poloidal ring graphite limiter, radius 0.196 m and 64 small graphite tiles (each with _ 0.02 m2 plasma-facing surface area) at 0.21 m. A series of standard test discharges in hydrogen was run, prior to boronization, at 1.1 T B,, 100 kA I, and 1.5 to 2.0 X 10’” mm3 average line-of-sight density. Typical waveforms are shown in fig. 3a. Detailed measurements were made with all available dianostics and
H. G. Esser et al. / Boronization of COMPASS
220 Table 2 COMPASS standard discharges Wall Gas Shot no. B,(T) Current &A) Loop volts (V) T, (eV)/(SiLi) Ti (eV) (10” m-7) Wall CII (a.u.) Wall 011 (a.u.) Wall Cr I (a.u.) Wall H y (a.u.) BIV PrBd(kWs) E,
Prad
/Pohm~c
X-ray anom. (SBD array) Z,ff
UB
Bl
H2
H2
2823 + 1.1 100 2.4 730 * 20 190 +10 1.7 580 270 150 300
2875 + 1.1 105 1.3 690 *30 250 * 1.5 1.6 120 <4 <4 650 3000 40 0.3 2.1 1.1
180 0.75 75.6 2.9
Bl He 3225 + 1.1 100 1.75 646 &23 210 k 25 1.6 300 40 90 40 2900 87 0.5 n/a 1.5
Bl
Bl
B2
B2
H?
H2
H2
D2
4338 + 1.1 95
4874 + 1.1 100 1.5 n/a _
4215 - 1.1 100 1.7 600 +20 110 + 10 1.6 150 50 12 270 900 n/a n/a 14.1 1.3
1.75 600 *50 130 & IO 1.4 205 54 21 430 n/a n/a n/a 26.6 1.4
1.75 80 5 <4 400 3000 _
5757 + 1.1 100
1.45 750 *50 155 +15 1.8 80 IO <4 180 3000 _
_ 1.6
UB: unboronized; Bl, B2: first and second boronizations.
values of characterising parameters are given in table 2. These parameter values correspond to a well-conditioned torus, in that the vacuum vessel had been baked, many hours ( > 100) of hydrogen GDC had been imple-
mented and several hundred tokamak shots had been run since the last vacuum opening. However table 2 shows that the resulting plasmas were relatively poor, with the high Z,, value (calculated from the measured
Fig. 4. Comparison of visible line emission, centred at 432.5 nm, in standard hydrogen discharges (1, = 100 kA, B, = 1.1 T, <= 1.8~ 10” mm”) before and after the first boronization.
221
H.G. Esser et al. / Boronization of COMPASS
loop voltage and central electron temperature (SiLi), assuming a parabolic-squared temperature profile, Spitzer resistivity plus neo-classical corrections and Z,, independent of radius) indicating a high impurity content. Visible line emission spectroscopy was used to separately view the limiter and wall regions of the tokamak. Measurements were make using a calibrated medium resolution 0.6 m SPEX monochromator fitted with a diode array, collecting line emission data over a 12.5 nm spectral region centred at 432.5 nm. This instrument provided time resolved (17 ms) chord integrated line emissions from 011 (435.1 nm), CII (426.7 nm), Fe I (427.2 nm), Cr I (425.4 nm) and H y (434.0 nm), see fig. 4a. Photon efficiencies (number of ionisations per photon) are available for all these lines [lo], but apart from hydrogen are strongly temperature dependent. In this phase of COMPASS operation no measurements of boundary region plasma parameters were made and hence the measured photon fluxes could not be converted to absolute impurity influxes with any degree of accuracy. However the rate of change of photon efficiency with temperature is quite similar for all the impurity lines (apart from thydrogen) and hence relative influxes can be derived. It should be noted that the singly ionised or neutral states observed may well be located within the scrape-off layer, with a speciesdependent probability of penetrating to the main body of the plasma. Hence the relative influxes calculated from the chord integrated line emissions may not necessarily reflect the content of the impurity influx into the main plasma. With this caveat the line emission data of table 2 yield influx ratios of approximately 1: 3 : 0.05 for C : 0 : Cr, respectively, indicating that oxygen was probably the dominant impurity in these preboronized plasmas. The high loop voltage and impurity content resulted in substantial hard X-ray emission (energy > 250 keV) from these plasmas, fig. 3. 3.2. After boronization 3.2.1.
