1. Design of Interferometer System
on Versatile Experiment Spherical Torus (VEST) at Seoul National University
D.H. Choi, Y.H. An, K.J. Chung and Y.S. Hwang Department of Nuclear Engineering, Seoul National University, Seoul, Korea Center for Advance Research in Fusion reactor Engineering (CARFRE) Seoul National University, Seoul, Korea 15th International Symposium on Laser-Aided Plasma Diagnostics (LAPD) October 9th-13th, ShineVille Resort, Jeju, Korea.

2. Introduction VEST: the first Spherical Torus in Korea
Objectives
Basic research on a compact, high-β ST (Spherical Torus) with elongated chamber in partial solenoid configuration
Study on innovative start-up, current drive, divertor, etc
Research topics
Magnetic reconnection
Sequential double null merging
Divertor
Non-inductive current drive
Specifications
Initial
Phase
Future
Chamber Radius [m]
0.8 : Main Chamber,
0.6 : Upper & Lower Chambers
Chamber Height [m]
2.4
Toroidal B Field [T]
0.1
0.3
Major Radius [m]
0.4
Minor Radius [m]
Aspect Ratio
> 1.3
Plasma Current [kA] 30
100
Safety factor, qa 74
6.7
Design of Interferometer System on Versatile Experiment Spherical Torus (VEST) at Seoul National University
D.H. Choi, LAPD2011

3. Introduction VEST : Double Null Merging Start-up with Partial Solenoids
Relatively smaller space for central solenoid in ST
→ Hard to supply sufficient magnetic flux
→ Two partial solenoid coils are installed at both vertical ends of a center stack.
Partial solenoid operation :
Inherits the merits of solenoid start-up
Possible to maintain low aspect ratio
→ Effective start-up method in Spherical Torus
→ Initial operation of the VEST will be focusing on double null merging start-up schemes.
→ it requires vertical profiles of plasma parameters
→ A vertically movable interferometer system has been designed for the diagnostics of plasma densities during the merging start-up phase of the VEST.
Design of Interferometer System on Versatile Experiment Spherical Torus (VEST) at Seoul National University
D.H. Choi, LAPD2011

4. System description Overall interferometer system arrangement on the VEST
Interferometer system is divided into two parts;
microwave electronics system and beam focusing system.
microwave electronics system : injects microwave beam, receives phase shifted beam,
and generate output signal to measure plasma density with electronic circuits. Two 94GHz Gunn Oscillators are used in a heterodyne configuration.
beam focusing system : beam focusing system has been designed for effective beam
reception and enhanced spatial resolution, critical for a vertically movable
system.
Design of Interferometer System on Versatile Experiment Spherical Torus (VEST) at Seoul National University
D.H. Choi, LAPD2011

5. Microwave electronics system
Frequency selection
Cutoff frequency of the VEST
plasma is 20.2GHz
Calculated beam position variation by refraction decreases below 1.8cm as microwave beam frequency
exceeds 94GHz.
→ The available 94GHz
microwave source is chosen
for the VEST interferometer
system after considering those
limits.
Microwave electronics
A voltage-controlled Gunn oscillator and a mechanically-controlled Gunn oscillator, both operating at a frequency of 94GHz, are used in a heterodyne configuration.
Two mixers generate reference signal and probe signal with an intermediate frequency of 40MHz
Signals are transmitted to oscilloscope or phase detector by coaxial cable.
Overall microwave components are located in aluminum shielding box and located more than 50cm away from the device in order to protect microwave components from the magnetic field.
Design of Interferometer System on Versatile Experiment Spherical Torus (VEST) at Seoul National University
D.H. Choi, LAPD2011

6. Beam focusing system planoconvex lens and concave mirror.
Beam focusing system is designed for the enhancement of spatial resolution, which is critical for a vertically movable system, and effective beam reception by focusing a diverging beam.
A simple beam focusing system is considered with two teflon planoconvex lenses
in front of the horn antennas and an inner concave reflecting mirror inside the vacuum vessel.
Due to the geometrical limitations of the device, a beam focusing system will be
installed outside the chamber, between
horn antenna and window,
Gaussian optics formulae are employed to calculate design parameters of optical components;
planoconvex lens and concave mirror.
→ These components will be designed for focusing broad beam to form a beam waist at the plasma center with the planoconvex lens and return through the same beam path with the concave mirror.
Design of Interferometer System on Versatile Experiment Spherical Torus (VEST) at Seoul National University
D.H. Choi, LAPD2011

