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Control of the magnetic configuration in the Heliotron-E device
Journal of Nuclear Materials 176& 177 (1990) 1070-1075 North-Holland
1070
Control of the magnetic configuration
in the Heliotron-E
device
T. Mizuuchi ‘, H. Matsuura 2, A. Komori 3, M. Harada ‘, S. Nagai 3, H. Zushi ‘, F. Sano ‘, K. Kondo I, S. Sudo I, M, Sato ‘, M. Nakasuga ‘, Y. Kawai 3 and T. Obiki ’ ’ Plasma Physics orator, Kyoto University, Gokasha, Uji, Kyota 611, Japan ’ College of Engineering Un~veTsi~ of Osaka Prefect~e, Mouse-cho 4-804, Sakai Osaka, Japan 3 lnterd~scip~ina~ Graduate School of Engineering Sciences, Kyushu University, Kasugakoen 6-1, Kasuga, Fukuoka Japan
The magnetic configuration of the Heliotron-E device was controlled by adding auxiliary toroidal and/or vertical fields. The changes in the plasma edge and the “divertor trace” were experimentally studied by using a double probe, calorimeters, a thermal Li-beam probe and a laser Thomson scattering system. The variation of the vacuum configuration was also confirmed by the “stellarator diode method” with a small hot cathode. It was found that the observed change in the edge region basically agreed with that expected from the line-tracing calculation.
The magnetic ~nfiguration of the Heliotron-E device is characterized by two parameters, a* = B,/B, where 3, is the toroidal field proand @*= B,/B,, duced by TF coils, B, is the vertical field and B, is the helical field on the axis [1,2]. (a*, p*) i= (0.0, -0.185) is the standard configuration. The variation of /3* (-0.192 s /3* I -0.172) mainly causes the shift of the magnetic axis. That of a* (-0.1 i5 a* 5 +O.lS) mainly changes the size of the magnetic surface. The rotational transform, magnetic shear and magnetic hill/well configuration are also changed in (a*, j3 * ) space. Moreover, the magnetic limiter configuration with the “divertor layer [3]” turns to the “wall limiter” config~ation as a* increases. Since the loss cone for trapped particles, the stability of the plasma and plasma-wall interactions are strongly coupled with these properties of the flux surface, experiments in various (a*, j3 * ) conditions are very instructive to understand the plasma confinement, MHD activities and edge plasma phenomena. The details of the experimental observations on the core plasma in various configurations are reported elsewhere [4]. One of the important observations for the core plasma is that the good operation “window” from a view point of the gross energy confinement is found at (a* - co.05, #3* - -0.192). It is interesting to study the role of the edge region in the improvement of the core plasma performance. On the other hand, there are
many rational surfaces in the outer region since the Heliotron-E device is a high shear system. This means that the outer flux surfaces are in danger of being broken by a small error field. From this point of view, it is necessary to experimentally confirm whether the magnetic configuration can be controlled as expected. In this paper, the effects of (a*, /3* ) modification on the edge region are discussed based on a vacuum magnetic field measurement and edge plasma measurements.