Density control
and recycling
behaviour
Initial attempts to run tokamaks immediately after boronization resulted in short pulses (< 10 ms) with a very high density not amenable to control by reducing the prefill, indicating very high recycling and wall retention. GDC in He was then used to remove hydrogen from the walls. Fig. 5 shows the time history of the RGA mass 2 signal (H,) during the 15 min glow in 3 X 10m3 mbar He, at 1 A, 400 V. It can be seen that the hydrogen signal rapidly falls with an e-folding time of approximately 2.2 min.
0 0 0
I
I” 10-6.
0
5 Time
(mins)
Fig. 5. Time histow of H, partial pressure during He GDC (3 X 10m3 mbar, 400 V, 1 A).
Following the He GDC the resulting tokamak discharges were of long duration but with very low density (< 1 x 10” mP3) and loop voltage I 0.2 V. Nevertheless the complete absence of hard X-rays indicated a slide-away rather than runaway regime. The gas prefill and gas puff rates were then increased shot-by-shot for a sequence of six discharges, each shot showing only a slight increase in density until standard shot conditions were reached. The subsequent shot however, with the same gas valve program, featured rapidly rising density. All further discharges in this series had densities rising to disruption, despite reducing the gas puff to zero. After 5 min GDC in He, strongly pumping walls were again produced and the same discharge behaviour cycle was repeated, with one discharge of the required density produced per cycle of _ six shots. This recycling behaviour (strongly pumping walls after He GDC followed by strong wall outgassing after a series of discharges) is similar to the behaviour observed on DIII-D after carbonisation [II]. In DIII-D density control was only possible by means a wall conditioning procedure involving accurately timed GDC sequences in hydrogen and helium between each tokamak discharge. The large dimensions of the DIII-D vacuum vessel enabled stable low pressure GDC plasmas to be initiated without application of RF by means of an electrode positioned at the wall radius and there
222
H.G. Esser et al. / Boronization of COMPASS
was little decrease in the shot repetition frequency. On COMPASS the GDC system was not sufficiently well automated to enable this procedure to be used. Hydrogen discharges with reasonable density control could only be obtained by He GDC for 5 min followed by several low density tokamak shots. The next 2 or 3 discharges would have reasonable recycling conditions enabling the required density to be achieved by preprogramming the gas valve voltage waveform and then the cycle would have to be repeated. After using this procedure for approximately 100 tokamak discharges with no indication of any change in the recycling behaviour it was decided to abandon operation in hydrogen and change the working gas to helium. Good density control was immediately obtained, with no requirement for GDC. 3.2.2. Impurity behaciour Standard discharges in hydrogen after boronization showed a dramatic improvement in impurity content. Fig. 4b shows a comparison of line emission from standard discharges before and after boronization. After boronization there is a virtual absence of oxygen, chromium and iron influxes and a considerable reduction in carbon (X i). The BIV triplet at 282.4 nm was used for boron influx monitoring. The large reductions in intrinsic impurity emission are quantified in table 2 together with other derived and measured parameters. As a result of the reduced impurity influx Zeff falls from 2.9 (preboronized) to 1.1 (postboronized). The loop voltage decreases from 2.4 to 1.3 V (see fig. 3b) with a corresponding reduction in ohmic power input, but with little change in electron temperature, corresponding to - 2 X increase in global electron energy confinement time. The total impurity radiation falls from 75% of the ohmic input to 30%. Impurity radiation in the soft X-ray region is dramatically reduced, the soft X-ray diode array system (energy range l-20 keV), suffering a drop in signal intensity by a factor - 40. It can be see from table 2 that after boronization there is an appreciable increase in ion temperature, but no simple explanation is available for this observation. If the protons and impurities are considered as a single ionic fluid then the equipartition power input would be expected to decrease with Zen, leading to a decrease in Ti for unchanged xi. The observed increase in Ti suggests an improvement in ion energy confinement. In contrast to the impurity behaviour the hydrogen influx is observed to increase after boronization. Several H, monitors are installed on COMPASS and all
