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Results from the bundle divertor experiment on DITE with neutral beam heating
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Journal of Nuclear Materials 121 (1984) 210-221 North-Holland, Amsterdam
RESULTS FROM THE BUNDLE DIVERTOR EXPERIMENT ON DITE WITH NEUTRAL BEAM HEATING P.C. JOHNSON, K.B. AXON, J.E. BRADLEY, J. BURT, W.H.M. CLARK, SK. ERENTS, S.J. FIELDING, D.H.J. GOODALL, P.J. HARBOUR, J. HUGILL, P.J. LOMAS, G.M. MCCRACKEN, J.W. PARTRIDGE, B.A. POWELL and G. PROUDFOOT Wham
Laboratory,
Abingdoh
Oxon., OX14
3DB,
UK (Euratom/UKAEA
Fusion Association)
Characteristics of the operation of the DITE tokamak with the MkII bundle divertor and neutral beam heating are described. In these experiments the divertor was pumped by a liquid helium cooled cryopump, and fuelling was by gas input into the tokamak vacuum vessel. Generally a fuelling rate of 1-2 x 10” atoms/s is required to sustain a constant density with the divertor operating, and about 50% of the gas input flows to the divertor. Global power balances have been carried out using instrumentation distributed around the tokamak and in the divertor. It is found that the wall loading exhibits substantial toroidal and poloidal asymmetries, with a high loading near the gas feed when the divertor is operating. The percentage power exhaust to the divertor is highest, up to 7046,in ohmic discharges with the gas feed turned off and the density falling. With 1.4 MW of neutral beam heating, about 24% of the power is seen at the divertor plates. Typically, between 70% and 80% of the input power can be countedfor. Measurements in the scrape-off plasma show that the density is about 1 X lOI mm3 at 30 to 40 mm outside the separatrix, when the line average density in the tokamak is 2-2.5X1O’9 m- 3. However there is less than 10% reionisation of recycled neutrals in the divertor, as determined from H, measurements. In addition, the strong coupling observed between the particle and power flow to the divertor suggests that the exhaust is convective, and this may be a consequence of the strong mirror field characteristic of the bundle divertor magnetic geometry.
1. Introduction The MkII bundle divertor (11 was first operated on DITE in April 1982. The initial results in ohmic discharges [2] demonstrated particle and energy exhaust to the divertor at levels up to 50% of the inputs from hydrogen fuelling and joule heating. These initial experiments were carried out at a toroidal field of 2 T. In recent work [3-61 a toroidal field of 2.6 T has been used routinely. Neutral beam heating has been added at power levels of up to 1.6 MW. Effort has been concentrated largely on improving the total energy balance, using instrumentation distributed around the torus and in the divertor, and on measurements of plasma parameters in the boundary (outside the separatrix) and the exhaust (in the divertor) in order to develop an understanding of the exhaust regimes of ohmically heated and neutral beam heated discharges. The layout of this paper is as follows. Section 2 contains a brief description of the DITE IA experiment 0022-3115/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
with the bundle divertor and the neutral beam injection system. Section 3 starts with a description of a reference discharge which has been used for much of the recent work. Comments are then made on the effects which limit the range of operating parameters in current and density. Global particle and energy balances are discussed in section 4. Section 5 deals with the experimental measurements of parameters in the boundary and exhaust plasma. The discussion and conclusions are in sections 6 and 7. Impurity control is not addressed as a separate topic in this paper. In all of the work reported here, fuelling was by gas input into the tokamak torus and the divertor was pumped by a liquid-helium cooled cryopump.
2. DITJZ IA The main parameters of the DITE IA tokamak with the MkII bundle divertor and the neutral beam injecB.V.
P. C. Johnson et al. / Results from the bundle dtuertor experiment on DITE Table 1 Parameters of the DITE IA tokamak, the ML11bundle divertor and the neutral beam injection system Parameter Tokamak Major radius, R, Limiter (minor) radius, a,_ Toroidal field, Bc Plasma current, Ip Limiters Cleaning Bundle divertor MkII Operating toroidal field Pulse.length Separatrix major radius, R, Nominal minor radius to separatrix, a, Area of boundary plasma connected directly to one divertor plate, A,, Average connection length, plate to plate, qo Mirror ratio, R M Divertor plates Pumping Neutral beam system b, Number of beam lines Number of source per line Energy of full energy component Total power, P,
Pulse length, tg Gas
Value 1.17 m 6 0.26 m ( 2.7 T ( 250 kA Uncoated graphite RF assisted glow discharge, mainly in H, ( 2.7 T 300 ms ‘) at 2.6 T 1.37-1.38 m 0.20-0.21 m
= 0.012 m2 7 =2 Graphite, uncoated LH, cryopump 4 2 24-30 keV 1.8 MW 50 ms Hydrogen
‘) The design figure was 200 ms at 2.7 T and this has been stretched to 300 ms at a slightly lower field. b, Full system at 2.4 MW, 120 ms currently being implemented.
tion system are given in table 1. The main changes to the tokatnak since the MkII bundle divertor has been installed are in the graphite limiters (2 poloidal aperture
limiters with 34” gaps top and bottom) and the use of RF assisted DC glow discharge cleaning (mainly in H,) for cleaning, with no gettering. The MkII divertor cryopump is used to pump the torus, in addition to turbo-molecular pumps. The main constituent of the residual gas (except for hydrogen) is mass 18 (H,O) at a pressure of l-2 X lo-* mb. The MkII divertor has operated well at conditions close to the design parameters. It has been found neces-
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sary for experimental
work to increase the pulse length from 200 ms to 300 ms, at a slightly reduced field, and at the expanse of greater ripple on the capacitor bank power supply. The neutral beam system now has 4 beam lines and the full 120 ms pulse at 2.4 MW is currently being implemented.
3. General features of dh!rted disclmp 3. I. The reference discharge The time evolution of a diverted discharge with neutral beam heating is shown in fig. 1. This discharge was used for the major part of the divertor work reported here, and will be referred to as the reference discharge in the remainder of the paper. The divertor field is energised when the toroidal field has reached a steady value. Then the plasma current is established in an equilibrium which is displaced = 20 mm from the minor axis towards the inner limiter (displacement here refers to the displacement of the geometric centre of a flux surface of radius 0.26 m, i.e., equal to the limiter radius). This displacement effectively inhibits divertor action. At the toroidal field of 2.6 T used for the discharge in fig. 1, the plasma current ZP following initiation is about 100 kA and line density Ee is in the range 0.8 X 1 X 1019 rnm3. Following discharge initiation, the density and current are raised. Inhibiting divertor action, by changing the equilibrium position, has been found necessary in order to be able to increase the density rapidly by fuelling in the tokamak chamber. In an equilibrium centered from the start of a pulse, very high fuelling rates are required to raise the density, and this can lead to unstable and irreproducible discharges. While the density and current are being increased, at 40 ms into the pulse the equilibrium is allowed to move towards the centered position (centered on the minor axis) over a 50 ms period. Divertor action commences towards the end of this period, accompanied by an abrupt cessation of the density ramp. An equilibrium centered on the limiter aperture maximises the useful width of the divertor scrape-off layer on the inboard side (R 4 R,) of the tokamak plasma. In order to maintain a constant density of A, = 2 x 1019 at Zr,= 140 kA in the joule heated phase of the discharge up to 150 ma, a gas input of 1.5 x 1021 H-atoms/s is required. From 150 ms, neutral beam injection is applied. This results in a 30-408 increase in density.