2. Experimental methods and results
The change of the vacuum magnetic configuration was directly studied by using the “stellarator diode method” developed by A.G. D&ii et al. [5], which is based on measurement of the emission current from a small hot cathode scanned in the chamber. In this method, the emission current at a fixed bias voltage is considered as a measure of the field line, mainly relating to the length of the field line to the wall L,,.R. Takahashi et al. applied this method to the Heliotron-E device for the first time and showed that this method was useful especially to decide the position of the “actual” outermost flux surface [6]_Almost the same procedure as the previous work was used here except for two points; (1) the base pressure was set at about 3 X loo4
0022-3115/90/$03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland)
T. Mizuuchi et al. / Control of the magnetic configuration in Heliotron-E
Pa (He gas, the mean free path - 100 m) in order to block plasma production due to the pulse operation of the field (B - 0.4 T), (2) the cathode was scanned along the major radius at No. 5.5 section of the torus. Fig. 1 shows the emission current at the fixed bias voltage of 15 V for some different configurations as a function of the cathode position. The rotational transforms obtamed by numerical field tracing are also plotted. We can find some characteristic points in each radial profile of the emission current, R,,, R,,, R,, and RI.,. In the region of R > R,,, the emission current is almost constant. It becomes to decrease when the cathode is inserted to the region of R < Rdl. In the region of R,, > R ’ R,,, however, the current drop is stil small. After the cathode was moved into the region of R < R,,, the
(a*=
O.O@=-0.185)
emission current rapidly decreased. According to the discussion in ref. [6], this rapid decrease suggests that the radius of the actual outermost surface is R,, at least in this weak field condition. Sub-peaks or plateau regions were observed at R,., and R,,,. As in the figure, the positions of R,,, and R,,, are related with that of rational surfaces of 1/2a - 2.0 and 1.5, respectively. The position of R,, also corresponds to the position of the rational surface of 1/27~ = 2.5 except for a* = 0.1 case. This means that the magnetic configuration is changed as expected from the calculation. These plateau regions suggest the existence of a magnetic island at the rational surface. In order to study the effect of the magnetic island on this measurement, an error field with a dominant mode of m/n = l/l was imposed to
(+O.l,-0.185)
100
1071
100
(-O.l ,-0.185) 1s :./-T
OS1 - .
. R-R0 (m)
0.8
01~~ .
0.2
0.3 R-R0 (m)
.
O-1 %r--G-4? . . R-R0 (m)
( 0.0,-0.185)
R-R0 (m)
R-R0 (m)
Fig. 1. Radial dependence of the emission current for different field configurations. obtained by numerical calculation.
The real curve is the rotational transform
1072
T. Mizuuchi et al. / Control of the magnetic configuration #45951-#45972
in Heliotron-E #46111-#46124
(0.0,-0.18!i) 1
0.3 R-R0 (m)
0.4
.2 Obl
0.4
#46370-#46381
#46952-#46061
..
0.3 R-R0 (m)
(-0.1,-0.185) . 1
10
T 00 %J 51 I
O*b’........“.....““’ .2 0 .3 R-R0 (m)
0.4
0.
II R-R0 (m)
Fig. 2. Radial profile of the edge density measured with a thermal Li-beam p\obe (solid line) and a double probe (O), which is about 115 o apart from the former along the torus.
the standard condition. The results is shown in fig. le, where the rotational transform at the standard condition is also plotted. The wide plateau region appeared around L/~T - 1 (compare with fig. la). From these observations, it can be concluded that the rational
surfaces are somewhat broken to the magnetic islands. Since the gradient of the current profile is large enough, the deformation of the flux surface seems to be restricted around the low mode rational surfaces. At b* = -0.192, the sub-peak of the emission current
T. Mizuuchi et al. f Control of the magnetic conjuration in ~e~~~tron-E
1073
#45532-#45652 r/o