indicated an approximate doubling in signal. The particle influx from recycling is considerably greater than the gas puff rate (> 10 x , corresponding to a coefficient of recycling of > 0.9) for these standard shot conditions and hence the increase in the H, remote from the gas feed position corresponds to a proportionate increase in the total hydrogen ionisation source term. Since the density is unchanged this would seem to suggest a decrease in particle confinement time by a factor 2. However the high Z,, value prior to boronization indicates that the impurities were making a substantial contribution to the electron density. If we assume for the sake of simplicity that the dominant impurity both before and after boronization is carbon, in the fully-stripped ionisation state, then the measured Z,, values give proton to electron density ratios of 0.56 and 0.9, respectively. The measured increase in hydrogen recycling influx is therefore consistent with this increase in discharge proton content, within the approximations of the simple analysis and lack of detailed experimental data, without invoking a decrease in particle confinement. 3.2.3. Helium discharges Helium had not been used as the working gas in COMPASS prior to boronization and hence comparison data are not available. Table 2 shows data from a set of standard helium discharges in the boronized torus following - 1.50 hydrogen and - 210 helium discharges (of typical duration - 200 ms). A considerable increase in impurity influx can be seen when compared with hydrogen discharges, almost certainly due to increased sputtering yields, but the impurity influxes are substantially lower than in preboronized hydrogen discharges. The BIV signal initially increased (X 1.7) after the change-over to helium indicating an enhancement in the erosion rate of the layer. The Z,, value of 1.5 indicates considerable contamination of the discharge by hydrogen, although it should be noted that the calculated Zeff is dependent on the particular temperature profile assumed and Zen values closer to 2 can be obtained by choosing different profiles. There is however a substantial reduction in H y, corresponding to the hydrogen recycling flux being considerably reduced, and good density control is obtained in all shots. During this series of helium discharges the torus was let up to atmosphere because of a diagnostic window failure. After 6 h of GDC in He good operating conditions were recovered. This behaviour is to be compared with the experience of preboronization vacuum openings where tens of hours of GDC followed by
223
H.G. Esser et al. / Boronization of COMPASS
Table 3 Boronization glow parameters
Ignites
Extinguishes
I I
I I
Volts
First boronization Gas Pressure Current
Voltage Duration
Second boronization 5% B(CH,),, 95% He
B(CH,L * 10-l mbar 1-15 mA 200-400 v _ 15 min
9x lo-” mbar
400 mA 375 v 115 min
225v
I I 0.4A
\-----7;
impurity fluxes. A significant reduction in B IV signal is also seen corresponding to film erosion/ removal.
I I
I 6x10
-’
IO -’
3x10-2
Pressure
(mb)
Fig. 6. Operating diagram for second boronization glow discharge, 0.5 X 10e3 mbar B(CH,), +variable He partial pressure.
several tokamak conditioning shots would be required. This resilience of the boronized layer to air exposure and its reactivation by plasma contact have been reported for all the boronized tokamaks ([1,4,5,7]). 3.2.4. Further hydrogen discharges Following more than 400 discharges in helium, hydrogen was again used as working gas. Good density control was now possible for tens of consecutive shots, with - 5 min of He GDC leading to recovery after eventual loss of density control. Low Z,, and loop voltage were observed, only slightly higher than in discharges immediately following boronization. A gradual deterioration in discharge quality with shot number was observed and it can be seen from table 2 that after a total of 1500 discharges (I, - 100-200 kA, n = (l-5) X 10” rnm3, T, - 700 eV, Ti - 150 eV) following boronization significant increases were apparent in all
4. Second boronization 4.1. Procedure Re-boronization of COMPASS was implemented after a substantial vacuum break for diagnostic installation. The vacuum vessel was baked to - 100 ’ C for 8 h followed by 50 h of GDC in H, and 30 min in He. Following the lack of control over the glow discharge parameters during the first boronization, an admixture of B(CH,), and He was used for the second boronization. Prior to running the boronization discharge the B(CH,), flow rate was set to give a partial pressure of 0.5 x lo-” mbar. The He flow rate was adjusted to give the ignition pressure of - 10-l mbar, and then reduced to as low a value as possible compatible with the required current to sustain the discharge. Fig. 6 shows the observed operating regime in discharge current/voltage versus pressure space. The transition from the high-current low-voltage high-pressure regime to the low-current high-voltage low-pressure mode as the He pressure was reduced was very sudden and usually resulted in the complete extinguishing of the glow, as the resistance of the discharge sharply increased. The glow could only be sustained by a rapid increase in
Table 4 A posteriori analysis of films - second boronization Mass thickness (pg/Cm*)
Sample
type Stainless Stainless Stainless stainless
steel steel steel steel
1b 2b 1’ 2’
Boron
Carbon
10.12 10.22 9.68 9.82
24.23 24.41 23.58 24.05
a Assumes film density of 1.2 g/cm3. ‘L Correspond to different measurement
positions on same sample.
