ICRF Mode Conversion Flow Drive on Alcator C-Mod and
Projections to Other Tokamaks
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Lin, Y. et al. “ICRF Mode Conversion Flow Drive on Alcator CMod and Projections to Other Tokamaks.” Gent (Belgium), 2009.
57-64. ©2009 American Institute of Physics
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http://dx.doi.org/10.1063/1.3273817
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Detailed Terms
ICRF Mode Conversion Flow Drive on Alcator
C-Mod and Projections to Other Tokamaks
Y. Lin, J.E. Rice, S.J. Wukitch, M.J. Greenwald, A.E. Hubbard, A. InceCushman, L. Lin, E.S. Marmar, M. Porkolab, M.L. Reinke, N. Tsujii, and
J.C. Wright
MIT, Plasma Science and Fusion Center, Cambridge, MA 02139, USA
Abstract. Plasma flow drive via ICRF mode conversion (MC) has been demonstrated on
Alcator C-Mod. The toroidal rotation in these D('He) MC plasmas is typically more than twice
above the empirically determined intrinsic rotation scaling in ICRF minority heated plasmas. In
L-mode plasmas at 3 MW ICRF power input, up to 90 km/s toroidal rotation and 2 km/s
localized ( r/a ~ 0.4) poloidal rotation has been observed. The MC ion cyclotron wave (ICW)
was detected by a phase contrast imaging system in heterodyne setup. Through TORIC 2-D full
wave simulation, and comparison with other experimental evidence, we hypothesize that the
interaction between the MC ICW and the 'He ions may be the mechanism for the observed MC
flow drive. TORIC simulation suggests that similar flow drive scenario may be realized on JET
D('He) plasmas. The promising scenarios on ITER are the inverted minority scenario (T)D and
high field launch for T-D-('He) plasma. In non-radioactive phase, these correspond to ('He)-H
and ''He('He) plasmas respectively.
Keywords: ICRF, flow drive, rotation, mode conversion, Alcator C-Mod, JET, ITER
PACS: 52.55.Fa, 52.53-q, 52.35.Hr, 52.50.Qt
INTRODUCTION
Flow drive via externally launched electromagnetic waves has been widely
identified as a high leverage tool that, if successful, can produce great benefits for
ITER and reactors. Flow drive using ICRF waves, both fast waves and slow waves,
have been studied previously. Fast magnetosonic wave (fast wave, or FW) flow drive
has been studied on JET [1], but the effect was weak. Because of the momentum of
RF waves is inversely proportional to the wave propagation velocity at the same
power, slow waves are preferred for flow drive. Slow waves can be launched directly
from the plasma edge for flow drive, for example, Alfven wave in Phaedrus-T
tokamak [2], direct-launch ion Bernstein wave (IBW) experiments on PBX-M and
TFTR [3, 4]. However, because the interaction between antenna and edge plasma can
cause serious impurity problems, direct launch of slow ICRF wave is generally not a
practical option for high power experiments. To utilize high power fast wave antenna
for flow drive, we can generate slow wave inside the plasma using the mode
conversion (MC) scheme. In a multi-species plasma, mode conversion occurs near the
so-called ion-ion hybrid surface (MC surface), ny^ = S, where ny is the parallel index of
refraction of the wave, and 5* is a Stix' parameter [5]. When the fraction of the
CPl 187, Radio Frequency Power in Plasmas
edited by V. Bobkov and J.-M. Noterdaeme
©2009 American Institute of Physics 978-0-7354-0753-4/09/$25.00
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minority species is small, the MC surface is very close to the ion cyclotron (IC)
resonance layer of the minority species, and the fast wave is absorbed completely at
the IC resonance. When the fraction is large, two slow waves may appear near the MC
surface [6, 7]: the MC IBW appears near the mid-plane propagates towards the high
field side and deposits power to electrons through Landau damping; the MC ion
cyclotron wave (ICW) propagates towards the low field side and deposits power to
electrons through Landau damping and to the minority ions through IC resonance
absorption. Previously, preliminary evidence of MC poloidal flow drive has been
reported on TFTR [8] and JET [9]. In this paper, we report the first detailed
observation of MC flow drive in both toroidal (Y^) and poloidal (Ve) directions in
tokamaks[10, II, 12].
