Download Likin_Poster_APS 2003 - HSX - University of Wisconsin–Madison

Electron Cyclotron Heating at B = 0.5 T in HSX
2
C.Deng ,
K.M.Likin, A.F.Almagri, D.T.Anderson, F.S.B.Anderson, J.Canik,
1
C.Domier , H.J.Lu, J.Radder, S.P.Gerhardt, J.N.Talmadge, K.Zhai
HSX Plasma Laboratory, University of Wisconsin, Madison, USA; 1UC-Davis, USA; 2UCLA, USA
 All channels have been calibrated on a bench. In
experiment, the ECE data have been benchmarked with
respect to the Thomson Scattering
R
0
0.5
R, m
Low Ne: h = 14 %
Effective Plasma Radius
Pin = 100 kW
High Ne: h = 50 %
R, m
where
1.3 Bi-Maxwellian Plasma
0.4
Bulk: nb ~ (1 –
r 2)
Tail: nt ~ exp(–4r2)
0.2
0
0
0.2
0.4
0.6
0.8
Effective plasma radius
1
Electron Temperature Profiles
Te, keV
6
Tail: Tt ~ exp(–4r2)
4
2
Bulk: Tb ~ exp(–
0 4r2)
0
0.2
0.4
0.6
0.8
Effective plasma radius
1
Te(0) = 0.4 keV
First pass
Two passes
Effective Plasma Radius
Single-pass absorption
1
0.8
Bi-Maxwellian
0.6
0.4
Te = 0.4 keV
0.2
0
0
0.5
1
1.5
2
2.5
#6
te 
3 me (kTe )
Each antenna is
an open ended
waveguide followed
by attenuator
4 2 ne e 4
3
Line Average Density, 1018 m-3
 Absorption versus plasma density
is calculated at constant Te in
Maxwellian plasma and based on
the TS and ECE data in
bi-Maxwellian plasma
 Owing to high non-thermal
electron population at a low
plasma density the absorption can
3.5 be high enough
#5
Attenuator
3/ 2
 Second Pass: Rays are reflected
from the wall and back into the
plasma, the absorption is up to
70% while the profile does not
broaden
Ne = 2·1018 m-3
Absorption, %
ne, 1018 m-3
0.6
 Model upon bi-Maxwellian
distribution function is used to
explain the enhanced stored
energy and the high
absorption efficiency at low
plasma density
 The density and temperature
profiles are taken from TS,
ECE and interferometer
measurements
 At 0.5·1018 m-3 the plasma
stored energy is 21 J due to
super-thermal tail and 5 J due
to bulk plasma and the singlepass absorption is about 0.5
 Corresponds to large hard Xray emission (poster by
Abdou)
Effective Plasma Radius
Absorption
Plasma Density Profiles
Pin
 At low plasma density the energy
that electrons can gain between
collisions is higher than at high
plasma density because of high
power per particle and longer
collision time:
pabs (r )
En(r ) 
t e ( r )
ne (r )
Low Ne: h = 14 %
0.5
1
1.5
2
1018
2.5
m-3
0
0.2
0.4
0.6
Effective plasma radius
 Electron temperatures measured by Thomson Scattering and
ECE are in a good agreement only at high plasma density
Amplifier
mw Detector
Quartz
Window
MD #1
MD #2
MD #3
MD #4
0
1
2
Line Average Density,
3
1018
4
m-3
MD #3
MD #4
Line Average Density,
1.E-03
3
1018
In QHS mode the growth
rate has been measured at
different launched power
levels. The growth rate drops
with decreasing of RF power
and its maximum is shifted
towards lower gas pressure
X-mode: 40 kW
MD #2
2
1.E-04
12000
MD #1
1
1.E-05
In anti-Mirror mode the
gas breakdown occurs with
a very low growth rate
4.2 Growth rate vs. RF Electric Field
1
0.8
0.6
0.4
0.2
0
0
1.E-06
Gas Pressure, Torr
Mirror
1
0.8
0.6
0.4
0.2
0
In QHS mode the growth
rate is twice that in Mirror
4000
0
1.E-07
3.2 Results of Measurements
QHS
8000
Growth rate is determined
from exponential fit to the
interferometer central
chord signal
QHS
Mirror
anti-Mirror
4
X-mode: 30 kW
X-mode: 20 kW
8000
O-mode: 40 kW
4000
m-3
RF Power is absorbed with high efficiency in a few passes
through the plasma column for a wide range of plasma densities
At low plasma density the efficiency remains high due to the
absorption on super-thermal electrons, in QHS their population
is higher than in Mirror
0
1.E-07
1.E-06
1.E-05
1.E-04
Gas Pressure, Torr
1.5
With ordinary mode the
growth rate is similar to that
1.E-03
with X-mode at a low power
level
 High electric field in front of the launching antenna causes the
gas to break down at a higher rate
2.0
1018
2.5
m-3
 Absorbed power is almost
independent of plasma density
40
30
 Radiated power rises with
plasma density
P a bs- QH S
20
P a bs- M i r r or
10
P r a d- M i r r or
P r a d- QH S
0
0.0
0.5
1.0
1.5
1018
2.0
m-3
Line average density,
Energy Confinement Time
2.0
2.5
 Energy confinement time is
defined from the experimental
data:
W
tE 
QHS
1.5
Mirror
1.0
0.5
0.0
0.0
0.5
1.0
1.5
Line average density,
1018
2.0
m-3
E
Pabs  Prad
 At 1.9·1018 m-3 the energy
confinement time is by a factor
of 1.5 higher in QHS as
2.5
compared to Mirror
5.2 ASTRA Code
4.1 Growth Rate vs. Mode of Operation
12000
1.0
Absorbed and Radiated Power
0.8
3.1 Experimental Lay-out
#1
0.5
Line average density,
Motivation: (1) to study the particle confinement
(2) to study the physics of plasma
breakdown by X-wave at the second
harmonic of wce
#2
10
0
Line average density,