around the r/2= = 2 surface was reduced. Owing to this, the level of the emission current at the surface of 1/2?t < 2 is lower than that of the standard configuration. 2.2. Radial profiles of the edge plasma In this section, the edge of ECH plasma is investigated. The range of the core plasma parameters used here were B - 1.7-1.9 T, Ze - 1 X lOi mm3, T& - 1 keV and P,, - 0.45 MW. The change in the radial profile of the edge plasma was studied by using a thermal Li-beam probe at No. 5.5 poloidal section, a double probe at No. 17.5 (about 115“ apart from No. 5.5) and a laser Thomson scattering system at No. 24.5 (about 180 * apart from No. 5.5). Owing to the limitation in access, the former two methods were applied mainly to the separatrix region (r/a > 1) and the latter was to the inside region (r/a c 1). Fig. 2 shows typical density profiles for different configurations. In the estimation of density from the double probe signal, the geometrical surface area of the probe tips was used without any correction. Although we must keep in mind the restriction on the data treatment due to the accessibility limit of these methods for irmer region, a change in the density gradient seems to exist at a radius of R,. In the outer region of R ;r R,, the density exponentially dropped with an eholding length X, - 0.01 m. In the region of RL -CR c R,, the density exponentially decreased with an e-holding length X, - 0.03-0.04 m at @* = -0.185. On the contrary, at B* = - 0.192 and -0.197 a hump in the density profile was observed with the both measurements. The same tendency was observed also in the Li-beam probe data at negative u* case. But it was not clear in the double probe data. As in the figures, the basic character of the radial dependence is almost the same for two profiles obtained by the independent methods at the different cross sections. The value of R, increases with increasing a* (see fig. 5). Ahhough the u* dependence of X, was not clear, it seems to increase for positive a*. Since the density fluctuation around R, measured with the Li-beam probe was increased by about a factor two for a* 2 0.1 compared to that for a* < 0.1, the increase of X, might be related with the increase of this fluctuation. The rather flat profile of the edge temperature in the separatrix region seems not to be affected by a*-modification. The electron tem~rature T, and density n, by the laser Thomson scattering at spatially fixed two positions, named B13R5 (R -R, = 0, Z d 0.257 m) and B13B.2 (R - R, = 0.06 m, 2 = 0.257 m), are shown as a
“jO _ 1.05 1
0.98 I
0.95 r’a 0.86 I
0.81
I
0.79 1
0.3-
“O
s 0.2d c
n*
o.t-
T. T
01
’
-0.1
,
I
-0.05
0
0.05
0.1
’
lo
0.15
u’
Fig. 3. a*-dependence of the electron temperature (0: B = 1.9 T, A: B = 1.76 T) and density (0: B =1.9 T, A: B = 1.76 T) at two spatially fixed points measured with a laser thomson scattering system which is about 180” apart from the section of the Li-beam probe along the torus.
function of a* at fixed /I* of -0.185 in fig. 3a and 3b, respectively. The line density was controlled to keep ahnost the same value. The normalized radius of the measured point, r/a, is denoted on the top of each figure. As increasing a*, the measured point moves more deeply into the core region because the average plasma radius is increased. Assuming the profile similarity, T, should change as the dashed line in the figure. For a* < 0, the observed T, seems to change along this line and it decreased to a lower level than the detectable limit when the measuring point moved to the outermost surface (r/a = 1). For a* > 0, however, T, stayed at lower value than the expected one. In this case, PZ*also did not increase. On the contrary, the increase of n, was observed as a* increased for the NBI plasma at /3* = -0.192. In this case, however, the line density also increased.
1074
__ ?
8 s
2
F
a
!I . ’
20:
*
.,%9”=-o.li35
I
dhwtor
I
$j IO-
trace
i
I
“0 z
; r=0.3!% (m)
Fig. 4. ‘The poloidal position (8) of the divertor trace as a function of a* estimated by the heat load measurements (I) and by the field-tracing calculations (shaded area).
2.3. Change in the divertor trace
Since the effects of the fi* control on the “divertor trace [3r are reported in refs. [3,7], the effects of a* control are discussed here. The distribution of the heat load on the wall was measured with the multi channel calorimeter array. As discussed in ref. 131, the observed peak position of the heat load is consistent with the calculated position of the ‘“divertor trace” defined from a criterion of the field line length. Fig. 4 shows the peak position of the heat load for various a* conditions. The bar in the figure denotes the size of the detector. The calculated “divertor trace” is also shown in the figure (the shaded region), which is in good agreement with the observed one. The ion saturation current profile near the wall and the divertor trace were also in good agreement. The peak value of the heat load was not constant for a* scan. It seems that this change of the observed peak value is mainly due to the “mismatch” of the “true” peak position and the calorimeter positions since these was a rather wide space (- 0.01 m) between adjacent calorimeters.