# MC D MH
- D( 'He) Mode Convers[on
• - - D(H) Minority Heating
^U
•
ou
80
•
• •
60
°DEP
.
40
20
0.8
1.0
1.2
1.4
1.6
t[s]
FIGURE 1. (a) Central V,^ after rf power application at
0.75 sec in an MC plasma (red solid) and an MH plasma
(blue dashed); (b) Te and ne traces; (c) rf power traces.
(")
n ,6
0 20 40 60 80 100 120 •
AW/Ip[kJ/PvlA]
FIGURE 2. Rotation vs. empirical
intrinsic rotation scaling AW/Ip.
EXPERIMENTAL OBSERVATION
On Alcator C-Mod, the rotation in ICRF minority heated (MH) plasmas has been
shown empirically to scale with AWp/Ip, similar to Ohmic heated plasmas, thus it is
thought to be intrinsic plasma rotation [13, 14]. To identify externally driven flow by
MC heating, we compare the rotation (both toroidal and poloidal) in MC heated
plasmas and MH plasmas. In this experiment, MH plasmas are heated by 80 MHz
ICRF power with H as the minority in majority D plasmas. For the MC plasmas, we
used 50 MHz ICRF power, and at modest ^He fraction (nsuJrie ~ 8-12%) in D majority
plasmas. All other plasma parameters are similar, hue averaged density Ue ~ 1.3x10^°
m"^, Bto ~ 5.1 T and Ip = 800 kA. In such a setup, the H cyclotron resonance in MH
plasmas is at the same location as the MC surface in MC plasmas. The plasmas are in
up-single-null L-mode, with VB drift in unfavourable direction for H-mode, to avoid
strong intrinsic rotation associated with H-mode. In Fig. I, Y^, Te, Ue and Prf traces of
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an MC plasma and an MH plasma are compared. Strong (up to 90 km/s) toroidal
rotation of impurity ions (Ar'^^ and Ar'^^) in the co-current direction has been
observed by high-resolution x-ray spectroscopy [15] in the MC plasma, but the
rotation change in the MH plasma is much smaller (< 30 km/s), consistent with the
empirical scaling of small AWp in L-mode. In Fig. 2, the change of central Y^ in a
number of MC plasmas and MH plasmas in this experiment are plotted. AV(^ in MC
plasmas is generally at least a factor of 2 higher than the empirically determined
intrinsic plasma rotation scaling (Fig. 2), and scales with the applied rf power (< 30
km/s per MW). The rotation rises near the core first and the profile is broadly peaked.
Spatially (0.3 < r/a < 0.6) localized poloidal rotation Ve in the ion diamagnetic drift
direction (~2 km/s at 3 MW) is also observed in MC plasmas, and similarly increases
with rf power, while the poloidal rotation in MH plasmas is smaller than the diagnostic
sensitivity (Fig. 3). The rotation also exists in the main ions, as shown in the Doppler
broadening of the turbulence spectra measured by a phase contrast imaging system
[16] (Fig. 4). Changing the toroidal phase of the antenna does not affect the rotation
direction, and it only weakly affects the rotation magnitude. The rotation is also
sensitive to the relative location of the MC layer vs. magnetic axis, and it is largest
when both MC and ^He IC layers are near the axis.
Mode Conversion
1.0
-I.Ql
0.0 0.2 0.4
0.6 O.S l.OO.n 0.2 0.4
0.6 0.8 t.Q
FIGURE 3. Poloidal velocity profiles at
different RF power levels: (a) MC plasma; (b)
MH plasma.
1.2
t(sec)
FIGURE 4. (a) Main ion rotation indicated by the
Doppler broadening of PCI fluctuations, (b) Trace
of impurity AVg; (c) RF power.
