1. at diagnostic position
Effects of Discharge Chamber Length on the Negative Ion Generation in Volume-Produced H- Ion Source
Bongki Jung1, Jeong-jeung Dang1, Yoosung Kim1, Kyoung-Jae Chung2, and Y. S. Hwang1,2
1Department of Nuclear Engineering, Seoul National University, Korea, 2Center for Advance Research in Fusion Reactor Engineering, Korea
Research Highlight
Significant enhancement of H- ion beam current is achieved by increasing the electron temperature in the source region through the reduction of the discharge chamber length by 30% (11 cm  7.5 cm), which is the simplest way to control the electron temperature.
Influence of Discharge Chamber Length on Plasma Properties
Experimental Setup
Electron density increases mostly with RF power as well as pressure
☞ Two discharge chambers with different lengths of 7.5 cm and 11 cm are used.
☞ Diagnostics: Langmuir probe, Laser photo-detachment.
☞ H- beam current measurement with a Faraday cup.
𝑬𝒙𝒕𝒓𝒂𝒄𝒕𝒊𝒐𝒏 𝒓𝒆𝒈𝒊𝒐𝒏
𝑯𝒆𝒂𝒕𝒊𝒏𝒈 𝒓𝒆𝒈𝒊𝒐𝒏
H- density
ne,Te,Vp
Langmuir Probe
Nd-YAG Laser
𝑺𝒉𝒐𝒓𝒕 𝒄𝒉𝒂𝒎𝒃𝒆𝒓 𝒍𝒆𝒏𝒈𝒕𝒉
𝑳𝒐𝒏𝒈 𝒄𝒉𝒂𝒎𝒃𝒆𝒓 𝒍𝒆𝒏𝒈𝒕𝒉
Electron temperature depends only on pressure and geometry, but not on RF power
SmCo magnet
: Cusp Field
In Vac. Cooling Chanel
Quartz Window
RF Anntena V
OSC
Isolation
Amplifier
2cm
1cm
: Dipole Field
Measured Filter Field
at diagnostic position
(200Gauss)
𝑯𝒆𝒂𝒕𝒊𝒏𝒈 𝒓𝒆𝒈𝒊𝒐𝒏
𝑬𝒙𝒕𝒓𝒂𝒄𝒕𝒊𝒐𝒏 𝒓𝒆𝒈𝒊𝒐𝒏
Electron temperature needs to be reduced by increasing the filter magnetic field
Electron temperature is increased by reducing the chamber length
Analysis with Particle Balance Model
Enhancement of H- Ion Generation
H- Density Measurement by PHD
H- Ion Current Measurement by FC
Particle balance equations*
H (N1) 2N2nea3 + 2n2nea4 + n2nea5 + n2N2a6 + n3nea7 + 2n3nea8+ (n1/t1(Te)) + (n3/t3(Te))
- N1nea1 - g(N1/T1) = 0
H1+ (n1) N1nea1 + n2nea5 + n3nea8 - (n1/t1(Te)) = 0
H2+ (n2) N2nea2 - n2nea4 - n2nea5 - n2N2a6 - (n2/t2(Te)) = 0
H3+ (n3) n2N2a6 - n3nea7 - n3nea8 - (n3/t3(Te)) = 0
Charge conservation equation
ti (Te): containment time for each ion species for electron temperature
( i=1, 2, 3 )
T1 : transit time of H atoms across the chamber ( = 4(V/A)/v0 )
g : recombination factor for H atoms at the wall
V/A : volume to surface ratio of the source chamber
p : hydrogen gas pressure
v0 : mean velocity of H atoms
n1 + n2 + n3 = ne
Particle conservation equation
N2 + (1/2)N1 + (1/2)n1 + n2 + (3/2)n3 = p/(kBT0)
Wall loss of H atoms
Based on the result of negative ion density by PHD diagnostics, negative ion density is compared to negative ion beam current.
Negative ion production and extracted ion beam currents increase with short chamber length case, but decrease in lower operating pressure due to relatively higher electron temperature in extraction region.
For long chamber
R = 5 cm, L = 11 cm
 V/A = 3.43 𝑹
Wall loss of ions
Discharge chamber with cusp B-field 𝑳
For short chamber
R = 5 cm, L = 7.5 cm
 V/A = 3.0
Conclusion
* Osamu Fukumasa et al., J. Phys. D: Appl. Phys. 18 (1985)
Shortening the discharge chamber length significantly increases negative ion production and extracted ion beam currents by increasing electron temperature in volume-produced negative hydrogen ion sources.
However, negative ion production decrease in lower operating pressure with the short discharge chamber length due to increase of electron temperature in extraction region as well as heating region.
Further increase of negative ion generation is predicted with strengthening magnetic filter field configuration of the discharge chamber in the extraction region to reduce the electron temperature at the extraction region.
* R Zorat and D Vender 2000 J. Phys. D: Appl. Phys
Comparison between the particle balance model and the probe measurement
The simple model based on the global particle balance predicts the experimental results quite well.
The electron temperature of the discharge chamber with short length is always higher than that of longer one.
The discharge chamber with short length has an advantage for obtaining higher electron temperature at low operating pressure.
The 15th International Conference on Ion Sources (ICIS’13)
September 9-13, 2013, Chiba, Japan