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Modification of the scrape-off layer by edge plasma modes
245
Journal of Nuclear Materials 162-164 (1989) 245-250
North-Holland, Amsterdam
MODIFICATION
OF THE SCRAPE-OFF
LAYER BY EDGE PLASMA
MODES
M. AL1 MAHDAVI, ‘, D. HILL *, S.L. ALLEN *, N.H. BROOKS I, K.H. BURRELL ‘, T. CARLSTROM ‘, D. CONTENT 3, J. DeBOO’, P. GOHIL I, N. GOTTARDI 4, C.L. HSIEH ‘, G. HAAS 5, G. JACKSON l, N. OHYABU ‘, M.E. PERRY 3, T. PETRIE I, M. RENSINK *, D. SCHISSEL’, M. SHIMADA 6, R. SNIDER’, R. STAMBAUGH I, R. STOCKDALE ’ and T. TAYLOR
’
’ General Atomics, P. 0. Box 85608, San Diego, CA 92138, USA ’ Lawrence Livermore National Laboratory, p.o. Box 551 I, Livermore, CA 94550, USA 3 John Hopkins Uniuersiry, Baltimore, Maryland, USA ‘Joint European Undertaking, Abingdon, Oxon. OX14 3EA, United Kingdom ’ Max Planck Institute Fiir Plasma Physics, Boltzmannstrasse 2, D-8046, Garching, Fed. Rep. Germany 6 Japan Atomic Energy Research Insiitute, Japan
Key words: H-mode, edge plasma modes, transport, impurity confinement The improved energy confinement of the H-mode is invariably coupled to improved particle confinement. In other experiments, increased particle confinement resulted in impurity accumulation and severe power losses. In DIII-D, similar to the results of other experiments, H-mode confinement is associated with improved particle confinement. But in contrast to other devices, in most DIII-D H-mode plasmas there is no indication of impurity accumulation. In high current shots the impurity content of plasma during the H-mode is lower than the L-mode phase. Two factors most responsible for the low impurity content of DIII-D H-mode plasmas are the nearly flat or hollow electron density profiles and the presence of giant Edge Localized Modes (ELMS). Giant ELMS also cause large amplitude modulations in particle and power flow to the divertor target plates, resulting in large scale changes in the parameters of the the scrape-off layer plasma. Our analysis of temperature and density profiles during ELMS show that particle loss during ELMS is primarily due to perpendicular transport across the field lines.
The good confinement regime of auxiliary heated plasmas is usually identified by a characteristic drop in H,/D, light emission with periodic bursts of H,/D,, light, referred to as Edge Localized Modes (ELMS) by the ASDEX group [1,2]). In DIII-D, H, bursts are coupled to an order of magnitude increase in power flow to the divertor plates and a four fold broadening of the heat deposition profile on the divertor plates [3]. The large particle and power flow into the divertor plasma upsets the equilibrium conditions of the divertor plasma, resulting in a stronger plasma interaction with the divertor target plates. Two distinct types of ELMS have been observed in DIII-D: infrequent regularly spaced giant ELMS and more frequent very low amplitude grassy ELMS. Giant ELMS cause loss of a significant fraction (up to 20%) of the plasma energy, and their effect, as shown in fig. 1, is easily observable on most diagnostic traces, such as
0 Elsevier Science Publishers B.V. Physics Publishing Division)
0022-3115/89/$03.50
(North-Holland
diamagnetic flux, loop voltage, and impurity line intensities. During giant ELMS there is an order of magnitude increase in heat flow to the divertor plates, and a four fold broadening of the heat deposition profile on the target plates [3]. Grassy ELMS, on the other hand, are detectable only on the H, light traces and infrared camera images of the divertor target plates. While grassy ELMS are present during nearly all DIII-D H-mode plasmas, the presence of giant ELMS is dependent upon the position of the X-point. The dependence of ELM behavior on the vertical position of the X-point is shown in fig. 2 showing data from an H-mode shot in which the vertical position of the X-point was varied during the neutral beam injection. In the early H-mode phase where the X-point position is - 18 cm above the vessel floor (fig. 2, top trace), well separated ELMS are observed (fig. 2 middle trace). In the middle period of the H-mode, where the X-point height has been in-
246
M. Ali Mahdaui
1.0
LOOP VOLTAGE ’ I\ ,“-u><_,” .“^ _\_,. _,_. ‘.., ,, .