3. Discussion In the previous section, several characteristic radii in the radial dependences of the emission current and of the edge plasma density are found. We try to compare these values with the results of the field tracing calculation. Fig. 5 shows the radius of the outermost flux. surface R, determined from the field calculation as function of a*. R, for a* ~0.1 is restricted by the protection
limiter at two joint sections of the torus. From the view point of the vacuum field measurement, the radius of the outermost surface should be RdZ instead of R,. As discussed in the section 2.1, R,, is closely related with the position of the rational surface of 1/2~ = 2.5 (dashed line in the figure, R,.,). At a* = 0.1, however, this rational surface crosses the protection limiter. In this case, R,, becomes close to R,. On the other hand, from the view point of the plasma profile, the position of the outer edge of the core plasma seems to be R,. As in the figure, this point is closer to R, than to R,,. Moreover, at a* = 0.1, they are very close to the outer most surface position in the case without a protection limiter (dotted line in the figure, R&). If the origin of the perturbation field which causes the reduction of the size of the outermost flux surface in the field measurement is any residual field around the torus, Rd2 might move to R, in the plasma experiment condition since the effect of the residual error field is reduced. However, this cannot explain the a* = 0.1 case. Since plasma edge is determined by the balance of the perpendicular loss and the parallel loss to the limiter or the wall, if the collisions or the perpendicular diffusion increases, the effect of the protection limiter might be covered. Indeed, some data suggest the reduction of the plasma radius at the after-glow phase where the collision frequency decreases.
P'=-0.185
0.4
0rn2rn .
.
0.2
a’ Fig. 5. a* dependences of the characteristic radii in the edge region. R, is the calculated radius of the outer edge of the separatrix region, R, the calculated radius of the outermost flux surface taking account of the protection limiters, R,, the calculated radius of the rational surface of &/2?r= 2.5; R6 is R, without the protection limiters, Rp, Rf;’ are R, and R, in fig. 2 from the Li-beam probe data, RFP is R, in fig. 2 from the doubk probe data.
T. Mizuuchi
et al. / Control of the magnetic configuration
4. summary The effects of (a*, /3 * ) modification on the edge region are studied experimentally. They are summarized as follows: (1) The observed variation of the size of flux surface basically agrees with the results of the field cakulation. This means that the field structure in the edge region is controlled by the additional fields. (2) From the view point of the vacuum field measurement, the size of the outermost flux surface seems to be reduced to 95-97% of the ideal one due to the deformation of the edge surface. From the view point of the edge plasma profile, however, this reduction might be ignored owing to the perpendicular diffusion. (3) The radial decay length of the density in the separatrix region seems to increase for positive a*. This might be related with the increase of the fluctuation. The edge temperature seems not to affected by a* modification. and density in the ex(4) The electron temperature panded region by increasing a* did not much increase under the condition of a* > 0 at jI * = - 0.185. /I* < -0.185, (5) In the inward shifted configuration, the radial profile of the emission current became smoother than that in the other conditions. The clear hump in the radial profile of the edge plasma density was observed at the outer edge of the sep-
in Helioiron-E
1075
aratrix region in this configuration. It is not clear at present whether these observations is relating to the improvement of the confinement. Further investigation is necessary to understand the role of the edge region in the improvement of the plasma performance.
Acknowledgment The authors are grateful to the Heliotron experiment group for their excellent cooperation and helpful supports.
[l] K. Uo, J. Phys. Sot. Japan 16 (1961) 1380. [2] T. Obiki, et al., Fusion Technol. 17 (1990) 101. [3] T. Mizuuchi and the Heliotron-E group, J. Nucl. Mater. 162-164 (1989) 105. [4] K. Kondo et al., in: Proc. 17th Europ. Conf. on Controlled Fusion and Plasma Heating, June, 1990. [S] A.G. D&ii et al., Sov. J. Plasma Phys. 14 (1988) 160 [Fiz. Plazmy 14 (1988) 2791. [6] R. Takahashi et al., Jpn. J. App. Phys. 28 (1989) 2604. [7] T. Obiki et al., Plasma Physics and Controlled Nuclear Fusion Research 1988, Vol. 2 (IAEA, Vienna, 1989) p. 337. [8] T. Mizuuchi et al., J. Nucl. Mater. 121 (1984) 3.