P.C. Johnson et al. / Results from the bundle akwtor
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Timelms Fig. 1. ‘k reference diverted discharge. (a) Diwrtor coil current, (b) plasma current. (c) loop voltage, (d) line-average density over the full minor radius a = 0.26 m, (e) position of the centre of the outer flux surface relative to the minor axis, (f’) gas input programme. (g) neutral beam power input.
P. C. Johnson
et al. / Results
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OR DITE
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Fig. 2. Variation
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run. Conditions
otherwise
are as in fig. 1.
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After the injection phase, the gas input is terminated and the density allowed to fall. The equilibrium is moved back to the displaced position at = 20 mm inside the minor axis before the end of the divertor coil pulse at 300 ms. This avoids loading the divertor ducts as the divertor field decays. The central parameters of this discharge are c = 580 eV, Ti = 460 eV, PZ== 2.5 x 1019 mm3 during the ohmic phase and T. = 650 eV, T = 800 eV, nc = 3.4 x 1019 me3 during neutral beam injection. Z,, is 2-2.5. The main impurity constituent is oxygen, with slightly less carbon and only traces of metals. Without the divertor it is more difficult to control the density during neutral injection, and Z,, is higher (up to 4) and there is more radiation loss [3].
In the pulse shown in fig. 1, there is evidence of unstable behaviour (on the loop voltage signal) during the fully diverted phase. The general character is of a relaxation instability similar to repeated minor disruptions, with small inward displacements of the equilibrium. The origins of the instability are not understood. Unstable discharges are observed mainly at low density and at high current. In the latter case, the onset of the instability appears to be associated with qs = 3 at the separatrix radius, effectively preventing operation at high current. However, the exploration of operation at low q has not been exhaustive, and further work is required to find a means of operating at higher currents.
3.2. Unstable regimes
Many pulse have been obtained on DITE with the bundle divertor in which the line density has exceeded 5 = 4 X 1019 rnv3. However such high densities have nkt been achieved routinely. Divertor discharges affect the surface conditions the tokamak so that although high density discharges may occur at the start of an experimental run (see fig. 2), the discharges eventually settle down to the type described in section 3.1, with A, 4 3 X 1019 rnm3. The gas feed programs for the pulses in fig. 2 was the same as for the reference discharge. Increasing the gas input during the diverted phase of these discharges generally leads to unstable operation without a significant increase in density even during neutral beam injection.
The steady value of current reached in the reference discharge corresponds to qs at the separatrix radius a, of between 3 and 3.5 (qs is defined as qs = 2vraiB,/ /@,I,). qs varies with time during a ‘constant current’ phase of a diverted discharge for the following reasons: Ripple on the ohmic supply. Ripple on the divertor coil current which causes the separatrix to move. With the divertor coil pulse stretched to 300 ms as in fig. 1, this movement is 8-10 mm. Changes in equilibrium. Increasing /3s or li reduces the minor radius of flux surfaces just inside the separatrix.
3.3. Operation at high density
(a)
Timelms
(b) 8
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3p 2
0 0
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Fig. 3: Particle balance in the reference diverted discharge. (a) Total particle loss r,, (b) ionisation source in the divertor SD.
P. C. Johnson et al. / Results from 4.
Particle and energy control
4. I. Particle balances
The global particle balance in the torus for diverted discharges in DITE with the MkII divertor has been discussed in refs. [2] and [4]. The particle balance equation used has the form:
!g=s,+s,-r,-r,,
(1)
where N is the total number of plasma electrons calculated from the line density using model profiles. Ss is the ionisation source due to the cold gas feed, S, is the ionisation source from neutral beam particles, r,_ is the net loss to the limiters and To is the net loss to the divertor. Ss is derived from H, measurements made near the gas feed. Comparing Ss and the gas input flowrate yields a gas feed ionisation efficiency of about 50%. The total loss rate rr = r, + r, evaluated in this way for the reference discharge is illustrated in fig. 3. The main contribution to I’, is ro, with the magnitude of the limiter loss estimated to be r, = 1.25 x 102’ H-atoms/s [4]. r, rises as the equilibrium is displaced towards the minor axis, with a further small rise during neutral injection. Measurements of H, intensity have been made also in the divertor, as illustrated in fig. 3. There is a good correlation between the divertor H, signal and the derived particle exhaust. The ionisation source in the divertor, as deduced from the absolute calibration of the divertor H, signal, is < 10% of the exhaust flux. The instability mentioned in section 3.2 is observed as pulses in the H, emission in the divertor (fig. 3). The unstable behaviour occurs in the early part of the fully diverted phase of the discharge when there is only ohmic heating prior to neutral injection.
the bundle diuertor experiment on
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wide range of conditions. Thermal instrumentation has been fitted to most of the accessible parts of the DITE torus and divertor chamber in order to look at the spatial distribution of power/energy loading in an attempt to complete the measured balance between inputs and losses. The main loss terms in power/energy balances in diverted discharges are obtained as follows. The torus wall energy loading is estimated using the bellows segments on the torus as calorimeters. Each of 6 bellows is instrumented with up to 9 calibrated miniature thermistors giving toroidal and poloidal information. The total instantaneous power loading on the wall is given by the integrated power as measured on the bellows, assuming that the time variation is the same as that of the bolometer mentioned above. The limiter segments and the divertor neutraliser plates are each instrumented with several thermocouples to given the energy loading. An AGA 780 thermal imaging camera [7] modified to give a time response of 20 ms has been used to determine the variation of power loading with time. When observing one of the two divertor plates, the energy loading obtained directly from the calibrated thermal camera agrees closely with the thermocouple measurement. The thermal imaging camera has been used also with a view of one side (electron drift side) of one of the two sets of limiters. Bolometry in the divertor chamber indicates that the radiation and charge exchange loss in the divertor is small (Q 30 kW). This is not included in the following analysis. The toroidal distribution of the wall energy loading as derived from the bellows measurements is shown for ohmic diverted and non-diverted discharges in fig. 4. The diverted discharge is similar to the reference dis-
4.2. ‘Energy and power balances The energy and power balances in diverted discharges are also discussed elsewhere [2,3]. In the earlier work on ohmic discharges the diverted power was shown to be highest at low plasma density, and decreasing as the density is raised. A substantial fraction of the power input appeared neither in the exhaust to the divertor nor in the total radiation and charge exchange loss in the torus estimated using a single bolometer looking at a minor cross-section of the torus. In the later work [3], power and energy balances have been looked at in considerably greater detail for a
Fig. 4. Toroidal variation of the wall energy loading for ohmic, diverted and non-diverted discharges.The ‘Th’ symbols refer to the location of instrumented bellows.