1.0
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et al. / Modificniron
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,, ^,
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,
,
A 1800
2000
2200
2400 TIME
2600
2800
3000
(msec)
Fig. 1. Plot of various diagnostic signals, showing effects of giant ELMS: (a) loop voltage, (b) line integral density, (c) central bolometer chord, (d) divertor neutral pressure, (e) H,/D, line intensity, (f) nickel-XVII line intensity.
of the scrape-off
Ia_ver ty edge plasma modes
creased to - 22 cm above the vessel floor, the frequency of giant ELMS has increased. During the last period where the X-point was lowered to - 16 cm, giant ELMS have disappeared and only grassy ELMS can be seen. Observation of time delays on H, light signals, measured at several poloidal positions, show that giant ELMS originate in the outboard boundary of the plasma. The rise in the H, light occurs first at the outboard mid-plane of the plasma, followed by the outboard divertor, then the inner wall at mid-plane, and finally at the inboard leg of the divertor. The delay between the rise of the H, signal in the two legs of the divertor is - 300 /ASwhich is in agreement with the observed delay between the arrival times of the heat pulses to the target plates [3]. These observations, which place the origin of ELMS in the region of bad curvature, suggest that ELMS are caused by the ballooning mode instability. Further evidence for the ballooning nature of ELMS was produced by the results of recent DIII-D experiments [4] which show giant ELMS occur only when the
‘I l
I
BEFORE ELM DURfNG ELM
25.0
35
0.5
6.0
-0.3
-0.1
0.1
-0.3
-0.1
0.1
d.g
0.5
0.3
0.5
0.7
-1
PINJ (MW)
I -~
J.,,‘,,,,,,,,,, ,, ,, .,, ,,,,,,,,,, , LI’
1000
1500
2000
TIME
2500
3000
3 50 0
(msec)
Fig. 2. Dependence of the frequency and amplitude of the H, spikes (second trace) on the vertical position of the X-point (top trace). Increasing the X-point height increased the frequency of giant ELMS. For X-point position lower than 121 cm below the magnetic axis of the plasma, giant ELMS disappear, and only grassy ELMS can be seen.
-0.5
VERTICAL
POSflON
0.7
0.0
(m)
Fig. 3. Plot of the electron density and temperature profiles, as measured by a Thomson scattering system, along a vertical path 23 centimeters outside of the magnetic axis of the plasma, before and during a giant ELM. The electron density outside the separatrix has increased by a factor of three during the ELM.
M. Ali Mohdoui et al. / Modification of the scrape-off layer by edge plasm0 modes 0
m ?
r ~~
o
jJ.5
00 -0.5
l
Q
PI
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-0.1
6.1
-0.3
-0.1
0.1
VERTICAL
BEFORE ELM 1 ms AFTER ELM
0.3
is
0.3
0.6
POSITION
0.7
0;s
'.l
0.7
0.8
1.1
(m)
Fig. 4. Comparison of the electron density and temperature profiles before and 1 ms after a giant ELM. After the ELM, a large fraction of the edge plasma particles are lost, while the temperature profile remains nearly unchanged.
edge pressure gradient exceeds the threshold for the ideal ballooning mode instability. Giant ELMS expel a large fraction of the edge plasma particles, resulting in a factor of three increase in the particle density in the scrape-off layer. The density profiles before and during an ELM are compared in fig. 3, showing the density rise outside the separatrix surface during the ELM. The profiles in fig. 3 are measured along a vertical chord 23 cm outside the magnetic axis of the plasma. The density profiles before and 1 ms after the termination of the ELM are compared in fig. 4. In the post-ELM profile of fig. 4, the electron density outside of the separatrix has returned to its pre-ELM value; whereas, in the outer half of the main plasma, electron density is reduced by one third. In contrast, the electron temperature profiles (fig. 4) remain nearly unchanged throughout an ELM. Although the evidence for ideal ballooning mode triggering of ELMS is very strong, a further mechanism may be required to explain the observed large heat and particle fluxes. There are two candidate classes of mechanisms; (1) electrostatic fluctuation-induced particle flux across the separatrix into the region of the open flux surfaces where electron heat conductivity transports
241
heat to the divertor plates, and (2) development of magnetic islands or ergodization of the field lines connecting a region of the main plasma to the open field lines. Electrostatic fluctuation-induced particle transport appears more consistent with the Thomson scattering data of figs. 3 and 4, where the temperature profile remains nearly unaffected by the ELM while the density profile is drastically modified. A simple calculation (see Appendix A) shows that for the profiles of figs. 3 and 4, a purely convective heat transport results in < 20% variation in T, within the range of the the disturbance. In the scrape-off layer, the influx of hot particles causes the particle density and power flow to the divertor to increase by an order to magnitude, however due to the very large parallel electron heat conductivity the downstream electron temperature can rise at most by a factor of two [5]. The second category of mechanisms could also explain the density rise in the scrape-off layer and high heat flux to the divertor plates. However, in contradiction with the Thomson scattering data of fig. 3, formation of magnetic islands or ergodization of field lines of such a magnitude as to explain the observed particle loss rate would necessitate a a large drop in the electron temperature within the closed flux surfaces. In DIII-D, similar to other experiments, the H-mode is associated with improved particle confinement [2], which results in an uncontrolled density rise. However, in contrast to the other experiments [6,7], in most DIII-D H-mode plasmas the impurity content of the plasma does not increase. Two factors most responsible for the low impurity content of the DIII-D H-mode plasmas are the nearly flat or hollow density profiles and the giant ELM activity. Both impurity and particle transport during H-mode show a strong current dependence [8]. With increasing plasma current, the rate of density rise increases, the edge plasma density gradient increases and the electron density profile becomes flatter and eventually hollow in the center (fig. 5). In DIII-D, similar to other machines [7], impurities tend to peak relative to the electron density profile, thus centrally peaking moderately at low I, and becoming hollow at higher currents. Impurity concentration profiles during the quiescent phase of the H-mode deduced from plasma radiation measurements using a 21-channel poloidal array of bolometers and a 16-channel tangential array of visible bremsstrahlung sensors [9] are shown in fig. 6. For ZP = 0.8 MA, where the electron density profile is slightly centrally peaked, profiles of nickel and carbon concentrations are also peaked at the center, whereas for ZP= 2.0 MA plasma, impurity concentration profiles are very hollow.
