Transcript Likin_APS 2003 - HSX - University of Wisconsin

Electron
Cyclotron Heating
at B = 0.5 T in HSX
K.M.Likin, A.F.Almagri, D.T.Anderson,
F.S.B.Anderson, J.Canik, C.Deng2, C.Domier1,
H.J.Lu, J.Radder, S.P.Gerhardt, J.N.Talmadge, K.Zhai
University of Wisconsin-Madison, USA
1UC-Davis, USA; 2UCLA, USA
RF Heating in HSX
 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
1
B 3
R
E
k
B
grad|B|
R
HSX configurations
Pin
a.u.
Normalized mod|B| along axis
Toroidal angle, degrees
 QHS has a helical axis of
symmetry and a very low level of
neoclassical transport
 Mirror configurations in HSX are
produced with auxiliary coils in
which an additional toroidal
mirror term is added to the
magnetic field spectrum
 In Mirror mode the term is added to the main field at the
location of launching antenna and
In anti-Mirror it is opposite to the main field
 Predicted global neoclassical confinement is poor in both
Mirror configurations
Ray Tracing Calculations
3-D Code is used to estimate absorption in HSX plasma
First Pass
Second Pass
Z, m
Z, m
 First pass: small refraction
because wave vector is
almost parallel to grad|n|
and grad|B|
R, m
 Second pass: high ray
refraction due to wide
beam with 20o divergence
R, m
Absorbed Power Profile (1)
 Single-pass absorbed power profile is
Ne = 2·1018 m-3
pretty narrow (< 0.2ap)
Te(0) = 0.4 keV
 Second Pass: Rays are reflected from
First pass
the wall and back into the plasma, the
Two passes
absorption is up to 70% while the
Effective Plasma Radius
profile does not broaden
 Absorption versus plasma density is
Single-pass absorption
calculated at constant Te in
1
0.8
Maxwellian plasma and based on the
Te from exp.
0.6
TS and ECE data in bi-Maxwellian
0.4
plasma
T
=
0.4
keV
e
0.2
 Owing to high non-thermal electron
0
0 0.5 1 1.5 2 2.5 3 3.5
population at a low plasma density the
Line Average Density, 10 m
absorption can be high enough
Absorption
Absorption, %
Absorbed Power Profile
18
-3
Absorbed Power Profile (2)
High Ne: h = 50 %
Low Ne: h = 14 %
En, keV
Absorption, %
Pin = 100 kW
Effective Plasma Radius
High Ne: h = 50 %
Low Ne: h = 14 %
Effective Plasma Radius
At low plasma density the energy that electrons can gain
between collisions is higher than at high plasma density
because high power per particle and longer collision time:
pabs (r )
En(r ) 
 e ( r )
ne (r )
Measurements of
RF Power Absorption
#4
#6
 Six absolutely calibrated microwave
detectors are installed around the HSX
#2
at 6, 36, 70 and  100 (0.2 m, 0.9
#1
m, 1.6 m and 2.6 m away from RF
Pin
power launch port, respectively). #3
and #5, #4 and #6 are located
symmetrically to the RF launch
Top view
#3
#5
Attenuator
Each antenna is an
open ended waveguide
followed by attenuator
Quartz
Window
Amplifier
mw Detector
Multi-Pass Absorption
1
0.8
0.6
0.4
0.2
0
Mirror
MD #1
MD #2
MD #3
MD #4
0
1
2
3
4
Line Average Density, 1018 m-3
Absorption
Absorption
QHS
1
0.8
0.6
0.4
0.2
0
MD #1
MD #2
MD #3
MD #4
0
1
2
3
4
Line Average Density, 1018 m-3
 RF Power is absorbed with high efficiency in a few passes
through the plasma column in the wide range of plasma
density
 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
Neutral Gas Breakdown
Motivation: (1) to study the particle confinement
(2) to study the physics of plasma breakdown
by X-wave at the second harmonic of wce
Growth rate, sec.-1
12000
8000
QHS
Mirror
anti-Mirror
4000
0
1.E-07
1.E-06
1.E-05
1.E-04
Gas Pressure, Torr
1.E-03
 Growth rate is determined
from exponential fit to the
interferometer central chord
signal
 In QHS mode the growth rate
is twice as that in Mirror
 In anti-Mirror mode the gas
breakdown occurs with a
very low growth rate
Growth rate vs.