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INPUT ENERGY
166kJ
Time
OUTPUT
(ms)
ENERGY 123kJ
DIVERTOR 80 - 280ms
Fig. 5. Global power balance for a neutral beam heated diverted discharge, as in fig. 1.
charge but without neutral beam injection. The limiter discharge has a centered equilibrium for the whole pulse, but is otherwise similar to the diverted discharge in density and current. The wall energy loading is localised near the limiters in the non-diverted discharge. This is reduced in the diverted discharge showing that the recycling and possibly localised impurity sources at the limiter are reduced by the action of the divertor. However, the wall loading in a diverted discharge is highest near the gas feed. The fuelling rate is about a factor 5 higher to sustain constant density in the diverted discharge than in the limiter discharge, and the increase in the local wall loading associated with this is
most probably due to charge exchange losses. A spectral survey in the region of the gas input showed no evidence there of local sources of impurities. Similar spatial distributions are seen with neutral beam injection. Clearly the use of local bolometry as a measure of the total losses to the wall is inadequate in the face of such strong variations around to torus. A global power balance for a discharge similar to the refemaz discharge (but with H injected into deuterium) is presented in fig. 5. The changes in the internal energy derived from /3# or profile data are small in these discharges, and this is not included in the figure. Also, the current run-down phase of the discharge-s is not
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P. C. Johnson et 01, / Results from the bundle divertor experiment on DITE
included. During the decay of the plasma current, the power to the limiters is consistently small. During neutral beam injection at a power level of 1.4 MW, the power loading on the divertor plates and the limiters is comparable. The divertor here takes approximately 24% of the total power input. The sum of the divertor, limiter and wall losses accounts for about 10% of the power input. The highest fractional power loading on the divertor plates, which can be as high as 70% of the power input [3], occurs however in the ohmic phase after neutral injection when there is no external gas feed into the tokamak and the density is falling. It is worth noting here that the divertor plate power loading is reduced considerably when the density is rising during the diverted phase of the discharge, as discussed in section 3.3. It would appear that the earlier work on power balances [2] is indicative of a scaling of power exhaust with dnJdf rather than simply with density. In ohmic diverted discharges about 80% of the energy input can be accounted for. The corresponding figure for neutral beam heated discharges is about 70%. The uncertainties in the measurements reported are difficult to assess, and it is not clear whether the discrepancy between the input energy and the measured losses can be ascribed to measurement errors or losses to surfaces which are not instrumented. In all of the cases analysed, the input energy exceeds the measured losses. The thermal imaging camera shows that the power. loading on the limiter during a diverted discharge occurs principally small areas, fig. 6. This may be associ: ated with sets of lines of force in the bundle divertor
LIMITER
r3 MEDIAN --
--
t = 115ms
t = 176ms
Fig. 6. Distribution of power loading on the limiter in a diverted discharge showing variation with poloidal angle.
scrape-off layer which have a very long connection length to the divertor plates.
5. Conditions in the boudary
and exhaust plasma
5.1. Parameters in the boundaryplasma
A detailed account for the use of an array of smallarea Langmuir probes to measure the electron density, the electron temperature and the flow math number (assuming Ti = T,) is given in ref. [6]. A set of large probes [8,9] has also been used to derive these quantities, with the addition of a measurement of the local heat flux through the sheath in front of the probe. Roth sets of probes have been used to make measurements in the electron and ion drift directions. Further measurements of the plasma density have been made using a 2 mm microwave interferometer. Further data is available on profiles of n, (microwaves, Thomson scattering), T, (ECE, Thomson scattering, soft X-rays) and Ti (NPA) in the core plasma (within the separatrix radius) but this will not be described in detail. Results obtained in the reference discharge will be discussed here. There are three factors which complicate the comparison of the small and large probe data, the different spatial location of the probes, their different sixes and their differing time resolution. The probes are located at different port positions on the torus: the probe array is near the divertor entrance above and below the mid-plane and the large probes are near to the top of the machine at major radii of between 1.19 m and 1.3 m. It is clear from the measurements that there can be in diverted discharges a toroidal and poloidal variation in the plasma parameters in the boundary, as with the limiter loading. This is to be expected from the complex magnetic topology in the bundle divertor scrape-off layer. Also the interpretation of the data from the large probe is dependent on an accurate knowledge of the connection length along the field to the divertor plates or limiters because of the shielding effects of the large probe housing [9]. Finally, the large probes have been used with a low time response (2 20 ms) both for voltage scans and heat flux measurements whereas the small probes have a fast response ( f: 0.1 ms). The latter can respond better to the time variations in the plasma parameters [6] which are a consequence of the ripple on the divertor coil current in addition to the natural variations in parameters. Taking the data from the large probe which has the longest connection length to a divertor plate, when the screening effects of the probe are small, the basic data
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Values of plasma parameters during the injection heated phase of the discharge at a minor radius of 0.18 m (20 mm inside the separatrix) are T, = 250, Ti = 350 and nc = 1.9 x 1019 rne3. Clearly there is a steep gradient in T, between the separatrix and the limiter. Ti is greater than T, over the whole profile. 5.2. Parameters in the diverted plasma
240
250
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2m
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Fig. 7. Plasma parameters measured with probes in the divertor scrapoff plasma and the limiter shadow. The times of 137 ms and 187 ms are in the ohmic and injected phases of the references discharge, fig. 1. Averages are taken over 30 ms. The probes form pairs, with odd numbers facing the divertor. Probe pairs l-2 and 7-8 pass close to the edge of the flux bundle immediately connected to the divertor, whereas probe pairs 3-4 and 5-6 pass through this flux bundle.
(saturation
current and T,) obtained
The power density to one of the divertor plates (ion side) is obtained as a function of time using the thermal imaging camera [S]. For the reference discharge the maximum power density during neutral injection reaches 35 MW m-2, with most of the diverted plasma delivering a power density greater than 3 MW m-’ at that time. During the ohmic phase before injection the maximum power density is 7.5 MW mb2. Measurements with small probes in the diverted plasma are described in ref. (61. The saturated ion current density to a probe facing the flow is illustrated in fig. 8 for horizontal scans across the diverted plasma near the divertor plate (electron side) during periods in the reference discharge before and during neutral injection. The scans are from a region of low power density towards a region of high power density. Electron temperatures are also shown in the figure. They are comparable with measurements in the outer parts of the boundary plasma discussed earlier. The power density derived from the current density and the electron temperature, assuming a convected energy of lOkT, per
by the probes is
comparable. The variation with minor radius of T, and xc in the ohmic and neutral injected phases of the reference discharge obtained from the small probes is shown in fig. 7 (a detailed description of the geometry is given in [6]). The values of density shown agree well with those derived from microwave interferometer measurements. The most significant result here is that the plasma density is in the range 0.5-l X 1019 mm3 at a distance of 40 mm outside the separatrix radius (taken at the maximum of the divertor coil current). At this density, most of the ionisation of the dissociated hydrogen from the gas feed will take place in the scrape-off plasma and not inside the separatrix. The density falls with increasing minor radius in areas of the scrape-off layer which are closely connected to the divertor (particularly probe pairs 3 & 4 and 5 & 6 in fig. 7) but elsewhere it remains high out to the limiter radius. The electron temperature is in the range lo-20 eV near the limiter radius increasing slowly towards the separatrix.