M. Ali Mahdavi et al. / Modification
”
2.0
MA
*
1.25 MA 0.8
-0.5
MA
-0.3
-0.1
0.1
VERTICAL
0.3
0.5
POSITION
0.7
0.9
1.1
(ml
Fig. 5. Electron density profiles for H-mode plasmas with various plasma currents. With increasing I,, the edge density gradient increases and the density profile becomes hollow.
Large amplitude ELMS result in the loss of a large fraction of the edge particles and impurities. The loss of highly concentrated edge impurities has a net time averaged purifying effect on the plasma. Following an ELM, most of the expelled protons/deuterons, slowly
---_
--
nN, 0.8 MA
-
nc
‘\ \ \
0.0 MA
\
,’ ,*’ n,,, 2.0 MA
,,,’ ,,,’
,’
,.
,’
.__...........____________
._-’ ._____...--’
0.0
0.2
0.8
0.4
0.8
of the scrape-off layer by edge plasma modes
recycle back into the edge plasma, and within = 100 ms the edge plasma density returns to its pre-ELM value. In contrast. Ni and C, the dominant impurities in DIII-D, do not recycle, and thus ELM activity reduces the impurity content of the plasma. With increasing plasma current the impurity population of the H-mode plasmas moves closer to the separatrix, therefore the process of plasma purification becomes stronger with increasing plasma current. Modulation of impurity and electron density profiles explains [lo] the large fluctuations in impurity line intensities and total plasma radiation shown in fig. 1. During the quiescent period following an ELM, simultaneous peaking of impurities and plasma particles in the low temperature edge zone increases the radiative efficiency of the plasma. In contrast, shortly after an ELM, the electron density profile is centrally peaked, and thus impurities move inward into the hotter less efficiently radiating zone. The same process explains the vast difference between the radiative efficiencies of low current and high current plasmas. In summary, giant ELMS in DIII-D cause large amplitude fluctuations in particle and power flow to the divertor target plates, resulting in large scale changes in the parameters of the scrape-off layer and the divertor plasma. These changes in the parameters of the plasma in contact with material surfaces affect the strength of the divertor plasma interaction with the target plates and are important considerations for the design of the next generation tokamaks. Strong evidence has been presented for the ideal ballooning mode triggering of ELMS, however a further mechanism to explain the observed high heat and particle losses during ELMS may be required. We have shown that the DIII-D data are more consistent with the class of instabilities where particle transport is primarily perpendicular to the field lines. The evolution of the density profile, plays an important role on impurity behavior and radiative properties of the DIII-D H-mode plasma. With increasing plasma current, the line average electron density increases and the center of gravity of the electron density profile and impurity profiles move to the boundary of the plasma, greatly increasing the radiative efficiency of the plasma.. Nevertheless, a decrease in the impurity content of the plasma caused by giant ELM activity prevents a radiative collapse of the plasma.
I.0
r/a Fig. 6. Impurity concentrations deduced from the bolometer and the visible bremsstrahhmg data. At low I,, impurity profiles are centrally peaked but become hollow at higher currents.
