1. Development of Mach probe for the ion flow measurement in VEST
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
Department of Nuclear Engineering, Seoul National University, Seoul, Korea

2. Contents Motivation Introduction
Basic test for the diagnosis - Circuit test - Area calibration
Flow measurement at the linear device with B field curvature
Future design
Summary
Seoul National University
Fusion and Plasma Application Laboratory

3. 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
Seoul National University
Fusion and Plasma Application Laboratory

4. Motivation- Diagnosis for the ion flow
There are mainly 3 diagnostics to measure the ion flow.
 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.
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 long time to make it.
Theoretical models is needed
Give perturbation to the plasma
Probe cannot endure high temperature
Seoul National University
Fusion and Plasma Application Laboratory

5. 𝐽 𝑢𝑝 𝐽 𝑑𝑜𝑤𝑛 =exp⁡(𝐾 𝑣 𝑑 𝑐 𝑠 )= exp⁡(𝐾𝑀)
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 :
𝐽 𝑢𝑝 𝐽 𝑑𝑜𝑤𝑛 =exp⁡(𝐾 𝑣 𝑑 𝑐 𝑠 )= exp⁡(𝐾𝑀)
𝐾: Calibration factor
Seoul National University
Fusion and Plasma Application Laboratory

6. 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.
- Vacuum seal
- Area calibration
Convert the current signal to
the voltage and send it to
the oscilloscope.
- Circuit test
Tip: Receive signal
BNC cable
gnd
DAQ circuit
Seoul National University
Fusion and Plasma Application Laboratory

7. Experimental Setup for Circuit Test
Input Signal
Square wave
1kHz
Vsig= V
Check the circuit with increasing the voltage level.
Compare the voltage level between the 2 circuits
Seoul National University
Fusion and Plasma Application Laboratory

8. Basic test for the diagnosis - Circuit test & result
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
Seoul National University
Fusion and Plasma Application Laboratory

9. 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.
Probe lengths are smaller than ion-neutral collision length and larger than Debye length. (Probe area = 3x 10 −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
[T. Shikama at el., JNM ’06]
tip
Probe 2
(upstream)
Probe 1
(downstream)
B field
Ceramic insulator
(ceramic paste)
[In plasma]
[Top view]
Ceramic tube
Seoul National University
Fusion and Plasma Application Laboratory

10. Experimental setup- Area calibration (DC plasma)
Permanent
magnet arrays
forming cusp N S
Mach Probe N
W-Th
Filaments
Field-free &
Uniform Density
Plasma Region
- 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.
Seoul National University
Fusion and Plasma Application Laboratory

11. Basic test for the diagnosis - Area Calibration
-60 V (ion dominant region)
Source characteristic: 60 eV electron
Ion dominant region
(‘Area effect on ion current’ only)
Perturbation of ceramic paste on tip
(not uniformly distributed)
+ Response of the fast electrons
Area ratio S1/S2 : 1.13
The current ratio is not unity.
Area ratio could be determined: 𝑺 𝟏 : 𝑺 𝟐 = 1.13:1
Biasing voltage in this source << - 60 V (negative bias)
Seoul National University
Fusion and Plasma Application Laboratory

12. Measurement at linear device with B field curvature
B field in VEST and linear device with B field curvature are similar.
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.
VEST
(typical value)
Linear device with B field curvature
B field
0.1 T T
Electron density
1 x 1018 m-3 (edge)
2 x 1017 m-3
Electron temperature
20 eV (edge)
8 eV
Seoul National University
Fusion and Plasma Application Laboratory

13. Magnetic field coils forming resonance B field with curvature
Experimental setup- for linear device (ECH plasma)
Probe location
Te ~ 8 eV
ne ~ 2 x 1017 m-3
B ~ T
Operation pressure ~ 4 x 10-4 Torr, H2
(base 4 x 10-6 Torr)
ECH Microwave
Source 300W
Flow direction
[ECH Resonance region]
[11 cm]
B-field direction
Magnetic field coils forming resonance B field with curvature
Mach Probe
Seoul National University
Fusion and Plasma Application Laboratory

14. Measurement at linear device with B field curvature – Result
** Area ratio was considered
Current Ratio ~ 0.8
Ion dominant region
(Area effect only)
Ceramic paste perturbed the tip.
(not uniformly distributed)
Electron response is applied
Linear device generally has M ~ 0.3 (n= 𝑚 −3 , 𝑇 𝑒 =7.5 𝑒𝑉, Ar)
Considering the area difference 𝑺 𝟏 : 𝑺 𝟐 =1.13:1, 𝑱 𝒖𝒑 𝑱 𝒅𝒐𝒘𝒏 =𝟏.𝟒𝟏𝟑
According to Chung’s theory, K ~ 1.6, M ~ 0.2
[K. Nagaoka, et al J. Phys. Soc.Jpn. 70, 131, 2001]
[Chung, Hutchinson et al., PRA ’88]
Seoul National University
Fusion and Plasma Application Laboratory

15. 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]
Seoul National University
Fusion and Plasma Application Laboratory

16. Future design- Probe design for VEST
[30cm]
Ceramic tube
[40cm]
¼” sus tube
Wilson type
Vacuum seal
[Edge measurement]
[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.
Seoul National University
Fusion and Plasma Application Laboratory

17. 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.
Seoul National University
Fusion and Plasma Application Laboratory

18. 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.
Seoul National University
Fusion and Plasma Application Laboratory

19. 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.
Tungsten tip can endure high temperature.
Debye length~ 10 −4 𝑚 −3 , ion neutral collision length >> mm (too low 𝑛 𝑛 ).
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,
𝐼 𝑖𝑠 =−0.605𝑒 𝑛 0 𝑘 𝑇 𝑒 𝑚 𝑖 𝑆 = 40mA
(Assuming 𝑛= 𝑐 𝑚 −3 , Te=30eV, S=4∗ 10 −6 m −3 )
[L Okuz et al., PSST ’04]
Seoul National University
Fusion and Plasma Application Laboratory