RF electric field
 In QHS mode the growth rate
X-mode: 30 kW
has been measured at different
X-mode: 20 kW
launched power levels. The
O-mode: 40 kW
8000
growth rate drops with
decreasing of RF power and its
4000
maximum is shifted towards
lower gas pressure
0
 With ordinary mode the growth
1.E-07 1.E-06 1.E-05 1.E-04 1.E-03
rate is similar to that with XGas Pressure, Torr
mode at a low power level
 High electric field in front of the launching antenna makes the
gas to break down at higher rate
12000
Growth rate, sec.-1
X-mode: 40 kW
Plasma Density Scan
Stored Energy
WE , J
40
QH S
30
M i r r or
20
10
0
0.0
0.5
1.0
1.5
2.0
2.5
Line average density, 1018 m-3
Absorbed and Radiated Power
P, kW
40
30
P a b s- QH S
20
P a b s- M i r r o r
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
2.0
1018
Line average density,
Energy Confinement Time
2.0
E , msec.
2.5
m-3
QHS
1.5
Mirror
1.0
0.5
0.0
0.0
0.5
1.0
1.5
2.0
Line average density, 1018 m-3
2.5
 In both QHS and Mirror modes the
stored energy is about 20 J at high
plasma density ( > 1018 m-3)
 Absorbed power is almost
independent of plasma density
 Radiated power rises with plasma
density
 Energy confinement time is defined
from the experimental data:
WE
E 
Pabs  Prad
 At 1.9·1018 m-3 the energy
confinement time is by a factor of
1.5 higher in QHS as compared to
Mirror
ASTRA Code
ASTRA:QHS
Er=0
ASTRA: Mirror
 QHS thermal conductivity is
dominated only by anomalous
transport:  e   eneo   eanom
 A better model of
anomalous transport in HSX
is an Alcator-like dependency
(ne in units of 1018 m-3):
 e,anom
10.35 2

m /s
ne
Te(0) from Thomson scattering is roughly independent of
density. Consistent with  ~ 1/n model.
Stored energy should have linear dependence on density but
data clearly does not show this (see the previous slide).
Stored Energy Increases
Linearly with Power
ISS95 scaling
ASTRA:
Mirror
ASTRA: QHS
Fixed density of 1.5·1018 m-3
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
ECE diagnostic on HSX
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
 All channels have been calibrated on a bench. In
experiment, the ECE data have been benchmarked with
respect to the Thomson Scattering
Electron Temperature in QHS
 ECE temperature drops with plasma
r = - 0.2
density. Tece at r = 0.2 at low and high
r = + 0.2
plasma density differs from each
r = + 0.24
r = + 0.5
other by a factor of 8
 Electron temperatures measured by
Thomson Scattering and ECE are in a
good agreement only at high plasma
0.5
1
1.5
2
2.5
Line average density, 10 m
density (>1.7·1018 m-3)
18
-3
Te profile at 1.9 ·1018 m-3
ECE vs. TS at r = 0.2
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
1
ECE
TS
0.8
ECE
TS
Te, keV
0
Te, keV
Tece, keV
ECE Temperature
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0.6
0.4
0.2
0
0
0.5
1
1.5
2
2.5
Line average density, 1018 m-3
0
0.2
0.4
0.6
Effective plasma radius
0.8
Tece, keV
QHS vs Mirror
ECE Temperature
7
6
5
4
3
2
1
0
QHS
Mirror
0
0.5
1
1.5
2
Line average density, 1018 m-3
 ECE temperature in QHS and
Mirror configuration are almost
the same except at very low plasma
density (<0.6·1018 m-3)
 At low plasma density due to a
better confinement of trapped
particles the electrons can gain
2.5
more energy in QHS mode than in
Mirror
Bi-Maxwellian plasma
Plasma Density Profiles
ne, 1018 m-3
0.6
Bulk: nb ~ (1 – r2)
0.4
Tail: nt ~ exp(–4r2)
0.2
0
0
0.2
0.4
0.6
0.8
Effective plasma radius
Electron Temperature Profiles
Te, keV
6
Tail: Tt ~ exp(–4r2)
4
2
Bulk: Tb ~ exp(–4r2)
0
0
0.2
0.4
0.6
0.8
Effective plasma radius
 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
1
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 single-pass absorption is about
0.5
1
 Corresponds to large hard X-ray
emission (poster by Abdou)
Stored Energy and ECE at
Low Plasma Density
 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)
Stored Energy and ECE at
High Plasma Density
 No stored energy
crashes observed at
high plasma density
(>1.5 ·1018 m-3)
 Crashes appear to
be due to an
instability on
super-thermals
Stored Energy vs.
Gas Puffing Location
 At low plasma density the stored energy strongly depends on
gas fueling
N = 0.4·1018 m-3
e
Top
180° T
Middle
180° T
Middle
30° T
WE, J
Mini-flange
30° T
Time, sec.
 When the puffing valve is moved further away from the plasma axis, the
neutral density drops in the plasma centre 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
Summary (cont.)
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 higherpower, higher-density to observe differences
in central electron temperature between
Mirror and QHS