Appendix A We
distinguish
instabilities
to explain
between
two
the particle
classes
of candidate an ELM;
loss during
M. Ali Mahdavi et al. / ~od~~~atian of the scmpe-off layer by edge plasma modes
249
(1) an electrostatic fluctuation-induced particle flux, and (2) formation of magnetic islands or ergodization of field lines, allowing particle flow along the open flux surfaces into the scrape-off layer. Here we will show that the first class of instabilities does not necessitate a large change in the edge temperature profile; therefore it is compatible with the observation that the electron temperature profile does not change appreciably during an ELM. On the other hand, the second class of mechanism requires a substantial change in the electron temperature profile, and thus is inconsistent with the temperature profile of figs. 3 and 4. The equations of continuity and energy conservation for the case of perpendicular particle transport are;
In the case of parallel particle convection, we must include the parallel electron heat conduction term to the energy conservation equation, aW/at = - 30. kTr,, + v. q,,, In the collision-less regime the parallel conduction term is of the order of /(h4,/m,) q,, and, therefore, an order of magnitude greater than the convective term. However, as we shall see, the parallel connection lenght is very large and the collisional parallel electron heat conductivity, i.e., q,,= Kv,,T, should be used. The parallel scale length, estimated from the continuity equation, is
an,/af
where St is the time interval from the start of the ELM to the measurement time. Using the plasma parameters of fig. 3 again, the estimated value of ql,- KT/I{[ is an order of magnitude greater than the corresponding convective term. Thus, while the convective heat flux term has a - 20% effect on the boundary plasma temperature, inclusion of the parallel conduction term should result in a large and readily measurable modification of the edge plasma temperature profile. It is important to note that the above arguments do not preclude presence of open field lines, and only show that open field lines alone could not account for the observed particle losses during ELM events. In particular, in the profiles of fig. 3, there are three low temperature points between the radii of OS and 0.7 m, which cannot be attributed to the scatter in the data. Existence of a magnetic island at the plasma boundary could explain such a low temperature region in the interior of the plasma.
aW,‘&=
= -v.rl, -$v.kTr,,
(1) (2)
r, is the anomalous particle flw, k is the Bolzmann constant, and where for the sake of simplicity we have ignored the perpendicular beat conduction term and set T = T, = q. Within the separatrix, particle flux is modeled by r, = a(r - ri), where a is a function of time only, and r, is the innermost radius of the disturbance. This model is suggested by the density profile data. Integrating eqs. (I) and (2), over the time period from the start of the ELM to the measurement time during the ELM, and assuming that T is independent of time results in: Sn, = no -n, = ja dt and W, = W, - sk[(r - ri ) VT + T] Jo d t, where the subscripts 0 and f signify the pre-ELM and during the ELM values of the parameters. The plasma temperature at the measurements time is z’, = W,/($n,k) =4(.d~~+5/31(r-‘i)/r,+1](~-1)},
4i-
(nonon,)sf
kT 2sM, \i---
’
(I31
where +* is the temperature gradient scale length. According to the result of eq. (S), using the experimentally observed value of nJne - 0.8, the temperature of the inner most re8ion of the disturbance should drop by - 101, and increase by - 20% in the outermost re8ion. This result is consistent with the assumption of constant plasma temperature used in the above calculation. In the above calculation we ignored perpendicular heat conduction. If the perpendicular heat diffusivity during the ELM is the same as before the onset of the ELM, then its inclusion will have roughly an equal and opposing effect on the temperature profile, helping to maintain the temperature profile close to its pre ELM value. Therefore within the experimental error bars the profiles of fig. 4 are consistent with a purely convective heat transport.
The Authors wish to thank Drs. G. Janeschitz, J.K. Lee, G. Porter, and T. Simonen for useful and stimulating discussions. This work was performed under the auspices of the US Department of Energy by General Atom&, under Contract No. DE-AC03-84ER51044, and by Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48.
References [l] F. Wagner, Nucl. Mater. 121 (1984) 103. [2] M. Keilhacker et al, in: Proc. 10th Int. Conf. on Plasma Physics and Controlled Fusion Research, London, 1984, Vol. I, p. 71.
250
M. Ah Mahdavi et al. / Modrfication of the scrape-off lqw
[3] D.N. Hill, T.W. Petrie, Howl, DIII-D divertor
M. Ali Mahdavi, L. Lao and W. target heat flux measurements
during ohmic and neutral beam heating, submitted to Nucl. Fusion. [4] P. Gohil et al., Study Of ELMS in DIII-D and Comparision with Ballooning Theory, General Atomics Report GA-A19035, submitted to Phys. Rev. Lett. [5] M. Ali Mahdavi et al., Phys. Rev. Lett. 47 (1981) 1062. [6] G. Fussman, Study of impurity accumulation in the ASDEX tokamak, in: Proc. 14th European Conf. on Controlled Fusion and Plasma Physics, Madrid, Spain, 1987.
by edge plasmu modes
[7] E.R. Miiller et al., Nucl. Fusion 27 (1987) 1817. [8] N. Brooks et al., Impurity Behavior During DIII-D Hmode Plasmas, General Atomics Report GA-A19108, submitted to Nucl. Fusion. [9] D. Content et al., Impurity Profiles in DIII-D H-mode Discharges, General Atomics Report GA-Al9288 (1988). [lo] M. Perry et al., Modeling Impurity Transport in DIII-D H-mode Plasmas With Giant ELMS, General Atomics Report GA-A19286.
