1. Seok-geun Lee, Young-hwa An, Y.S. Hwang
The study on plasma potential profile for efficient negative hydrogen ion extraction in TCP negative ion source
Seok-geun Lee, Young-hwa An, Y.S. Hwang
August, 2008
Department of Energy Systems Engineering
Seoul National University

2. Outline Schematic of TCP ion source
Previous results and research motivation
previous work
motivation
Experiment Result
Experimental setup
Probe diagnostic for plasma parameter
- plasma temperature, density and potential
Beam extraction experiment
Summary & Conclusion

3. Schematic of TCP ion source
Volume production source
H- ion production reaction
1.High temperature electron region
Vibrational excited H2
e(~20eV) + H2 → e + H2(v”)
2.Low temperature electron region
Dissociative attachment
e(<1.0eV) + H2(v”≥ 5) → H- + H
Electron Detachment
H- + e(>1.0eV) → H + 2e
Importance of plasma density
and electron temperature control
Filter field
External
RF antenna
Dipole magnet
Bias electrode
Primary ionization region
H- formation region
Filter magnet
H- ion
Virtual Filter Magnet
Removal of high temperature electron
to create low temperature electron region
Suppression Dipole
Removal of electron from H- beam

4. Advantage of TCP hydrogen negative ion source
Beam extraction for 30min.(RF power:1kW)
Characteristics of H- ion source using RF TCP plasmas
Volume production H- ion source
Longtime operation by RF plasma source not filament
No contamination by external spiral antenna
Cs free H- ion source
Other negative hydrogen ion sources
Characteristic of beam in TCP ion source
Peak power efficiency : 1.2mA/kW
Beam current : 1.67mA
beam current is relatively low
but beam current per RF power is high

5. Beam extraction with various plasma conditions
Beam current by changing RF power, filter field strength
Beam current by changing gas flow in high filtering
As RF power is raised up, H- beam current increase. At the high RF power, H- beam current increase by increasing filter field. In high filter field condition, although plasma density reduction was high, reduction of electron temperature was sufficient with high filtering. As a result, in high RF power and high filter field condition, H- beam current is maximum value.
Beam current cannot be increased indefinitely by increasing RF power Because O-ring seal near air-cooled quartz window cannot survive with high RF power.

6. Optimum bias voltage by filter field strength at radius 4cm bias electrode
Bias electrode radius : 4cm.
Potential profile changes due to bias electrode voltage is affected by filter field strength and filter field profile. As filter field strength increases gradually, optimum bias voltage decreases.
At high filter field strength, optimum bias voltage is 20V while optimum bias voltage is 40V at low filter field strength.

7. Filter field profile on bias electrode
Electrode size : radius 4cm
Filter magnetic field in x axis S
x axis
Discharge chamber
Bias electrode
Chamber radius : 5cm N
x axis
Bias electrode radius : 4cm.
Electron moves along the filter field. At radius 4cm bias electrode, it is highly possible that filter field line moves to bias electrode.
(a) (b)
Schematic of filter field distribution on bias electrode by bias electrode size
(a) Radius 4cm bias electrode (b) Radius 2cm bias electrode

8. Filter field simulation
Maxwell 3D simulation
Radius 4cm
Radius 2cm
Filter magnetic field in x axis
Filter magnetic field in y axis

9. Experimental setup Experimental Condition
RF power : 1kW
Flow rate : 2sccm (pressure ~3mTorr)
Bias electrode size : radius 2cm, 4cm
Bias voltage : 0~30V (5V)
8cm
4cm
Bias electrode radius:4cm Bias electrode radius:2cm

10. Probe diagnostic result
Plasma potential
Electrode size : radius 4cm
Electrode size : radius 2cm
As bias voltage is raised up, plasma potential increases when bias electrode with the radius of 4cm is used. On the other hand, plasma potential is maintained to be 7V when the electrode with the radius of 2cm is used.
As reducing bias electrode size, filter field does not insect to bias electrode. And plasma potential is independent to bias voltage.

11. Probe diagnostic result
Plasma density
Electrode size : radius 4cm
Electrode size : radius 2cm
As bias voltage is raised up, plasma density is almost uniform with radius 4cm bias electrode.
But as bias voltage is raised up, plasma density increases with radius 2cm bias electrode.
When bias electrode radius is 2cm, plasma potential is lower than bias voltage.
Then, more ions remain in bulk plasma.

12. Probe diagnostic result
Plasma temperature
Electrode size : radius 4cm
Electrode size : radius 2cm
Electron temperature is controlled by filter field Bias electrode size is not affect to plasma temperature.
At 6cm from bias electrode, electron temperature decrease gradually. And electron temperature is below 1eV at 2cm from bias electrode

13. Beam extraction result
Beam current by electrode size
Current density at bias electrode
At bias electrode radius 2cm, more beam current extraction and lower optimum bias voltage.
beam extraction efficiency is improved by reducing electrode size.
Current density of bias electrode radius 2cm is higher than that of bias electrode radius 4cm. It shows that plasma density is high in front of bias electrode at radius 2cm.
Same experiment condition RF power: 1kW
Flow rate: 1.6sccm
Beam energy: 1.3keV

14. Summary & Conclusion Summary Conclusion
▪ Optimum bias voltage for maximum H- beam currents depends on plasma potential profile in front of extraction hole .
▪ Potential profile change due to bias electrode voltage is affected by filter field strength and filter field profile .
▪ At high filter field strength, optimum bias voltage is 20V while optimum bias voltage is 40V at low filter field strength.
▪ As bias voltage is raised up, plasma potential increases when bias electrode with the radius of 4cm is used.
▪ Plasma potential is maintained to be 7V when the electrode with the radius of 2cm is used .
▪ Optimum bias voltages for maximum H- beam currents dropped from 21V to 17V with the smaller bias electrode
▪ Beam currents with the smaller bias electrode also increase 1.33 times compared to the large bias electrode with the radius of 4cm.
Conclusion
▪ To obtain appropriate plasma potential structure in front of the extraction hole, filtering field line needs to be in parallel with the bias electrode to keep plasma potentials from following bias voltage.
▪ With reducing bias electrode, we obtain an effect that filter field is parallel with the bias electrode.

15. Thank You !

16. Production of H- ion H- ion production reaction
Filter magnetic field in z axis
H- ion production reaction
1.High temperature electron region
Vibrational excited H2
e(~20eV) + H2 → e + H2(v”)
2.Low temperature electron region
Dissociative attachment
e(<1.0eV) + H2(v”≥ 5) → H- + H
Electron Detachment
H- + e(>1.0eV) → H + 2e
Importance of plasma density
and electron temperature control
High temperature electron region
Two factor for beam extraction
Plasma condition
RF power, gas flow
Potential profile
filter magnet, bias electrode
Low temperature electron region