1070
Control of the magnetic configuration
in the Heliotron-E
device
T. Mizuuchi ‘, H. Matsuura 2, A. Komori 3, M. Harada ‘, S. Nagai 3, H. Zushi ‘, F. Sano ‘, K. Kondo I, S. Sudo I, M, Sato ‘, M. Nakasuga ‘, Y. Kawai 3 and T. Obiki ’ ’ Plasma Physics orator, Kyoto University, Gokasha, Uji, Kyota 611, Japan ’ College of Engineering Un~veTsi~ of Osaka Prefect~e, Mouse-cho 4-804, Sakai Osaka, Japan 3 lnterd~scip~ina~ Graduate School of Engineering Sciences, Kyushu University, Kasugakoen 6-1, Kasuga, Fukuoka Japan
The magnetic configuration of the Heliotron-E device was controlled by adding auxiliary toroidal and/or vertical fields. The changes in the plasma edge and the “divertor trace” were experimentally studied by using a double probe, calorimeters, a thermal Li-beam probe and a laser Thomson scattering system. The variation of the vacuum configuration was also confirmed by the “stellarator diode method” with a small hot cathode. It was found that the observed change in the edge region basically agreed with that expected from the line-tracing calculation.
The magnetic ~nfiguration of the Heliotron-E device is characterized by two parameters, a* = B,/B, where 3, is the toroidal field proand @*= B,/B,, duced by TF coils, B, is the vertical field and B, is the helical field on the axis [1,2]. (a*, p*) i= (0.0, -0.185) is the standard configuration. The variation of /3* (-0.192 s /3* I -0.172) mainly causes the shift of the magnetic axis. That of a* (-0.1 i5 a* 5 +O.lS) mainly changes the size of the magnetic surface. The rotational transform, magnetic shear and magnetic hill/well configuration are also changed in (a*, j3 * ) space. Moreover, the magnetic limiter configuration with the “divertor layer [3]” turns to the “wall limiter” config~ation as a* increases. Since the loss cone for trapped particles, the stability of the plasma and plasma-wall interactions are strongly coupled with these properties of the flux surface, experiments in various (a*, j3 * ) conditions are very instructive to understand the plasma confinement, MHD activities and edge plasma phenomena. The details of the experimental observations on the core plasma in various configurations are reported elsewhere [4]. One of the important observations for the core plasma is that the good operation “window” from a view point of the gross energy confinement is found at (a* - co.05, #3* - -0.192). It is interesting to study the role of the edge region in the improvement of the core plasma performance. On the other hand, there are
many rational surfaces in the outer region since the Heliotron-E device is a high shear system. This means that the outer flux surfaces are in danger of being broken by a small error field. From this point of view, it is necessary to experimentally confirm whether the magnetic configuration can be controlled as expected. In this paper, the effects of (a*, /3* ) modification on the edge region are discussed based on a vacuum magnetic field measurement and edge plasma measurements.
