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 151-744, Korea kyuminchoe@snu.ac.kr ICIS’13_TueP64

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

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.

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.

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)

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)]

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.

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.

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 13.56 MHz 2 helicon plasma injector 4 helicon plasma injector

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

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

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.

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

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

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.

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