Journal of Nuclear Materials 162-164 (1989) 245-250
North-Holland, Amsterdam
MODIFICATION
OF THE SCRAPE-OFF
LAYER BY EDGE PLASMA
MODES
M. AL1 MAHDAVI, ‘, D. HILL *, S.L. ALLEN *, N.H. BROOKS I, K.H. BURRELL ‘, T. CARLSTROM ‘, D. CONTENT 3, J. DeBOO’, P. GOHIL I, N. GOTTARDI 4, C.L. HSIEH ‘, G. HAAS 5, G. JACKSON l, N. OHYABU ‘, M.E. PERRY 3, T. PETRIE I, M. RENSINK *, D. SCHISSEL’, M. SHIMADA 6, R. SNIDER’, R. STAMBAUGH I, R. STOCKDALE ’ and T. TAYLOR
’
’ General Atomics, P. 0. Box 85608, San Diego, CA 92138, USA ’ Lawrence Livermore National Laboratory, p.o. Box 551 I, Livermore, CA 94550, USA 3 John Hopkins Uniuersiry, Baltimore, Maryland, USA ‘Joint European Undertaking, Abingdon, Oxon. OX14 3EA, United Kingdom ’ Max Planck Institute Fiir Plasma Physics, Boltzmannstrasse 2, D-8046, Garching, Fed. Rep. Germany 6 Japan Atomic Energy Research Insiitute, Japan
Key words: H-mode, edge plasma modes, transport, impurity confinement The improved energy confinement of the H-mode is invariably coupled to improved particle confinement. In other experiments, increased particle confinement resulted in impurity accumulation and severe power losses. In DIII-D, similar to the results of other experiments, H-mode confinement is associated with improved particle confinement. But in contrast to other devices, in most DIII-D H-mode plasmas there is no indication of impurity accumulation. In high current shots the impurity content of plasma during the H-mode is lower than the L-mode phase. Two factors most responsible for the low impurity content of DIII-D H-mode plasmas are the nearly flat or hollow electron density profiles and the presence of giant Edge Localized Modes (ELMS). Giant ELMS also cause large amplitude modulations in particle and power flow to the divertor target plates, resulting in large scale changes in the parameters of the the scrape-off layer plasma. Our analysis of temperature and density profiles during ELMS show that particle loss during ELMS is primarily due to perpendicular transport across the field lines.
The good confinement regime of auxiliary heated plasmas is usually identified by a characteristic drop in H,/D, light emission with periodic bursts of H,/D,, light, referred to as Edge Localized Modes (ELMS) by the ASDEX group [1,2]). In DIII-D, H, bursts are coupled to an order of magnitude increase in power flow to the divertor plates and a four fold broadening of the heat deposition profile on the divertor plates [3]. The large particle and power flow into the divertor plasma upsets the equilibrium conditions of the divertor plasma, resulting in a stronger plasma interaction with the divertor target plates. Two distinct types of ELMS have been observed in DIII-D: infrequent regularly spaced giant ELMS and more frequent very low amplitude grassy ELMS. Giant ELMS cause loss of a significant fraction (up to 20%) of the plasma energy, and their effect, as shown in fig. 1, is easily observable on most diagnostic traces, such as
0 Elsevier Science Publishers B.V. Physics Publishing Division)
0022-3115/89/$03.50
(North-Holland
diamagnetic flux, loop voltage, and impurity line intensities. During giant ELMS there is an order of magnitude increase in heat flow to the divertor plates, and a four fold broadening of the heat deposition profile on the target plates [3]. Grassy ELMS, on the other hand, are detectable only on the H, light traces and infrared camera images of the divertor target plates. While grassy ELMS are present during nearly all DIII-D H-mode plasmas, the presence of giant ELMS is dependent upon the position of the X-point. The dependence of ELM behavior on the vertical position of the X-point is shown in fig. 2 showing data from an H-mode shot in which the vertical position of the X-point was varied during the neutral beam injection. In the early H-mode phase where the X-point position is - 18 cm above the vessel floor (fig. 2, top trace), well separated ELMS are observed (fig. 2 middle trace). In the middle period of the H-mode, where the X-point height has been in-
246
M. Ali Mahdaui
1.0
LOOP VOLTAGE ’ I\ ,“-u><_,” .“^ _\_,. _,_. ‘.., ,, .
1.0
Cl.0 =I
” DENSITY
et al. / Modificniron
’ !
:_.
/ . .I
-,
““““’ ” P-RADIATION
,
,
_,
,, ”
! /
‘b.