2. Experimental methods and results
The change of the vacuum magnetic configuration was directly studied by using the “stellarator diode method” developed by A.G. D&ii et al. [5], which is based on measurement of the emission current from a small hot cathode scanned in the chamber. In this method, the emission current at a fixed bias voltage is considered as a measure of the field line, mainly relating to the length of the field line to the wall L,,.R. Takahashi et al. applied this method to the Heliotron-E device for the first time and showed that this method was useful especially to decide the position of the “actual” outermost flux surface [6]_Almost the same procedure as the previous work was used here except for two points; (1) the base pressure was set at about 3 X loo4
0022-3115/90/$03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland)
T. Mizuuchi et al. / Control of the magnetic configuration in Heliotron-E
Pa (He gas, the mean free path - 100 m) in order to block plasma production due to the pulse operation of the field (B - 0.4 T), (2) the cathode was scanned along the major radius at No. 5.5 section of the torus. Fig. 1 shows the emission current at the fixed bias voltage of 15 V for some different configurations as a function of the cathode position. The rotational transforms obtamed by numerical field tracing are also plotted. We can find some characteristic points in each radial profile of the emission current, R,,, R,,, R,, and RI.,. In the region of R > R,,, the emission current is almost constant. It becomes to decrease when the cathode is inserted to the region of R < Rdl. In the region of R,, > R ’ R,,, however, the current drop is stil small. After the cathode was moved into the region of R < R,,, the
(a*=
O.O@=-0.185)
emission current rapidly decreased. According to the discussion in ref. [6], this rapid decrease suggests that the radius of the actual outermost surface is R,, at least in this weak field condition. Sub-peaks or plateau regions were observed at R,., and R,,,. As in the figure, the positions of R,,, and R,,, are related with that of rational surfaces of 1/2a - 2.0 and 1.5, respectively. The position of R,, also corresponds to the position of the rational surface of 1/27~ = 2.5 except for a* = 0.1 case. This means that the magnetic configuration is changed as expected from the calculation. These plateau regions suggest the existence of a magnetic island at the rational surface. In order to study the effect of the magnetic island on this measurement, an error field with a dominant mode of m/n = l/l was imposed to
(+O.l,-0.185)
100
1071
100
(-O.l ,-0.185) 1s :./-T
OS1 - .
. R-R0 (m)
0.8
01~~ .
0.2
0.3 R-R0 (m)
.
O-1 %r--G-4? . . R-R0 (m)
( 0.0,-0.185)
R-R0 (m)
R-R0 (m)
Fig. 1. Radial dependence of the emission current for different field configurations. obtained by numerical calculation.
The real curve is the rotational transform
1072
T. Mizuuchi et al. / Control of the magnetic configuration #45951-#45972
in Heliotron-E #46111-#46124
(0.0,-0.18!i) 1
0.3 R-R0 (m)
0.4
.2 Obl
0.4
#46370-#46381
#46952-#46061
..
0.3 R-R0 (m)
(-0.1,-0.185) . 1
10
T 00 %J 51 I
O*b’........“.....““’ .2 0 .3 R-R0 (m)
0.4
0.
II R-R0 (m)
Fig. 2. Radial profile of the edge density measured with a thermal Li-beam p\obe (solid line) and a double probe (O), which is about 115 o apart from the former along the torus.
the standard condition. The results is shown in fig. le, where the rotational transform at the standard condition is also plotted. The wide plateau region appeared around L/~T - 1 (compare with fig. la). From these observations, it can be concluded that the rational
surfaces are somewhat broken to the magnetic islands. Since the gradient of the current profile is large enough, the deformation of the flux surface seems to be restricted around the low mode rational surfaces. At b* = -0.192, the sub-peak of the emission current
T. Mizuuchi et al. f Control of the magnetic conjuration in ~e~~~tron-E
1073
#45532-#45652 r/o