Atom ratios
Calculated a layer thickness (nm)
0.46 0.46 0.46 0.46
286 288 271 282
H.G. Esser et ~11./ Boronizationof COMPASS
224 50 r
Le.07
4687
Time
imsl
Time
lmsl
Fig. 7. Electron density (n,) and gas puff programme (@) versus time in a sequence of hydrogen discharges following the second boronization. The plasma current is constant at 100 kA from 50-250 ms.
supply voltage simultaneous with the discharge mode transition. Table 3 summarises the glow parameters for the second boronization and compares them with the previous parameters. Probe-introduced samples were again used for film analysis. These showed a measured thickness of 170 nm and a B/C ratio of 0.45, see table 4. 4.2. Plasma performance 4.2. I. Density control, recycling and isotope exchange The initial performance of the tokamak after this boronization closely duplicated that after the first boronization. Helium GDC (15 min) was used to remove excess hydrogen from the walls. For tokamak operation in hydrogen this resulted in strongly pumping walls with Iow density slide-away discharges despite strong gas puffing. Repeating the same puff led to higher density shot by shot, with the density showing rapid pump-out after termination of the gas puff, even at high densities (fig. 7). After a number of discharges (- 10) the density would rise uncontrollably with prefill only. Discharges with helium prefill produced high densities even with no gas puff. Neutral particle analyser (NPA) and spectroscopic diagnostic data showed substantial hydrogen recycling in these plasmas. Dis-
charges with hydrogen prefill and helium gas puff resulted in more recycling than the use of hydrogen alone and it was noticeable that when the helium gas puff was turned off there was no signi~cant density pumpout. The observed density behaviour points to the presence of a large reservoir of hydrogen in the deposited films. Glow cleaning in helium results in a depletion of the near-surface portion of this reservoir, the released hydrogen being pumped away. The wall surface can then absorb hydrogen until the depleted layer is refilled, either by tokamak operation in hydrogen or else by diffusion from the deeper regions of the films. Tokamak operation in helium also evolves hydrogen from the walls but the released gas is absorbed into the plasma and leads to an unwanted density increase. The helium is not pumped by the walls and is completeiy recycled. This behaviour is consistent with the concept of a large reservoir of hydrogen in the boronized layer not accessible to depletion by GDC and constantly replenishing the near-surface layer by diffusion. It is possible to get a very approximate figure for the replenishment rate by monitoring the hydrogen partial pressure during He GDC. As described above this shows a rapid decrease over a time scale - minutes, corresponding to the removal of - IO”’ H2 molecules or - 2 monolayers, to a constant level (see fig. 7). We assume that it is held at this value, - 5 X 10” mol/s, by an equilibrium between the rate at which hydrogen is removed from the layer (equal to the rate at which it diffuses into the near-surface region) and the rate of removal from the vacuum vessel by the pumps. The time-scale for replenishment of the near-surface region, after He GDC conditioning, is - 3 h, which is consistent with the observation that when He GDC is carried out the end of an operational week the first tokamaks of the following week exhibit high recycling. It is consistent also with the time period in an operating day over which low recycling persists after He GDC (although this ignores the effect of tokamak discharges on the wall loading). During this period of time a closed loop density feedback system, operating from the 2 mm microwave interferometer, was installed, leading to much better control over plasma behaviour. Short duration He GDC (- 5 min) was still required to recondition the walls prior to commencing a day’s tokamak programme, and also 1 or 2 times during the day. After operation for some 150 shots in hydrogen, with occasional discharges in helium, the working gas was changed to deuterium. Mass resolved NPA measurements showed that for several tens of discharges
H.G. Esser et al. / Boronization of COMPASS
D,/H, agreed with the NPA nD/nH ratio to within 20%. On TCA changeover of the working gas isotope species was investigated after boronization [6]. They report that only a few discharges were required to reduce the concentration of hydrogen to < 5% (measured spectroscopically)
1.6
1.4
1.2
=
ee
1
225
4.2.2. Impurity behauiour The effect of the second boronization on impurity behaviour was similar to that observed previously. The oxygen, chromium and iron influxes were reduced to below the experimental measurement threshold. Carbon was substantially reduced and the hydrogen influx approximately doubled (see table 2). Low voltage, Z,, and total impurity radiation resulted. When the working gas was changed to deuterium only a slight increase in impurity influxes was observed.
