1. The 15th International Conference on Ion Sources (ICIS’13)
Development of the RF Ion Source with Multi-Helicon Plasma Injectors for Neutral Beam Injection System of VEST
Sept. 10th, 2013
Kyumin Choe, Bongki Jung,
Kyoung-Jae Chung, and Y. S. Hwang
Dept. of Nuclear Engineering, Seoul National University 1 Gwanak-ro, Gwanak-gu, Seoul , Korea
ICIS’13_TueP64

2. VEST(Versatile Experiment Spherical Torus) at SNU
VEST - the First Spherical Torus in Korea
Objectives
Basic research on a compact, high- ST (Spherical Torus)
with elongated chamber in partial solenoid configuration
Study on innovative partial solenoid start-up, divertor, etc
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
(40 kA achieved)
100
Safety factor, qa
7.4
6.7
* Elongation : 3.3 aussmed
Requirement of NBI for VEST : 30keV & 10A

3. Determination of Plasma Parameters of Ion Source
30kV: 2.3mm
50kV: 5mm
Helicon plasma is the most appropriate for obtaining high plasma density. But, poor uniformity of the helicon plasma must be resolved.

4. Advantage Disadvantage Advantages and Disadvantages of Helicon Plasma
as an Ion Source for NBI System
Advantage
Higher plasma density (~1×1013[#/cm3]) can be achieved with helicon plasma than inductively coupled plasma (~1×1012 [#/cm3]) at identical input RF power, and high monatomic fraction (90%) can be obtained in helicon plasma
Disadvantage
Intrinsically helicon plasma has density gradient along the radial profile of discharge region. It indicates helicon plasma has low uniformity in discharge region.

5. Previous Experience on Multi-helicon Plasma Source
at Seoul National University
Large-area high-density uniform plasma with multi-helicon plasma
6 helicon plasma injector is attached to process chamber and antennas are connected in parallel to RF power
M = 1 mode Nagoya type III antenna shows the best performance
Uniformity of 5% is achieved.
“Development of a novel large-area high-density uniform plasma source with multiple helicon plasma injectors”
by Seung Ho Han (2004)

6. Design Consideration for New Multi-Helicon Ion Source
Optimal B Field and Antenna configuration with Permanent Magnet for Helicon Ion Source
It is reported that short Antenna length is effective for higher plasma density in transport region with helicon injectors by using Permanent Magnet
[F.F. Chen, POP 16(2009)]

7. Magnetic Field Optimization of Multi-helicon plasma source
Design of Multi-Helicon Ion Source
Magnetic Field Optimization of Multi-helicon plasma source
Positioning the magnet behind the plasma generation region shows good magnetic field configuration
100mm
~100G Region
~200G Region
Serial type connection antenna can apply.(in calculation)
13.56MHz RF Power can be used for helicon mode transition condition with permanent magnet.(~250G, Low-hybrid resonance)
Large Area uniformity can be achieved.
Magnetic field can be changed mechanically.

8. Design of Multi-Helicon Ion Source
Multi-helicon plasma source with Permanent Magnets
High Density Helicon Plasma
Modified large-area, high-density and uniform hydrogen plasma with multi-helicon injectors is conceptually designed.

9. Multi-Helicon Plasma Chamber
Experimental Setup
Multi-Helicon Plasma Chamber
Langmuir
probe for
diagnostics
Rectangular plasma chamber with 2 or 4 helicon plasma injectors
4 ring-type permanent magnets are arranged to make cusp magnetic field
4 Nagoya type III antennas are connected serially and operated by one matching system and one RF power of MHz
2 helicon plasma injector
4 helicon plasma injector

10. Mode Transition to Helicon Plasma
Operating Characteristics of Multi-helicon Plasma Source
Mode Transition to Helicon Plasma
4-helicon plasma injector
Helicon mode transition does not occur at the same time but successively injector by injector
2-helicon plasma injector

11. Plasma Density of Multi-Helicon Plasma
Plasma Characteristics in the Middle of Diffusion Chamber
Plasma Density of Multi-Helicon Plasma
Gas : Argon
Operating pressure : 35mTorr
Electron temperature (Te) : about 2eV
Highest density
4-helicon : 3.0×1018m-3
2-helicon : 2.6×1018m-3
Plasma density increases with RF power.
When first helicon mode transition occurs, plasma density increases abruptly.
Quartz near antenna was overheated at high power, so we cannot power up more.
Because of quartz overheating, the plasma density of the full helicon mode cannot measured.
First helicon mode transition
of 4-helicon plasma injector
Second transition
Third transition
of 2-helicon plasma injector
Second
transition

12. Conclusion and Future Work
Using multi-helicon plasma injector with permanent magnets, the high-density argon plasma over 1018 m-3 is achieved in the diffusion region. A camera image shows good spatial uniformity in the diffusion region, but plasma uniformity needs to be measured with a Langmuir probe array.
Non-simultaneous onset of helicon mode for each source is observed, and it is believed to come from the unbalance of power distribution to each helicon plasma injector. Parallel connection of multiple helicon injectors needs to be explored to resolve the problem.
Generation of multi-helicon plasma with hydrogen gas will be carried out as a next step. Also, the beam extraction and acceleration system in pulsed mode will be designed and fabricated.

13. Magnetic Field Optimization of Multi-helicon plasma source
Design of Multi-Helicon Ion Source
Magnetic Field Optimization of Multi-helicon plasma source
100mm
~100G Region
Intrinsically plasma Loss is relatively Large in this magnetic configuration for plasma uniformity.
Relatively Uniform B region

14. Design Parameter of Multi-helicon plasma source
Design of Multi-Helicon Ion Source
Design Parameter of Multi-helicon plasma source
Design Parameter
Axial B field
100~300, 900 [Gauss]
Driving Frequency
13.56MHz, 60MHz
Antenna Length & Type
30 ~ 100mm, Half helical, single turn or Nagoya III
Plasma Facing Material
SUS, Aluminum, Al2O3

15. Abstract
Despite of high plasma density, helicon plasma has not yet been applied to a large area ion source such as a driver for NBI system due to intrinsically poor plasma uniformity in the discharge region.
In this study, a RF ion source with multi-helicon plasma injectors for high plasma density with good uniformity has been developed for the neutral beam injection (NBI) system of Versatile Experiment Spherical Torus (VEST) at Seoul National University.
The ion source consists of the rectangular plasma expansion chamber (120*120*120 mm), four helicon plasma injectors with ring-type permanent magnets and RF power system.
Main feature of the source is plasma confinement in the cusp magnetic field configuration which is generated by permanent magnets for helicon plasma injectors.
To optimize plasma density and uniformity of the ion source, their dependency on operating conditions such as pressure, input power, RF antenna type is investigated.

16. Langmuir Probe Diagnostics
Experimental Setup
Langmuir Probe Diagnostics
13.56MHz RF compensated probe
Data number : 500~2000
Triggered mode preferable
Average mode : 16~64
Graphite resistor
5~50ohm for Vp, Te
1kohm for Iis, Vf
AD210
Generally, 15V input limit
Noise reduction required
Powered by SMPS
Gain : 10 (BOP 100), 20 (BOP200)
Voltage controlled mode
Must be grounded
Offset voltage available
Sweep Frequency : 1~100Hz
Sawtooth or Triangular signal