Transcript **** 1 - sunist

The Second A3 Foresight Workshop on Spherical Torus (ST)
Tsinghua University, Beijing, China, Jan. 6-8, 2013
Development of Mach probe for
the ion flow measurement in VEST
S.M. Yang, N.K. Kim, H.Y. Lee, D.H. Na, J.G. Jo, J.W. Lee, M.G Yoo,
G.H. Kim, Y.S. Hwang and Yong-Su Na*
*Corresponding author’s E-mail: [email protected]
Department of Nuclear Engineering, Seoul National University, Seoul, Korea
Contents
1. Motivation
2. Introduction
3. Basic test for the diagnosis
- Circuit test
- Area calibration
4. Flow measurement at the linear device with B field curvature
5. Future design
6. Summary
2
Seoul National University
Fusion and Plasma Application Laboratory
Motivation– Toroidal rotation at the effect of the edge
• Toroidal rotation is found to have an impact on global energy or particle
confinement.
• Toroidal rotation is beneficial on transport and MHD instability in tokamak
plasmas.
ex) suppression of turbulence, stabilization of the resistive wall mode
• The core momentum transport is not fully understood and it is known that the
core rotation is heavily influenced from the flux at the edge.
• Rotation at the edge can be one of the important factors to understand the
toroidal rotation
 Measure ion velocity at the edge of the VEST
3
Seoul National University
Fusion and Plasma Application Laboratory
Motivation- Diagnosis for the ion flow
• There are mainly 3 diagnostics to measure the ion flow.
CXRS
XICS
Mach probe
Principle
Doppler shift
(line emission)
Doppler shift
(X-ray)
Ratio of ion saturation currents
Pros
No perturbation to the plasma
Simpler to make
Cheaper than other diagnostics
Appropriate for low temp. & dens.
cons
NBI is needed
Difficult and takes
Theoretical models is needed
long time to make it. Give perturbation to the plasma
Probe cannot endure high temperature
 Among these, Mach probe can be the best choice for the ion flow
measurement at the edge of the VEST, which has relatively low temperature and
density.
4
Seoul National University
Fusion and Plasma Application Laboratory
Introduction - What is the Mach Probe?
• Measures the ion saturation current with 2 tips.
Plasma Flow Direction
[E Ko at el., PPCF ’06]
• If there’s any ion flow, ion saturation current at each tip has to be identical.
• In most theoretical models, flow is estimated by taking the ratio of
ion saturation currents :
𝐽𝑢𝑝
𝐽𝑑𝑜𝑤𝑛
=
𝑣𝑑
exp(𝐾 )=
𝑐𝑠
exp(𝐾𝑀)
𝐾: Calibration factor
5
Seoul National University
Fusion and Plasma Application Laboratory
Introduction - Mach probe and DAQ circuit part
Mach probe part
DAQ circuit part
• Get the ion saturation current from a
plasma and transmit the signal to the
circuit.
• Convert the current signal to
the voltage and send it to
the oscilloscope.
- Vacuum seal
- Area calibration
- Circuit test
Tip: Receive signal
BNC cable
BNC cable
6
DAQ circuit
Seoul National University
Fusion and Plasma Application Laboratory
Experimental Setup for Circuit Test
Input Signal
Square wave
1kHz
Vsig= 0 - 2 V
1.
2.
7
Check the circuit with increasing the voltage level.
Compare the voltage level between the 2 circuits
Seoul National University
Fusion and Plasma Application Laboratory
Basic test for the diagnosis - Circuit test & result
2
Measured voltage(V)
1
0
Reference
Circuit 1
-1
-2
-2
Circuit 2
-1
0
1
2
Applied voltage(V)
• Since Mach probe compares the ion saturation current between two tips, the
voltage measured by a circuit using reference voltage has to be identical.
 Both circuits gave almost same voltage.
Circuit 1 and 2 are identical
8
Seoul National University
Fusion and Plasma Application Laboratory
Basic test for the diagnosis - Design of Mach probe
• Cylindrical tungsten tips are chosen since the geometrical effect of probe tips
are known to be small in weakly magnetized plasma.
[T. Shikama at el., JNM ’06]
• Probe lengths are smaller than ion-neutral collision length and larger than
Debye length. (Probe area = 3x10−6 m−3, expected ion saturation current = 30mA)
• To measure the parallel flow(which supposed to have higher velocity) tip
direction is fixed as follows
B field
Probe 2
(upstream)
Ceramic insulator
(ceramic paste)
[In plasma]
9
Ceramic insulator
(ceramic paste)
Probe 1
(downstream)
tip
Ceramic tube
[Top view]
Seoul National University
Fusion and Plasma Application Laboratory
Experimental setup- Area calibration (DC plasma)
N
Mach Probe
S
W-Th
Filaments
Field-free &
Uniform Density
Plasma Region
Permanent
magnet arrays
forming cusp
N
- 60 V
• Hot filaments generate electrons in random direction, consider this doesn’t
have any ion flow.
• Ion saturation current in the two tips need to be same if the area of those are
same.
10
Seoul National University
Fusion and Plasma Application Laboratory
Basic test for the diagnosis - Area Calibration
-60 V (ion dominant region)
Source characteristic: 60 eV electron
2.0
CurrentRatio S1/S2
1.8
1.6
1.4
1.2
Ion dominant region
(‘Area effect on ion current’ only)
Area ratio S1/S2 : 1.13
1.0
Perturbation of ceramic paste on tip
(not uniformly distributed)
+ Response of the fast electrons
0.8
0.6
0.4
0.2
0.0
-90
-80
-70
-60
-50
-40
-30
-20
Biasing Voltage
• The current ratio is not unity.