MC WAVE DETECTION, TORIC SIMULATION AND
MOMEMTUM TRANSPORT MODELING
In the MC plasmas, the MC ICW has been detected by the PCI system in
heterodyne setup (Fig. 5) [6]. The spatial location of the wave (~ 4 cm on the high
field side (HFS) of the He IC layer) and the kR value (3-7 cm"') agree with those
calculated from dispersion equation and previous MC experiments in D(^He) at the
same frequency and similar B field [7]. We calculate the RF power deposition using 2D full wave TORIC code [17,18] with experimental parameters and equilibrium. The
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direct electron heating profile agrees with break-in-slope analysis of Te signals from
ECE measurement. In the plasmas with strong flow drive, we find that there is a
significant portion of rf power deposition by the MC ICW to the ^He ions via
cyclotron resonance (Fig. 6-(a)). The power of the MC ICW is deposited on ^He ions
in the vicinity of the MC layer, and has a rather broad feature in the region of 0.2 < r/a
< 0.6 after integrated along flux surfaces (Fig. 6-(b)). Because of the MC ICW has a
parallel wavenumber ky much larger than that of fast wave (a factor of 4-5 in this
case), such strong ion interaction of the MC ICW is expected due to a much broader
Doppler width. We hypothesize that the interaction between the MC ICW with the ^He
ions is the main cause of the observed toroidal and poloidal flow.
1.6(jl
QJ 1
kj^> Oj propagating
towards LFS
1.4-
<
1.2-
1.0-
0.8
/
!
MC ICW
1
'
: (a)
64 65 66 67 68 69 70
R (cm)
/
MCICW
(b)
-10 -5 0 5 10
k_(l/cm)
FIGURE 5. MC ICW detected by PCI line integrated
density fluctuation at the heterodyne RF frequency: (a)
vs. R and t. (b) vs. ICR and t.
(a) ExpenmenI
FIGURE 6. TORIC simulation on MC
ICW power deposition to 'He ions: (a) 2-D
plot; (b) flux surface averaged.
^h) Simulation
FIGURE 7. Surface plots of AV^ =V^ -V^ (Prf = 0) vs. r/a and time, (a) Experimental data; (b)
Momentum transport modelling.
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However, the toroidal rotation depends on many closely related parameters
including momentum transport. Assuming a toroidal force proportional to the
deposition profile shown in Fig. 6-(b), we can reproduce the velocity profile and
temporal evolution by solving the transport equation in cylindrical coordinates
including a momentum diffusion coefficient x^ and a pinch velocity Vpmch- In Fig. 7,
good agreement is shown between the experimentally measured Y^ (same MC
discharge as in Fig. 1) and that from transport modelling using x^ = 0.1 m^/s and
inward Vpmch = -2.0x(r/a) (m/s). Although the modelling cannot unambiguously
determine the force term, we can estimate the effective driving force to be about 0.03
to 0.05 N per MW ICW power in order to match the experimentally measured rotation
evolution.
The MC ICW-^He interaction flow drive hypothesis is consistent with our
experimental observation of ^He dependence of flow drive: At either low ^He (< 4%)
fraction (FW minority heating) or high ^He (> 20%) fraction (MC electron heating),
the rotation is no more than the intrinsic rotation. The hypothesis is also consistent
with the similarity of the power deposition and the observed flow drive efficiency vs.
the MC layer location shown in Fig. 8. In this figure, we compare the B field
dependence of the TORIC calculated fraction of RF power to MC ICW to ^He ions
and the experimental measured flow drive efficiency. The dependence in the power
deposition can be interpreted by the Bpoi and Ti contribution that is required by the
mode conversion to ICW [19, 20, 21]. Both data show a peak when the MC surface
and IC resonance are near the axis. A detailed scan of ^He fraction and B field scan
will help clarify the role of the MC ICW in flow drive.