around the r/2= = 2 surface was reduced. Owing to this, the level of the emission current at the surface of 1/2?t < 2 is lower than that of the standard configuration. 2.2. Radial profiles of the edge plasma In this section, the edge of ECH plasma is investigated. The range of the core plasma parameters used here were B - 1.7-1.9 T, Ze - 1 X lOi mm3, T& - 1 keV and P,, - 0.45 MW. The change in the radial profile of the edge plasma was studied by using a thermal Li-beam probe at No. 5.5 poloidal section, a double probe at No. 17.5 (about 115“ apart from No. 5.5) and a laser Thomson scattering system at No. 24.5 (about 180 * apart from No. 5.5). Owing to the limitation in access, the former two methods were applied mainly to the separatrix region (r/a > 1) and the latter was to the inside region (r/a c 1). Fig. 2 shows typical density profiles for different configurations. In the estimation of density from the double probe signal, the geometrical surface area of the probe tips was used without any correction. Although we must keep in mind the restriction on the data treatment due to the accessibility limit of these methods for irmer region, a change in the density gradient seems to exist at a radius of R,. In the outer region of R ;r R,, the density exponentially dropped with an eholding length X, - 0.01 m. In the region of RL -CR c R,, the density exponentially decreased with an e-holding length X, - 0.03-0.04 m at @* = -0.185. On the contrary, at B* = - 0.192 and -0.197 a hump in the density profile was observed with the both measurements. The same tendency was observed also in the Li-beam probe data at negative u* case. But it was not clear in the double probe data. As in the figures, the basic character of the radial dependence is almost the same for two profiles obtained by the independent methods at the different cross sections. The value of R, increases with increasing a* (see fig. 5). Ahhough the u* dependence of X, was not clear, it seems to increase for positive a*. Since the density fluctuation around R, measured with the Li-beam probe was increased by about a factor two for a* 2 0.1 compared to that for a* < 0.1, the increase of X, might be related with the increase of this fluctuation. The rather flat profile of the edge temperature in the separatrix region seems not to be affected by a*-modification. The electron tem~rature T, and density n, by the laser Thomson scattering at spatially fixed two positions, named B13R5 (R -R, = 0, Z d 0.257 m) and B13B.2 (R - R, = 0.06 m, 2 = 0.257 m), are shown as a
“jO _ 1.05 1
0.98 I
0.95 r’a 0.86 I
0.81
I
0.79 1
0.3-
“O
s 0.2d c
n*
o.t-
T. T
01
’
-0.1
,
I
-0.05
0
0.05
0.1
’
lo
0.15
u’
Fig. 3. a*-dependence of the electron temperature (0: B = 1.9 T, A: B = 1.76 T) and density (0: B =1.9 T, A: B = 1.76 T) at two spatially fixed points measured with a laser thomson scattering system which is about 180” apart from the section of the Li-beam probe along the torus.
function of a* at fixed /I* of -0.185 in fig. 3a and 3b, respectively. The line density was controlled to keep ahnost the same value. The normalized radius of the measured point, r/a, is denoted on the top of each figure. As increasing a*, the measured point moves more deeply into the core region because the average plasma radius is increased. Assuming the profile similarity, T, should change as the dashed line in the figure. For a* < 0, the observed T, seems to change along this line and it decreased to a lower level than the detectable limit when the measuring point moved to the outermost surface (r/a = 1). For a* > 0, however, T, stayed at lower value than the expected one. In this case, PZ*also did not increase. On the contrary, the increase of n, was observed as a* increased for the NBI plasma at /3* = -0.192. In this case, however, the line density also increased.
1074
__ ?
8 s
2
F
a
!I . ’
20:
*
.,%9”=-o.li35
I
dhwtor
I
$j IO-
trace
i
I
“0 z
; r=0.3!% (m)
Fig. 4. ‘The poloidal position (8) of the divertor trace as a function of a* estimated by the heat load measurements (I) and by the field-tracing calculations (shaded area).
2.3. Change in the divertor trace
Since the effects of the fi* control on the “divertor trace [3r are reported in refs. [3,7], the effects of a* control are discussed here. The distribution of the heat load on the wall was measured with the multi channel calorimeter array. As discussed in ref. 131, the observed peak position of the heat load is consistent with the calculated position of the ‘“divertor trace” defined from a criterion of the field line length. Fig. 4 shows the peak position of the heat load for various a* conditions. The bar in the figure denotes the size of the detector. The calculated “divertor trace” is also shown in the figure (the shaded region), which is in good agreement with the observed one. The ion saturation current profile near the wall and the divertor trace were also in good agreement. The peak value of the heat load was not constant for a* scan. It seems that this change of the observed peak value is mainly due to the “mismatch” of the “true” peak position and the calorimeter positions since these was a rather wide space (- 0.01 m) between adjacent calorimeters.