16,
,
20 Horizontal
3
40 pocilion
60
60 in diverlor
chamber.
d/mm
Fig. 8. Ion saturation current density and electron temperature variations along a scan in the diverted plasma for the reference discharge. The fines of force connected to the separatrix are to the right of the points in the figure: however the exact position is not known.
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P. C. Johnson et al. / Results ftom the bundle diwrtor experiment an DITE
particle, varies from 1 MW mm2 to 24 MW rne2, which is comparable with values obtained from thermography. The data is insufficient to give an accurate measurement of the total particle flux in the exhaust. However the particle flux density derived from the particle balance assuming that the effective area of each of the exhaust channels is about 0.01 m2 (table l), gives values (3-4 X 1O23 me2 s-* for the reference discharge) in reasonable agreement with the probe data. The convected energy per particle estimated from the particle balance and diverted power [3] is often much higher, up to 1.5 keV/particle, than the m~mum of 0.3 keV/particle derived from this probe data.
6. Discussion
Many of the features of diverted discharges in DITE can be understood if *the density in the 40 mm region outside the separatrix is 2 1 x 1019 me3 at a temperature between 10 and 30 eV. Such a boundary plasma is
approaching the region of interest for large devices where a thick, dense scrape-off plasma is postulated to be required to protect the first wall from charge exchange particles. One consequence of a high density boundary plasma is a poor fuelling of the tokamak plasma by gas puffing as most of the ionisation of H, dissociation products will take place in the boundary. An inabi~ty to raise the density has been one of the consistent features of experimental work with the MkII bundle divertor as described earlier. The radial distribution of radiation losses in the tokamak during diverted discharges, fig. 9, shows a peak in the region between the limiter and the separatrix, and this may also be explained by a high edge density. The main terms in a detailed radial power balance for a neutral beam heated discharge are listed in table 2. To this list must be added parallel flow losses to the divertor in the region outside the separatrix, and to the limiter outside the limiter radius. In order to place any greater significance on a global measurement of power BOLOMETER
ARRAY
LINE OF SIGHT INTENSI~ REFERENCE
DIVERTOR
DISCHARGE
Fig. 9. The spatial distribution of line-of-sight bolometer signal from the IO-channel bolometer array for the reference divertor discharge.
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Table 2 List of main terms in the plasma power balance for a neutral beam heated di8cluuge Component
Term
Ohmic heating
electron heating
Fast ions
charge exchange electron heating ion heating orbit losses divertor ripple losses [ lo,1 l]
Plasma ions
equipartition charge exchange ion thermal conduction convection
Plasma electrons
equipartition radiation electron thermal conduction convection
unload efficiency it is important to make estimates of the magnitudes of all of these terms. This includes the boundary plasma where the processes listed still apply especially when the edge density is high. In the reference discharge, where Ti z T, during neutral injection, it is evident that most of the power delivered to the boundary plasma will be in the electrons and that radiation losses from this region play an important part in the boundary energy balance. Charge exchange loss from plasma ions in the boundary plasma is also an important term. Some losses occur through orbit interacting with the divertor ripple maxima [lO,ll]. The divertor plate8 and limiters take the power remaining after all the other losses have been subtracted. Work is in progress in this area and will be reported in a later publication. The presence of a potential barrier to the electrons in a magnetic mirror is well known, and applies also to the bundle divertor [S]. This potential barrier has the effect of accelerating plasma to supersonic speeds whilst maintaining charge neutrality. If the ions are collision-free over the scale length of the mirror magnetic field then the effect of the divertor mirror is to select only the mirror loss cone for transmission to the divertor. The net effect is to produce a supersonic diverted plasma which has a density somewhat lower than that in the boundary plasma. This is consistent with the probe measurements made in the divertor on the reference discharge (61 where the Mach number is shown to be
greater than unity and the electron density less than 5 X lo’* mT3. The low re-ionisation of recycled neutrals in the divertor and the low radiation/charge exchange loss from the diverted plasma can be explained on the basis of a low plasma density in the divertor region. Again, work is in progress in this area.
7. Conclusions Experiments with the MkII bundle divertor on DITE have shown good particle diversion and good power diversion particularly with reduced gas feed in the main chamber. The radiation loss from the torus and the Z,,, are lower with the divertor operating than in limiter discharges. The wall power loading has been shown to vary substantially over different areas of the torus. In particular, due to the high gas feed necessary to fuel the exhaust the power loading is highest near the gas input in diverted discharges. The power loading near the limiters is reduced with the divertor operating. Measurements in the boundary region between the separatrix and the limiter show that the density is greater than 1 X 1019 mm2 over much of the scrape-off layer. The electron near the limiter edge is in the range lo-20 eV. Most of the ionisation of the hydrogen fuel will take place in the scrape-off layer.at such a value of density, and this could explain the difficulty in fuelling the tokamak in divertor experiments. The high radiation loss from the boundary may also be a consequence of the high edge density. The diverted plasma exhibits minimal re-ionisation of hydrogen recycled in the divertor. The exhaust regime is convective and the flow, as inferred from probe measurements, is supersonic. The effect of the bundle divertor magnetic mirror on the flow into the divertor will be introduce a potential barrier to the electrons. This accelerates the ions to supersonic speeds and lead to a density in the exhaust to the divertor plates which is lower than in the scrapeoff layer.
Acknowledgements The author8 are indebted to the DITE technical staff under Mr. R.W. Storey and mr. R.E. Bradford for machine operations, to the scientific staff for work on diagnostic8 and to the Group Leader, Dr. J.W.M. Paul for his continual encouragement and support.
P.C. Johnson et al. / Results from the bundle divertor experiment on DITE
References [l] P.J. Harbour, M.F.A. Harrison, A.D. Sanderson, P.E. Stott, The application of bundle divertors to experimental tokamaks and reactors and some consequences of collisionless exhaust flow to divertor targets, Proc. 7th intern. Conf. on Plasma Physics and Controlled Nuclear Fusion Research, Innsbruck, 1978; IAEA, Vienna Vol III, p. 431. [2] K.B. Axon et al., Results from the DITE experiment, Proc. 9th Intern. Conf. on Plasma Physics and Controlled Nuclear Fusion Research, Baltimore, 1982; IAEA. [3] W.H.M. Clark et al., Energy and power balances in the DITE tokamak, to be published in Proc. 11th European Conf. Controlled Fusion and Plasma Physics, Aachen, 1983. [4] S.J. Fielding, D.H.J. Goodall, J. Hugill and P.C. Johnson, Characteristics of the particle and energy exhaust into the DITE bundle divertor, to be published in Proc. 11th European Conf. Controlled Fusion and Plasma Physics, Aachen, 1983. [5] P.C. Johnson et al., Recent results on the DITE bundle
[6]
[7] [8]
[9]
[lo]
[ll]
221
divertor, to be published in Proc. 11th European Conf. Controlled Fusion and Plasma Physics, Aachen, 1983. P.J. Harbour and G. Proudfoot, Mach number deduced from probe measurements in the divertor and boundary layer of DITE, J. Nucl. Mater. 121 (1984) 222 (these proceedings). Infrared system AB, Box 3, S-18211 Danderyd, Sweden. P.C. Stangeby, G.M. McCracken, S.K. Erents, J.E. Vince. and R. Wilder, Edge measurements of T,, Ti, n, E, on the DITE tokamak using a biased power bolometer, J. Vat. Sci. Technol. A (1983) 1. P.C. Stangeby, The disturbing effect of probes inserted into edge plasmas of fusion devices, J. Nucl. Mater. 121 (1984) 36 (these proceedings). P.C. Johnson, B.S. Mudford and J.G. Cordey, Neutral beam injection modelling for DITE with the MkII bundle divertor, to be published in Proc. 11th European Conf. Controlled Fusion and Plasma Physics, Aachen, 1983. L.M. Hively et al., Constrained ripple optimisation of bundle divertors, Nucl. Technol. Fusion 2 (1982) 372-391.