,, ^,
I_ ^
_’
., ,, )
,
,
A 1800
2000
2200
2400 TIME
2600
2800
3000
(msec)
Fig. 1. Plot of various diagnostic signals, showing effects of giant ELMS: (a) loop voltage, (b) line integral density, (c) central bolometer chord, (d) divertor neutral pressure, (e) H,/D, line intensity, (f) nickel-XVII line intensity.
of the scrape-off
Ia_ver ty edge plasma modes
creased to - 22 cm above the vessel floor, the frequency of giant ELMS has increased. During the last period where the X-point was lowered to - 16 cm, giant ELMS have disappeared and only grassy ELMS can be seen. Observation of time delays on H, light signals, measured at several poloidal positions, show that giant ELMS originate in the outboard boundary of the plasma. The rise in the H, light occurs first at the outboard mid-plane of the plasma, followed by the outboard divertor, then the inner wall at mid-plane, and finally at the inboard leg of the divertor. The delay between the rise of the H, signal in the two legs of the divertor is - 300 /ASwhich is in agreement with the observed delay between the arrival times of the heat pulses to the target plates [3]. These observations, which place the origin of ELMS in the region of bad curvature, suggest that ELMS are caused by the ballooning mode instability. Further evidence for the ballooning nature of ELMS was produced by the results of recent DIII-D experiments [4] which show giant ELMS occur only when the
‘I l
I
BEFORE ELM DURfNG ELM
25.0
35
0.5
6.0
-0.3
-0.1
0.1
-0.3
-0.1
0.1
d.g
0.5
0.3
0.5
0.7
-1
PINJ (MW)
I -~
J.,,‘,,,,,,,,,, ,, ,, .,, ,,,,,,,,,, , LI’
1000
1500
2000
TIME
2500
3000
3 50 0
(msec)
Fig. 2. Dependence of the frequency and amplitude of the H, spikes (second trace) on the vertical position of the X-point (top trace). Increasing the X-point height increased the frequency of giant ELMS. For X-point position lower than 121 cm below the magnetic axis of the plasma, giant ELMS disappear, and only grassy ELMS can be seen.
-0.5
VERTICAL
POSflON
0.7
0.0
(m)
Fig. 3. Plot of the electron density and temperature profiles, as measured by a Thomson scattering system, along a vertical path 23 centimeters outside of the magnetic axis of the plasma, before and during a giant ELM. The electron density outside the separatrix has increased by a factor of three during the ELM.
M. Ali Mohdoui et al. / Modification of the scrape-off layer by edge plasm0 modes 0
m ?
r ~~
o
jJ.5
00 -0.5
l
Q
PI
-Ii.3
-0.1
6.1
-0.3
-0.1
0.1
VERTICAL
BEFORE ELM 1 ms AFTER ELM
0.3
is
0.3
0.6
POSITION
0.7
0;s
'.l
0.7
0.8
1.1
(m)
Fig. 4. Comparison of the electron density and temperature profiles before and 1 ms after a giant ELM. After the ELM, a large fraction of the edge plasma particles are lost, while the temperature profile remains nearly unchanged.
edge pressure gradient exceeds the threshold for the ideal ballooning mode instability. Giant ELMS expel a large fraction of the edge plasma particles, resulting in a factor of three increase in the particle density in the scrape-off layer. The density profiles before and during an ELM are compared in fig. 3, showing the density rise outside the separatrix surface during the ELM. The profiles in fig. 3 are measured along a vertical chord 23 cm outside the magnetic axis of the plasma. The density profiles before and 1 ms after the termination of the ELM are compared in fig. 4. In the post-ELM profile of fig. 4, the electron density outside of the separatrix has returned to its pre-ELM value; whereas, in the outer half of the main plasma, electron density is reduced by one third. In contrast, the electron temperature profiles (fig. 4) remain nearly unchanged throughout an ELM. Although the evidence for ideal ballooning mode triggering of ELMS is very strong, a further mechanism may be required to explain the observed large heat and particle fluxes. There are two candidate classes of mechanisms; (1) electrostatic fluctuation-induced particle flux across the separatrix into the region of the open flux surfaces where electron heat conductivity transports
241
heat to the divertor plates, and (2) development of magnetic islands or ergodization of the field lines connecting a region of the main plasma to the open field lines. Electrostatic fluctuation-induced particle transport appears more consistent with the Thomson scattering data of figs. 3 and 4, where the temperature profile remains nearly unaffected by the ELM while the density profile is drastically modified. A simple calculation (see Appendix A) shows that for the profiles of figs. 3 and 4, a purely convective heat transport results in < 20% variation in T, within the range of the the disturbance. In the scrape-off layer, the influx of hot particles causes the particle density and power flow to the divertor to increase by an order to magnitude, however due to the very large parallel electron heat conductivity the downstream electron temperature can rise at most by a factor of two [5]. The second category of mechanisms could also explain the density rise in the scrape-off layer and high heat flux to the divertor plates. However, in contradiction with the Thomson scattering data of fig. 3, formation of magnetic islands or ergodization of field lines of such a magnitude as to explain the observed particle loss rate would necessitate a a large drop in the electron temperature within the closed flux surfaces. In DIII-D, similar to other experiments, the H-mode is associated with improved particle confinement [2], which results in an uncontrolled density rise. However, in contrast to the other experiments [6,7], in most DIII-D H-mode plasmas the impurity content of the plasma does not increase. Two factors most responsible for the low impurity content of the DIII-D H-mode plasmas are the nearly flat or hollow density profiles and the giant ELM activity. Both impurity and particle transport during H-mode show a strong current dependence [8]. With increasing plasma current, the rate of density rise increases, the edge plasma density gradient increases and the electron density profile becomes flatter and eventually hollow in the center (fig. 5). In DIII-D, similar to other machines [7], impurities tend to peak relative to the electron density profile, thus centrally peaking moderately at low I, and becoming hollow at higher currents. Impurity concentration profiles during the quiescent phase of the H-mode deduced from plasma radiation measurements using a 21-channel poloidal array of bolometers and a 16-channel tangential array of visible bremsstrahlung sensors [9] are shown in fig. 6. For ZP = 0.8 MA, where the electron density profile is slightly centrally peaked, profiles of nickel and carbon concentrations are also peaked at the center, whereas for ZP= 2.0 MA plasma, impurity concentration profiles are very hollow.