PROJECTION TO OTHER TOKAMAKS AND ITER
One of the parameters that affect the ICRF physics is the ratio of perpendicular
wavelength and the machine size, i.e., kj_R ~ COR/VA °^ Ue^'^R- This ratio determines
the amount of RF power available for mode conversion by tunneling the evanescent
layer between the L-cutoff layer and the MC surface. The MC physics on ITER is
expected to be very different because the major radius of ITER is roughly 9 times of
that of C-Mod while plasma densities are similar. The high temperature (~20 keV) of
ITER plasma will significantly increase the fast wave absorption on electrons
independent of minority or MC heating [22]. Therefore, it is critical to verify the MC
flow drive method on other tokamaks, especially existing larger tokamaks, in order to
establish its applicability on ITER.
We have studied the MC scenarios on JET D(^He) plasmas using TORIC, and
performed parameters scans including Bt, Ip, and ^He fraction. ^He fraction has been
shown to be the most sensitive parameter, and our study suggests at nsuJue ~ 10-15%,
more than 30% of total RF power can be deposited to ions through the MC ICW.
Data-mining of previous JET experiments has indeed found similar dependence of
plasma toroidal rotation vs. ^He fraction in internal transport barrier (ITB) reversed
shear plasmas with both neutral beam heating and ICRF heating [23, 24]. Figure 9
shows the TORIC simulation on the fraction of the MC ICW power to the ^He ions vs.
the ^He fraction from TORIC simulation of one of these ITB plasmas. Future
experiment on JET will help understand the likely RF effect on toroidal rotation.
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Some preliminary evidence of poloidal flow drive has also been observed on TFTR,
where RF power correlated poloidal flow was observed between the MC layer and the
^He IC resonance in D-''He-(^He) plasmas [8]. In hindsight, it was possible that the
flow was caused by the MC ICW, similar mechanism as our observation.
The large size and much higher temperature on ITER create a very different
regime in terms of mode conversion physics. TORIC simulation shows that launching
fast wave from the low field side of the tokamak, like that planned on ITER, will
result in insignificant mode conversion in D-T-(^He) and "'HefHe) plasmas (Fig. 10(a)). On the other hand, if an ITER ICRF antenna were built to launch power from the
high field side, a substantial portion of power can be mode converted to slow waves,
and deposited directly to ^He ions, potentially driving plasma rotation (Fig. 10-(b)).
24 r
TORIC simulation
JET Pulse 59537, t = 46 sec
Flow Drive Efficiency
- AV^[l<m/s]<n > [10'°m-']/Prf[MW]
TORIC MC ICW 55
Ion Heating [%]
50
45
5.1
5.2
5.3
66
68
70
72
74 [em]
Major radius of-'He cyclotron resonance
61
63
65
67
69 [cm]
Major radius of mode conversion surface
F I G U R E 8. Bt dependence on C-Mod data and
comparison to T O R I C simulation. The M C and
IC locations are also indicated.
Counter-lp phase, averaged over toroidal numbers
nphi= 10, 13 and 16
F I G U R E 9.
T O R I C simulation on a JET
plasmas (Bt =3.45T , f = 33 M H z , Ip =2.8 M A ) .
Plotted are power of the M C I C W to ' H e ions as
a percentage of total power vs. ' H e fraction.
For the normal LFS launch on ITER, mode conversion in inverted minority heating
plasmas may be good candidates for MC flow drive. Except some preliminary
evidence of sheared poloidal flow drive near the edge in (D)-^He plasma on JET (MC
IBW interaction with ions) [9], MC flow drive in the inverted minority heating
scenario has not been well studied. As shown in JET (^He)-H inverted minority
heating experiments, electron heating becomes dominant even at rather low ^He
fraction at 2.5% [25]. The situation on ITER may be different due to its much higher
temperature and wider Doppler broadening of the IC resonance. In Fig. II-(a), the
inverted minority case of a (T)-D plasma is shown. By moving the D IC layer out of
the plasma, a significant amount of RF power is deposited to the T ions through the
MC ICW and IBW. With 70% RF power being deposited to the T ions via the slow
waves, this seems to be a promising scenario for flow drive. Another scenario involves
62
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the seeding of \ i to create (^Li)-D inverted minority heating in 50-50 D-T burning
plasma (Fig. 11-(b)). In this case, as much as 10% of total RF power may be deposited
to the \ i ions. In order to extrapolate this method to ITER, experimental study of MC
flow drive will be required in such inverted minority scenario, e.g., (^He)-H plasma,
by finding the small window of strong ion interaction of the MC waves.