• Area ratio could be determined: 𝑺𝟏 : 𝑺𝟐 = 1.13:1
• Biasing voltage in this source << - 60 V (negative bias)
11
Seoul National University
Fusion and Plasma Application Laboratory
Measurement at linear device with B field curvature
• B field in VEST and linear device with B field curvature are similar.
VEST
(typical value)
B field
Electron
density
Electron
temperature
Linear device with
B field curvature
0.1 T
0.0875 T
1 x 1018 m-3 (edge)
2 x 1017 m-3
20 eV (edge)
8 eV
• Parameters at the edge of VEST and linear device with B field curvature are
not significantly different.
• As a preliminary diagnosis of the Mach probe at the edge of the VEST,
measurement at the linear device with a B field curvature is done.
12
Seoul National University
Fusion and Plasma Application Laboratory
Experimental setup- for linear device (ECH plasma)
Probe location
B-field
direction
ECH Microwave
Source 300W
[11 cm]
Te ~ 8 eV
ne ~ 2 x 1017 m-3
B ~ 0.0875 T
Operation pressure ~ 4 x 10-4 Torr, H2
(base 4 x 10-6 Torr)
Flow
direction
[ECH Resonance region]
Mach Probe
Magnetic field coils forming resonance B field with curvature
13
Seoul National University
Fusion and Plasma Application Laboratory
Measurement at linear device with B field curvature – Result
1.8
Current Ratio S1/S2
1.6
** Area ratio was considered
1.4
1.2
1.0
Ion dominant region
(Area effect only)
Current Ratio ~ 0.8
0.8
Ceramic paste perturbed the tip.
(not uniformly distributed)
Electron response is applied
0.6
0.4
0.2
0.0
-70
-60
-50
-40
-30
-20
-10
0
Biasing Voltage(V)
• Linear device generally has M ~ 0.3 (n = 1017 𝑚−3 , 𝑇𝑒 = 7.5 𝑒𝑉, Ar)
• Considering the area difference 𝑺𝟏 : 𝑺𝟐 =1.13:1,
• According to Chung’s theory, K ~ 1.6, M ~ 0.2
14
𝑱𝒖𝒑
[K. Nagaoka, et al J. Phys. Soc.Jpn. 70, 131, 2001]
𝑱𝒅𝒐𝒘𝒏
= 𝟏. 𝟒𝟏𝟑
[Chung, Hutchinson et al., PRA ’88]
Seoul National University
Fusion and Plasma Application Laboratory
Discussion for the future design
• Since M ~ 0.2 is measured at linear device with magnetic field curvature,
we expect to have similar ion flow at the edge of VEST.
• For the measurement with a better precision, new ceramic design (Alumina) is
considered as in the below figure.
- Less area difference for each tip
- Uniform ceramic thickness will preserve uniform sheath for each
the probe.
- Two probes can be located in parallel (angle accuracy).
[ceramic for the
tips]
15
Seoul National University
Fusion and Plasma Application Laboratory
Future design- Probe design for VEST
Wilson type
Vacuum seal
[Edge measurement]
[30cm]
[40cm]
Ceramic tube
¼” sus tube
[Core measurement]
• A Wilson type vacuum seal is used for the radial scan.
• Length of the probe is determined for the measurement at the core for the
future.
16
Seoul National University
Fusion and Plasma Application Laboratory
Summary
• For the measurement of ion flow at the edge of the VEST, the Mach probe is
selected for the low density & temperature plasma.
• Basic test for the Mach probe is done.
- Circuit test
- Area calibration
• As a preliminary measurement, the flow at the linear device with B field
curvature is measured which expect to have similar parameters as in the edge of
the VEST.
• The future design for ceramics for the tip is made.
- Less area difference for each tip
- Uniform ceramic thickness will preserve uniform sheath for each probe.
- Two probes can be located in parallel (angle accuracy).
• To do a radial scan, vacuum type(Wilson seal) is decided with a sufficient length.
17
Seoul National University
Fusion and Plasma Application Laboratory
Back up- Mach probe models
[Chung., PSST ’12]
Model by Mott-Smith(1926) and Hudis(1970) is older than other models
Other unmagnetized model ranged 1.0~1.34
 K can be lower than this model since strong magnetic field condition is not
achieved.
 But, tendency can be seen since exponential form is still conserved.
18
Seoul National University
Fusion and Plasma Application Laboratory
Back up- Design of the probe tips
• The length between two probes considered to be have a small effect by
experimentally comparing double sided Langmuir probe and Mach probe.
[L Okuz et al., PSST ’04]
• Tungsten tip can endure high temperature.
• Debye length~ 10−4 𝑚−3 , ion neutral collision length >> mm (too low
𝑛𝑛 ).
• Geometrical effect is explained with comparing MP, DLP and SP with
different ion collection angle. (100°, 60°, 20° respectively)
• Expected ion saturation current is estimated by Bohm flux,
𝑘𝑇𝑒 0.5
𝐼𝑖𝑠 = −0.605𝑒𝑛0
𝑆 = 40mA
𝑚𝑖
(Assuming 𝑛 = 1012 𝑐𝑚−3 , Te = 30eV, S = 4 ∗ 10−6 m−3 )
19
Seoul National University
Fusion and Plasma Application Laboratory