3. Discussion In the previous section, several characteristic radii in the radial dependences of the emission current and of the edge plasma density are found. We try to compare these values with the results of the field tracing calculation. Fig. 5 shows the radius of the outermost flux. surface R, determined from the field calculation as function of a*. R, for a* ~0.1 is restricted by the protection
limiter at two joint sections of the torus. From the view point of the vacuum field measurement, the radius of the outermost surface should be RdZ instead of R,. As discussed in the section 2.1, R,, is closely related with the position of the rational surface of 1/2~ = 2.5 (dashed line in the figure, R,.,). At a* = 0.1, however, this rational surface crosses the protection limiter. In this case, R,, becomes close to R,. On the other hand, from the view point of the plasma profile, the position of the outer edge of the core plasma seems to be R,. As in the figure, this point is closer to R, than to R,,. Moreover, at a* = 0.1, they are very close to the outer most surface position in the case without a protection limiter (dotted line in the figure, R&). If the origin of the perturbation field which causes the reduction of the size of the outermost flux surface in the field measurement is any residual field around the torus, Rd2 might move to R, in the plasma experiment condition since the effect of the residual error field is reduced. However, this cannot explain the a* = 0.1 case. Since plasma edge is determined by the balance of the perpendicular loss and the parallel loss to the limiter or the wall, if the collisions or the perpendicular diffusion increases, the effect of the protection limiter might be covered. Indeed, some data suggest the reduction of the plasma radius at the after-glow phase where the collision frequency decreases.
P'=-0.185
0.4
0rn2rn .
.
0.2
a’ Fig. 5. a* dependences of the characteristic radii in the edge region. R, is the calculated radius of the outer edge of the separatrix region, R, the calculated radius of the outermost flux surface taking account of the protection limiters, R,, the calculated radius of the rational surface of &/2?r= 2.5; R6 is R, without the protection limiters, Rp, Rf;’ are R, and R, in fig. 2 from the Li-beam probe data, RFP is R, in fig. 2 from the doubk probe data.
T. Mizuuchi
et al. / Control of the magnetic configuration
4. summary The effects of (a*, /3 * ) modification on the edge region are studied experimentally. They are summarized as follows: (1) The observed variation of the size of flux surface basically agrees with the results of the field cakulation. This means that the field structure in the edge region is controlled by the additional fields. (2) From the view point of the vacuum field measurement, the size of the outermost flux surface seems to be reduced to 95-97% of the ideal one due to the deformation of the edge surface. From the view point of the edge plasma profile, however, this reduction might be ignored owing to the perpendicular diffusion. (3) The radial decay length of the density in the separatrix region seems to increase for positive a*. This might be related with the increase of the fluctuation. The edge temperature seems not to affected by a* modification. and density in the ex(4) The electron temperature panded region by increasing a* did not much increase under the condition of a* > 0 at jI * = - 0.185. /I* < -0.185, (5) In the inward shifted configuration, the radial profile of the emission current became smoother than that in the other conditions. The clear hump in the radial profile of the edge plasma density was observed at the outer edge of the sep-
in Helioiron-E
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aratrix region in this configuration. It is not clear at present whether these observations is relating to the improvement of the confinement. Further investigation is necessary to understand the role of the edge region in the improvement of the plasma performance.
Acknowledgment The authors are grateful to the Heliotron experiment group for their excellent cooperation and helpful supports.
[l] K. Uo, J. Phys. Sot. Japan 16 (1961) 1380. [2] T. Obiki, et al., Fusion Technol. 17 (1990) 101. [3] T. Mizuuchi and the Heliotron-E group, J. Nucl. Mater. 162-164 (1989) 105. [4] K. Kondo et al., in: Proc. 17th Europ. Conf. on Controlled Fusion and Plasma Heating, June, 1990. [S] A.G. D&ii et al., Sov. J. Plasma Phys. 14 (1988) 160 [Fiz. Plazmy 14 (1988) 2791. [6] R. Takahashi et al., Jpn. J. App. Phys. 28 (1989) 2604. [7] T. Obiki et al., Plasma Physics and Controlled Nuclear Fusion Research 1988, Vol. 2 (IAEA, Vienna, 1989) p. 337. [8] T. Mizuuchi et al., J. Nucl. Mater. 121 (1984) 3.