P,,, by =He at 0,_^
Pats by ^He at 12 ,1,3
100
I D-He3
I Hybrid
I
,
|'^c,He!
W/ciTf/MW,t
I
a.
I
0
-60 -40 - 2 0
0
20
40
-100
-60 -40 -20
0 20 40
>f = R - fV«« (cm)
( a ) nT/ne - 4 0 %
PQwer partition
(b)
nD^ne - 4 0 %
3 0 % to He ions
nHe3/ne - 1 0 % (^ia FW)
f - 53 MHz
6 8 % to electrons
nit = 27
LFS launch
ITER Configuration: BtO = 5.3 T, Ip = 13 MA,Ti
nT/ne = 46%
Power partitinn
nD/ne ^ 45%
2 6 % to He3 ions
nHe3/ne = 4 %
(via ICW+IBW>
f ^ 53 MHz
6 1 % t n electron?
n4 = 27
HFS launch
- 2 0 keV, Te = 24 keU, neO = le20 m ^
FIGURE 10. ITER simulation on normal MC scenarios. Contours are RF power to 'He ions in D-T(He3) plasmas, (a) LFS launch, (b) HFS launch.
Pabs b y T a t
"c,T
Pjbs b y \ i
100
1
A
ilJCW
so •
^
a.
S'
0
at^cLi
' ' ' ' ' l"
1
1
1
1
1
1
I
W/crrf/MW,,,
1
./:
licw
^J
Hybrid
-100
-20
1.
0
20 4 0
60 SO
3.8
1.S
0.67
0.28
0.12
f!c,u
-100
-20
1 D-Li
' Hybrid
t
20 40
60
80
X - R - R|^,
( a ) riT/ne = S%
nD/ne - 9 2 %
f ^ 27 MHz
nif. = 27
Power partition
70% to T ions
(via ICW + IBW)
30% to electron?
f b j nT/ne - 50%
nD/re^42.5%
nL(/ne = 2.5%
1= = 34 MHz
n(t = 27
ITER Configuration: BtO - S3 T. Ip ^ 15 MA,Ti ^ Te ^ 10 keV, neO ^
Fast wave launched from low-field side
Power partition
4B% to D ions
43% to electron?
9 % to Li ions
(via ICW + IBW)
le20 m"^
FIGURE 11. ITER simulation on inverse minority heating plasmas. Contours are RF power to
minority ions, (a) (T)-D plasma, (b) T-(\i)-D plasma.
63
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DISCUSSION
Since slow waves carry larger momentum than fast waves at the same power
density, slow waves, like direct launched IBW and MC wave have long been predicted
to be potentially able to drive plasma flows. Our experimental observation seems to be
higher than a previous simulation on flow drive with MC ICW [26], but in reasonable
agreement with analytical approximation in Ref [27]. Theories indicate RF drive
rotation would show clear antenna phase (wave toroidal direction) dependence. This is
inconsistent with our preliminary experimental evidence. Both experimental and
theoretical work will be required in order to further understand the RF force, or an
equivalent mechanism that transports momentum against the Y^ gradient, and
extrapolate to other tokamaks and ITER.
ACKNOWLEDGMENTS
The authors thank the Alcator C-Mod operation and ICRF groups. This research
utilized the MIT Plasma Science and Fusion Center Theory Group parallel
computational cluster. This work is supported at MIT by U.S. Department of Energy
Cooperative agreement No. DE-FC02-99-ER54512.
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