Journal of Nuclear Materials 121 (1984) 210-221 North-Holland, Amsterdam
RESULTS FROM THE BUNDLE DIVERTOR EXPERIMENT ON DITE WITH NEUTRAL BEAM HEATING P.C. JOHNSON, K.B. AXON, J.E. BRADLEY, J. BURT, W.H.M. CLARK, SK. ERENTS, S.J. FIELDING, D.H.J. GOODALL, P.J. HARBOUR, J. HUGILL, P.J. LOMAS, G.M. MCCRACKEN, J.W. PARTRIDGE, B.A. POWELL and G. PROUDFOOT Wham
Laboratory,
Abingdoh
Oxon., OX14
3DB,
UK (Euratom/UKAEA
Fusion Association)
Characteristics of the operation of the DITE tokamak with the MkII bundle divertor and neutral beam heating are described. In these experiments the divertor was pumped by a liquid helium cooled cryopump, and fuelling was by gas input into the tokamak vacuum vessel. Generally a fuelling rate of 1-2 x 10” atoms/s is required to sustain a constant density with the divertor operating, and about 50% of the gas input flows to the divertor. Global power balances have been carried out using instrumentation distributed around the tokamak and in the divertor. It is found that the wall loading exhibits substantial toroidal and poloidal asymmetries, with a high loading near the gas feed when the divertor is operating. The percentage power exhaust to the divertor is highest, up to 7046,in ohmic discharges with the gas feed turned off and the density falling. With 1.4 MW of neutral beam heating, about 24% of the power is seen at the divertor plates. Typically, between 70% and 80% of the input power can be countedfor. Measurements in the scrape-off plasma show that the density is about 1 X lOI mm3 at 30 to 40 mm outside the separatrix, when the line average density in the tokamak is 2-2.5X1O’9 m- 3. However there is less than 10% reionisation of recycled neutrals in the divertor, as determined from H, measurements. In addition, the strong coupling observed between the particle and power flow to the divertor suggests that the exhaust is convective, and this may be a consequence of the strong mirror field characteristic of the bundle divertor magnetic geometry.
1. Introduction The MkII bundle divertor (11 was first operated on DITE in April 1982. The initial results in ohmic discharges [2] demonstrated particle and energy exhaust to the divertor at levels up to 50% of the inputs from hydrogen fuelling and joule heating. These initial experiments were carried out at a toroidal field of 2 T. In recent work [3-61 a toroidal field of 2.6 T has been used routinely. Neutral beam heating has been added at power levels of up to 1.6 MW. Effort has been concentrated largely on improving the total energy balance, using instrumentation distributed around the torus and in the divertor, and on measurements of plasma parameters in the boundary (outside the separatrix) and the exhaust (in the divertor) in order to develop an understanding of the exhaust regimes of ohmically heated and neutral beam heated discharges. The layout of this paper is as follows. Section 2 contains a brief description of the DITE IA experiment 0022-3115/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
with the bundle divertor and the neutral beam injection system. Section 3 starts with a description of a reference discharge which has been used for much of the recent work. Comments are then made on the effects which limit the range of operating parameters in current and density. Global particle and energy balances are discussed in section 4. Section 5 deals with the experimental measurements of parameters in the boundary and exhaust plasma. The discussion and conclusions are in sections 6 and 7. Impurity control is not addressed as a separate topic in this paper. In all of the work reported here, fuelling was by gas input into the tokamak torus and the divertor was pumped by a liquid-helium cooled cryopump.
2. DITJZ IA The main parameters of the DITE IA tokamak with the MkII bundle divertor and the neutral beam injecB.V.
P. C. Johnson et al. / Results from the bundle dtuertor experiment on DITE Table 1 Parameters of the DITE IA tokamak, the ML11bundle divertor and the neutral beam injection system Parameter Tokamak Major radius, R, Limiter (minor) radius, a,_ Toroidal field, Bc Plasma current, Ip Limiters Cleaning Bundle divertor MkII Operating toroidal field Pulse.length Separatrix major radius, R, Nominal minor radius to separatrix, a, Area of boundary plasma connected directly to one divertor plate, A,, Average connection length, plate to plate, qo Mirror ratio, R M Divertor plates Pumping Neutral beam system b, Number of beam lines Number of source per line Energy of full energy component Total power, P,
Pulse length, tg Gas
Value 1.17 m 6 0.26 m ( 2.7 T ( 250 kA Uncoated graphite RF assisted glow discharge, mainly in H, ( 2.7 T 300 ms ‘) at 2.6 T 1.37-1.38 m 0.20-0.21 m
= 0.012 m2 7 =2 Graphite, uncoated LH, cryopump 4 2 24-30 keV 1.8 MW 50 ms Hydrogen
‘) The design figure was 200 ms at 2.7 T and this has been stretched to 300 ms at a slightly lower field. b, Full system at 2.4 MW, 120 ms currently being implemented.
tion system are given in table 1. The main changes to the tokatnak since the MkII bundle divertor has been installed are in the graphite limiters (2 poloidal aperture
limiters with 34” gaps top and bottom) and the use of RF assisted DC glow discharge cleaning (mainly in H,) for cleaning, with no gettering. The MkII divertor cryopump is used to pump the torus, in addition to turbo-molecular pumps. The main constituent of the residual gas (except for hydrogen) is mass 18 (H,O) at a pressure of l-2 X lo-* mb. The MkII divertor has operated well at conditions close to the design parameters. It has been found neces-
211
sary for experimental
work to increase the pulse length from 200 ms to 300 ms, at a slightly reduced field, and at the expanse of greater ripple on the capacitor bank power supply. The neutral beam system now has 4 beam lines and the full 120 ms pulse at 2.4 MW is currently being implemented.