7. Beam focusing system Design of optical components
Lens design
geometrical constraints for a lens design
①The first constraint is that d2 must exceed 200mm,
→ available beam waist radius and lens curvature radius are
determined from (a).
→ available range of d1 and lens curvature radius in (b).
②beam radius at mirror should be shorter than 40mm
→ available beam waist range at the plasma center from (c).
③beam radius at lens should be less than 30mm, a quarter of
the width of the vacuum window.
→ This beam radius gives proper range of d1.
→ From all these constraints, lenses with curvature radius of 43mm
are found to be suitable for the VEST interferometer system
Mirror design
spherical concave mirror with the curvature radius of 540mm is
Selected to locate focal point at plasma center (270mm from the mirror).
Since inner reflecting mirror is installed on the inner wall of the
VEST chamber, it can contact with plasma
→ it should be installed either behind the limiter
or play a role of limiter itself.
Design of Interferometer System on Versatile Experiment Spherical Torus (VEST) at Seoul National University
D.H. Choi, LAPD2011

8. Beam focusing system Beam analysis on the designed focusing system
Beam radius and beam intensity ratio along the beam path are calculated with the designed beam focusing system.
After beam transmits the lens, the first beam waist is formed at the plasma center and beam radius at the mirror becomes less than 25mm, 5 times less than that without focusing system
Beam intensity ratio increases after it transmits through the lens and its intensity ratio at the mirror is about 30 times larger than that without the focusing system.
→ With the beam focusing
system, both spatial resolution
and beam intensity are expected to be enhanced significantly.
Design of Interferometer System on Versatile Experiment Spherical Torus (VEST) at Seoul National University
D.H. Choi, LAPD2011

9. Beam focusing experiments Experimental setup
Before installing the interferometer system on the device, preliminary experiments have been done with the beam focusing system on a horizontal plane model reflecting a real-sized vacuum vessel
While microwave system and the horizontal plane model of the device are fixed at corresponding locations, d1 is changed by moving lens
position along the beam path,
→ then also d2 and the beam waist vary. Therefore the value of d1+d2, which is the distance from antenna to the first beam waist, will be varied from 350mm to different values.
Design of Interferometer System on Versatile Experiment Spherical Torus (VEST) at Seoul National University
D.H. Choi, LAPD2011

10. Beam focusing experiments Experimental results and discussion
As moving lens from antenna to window, beam intensity ratio shows maximum value at around
91mm and it is not much different from the calculated optimum value
→ This confirms that Teflon lenses are working well as focusing elements.
The reason for the variation is considered to be the difference between positions of first beam waist and reflected beam waist.
varying value of d1 by moving lenses along
the beam path also varies d2, and then d1+d2,
the distance between antenna and first beam
waist, also is varied then the first beam waist
will not be located at the plasma center
as calculated
locating reflected beam waist at the same position of the first beam waist is available only when the first beam waist is located at plasma center
the positions of two beam waists are different when d1 has not a value of 108mm.
Design of Interferometer System on Versatile Experiment Spherical Torus (VEST) at Seoul National University
D.H. Choi, LAPD2011

11. Beam focusing experiments Experimental results and discussion
Blocking plate, which hardly transmits microwave, is vertically moved downside at fixed intervals of 5mm, then decreased signals represent blocked beam intensity with the same intervals.
→ Beam intensity profile is obtained by differentiating polynomially-fitted signals.
Measured beam radius from the Gaussian beam intensity profile is compared with the calculated beam radius
→ showing reasonable agreement in overall behaviors.
beam waist is located near plasma center where it is designed to place beam waist.
→ This beam focusing system performs well as expected
by placing beam waist at the plasma center.
Design of Interferometer System on Versatile Experiment Spherical Torus (VEST) at Seoul National University
D.H. Choi, LAPD2011

12. Summary and future works
A 94GHz heterodyne interferometer system has been designed for plasma density measurements of the VEST, and also optical components of the beam focusing system are designed suitably under geometrical limitations of the device.
Experimental results with the beam focusing system show reasonable agreements with Gaussian beam optics calculations, and the designed focusing system is confirmed to place beam waist at plasma center as designed.
Future works
With this focusing system, an actual interferometer system with good spatial resolution will be installed and operated on the VEST device with a vertically moving system implemented.
Line integrated plasma densities at several horizontal planes will be measured by scanning vertically in repeating identical shots and vertical density profiles will be obtained by applying an appropriate inversion technique.
Design of Interferometer System on Versatile Experiment Spherical Torus (VEST) at Seoul National University
D.H. Choi, LAPD2011