M. Ali Mahdavi et al. / Modification
”
2.0
MA
*
1.25 MA 0.8
-0.5
MA
-0.3
-0.1
0.1
VERTICAL
0.3
0.5
POSITION
0.7
0.9
1.1
(ml
Fig. 5. Electron density profiles for H-mode plasmas with various plasma currents. With increasing I,, the edge density gradient increases and the density profile becomes hollow.
Large amplitude ELMS result in the loss of a large fraction of the edge particles and impurities. The loss of highly concentrated edge impurities has a net time averaged purifying effect on the plasma. Following an ELM, most of the expelled protons/deuterons, slowly
---_
--
nN, 0.8 MA
-
nc
‘\ \ \
0.0 MA
\
,’ ,*’ n,,, 2.0 MA
,,,’ ,,,’
,’
,.
,’
.__...........____________
._-’ ._____...--’
0.0
0.2
0.8
0.4
0.8
of the scrape-off layer by edge plasma modes
recycle back into the edge plasma, and within = 100 ms the edge plasma density returns to its pre-ELM value. In contrast. Ni and C, the dominant impurities in DIII-D, do not recycle, and thus ELM activity reduces the impurity content of the plasma. With increasing plasma current the impurity population of the H-mode plasmas moves closer to the separatrix, therefore the process of plasma purification becomes stronger with increasing plasma current. Modulation of impurity and electron density profiles explains [lo] the large fluctuations in impurity line intensities and total plasma radiation shown in fig. 1. During the quiescent period following an ELM, simultaneous peaking of impurities and plasma particles in the low temperature edge zone increases the radiative efficiency of the plasma. In contrast, shortly after an ELM, the electron density profile is centrally peaked, and thus impurities move inward into the hotter less efficiently radiating zone. The same process explains the vast difference between the radiative efficiencies of low current and high current plasmas. In summary, giant ELMS in DIII-D cause large amplitude fluctuations in particle and power flow to the divertor target plates, resulting in large scale changes in the parameters of the scrape-off layer and the divertor plasma. These changes in the parameters of the plasma in contact with material surfaces affect the strength of the divertor plasma interaction with the target plates and are important considerations for the design of the next generation tokamaks. Strong evidence has been presented for the ideal ballooning mode triggering of ELMS, however a further mechanism to explain the observed high heat and particle losses during ELMS may be required. We have shown that the DIII-D data are more consistent with the class of instabilities where particle transport is primarily perpendicular to the field lines. The evolution of the density profile, plays an important role on impurity behavior and radiative properties of the DIII-D H-mode plasma. With increasing plasma current, the line average electron density increases and the center of gravity of the electron density profile and impurity profiles move to the boundary of the plasma, greatly increasing the radiative efficiency of the plasma.. Nevertheless, a decrease in the impurity content of the plasma caused by giant ELM activity prevents a radiative collapse of the plasma.
I.0
r/a Fig. 6. Impurity concentrations deduced from the bolometer and the visible bremsstrahhmg data. At low I,, impurity profiles are centrally peaked but become hollow at higher currents.