3. General features of dh!rted disclmp 3. I. The reference discharge The time evolution of a diverted discharge with neutral beam heating is shown in fig. 1. This discharge was used for the major part of the divertor work reported here, and will be referred to as the reference discharge in the remainder of the paper. The divertor field is energised when the toroidal field has reached a steady value. Then the plasma current is established in an equilibrium which is displaced = 20 mm from the minor axis towards the inner limiter (displacement here refers to the displacement of the geometric centre of a flux surface of radius 0.26 m, i.e., equal to the limiter radius). This displacement effectively inhibits divertor action. At the toroidal field of 2.6 T used for the discharge in fig. 1, the plasma current ZP following initiation is about 100 kA and line density Ee is in the range 0.8 X 1 X 1019 rnm3. Following discharge initiation, the density and current are raised. Inhibiting divertor action, by changing the equilibrium position, has been found necessary in order to be able to increase the density rapidly by fuelling in the tokamak chamber. In an equilibrium centered from the start of a pulse, very high fuelling rates are required to raise the density, and this can lead to unstable and irreproducible discharges. While the density and current are being increased, at 40 ms into the pulse the equilibrium is allowed to move towards the centered position (centered on the minor axis) over a 50 ms period. Divertor action commences towards the end of this period, accompanied by an abrupt cessation of the density ramp. An equilibrium centered on the limiter aperture maximises the useful width of the divertor scrape-off layer on the inboard side (R 4 R,) of the tokamak plasma. In order to maintain a constant density of A, = 2 x 1019 at Zr,= 140 kA in the joule heated phase of the discharge up to 150 ma, a gas input of 1.5 x 1021 H-atoms/s is required. From 150 ms, neutral beam injection is applied. This results in a 30-408 increase in density.
P.C. Johnson et al. / Results from the bundle akwtor
212
(a)
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, , ,J, 200
,,,,, , 250
) , 300
Timelms Fig. 1. ‘k reference diverted discharge. (a) Diwrtor coil current, (b) plasma current. (c) loop voltage, (d) line-average density over the full minor radius a = 0.26 m, (e) position of the centre of the outer flux surface relative to the minor axis, (f’) gas input programme. (g) neutral beam power input.
P. C. Johnson
et al. / Results
fromthe
bundle divertor experiment
213
OR DITE
21731
i;
u.
OO
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*
”
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.“““I
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&I-
ii
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lime I ms
Fig. 2. Variation
in density
during
a sequence
of pulses at the beginning
of an experimental
run. Conditions
otherwise
are as in fig. 1.
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P. C. Johnson et al. / Results from the buna% bertor experiment on DITE
After the injection phase, the gas input is terminated and the density allowed to fall. The equilibrium is moved back to the displaced position at = 20 mm inside the minor axis before the end of the divertor coil pulse at 300 ms. This avoids loading the divertor ducts as the divertor field decays. The central parameters of this discharge are c = 580 eV, Ti = 460 eV, PZ== 2.5 x 1019 mm3 during the ohmic phase and T. = 650 eV, T = 800 eV, nc = 3.4 x 1019 me3 during neutral beam injection. Z,, is 2-2.5. The main impurity constituent is oxygen, with slightly less carbon and only traces of metals. Without the divertor it is more difficult to control the density during neutral injection, and Z,, is higher (up to 4) and there is more radiation loss [3].
In the pulse shown in fig. 1, there is evidence of unstable behaviour (on the loop voltage signal) during the fully diverted phase. The general character is of a relaxation instability similar to repeated minor disruptions, with small inward displacements of the equilibrium. The origins of the instability are not understood. Unstable discharges are observed mainly at low density and at high current. In the latter case, the onset of the instability appears to be associated with qs = 3 at the separatrix radius, effectively preventing operation at high current. However, the exploration of operation at low q has not been exhaustive, and further work is required to find a means of operating at higher currents.
3.2. Unstable regimes
Many pulse have been obtained on DITE with the bundle divertor in which the line density has exceeded 5 = 4 X 1019 rnv3. However such high densities have nkt been achieved routinely. Divertor discharges affect the surface conditions the tokamak so that although high density discharges may occur at the start of an experimental run (see fig. 2), the discharges eventually settle down to the type described in section 3.1, with A, 4 3 X 1019 rnm3. The gas feed programs for the pulses in fig. 2 was the same as for the reference discharge. Increasing the gas input during the diverted phase of these discharges generally leads to unstable operation without a significant increase in density even during neutral beam injection.
The steady value of current reached in the reference discharge corresponds to qs at the separatrix radius a, of between 3 and 3.5 (qs is defined as qs = 2vraiB,/ /@,I,). qs varies with time during a ‘constant current’ phase of a diverted discharge for the following reasons: Ripple on the ohmic supply. Ripple on the divertor coil current which causes the separatrix to move. With the divertor coil pulse stretched to 300 ms as in fig. 1, this movement is 8-10 mm. Changes in equilibrium. Increasing /3s or li reduces the minor radius of flux surfaces just inside the separatrix.
3.3. Operation at high density
(a)
Timelms
(b) 8
-7
all
5
9
4
P
3p 2
0 0
50
100
150
200
250
300
Time Ims
Fig. 3: Particle balance in the reference diverted discharge. (a) Total particle loss r,, (b) ionisation source in the divertor SD.
P. C. Johnson et al. / Results from 4.
Particle and energy control
4. I. Particle balances
The global particle balance in the torus for diverted discharges in DITE with the MkII divertor has been discussed in refs. [2] and [4]. The particle balance equation used has the form:
!g=s,+s,-r,-r,,
(1)
where N is the total number of plasma electrons calculated from the line density using model profiles. Ss is the ionisation source due to the cold gas feed, S, is the ionisation source from neutral beam particles, r,_ is the net loss to the limiters and To is the net loss to the divertor. Ss is derived from H, measurements made near the gas feed. Comparing Ss and the gas input flowrate yields a gas feed ionisation efficiency of about 50%. The total loss rate rr = r, + r, evaluated in this way for the reference discharge is illustrated in fig. 3. The main contribution to I’, is ro, with the magnitude of the limiter loss estimated to be r, = 1.25 x 102’ H-atoms/s [4]. r, rises as the equilibrium is displaced towards the minor axis, with a further small rise during neutral injection. Measurements of H, intensity have been made also in the divertor, as illustrated in fig. 3. There is a good correlation between the divertor H, signal and the derived particle exhaust. The ionisation source in the divertor, as deduced from the absolute calibration of the divertor H, signal, is < 10% of the exhaust flux. The instability mentioned in section 3.2 is observed as pulses in the H, emission in the divertor (fig. 3). The unstable behaviour occurs in the early part of the fully diverted phase of the discharge when there is only ohmic heating prior to neutral injection.
the bundle diuertor experiment on
DITE
215
wide range of conditions. Thermal instrumentation has been fitted to most of the accessible parts of the DITE torus and divertor chamber in order to look at the spatial distribution of power/energy loading in an attempt to complete the measured balance between inputs and losses. The main loss terms in power/energy balances in diverted discharges are obtained as follows. The torus wall energy loading is estimated using the bellows segments on the torus as calorimeters. Each of 6 bellows is instrumented with up to 9 calibrated miniature thermistors giving toroidal and poloidal information. The total instantaneous power loading on the wall is given by the integrated power as measured on the bellows, assuming that the time variation is the same as that of the bolometer mentioned above. The limiter segments and the divertor neutraliser plates are each instrumented with several thermocouples to given the energy loading. An AGA 780 thermal imaging camera [7] modified to give a time response of 20 ms has been used to determine the variation of power loading with time. When observing one of the two divertor plates, the energy loading obtained directly from the calibrated thermal camera agrees closely with the thermocouple measurement. The thermal imaging camera has been used also with a view of one side (electron drift side) of one of the two sets of limiters. Bolometry in the divertor chamber indicates that the radiation and charge exchange loss in the divertor is small (Q 30 kW). This is not included in the following analysis. The toroidal distribution of the wall energy loading as derived from the bellows measurements is shown for ohmic diverted and non-diverted discharges in fig. 4. The diverted discharge is similar to the reference dis-
4.2. ‘Energy and power balances The energy and power balances in diverted discharges are also discussed elsewhere [2,3]. In the earlier work on ohmic discharges the diverted power was shown to be highest at low plasma density, and decreasing as the density is raised. A substantial fraction of the power input appeared neither in the exhaust to the divertor nor in the total radiation and charge exchange loss in the torus estimated using a single bolometer looking at a minor cross-section of the torus. In the later work [3], power and energy balances have been looked at in considerably greater detail for a
Fig. 4. Toroidal variation of the wall energy loading for ohmic, diverted and non-diverted discharges.The ‘Th’ symbols refer to the location of instrumented bellows.
