1. Study on Monatomic Fraction Improvement with Alumina Layer on Metal Electrode in Hydrogen Plasma Source
Bong-Ki Jung, Kyung-Jae Chung,
Jeong-Jeung Dang, Y. S. Hwang
ICIS 2011 Sept. 12th 2011, Giardini, Naxos, Italy
NUPLEX, Dept. of Nuclear, Seoul National University, San 56-1, Shillim-dong, Gwanak-gu, Seoul , Korea

2. Introduction & Research Motivation System Setup
Outline
Introduction & Research Motivation
System Setup
Monatomic Fraction in Hydrogen plasma
Monatomic beam fraction for various plasma-facing materials
Effects of alumina layer on aluminum plasma-facing electrode
Explanation of alumina layer effects on monatomic fraction in hydrogen plasma
Conclusion

3. Introduction : Hydrogen Ion Source for Applications
Proton accelerator
Neutral beam injector
Neutron generator
Ion Source Requirements
High Monatomic Ion Fraction
- Increasing energy per atom
High Current(Density)
- Helicon ion source
Long-life Time
- Radio frequency ion source

4. Research Motivation : Monatomic Fraction in Hydrogen Plasma
Previous work show variation of ion species in the hydrogen plasma related in boundary conditions (facing material, pressure) and plasma properties(density, temperature)1) 2) but effects of various wall material need to be considered more intensively.
For this reason, this research is focused on characterizing monatomic ratio for various materials in hydrogen plasma.
Reference :
1) Osamu Fukumasa et al. “Wall Effects on the percentage of Atomic Ions in Hydrogen Plasma”, Phys. Letters 100A (1984)
2) Chun-Ku Chen et al. “Modeling the discharge region of a microwave generated hydrogen plasma”, J. Phys. D: Appl. Phys. 32 (1999) 688–698.

5. System Setup : Helicon ion source & dipole magnet for measuring ion species
<Ion beam fraction measuring system>
Gas feeding
Solenoid Magnet
Quartz Tube
RF Antenna
Hydrogen Plasma
Replaceable plasma-facing electrode
Proton beam
45° dipole magnet (current controllable)
Faraday cup
Pico-ammeter H+
H2+
H3+
Collimator
Langmuir Probe Diagnostics
OES Diagnostics System
Replaceable plasma-facing electrode : Mo, S/S, Cu, Al
Gas: hydrogen, Power: ~1kW, Pressure: 50mTorr
Discharge Tube : inner diameter of 20mm (Quartz)

6. Ion Fraction Measurement for Various Materials
500W
900W
At the same pressure condition, experimental data shows monatomic fraction with aluminum electrode is higher than other materials.

7. Plasma-Assisted Growth of Aluminum Oxide layer
Mass spectrum
at the beginning of operation H+
H2+
H3+
H2O+
Generally, water(H2O+) peak level is observed at the beginning of operation and this impurity should be eliminated in the plasma ion source. However, H2O in hydrogen plasma can be a precursor for growing alumina (Al2O3) layer on the aluminum electrode.1) 2) 3)
Reference :
3) S. J. Yun et al. J. Vac. Sci. & Technol. A 15 (1997) 2993
4) K. Kukli et al. J. Vac. Sci. & Technol. A 15 (1997) 2214
5) G. S. Higashi et al. Appl. Phys. Lett. 55 (1989) 1963

8. Reference : 6) L. Schachter, et al, Rev. Sci. Instrum. 73, 570 (2002).
Increase of monatomic beam fraction with alumina covered on electrode structure.
Plasma
Quartz
Dielectric cover
Mo electrode
Compared monatomic fraction results with Al, Mo, Alumina covered Mo electrode shows that alumina layer affects monatomic fraction in hydrogen plasma.
Similar effects of alumina layer in previous results are reported, and increase of plasma density by high electron impact secondary electron emission, and enhanced confinement due to alumina layer on wall material were reasons of increasing highly charged ions in ECR source
Reference : 6) L. Schachter, et al, Rev. Sci. Instrum. 73, 570 (2002).

9. Diagnostic System Setup to measure Ne
Gas feeding
Solenoid Magnet
Quartz Tube
RF Antenna
Hydrogen Plasma
Langmuir Probe Diagnostics (planner type)
OES Diagnostics
Replaceable plasma-facing electrode (Al,Cu)
To check validity of increase of plasma density, OES and probe diagnostics are used in this experiments.

10. Plasma Density Diagnostics : Ha, Hb line Ratio & Ion Saturation Current for Al & Cu electrode
Ha / Hb ratio = 11
Ha Hb
Ha / Hb ratio = 10.5 Cu
OES and probe diagnostics results show plasma density increases with Al electrode than Cu electrode.
To clarify more intensively, results of monatomic fraction are compared with Al and Cu electrode at points of the identical plasma density value.

11. Ion Beam Fraction at the same Ne
However, monatomic fraction still has difference in almost comparable electron density in comparison of that with copper electrode (600W) and aluminum (500W)
Another result of ICP ion source shows less dependency of plasma density on monatomic fraction. 15
This result indicates another important parameter exists on monatomic fraction.
Reference :
7) J. H. Vainionpaa, et al. Rev. Sci. Instrum. 78, (2007).

12. Calculation of Ion Fraction for recombination factor
Modeling Results
- Plasma-facing material (g ) dependency
ne : 1E12 cm-3
Te : 5eV
Pressure : 50 mTorr
Dielectric material
Metal
Definition of recombination factor (g) :
Loss of H atom at the wall
in the particle balance equation
Basically, alumina and copper has very different surface recombination coefficient in consider of effect on ion species in hydrogen plasma
Computational modeling result show lower recombination factor increase monatomic fraction.
Reference
7) Chun-Ku Chen, et al., J. Phys. D: Appl. Phys. 32 (1999)
8) Roberto Zorat, et al., Plasma Sources Sci. Technol. 9 (2000)
9) Yoshikazu Okumura, et al., Rev. Sci. Instrum. 55 (1984) 1-7

13. Conclusion & Future work
Aluminum oxide layer can be deposited on Al electrode and the oxide layer affect increase of monatomic fraction in hydrogen plasma.
Alumina layer on alumina electrode affects increase of electron density as a important plasma parameter on high monatomic fraction in hydrogen plasma.
Lower surface recombination coefficient of alumina layer is more critical parameter on improvement of monatomic fraction in hydrogen plasma.
Although most chamber wall material is made of Quartz in helicon ion source, monatomic fraction depends on electrode material. Therefore electrode surface characteristic is more important on high monatomic fraction in hydrogen ion source.
In future, surface recombination characteristics of dielectric materials operating gas in various temperature could be investigated to increase monatomic fraction in hydrogen ion source.