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

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

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

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

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.

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.

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

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

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

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.

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.

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

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

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.

Thank You !

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