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P.C. Johnsan et ol. / Resdta from the bundle akrtor
experiment on DITE
INPUT ENERGY
166kJ
Time
OUTPUT
(ms)
ENERGY 123kJ
DIVERTOR 80 - 280ms
Fig. 5. Global power balance for a neutral beam heated diverted discharge, as in fig. 1.
charge but without neutral beam injection. The limiter discharge has a centered equilibrium for the whole pulse, but is otherwise similar to the diverted discharge in density and current. The wall energy loading is localised near the limiters in the non-diverted discharge. This is reduced in the diverted discharge showing that the recycling and possibly localised impurity sources at the limiter are reduced by the action of the divertor. However, the wall loading in a diverted discharge is highest near the gas feed. The fuelling rate is about a factor 5 higher to sustain constant density in the diverted discharge than in the limiter discharge, and the increase in the local wall loading associated with this is
most probably due to charge exchange losses. A spectral survey in the region of the gas input showed no evidence there of local sources of impurities. Similar spatial distributions are seen with neutral beam injection. Clearly the use of local bolometry as a measure of the total losses to the wall is inadequate in the face of such strong variations around to torus. A global power balance for a discharge similar to the refemaz discharge (but with H injected into deuterium) is presented in fig. 5. The changes in the internal energy derived from /3# or profile data are small in these discharges, and this is not included in the figure. Also, the current run-down phase of the discharge-s is not
217
P. C. Johnson et 01, / Results from the bundle divertor experiment on DITE
included. During the decay of the plasma current, the power to the limiters is consistently small. During neutral beam injection at a power level of 1.4 MW, the power loading on the divertor plates and the limiters is comparable. The divertor here takes approximately 24% of the total power input. The sum of the divertor, limiter and wall losses accounts for about 10% of the power input. The highest fractional power loading on the divertor plates, which can be as high as 70% of the power input [3], occurs however in the ohmic phase after neutral injection when there is no external gas feed into the tokamak and the density is falling. It is worth noting here that the divertor plate power loading is reduced considerably when the density is rising during the diverted phase of the discharge, as discussed in section 3.3. It would appear that the earlier work on power balances [2] is indicative of a scaling of power exhaust with dnJdf rather than simply with density. In ohmic diverted discharges about 80% of the energy input can be accounted for. The corresponding figure for neutral beam heated discharges is about 70%. The uncertainties in the measurements reported are difficult to assess, and it is not clear whether the discrepancy between the input energy and the measured losses can be ascribed to measurement errors or losses to surfaces which are not instrumented. In all of the cases analysed, the input energy exceeds the measured losses. The thermal imaging camera shows that the power. loading on the limiter during a diverted discharge occurs principally small areas, fig. 6. This may be associ: ated with sets of lines of force in the bundle divertor
LIMITER
r3 MEDIAN --
--
t = 115ms
t = 176ms
Fig. 6. Distribution of power loading on the limiter in a diverted discharge showing variation with poloidal angle.
scrape-off layer which have a very long connection length to the divertor plates.
5. Conditions in the boudary
and exhaust plasma
5.1. Parameters in the boundaryplasma
A detailed account for the use of an array of smallarea Langmuir probes to measure the electron density, the electron temperature and the flow math number (assuming Ti = T,) is given in ref. [6]. A set of large probes [8,9] has also been used to derive these quantities, with the addition of a measurement of the local heat flux through the sheath in front of the probe. Roth sets of probes have been used to make measurements in the electron and ion drift directions. Further measurements of the plasma density have been made using a 2 mm microwave interferometer. Further data is available on profiles of n, (microwaves, Thomson scattering), T, (ECE, Thomson scattering, soft X-rays) and Ti (NPA) in the core plasma (within the separatrix radius) but this will not be described in detail. Results obtained in the reference discharge will be discussed here. There are three factors which complicate the comparison of the small and large probe data, the different spatial location of the probes, their different sixes and their differing time resolution. The probes are located at different port positions on the torus: the probe array is near the divertor entrance above and below the mid-plane and the large probes are near to the top of the machine at major radii of between 1.19 m and 1.3 m. It is clear from the measurements that there can be in diverted discharges a toroidal and poloidal variation in the plasma parameters in the boundary, as with the limiter loading. This is to be expected from the complex magnetic topology in the bundle divertor scrape-off layer. Also the interpretation of the data from the large probe is dependent on an accurate knowledge of the connection length along the field to the divertor plates or limiters because of the shielding effects of the large probe housing [9]. Finally, the large probes have been used with a low time response (2 20 ms) both for voltage scans and heat flux measurements whereas the small probes have a fast response ( f: 0.1 ms). The latter can respond better to the time variations in the plasma parameters [6] which are a consequence of the ripple on the divertor coil current in addition to the natural variations in parameters. Taking the data from the large probe which has the longest connection length to a divertor plate, when the screening effects of the probe are small, the basic data
218
P.C. Johnson et al. / Results
from
the bundle divertor experiment on DITE
Values of plasma parameters during the injection heated phase of the discharge at a minor radius of 0.18 m (20 mm inside the separatrix) are T, = 250, Ti = 350 and nc = 1.9 x 1019 rne3. Clearly there is a steep gradient in T, between the separatrix and the limiter. Ti is greater than T, over the whole profile. 5.2. Parameters in the diverted plasma
240
250
260
270
2&o
2x1
260
2m
Minor radius/mm
Fig. 7. Plasma parameters measured with probes in the divertor scrapoff plasma and the limiter shadow. The times of 137 ms and 187 ms are in the ohmic and injected phases of the references discharge, fig. 1. Averages are taken over 30 ms. The probes form pairs, with odd numbers facing the divertor. Probe pairs l-2 and 7-8 pass close to the edge of the flux bundle immediately connected to the divertor, whereas probe pairs 3-4 and 5-6 pass through this flux bundle.