.o
0.8
0.6
0.4
5290
5260
5300
5. Summary Shot
Number
Fig. 8. Isotope ratio nr,/nH for a sequence of shots in D, (I, = 100 kA, B, = 1.1 T, K=(1.5-3.2)~ lOI m-‘) following 15 min He GDC, after the second boronization.
after the changeover hydrogen dominated the discharge (11,,/n~ < 0.5). The D/H ratio increased after He GDC and then decreased shot-by-shot. Fig. 8 shows this behaviour in a sequence of D, discharges immediately after He GDC, some 160 discharges after the working gas change-over, and it can be seen that no/n” is still low, u 1. This behaviour is again consistent with a large amount of hydrogen deep in the layer and a continually replenished near-surface region. The D/H ratio was eventually increased to 2-3 by 16 h of GDC in D, followed by 15 min GDC in He but after 100 shots had again fallen to < 2, where it remained for the rest of this operational period of COMPASS, involving some 1400 shots after the changeover to D,. It should be mentioned that the isotope ratio of the recycling influx is monitored by D,/H, spectroscopy both close to the limiter and 180” away toroidally. Both sets of observations yield D/H ratios considerably greater than the NPA no/n” value (by a factor _ 3). This is not understood at present, especially since during a short series of shots just prior to the second boronization deuterium puffing was used in hydrogen discharges and the measured
Trimethyl boron, a much less hazardous material than diborane, has been used to boronize COMPASS, a tokamak not previously conditioned by any film deposition technique. First boronization of COMPASS used a glow discharge in pure B(CH,),. The glow discharge was unstable and incapable of being operated at low pressure. The resulting deposited film was of low B/C ratio. A second boronization used an admixture of 5% B(CH,), and 95% He at a total pressure of 9 x 10-j mbar. The stable low pressure glow resulted in the deposition of a much thicker film with higher B/C ratio. A dramatic reduction in impurity influxes was observed following each boronization. The oxygen, chromium and iron line emissions were reduced to below the experimental measurement threshold, and carbon was reduced by a factor 5. As a result the Z eff, loop voltage and impurity radiation fraction were much lower than in similar preboronization plasmas, and the electron energy confinement time was approximately doubled. The observed hydrogen influx increase was consistent with the increased proton content of the plasmas, with no change in particle confinement. Impurity control lasts for several hundred discharges. Even after 1500 shots the plasmas had not deteriorated to the preboronized state. Rapid recov-
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ery to a well-conditioned torus was observed after vacuum openings. After each boronization, density control in hydrogen tokamak discharges was difficult to achieve, the torus walls acting either as a strong pump or a gas source, depending on the number of tokamak discharges since the last He GDC session. This was avoided after the first boronization by operating in He for several hundred shots. Subsequent discharges in hydrogen exhibited good density control and low impurity levels. The second boronization resulted in a much larger reservoir of hydrogen in the walls with only the near-surface region being depleted by He GDC. Tokamak operation in He led to density increase and lack of control. Density control was made possible by implementation of a closed-loop feedback system in conjunction with occasional He GDC. The plasma isotope mixture could not be changed arbitrarily. Due to the high hydrogen recycling flux, deuterium discharges were still heavily polluted with hydrogen several hundred shots after changing the working gas. It is planned to use dcuterated trimethyl boron in the next boronization of COMPASS (after installation of the D-vessel) when significantly isotopically-purer deuterium plasmas are expected to result, but ideally a solution to avoid the high gas retention in the films is required.
Acknowledgements We are grateful to D.C. Robinson and T.N. Todd for advice and encouragement during the period of this work, to the COMPASS Physics Team for diagnostic measurements and to the Technical and Engineering Sections for machine operations. Analysis of the boronized samples was performed by P. Karduck at GfE, Technical University Aachen.
References [l] J. Winter et [2] J. Winter et [3] J. Winter et [4] U. Schneider
al., J. Nucl. Mater. 162-164 (1989) 713. al., J. Nucl. Mater. 122 & 123 (1984) 1187. al., Nucl. Instr. and Meth. B23 (1987) 538. et al., J. Nucl. Mater. 176 & 177 (1991) 350.
[5] C. Hollenstein et al., J. Nucl. Mater. 176 & 177 (1991) 343. [6] T. Dudok de Wit, B.P. Duval, C. Hollenstein and B. Joyce, Proc. 17th EPS Conference on Controlled Fusion and Plasma Heating, Amsterdam, 1990, p. 1476. [7] H.F. Dylla, J. Nucl. Mater. 176 & 177 (1991) 337. [8] H.G. Esser, H.B. Reimer, J. Winter and D. Ringer, Fusion Technol. 1 (1988) 791. [9] J. Winter and H.G. Esser, J. Nucl. Mater. 176 & 177 (1991) 481. [lOI K.H. Behringer, J. Nucl. Mater. 1455147 (1987) 145. [II] G. Jackson et al., J. Nucl. Mater. 176 & 177 (1991) 311.