Appendix A We
distinguish
instabilities
to explain
between
two
the particle
classes
of candidate an ELM;
loss during
M. Ali Mahdavi et al. / ~od~~~atian of the scmpe-off layer by edge plasma modes
249
(1) an electrostatic fluctuation-induced particle flux, and (2) formation of magnetic islands or ergodization of field lines, allowing particle flow along the open flux surfaces into the scrape-off layer. Here we will show that the first class of instabilities does not necessitate a large change in the edge temperature profile; therefore it is compatible with the observation that the electron temperature profile does not change appreciably during an ELM. On the other hand, the second class of mechanism requires a substantial change in the electron temperature profile, and thus is inconsistent with the temperature profile of figs. 3 and 4. The equations of continuity and energy conservation for the case of perpendicular particle transport are;
In the case of parallel particle convection, we must include the parallel electron heat conduction term to the energy conservation equation, aW/at = - 30. kTr,, + v. q,,, In the collision-less regime the parallel conduction term is of the order of /(h4,/m,) q,, and, therefore, an order of magnitude greater than the convective term. However, as we shall see, the parallel connection lenght is very large and the collisional parallel electron heat conductivity, i.e., q,,= Kv,,T, should be used. The parallel scale length, estimated from the continuity equation, is
an,/af
where St is the time interval from the start of the ELM to the measurement time. Using the plasma parameters of fig. 3 again, the estimated value of ql,- KT/I{[ is an order of magnitude greater than the corresponding convective term. Thus, while the convective heat flux term has a - 20% effect on the boundary plasma temperature, inclusion of the parallel conduction term should result in a large and readily measurable modification of the edge plasma temperature profile. It is important to note that the above arguments do not preclude presence of open field lines, and only show that open field lines alone could not account for the observed particle losses during ELM events. In particular, in the profiles of fig. 3, there are three low temperature points between the radii of OS and 0.7 m, which cannot be attributed to the scatter in the data. Existence of a magnetic island at the plasma boundary could explain such a low temperature region in the interior of the plasma.
aW,‘&=
= -v.rl, -$v.kTr,,
(1) (2)
r, is the anomalous particle flw, k is the Bolzmann constant, and where for the sake of simplicity we have ignored the perpendicular beat conduction term and set T = T, = q. Within the separatrix, particle flux is modeled by r, = a(r - ri), where a is a function of time only, and r, is the innermost radius of the disturbance. This model is suggested by the density profile data. Integrating eqs. (I) and (2), over the time period from the start of the ELM to the measurement time during the ELM, and assuming that T is independent of time results in: Sn, = no -n, = ja dt and W, = W, - sk[(r - ri ) VT + T] Jo d t, where the subscripts 0 and f signify the pre-ELM and during the ELM values of the parameters. The plasma temperature at the measurements time is z’, = W,/($n,k) =4(.d~~+5/31(r-‘i)/r,+1](~-1)},
4i-
(nonon,)sf
kT 2sM, \i---
’
(I31
where +* is the temperature gradient scale length. According to the result of eq. (S), using the experimentally observed value of nJne - 0.8, the temperature of the inner most re8ion of the disturbance should drop by - 101, and increase by - 20% in the outermost re8ion. This result is consistent with the assumption of constant plasma temperature used in the above calculation. In the above calculation we ignored perpendicular heat conduction. If the perpendicular heat diffusivity during the ELM is the same as before the onset of the ELM, then its inclusion will have roughly an equal and opposing effect on the temperature profile, helping to maintain the temperature profile close to its pre ELM value. Therefore within the experimental error bars the profiles of fig. 4 are consistent with a purely convective heat transport.
The Authors wish to thank Drs. G. Janeschitz, J.K. Lee, G. Porter, and T. Simonen for useful and stimulating discussions. This work was performed under the auspices of the US Department of Energy by General Atom&, under Contract No. DE-AC03-84ER51044, and by Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48.
References [l] F. Wagner, Nucl. Mater. 121 (1984) 103. [2] M. Keilhacker et al, in: Proc. 10th Int. Conf. on Plasma Physics and Controlled Fusion Research, London, 1984, Vol. I, p. 71.
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M. Ah Mahdavi et al. / Modrfication of the scrape-off lqw
[3] D.N. Hill, T.W. Petrie, Howl, DIII-D divertor
M. Ali Mahdavi, L. Lao and W. target heat flux measurements
during ohmic and neutral beam heating, submitted to Nucl. Fusion. [4] P. Gohil et al., Study Of ELMS in DIII-D and Comparision with Ballooning Theory, General Atomics Report GA-A19035, submitted to Phys. Rev. Lett. [5] M. Ali Mahdavi et al., Phys. Rev. Lett. 47 (1981) 1062. [6] G. Fussman, Study of impurity accumulation in the ASDEX tokamak, in: Proc. 14th European Conf. on Controlled Fusion and Plasma Physics, Madrid, Spain, 1987.
by edge plasmu modes
[7] E.R. Miiller et al., Nucl. Fusion 27 (1987) 1817. [8] N. Brooks et al., Impurity Behavior During DIII-D Hmode Plasmas, General Atomics Report GA-A19108, submitted to Nucl. Fusion. [9] D. Content et al., Impurity Profiles in DIII-D H-mode Discharges, General Atomics Report GA-Al9288 (1988). [lo] M. Perry et al., Modeling Impurity Transport in DIII-D H-mode Plasmas With Giant ELMS, General Atomics Report GA-A19286.