(saturation
current and T,) obtained
The power density to one of the divertor plates (ion side) is obtained as a function of time using the thermal imaging camera [S]. For the reference discharge the maximum power density during neutral injection reaches 35 MW m-2, with most of the diverted plasma delivering a power density greater than 3 MW m-’ at that time. During the ohmic phase before injection the maximum power density is 7.5 MW mb2. Measurements with small probes in the diverted plasma are described in ref. (61. The saturated ion current density to a probe facing the flow is illustrated in fig. 8 for horizontal scans across the diverted plasma near the divertor plate (electron side) during periods in the reference discharge before and during neutral injection. The scans are from a region of low power density towards a region of high power density. Electron temperatures are also shown in the figure. They are comparable with measurements in the outer parts of the boundary plasma discussed earlier. The power density derived from the current density and the electron temperature, assuming a convected energy of lOkT, per
by the probes is
comparable. The variation with minor radius of T, and xc in the ohmic and neutral injected phases of the reference discharge obtained from the small probes is shown in fig. 7 (a detailed description of the geometry is given in [6]). The values of density shown agree well with those derived from microwave interferometer measurements. The most significant result here is that the plasma density is in the range 0.5-l X 1019 mm3 at a distance of 40 mm outside the separatrix radius (taken at the maximum of the divertor coil current). At this density, most of the ionisation of the dissociated hydrogen from the gas feed will take place in the scrape-off plasma and not inside the separatrix. The density falls with increasing minor radius in areas of the scrape-off layer which are closely connected to the divertor (particularly probe pairs 3 & 4 and 5 & 6 in fig. 7) but elsewhere it remains high out to the limiter radius. The electron temperature is in the range lo-20 eV near the limiter radius increasing slowly towards the separatrix.
16,
,
20 Horizontal
3
40 pocilion
60
60 in diverlor
chamber.
d/mm
Fig. 8. Ion saturation current density and electron temperature variations along a scan in the diverted plasma for the reference discharge. The fines of force connected to the separatrix are to the right of the points in the figure: however the exact position is not known.
219
P. C. Johnson et al. / Results ftom the bundle diwrtor experiment an DITE
particle, varies from 1 MW mm2 to 24 MW rne2, which is comparable with values obtained from thermography. The data is insufficient to give an accurate measurement of the total particle flux in the exhaust. However the particle flux density derived from the particle balance assuming that the effective area of each of the exhaust channels is about 0.01 m2 (table l), gives values (3-4 X 1O23 me2 s-* for the reference discharge) in reasonable agreement with the probe data. The convected energy per particle estimated from the particle balance and diverted power [3] is often much higher, up to 1.5 keV/particle, than the m~mum of 0.3 keV/particle derived from this probe data.
6. Discussion
Many of the features of diverted discharges in DITE can be understood if *the density in the 40 mm region outside the separatrix is 2 1 x 1019 me3 at a temperature between 10 and 30 eV. Such a boundary plasma is
approaching the region of interest for large devices where a thick, dense scrape-off plasma is postulated to be required to protect the first wall from charge exchange particles. One consequence of a high density boundary plasma is a poor fuelling of the tokamak plasma by gas puffing as most of the ionisation of H, dissociation products will take place in the boundary. An inabi~ty to raise the density has been one of the consistent features of experimental work with the MkII bundle divertor as described earlier. The radial distribution of radiation losses in the tokamak during diverted discharges, fig. 9, shows a peak in the region between the limiter and the separatrix, and this may also be explained by a high edge density. The main terms in a detailed radial power balance for a neutral beam heated discharge are listed in table 2. To this list must be added parallel flow losses to the divertor in the region outside the separatrix, and to the limiter outside the limiter radius. In order to place any greater significance on a global measurement of power BOLOMETER
ARRAY
LINE OF SIGHT INTENSI~ REFERENCE
DIVERTOR
DISCHARGE
Fig. 9. The spatial distribution of line-of-sight bolometer signal from the IO-channel bolometer array for the reference divertor discharge.
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P.C. Johnson et al. / Results from the bundle divertor experiment on DITE
Table 2 List of main terms in the plasma power balance for a neutral beam heated di8cluuge Component
Term
Ohmic heating
electron heating
Fast ions
charge exchange electron heating ion heating orbit losses divertor ripple losses [ lo,1 l]
Plasma ions
equipartition charge exchange ion thermal conduction convection
Plasma electrons
equipartition radiation electron thermal conduction convection
unload efficiency it is important to make estimates of the magnitudes of all of these terms. This includes the boundary plasma where the processes listed still apply especially when the edge density is high. In the reference discharge, where Ti z T, during neutral injection, it is evident that most of the power delivered to the boundary plasma will be in the electrons and that radiation losses from this region play an important part in the boundary energy balance. Charge exchange loss from plasma ions in the boundary plasma is also an important term. Some losses occur through orbit interacting with the divertor ripple maxima [lO,ll]. The divertor plate8 and limiters take the power remaining after all the other losses have been subtracted. Work is in progress in this area and will be reported in a later publication. The presence of a potential barrier to the electrons in a magnetic mirror is well known, and applies also to the bundle divertor [S]. This potential barrier has the effect of accelerating plasma to supersonic speeds whilst maintaining charge neutrality. If the ions are collision-free over the scale length of the mirror magnetic field then the effect of the divertor mirror is to select only the mirror loss cone for transmission to the divertor. The net effect is to produce a supersonic diverted plasma which has a density somewhat lower than that in the boundary plasma. This is consistent with the probe measurements made in the divertor on the reference discharge (61 where the Mach number is shown to be
greater than unity and the electron density less than 5 X lo’* mT3. The low re-ionisation of recycled neutrals in the divertor and the low radiation/charge exchange loss from the diverted plasma can be explained on the basis of a low plasma density in the divertor region. Again, work is in progress in this area.
7. Conclusions Experiments with the MkII bundle divertor on DITE have shown good particle diversion and good power diversion particularly with reduced gas feed in the main chamber. The radiation loss from the torus and the Z,,, are lower with the divertor operating than in limiter discharges. The wall power loading has been shown to vary substantially over different areas of the torus. In particular, due to the high gas feed necessary to fuel the exhaust the power loading is highest near the gas input in diverted discharges. The power loading near the limiters is reduced with the divertor operating. Measurements in the boundary region between the separatrix and the limiter show that the density is greater than 1 X 1019 mm2 over much of the scrape-off layer. The electron near the limiter edge is in the range lo-20 eV. Most of the ionisation of the hydrogen fuel will take place in the scrape-off layer.at such a value of density, and this could explain the difficulty in fuelling the tokamak in divertor experiments. The high radiation loss from the boundary may also be a consequence of the high edge density. The diverted plasma exhibits minimal re-ionisation of hydrogen recycled in the divertor. The exhaust regime is convective and the flow, as inferred from probe measurements, is supersonic. The effect of the bundle divertor magnetic mirror on the flow into the divertor will be introduce a potential barrier to the electrons. This accelerates the ions to supersonic speeds and lead to a density in the exhaust to the divertor plates which is lower than in the scrapeoff layer.
Acknowledgements The author8 are indebted to the DITE technical staff under Mr. R.W. Storey and mr. R.E. Bradford for machine operations, to the scientific staff for work on diagnostic8 and to the Group Leader, Dr. J.W.M. Paul for his continual encouragement and support.
P.C. Johnson et al. / Results from the bundle divertor experiment on DITE
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