Six absolutely calibrated
microwave detectors are installed
around the HSX at 6, 36, 70
and  100 (0.2 m, 0.9 m, 1.6 m
and 2.6 m away from RF power
launch port, respectively). #3 and
#5, #4 and #6 are located
symmetrically to the RF launch
20
0.0
4. Breakdown Studies
Top view
#3
M i r r or
0
3. Multi-Pass Absorption
#4
Absorption
 Second pass:
high ray refraction
due to wide
beam with
20o divergence
 Single-pass absorbed power
profile is pretty narrow (< 0.2ap)
High Ne: h = 50 %
Absorption, %
 First pass:
small refraction
because wave vector
is almost parallel to
grad|n| and grad|B|
0.4
0.2
0
0
0.5
1
1.5
2
2.5
18
-3
Line average density, 10 m
2.5
0.6
 Diamagnetic loop
shows the plasma
energy crashes at
low plasma density
 ECE signals are in
phase with the
energy crashes
 Also observed on
soft X-ray
emission (see
poster by
Sakaguchi)
 In both QHS and Mirror
modes the stored energy is
about 20 J at high plasma
density ( > 1018 m-3)
QH S
30
 At low plasma density due to a better confinement of trapped
particles the electrons can gain more energy in QHS mode than
 ECE temperature in QHS and Mirror configuration are almost
the same except at low plasma density (<0.6·1018 m-3)
in Mirror
1.2 Absorbed Power Profile
En, keV
Z, m
Z, m
Second Pass
2
 ECE temperature drops with plasma density. Tece at r = 0.2 at
low and high plasma density differs from each other by a
factor of 8
3-D Code is used to estimate absorption in HSX plasma
First Pass
1.5
Line average density, 1018 m-3
1. Ray Tracing Calculations
1.1 Ray Trajectories
1
40
ECE
TS
0.8
ECE
TS
Stored Energy
WE , J
r = + 0.5
1
Te, keV
r = + 0.24
QHS
Mirror
Te, keV
r = + 0.2
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
Te profile at 1.9 ·1018 m-3
5.4 Stored Energy Crashes
 QHS thermal conductivity is
dominated only by anomalous
transport
ASTRA:QHS
 Data is consistent with
Alcator-like scaling
where tE ~ n
 Te(0) from Thomson scattering
Er=0
is roughly independent of
ASTRA: Mirror
density. Consistent with  ~
1/n model.
Stored energy should have linear dependence on density but data
clearly does not show this (see above).
5.3 RF Power Scan
Fixed density of 1.5·1018 m-3
ISS95
scaling
ASTRA:
QHS
ASTRA:
Mirror
Difference in stored energy
between QHS and Mirror
reflects 15% difference in
volume
W ~ P in agreement with
 ~ 1/n model
 At lower density, stored energy is greater than predicted by
ASTRA code and TS disagrees with the model
45th Annual Meeting of the Division of Plasma Physics, October 27-31, 2003, Albuquerque, New Mexico
5.5 Stored Energy vs. Neutrals
At low plasma density the stored energy strongly depends on
gas fueling:
Ne = 0.4·1018 m-3
Mini-flange
30° T
Top
180° T
Middle
30° T
WE, J
grad|B|
r = - 0.2
7
6
5
4
3
2
1
0
ECE vs. TS at r = 0.2
P, kW
B
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
5.1 Plasma Density Scan
tE , msec.
k
ECE Temperature
ECE Temperature
Growth rate, sec.-1
E
2.2 Results of Measurements
Growth rate, sec.-1
1
B 3
R
Tece, keV
4-channel ECE
radiometer is used to
measure the electron
temperature in HSX
plasma: one channel is
put on the high field
side and 3 others on
the low field side. At
B=0.5 T (on-axis
heating) the effective
plasma radii in QHS
mode are as follows:
-0.2, 0.2, 0.24 and 0.5,
respectively
Tece, keV
 Microwave power at 28 GHz
produces and heats the plasma
at the second harmonic of wce
 Wave beam is launched from
the low magnetic field side and
is focused on the magnetic axis
with a spot size of 4 cm
 Wave beam propagates almost
along grad|B| and grad(ne) that
leads to a small ray refraction
 One can expect a sharp
absorbed power profile because
modB along the beam axis is
inverse to R3
5. Stored Energy Studies
2. ECE Diagnostic on HSX
2.1 Block diagram
Absorption
RF Heating in HSX
Middle
180° T
Time, sec.
When the puffing valve is moved further away from the plasma
axis, the neutral density drops in the plasma center where the
resonant RF-electron interactions take place. Electrons then
gain more energy between collisions because they suffer less
scattering on neutrals
Summary
The microwave multi-pass absorption efficiency is higher in
QHS and Mirror
(0.8-0.9) than in anti-Mirror (0.6)
Density growth rates at breakdown clearly indicate the
difference in particle confinement in different magnetic
configurations
Electron temperature increases linearly with absorbed power
up to at least 600 eV
ECE and TS data are in a good agreement at high plasma
density
At low plasma density the ECE radiometer measures a high
non-thermal electron population in QHS and Mirror
configurations; higher signal for QHS
ASTRA modeling shows the need for higher-power, higherdensity to observe differences in central electron temperature
between Mirror and QHS