1. Enhancement of Ion Beam Current with Layered-Glows in Constricted DC Plasma Ion Source
Yeong-Shin Park and Y. S. Hwang
International Conference on Ion Sources 2009
September 21st 2009 Gatlinburg, Tennessee, USA
NUPLEX, Dept. of Nuclear Eng., Seoul National University Gwanak _ 599 Gwanak-ro, Gwanak-gu, Seoul , Korea
PYS_ICIS_CDC-Jun09

2. Abstract
High current mode in operating constricted DC plasma ion source has been discovered and investigated. The constricted DC plasma could be sustained at higher current than in conventional operation mode (lower current) at fixed discharge voltage. Phenomenally, several discrete layered-glows are created between a small anode glow and a cathode glow. The layers are thin and divided by dark spaces in which the charged particle could be accelerated. In this high current mode, ion beam current density is about 100 times higher than in the conventional mode at same voltage. It is noteworthy that lower gas pressure is desirable to sustain the layered-glow mode, which is also profitable for ion source in the view of differential pumping. Ion current density exceeds 300 mA/cm2 at 175 W power where ion density can be estimated over 3.7x1012 cm-3.

3. Previous Work
Constricted DC plasma (hollow anode discharge, plasma ball) : A. Anders, V. Miljevic
Constricted DC plasma based on DC discharge with hollow anode having small area
Relatively high pressure and high voltage (~1kV) and low current density.
Spatial potential profile, anode sheath, double layer, and sheath expansion were measured.
Electron and ion source using the constricted DC plasma was fabricated.
► General design of hollow anode discharge[1]
► Conceptual figure of Plasma Ball[2]
► Spatial distribution of plasma density[1]
[1] A. Anders and S. Anders, Plasma Sources Sci. Technol. 4, 571 (1995), [2] A. Anders and M. Kühn, Rev. Sci. Instrum. 69, 1340 (1998)

4. Motivation and Outline of the Research
High Current Operation Mode
Conventional Constricted
DC Plasma
Contributions
of the
Research
Constricted DC plasma is a simple, RF hazard free and compact ion source.
It is used as a plasma igniter or an electron source as well as an ion source.
The particle beam current density is small and the operating pressure is high.
To increase power efficiency.
To operate constricted DC plasma ion source in high discharge current mode in order to achieve high current ion beam.
Low discharge voltages and low operating pressure would be desirable.
High current density ion beam could be supplied with the enhanced constricted DC plasma ion source.
It is possible to construct DC plasma ion source having high power efficiency.

5. Experimental Setup Gas : H2, (He) Flow rate : 3 ~ 100 sccm
Area of Cathode : 9.6 cm2
Area of Anode : cm2
Area Ratio : 0.33 %
Discharge Gap : 100 mm
Tube Diameter : 35 mm
Discharge Voltage : 0~1 kV
Discharge Current : 0~500 mA
Extraction Voltage : 0~7 kV
Ion beam

6. Helium Discharge shows Characteristics of Conventional Constricted DC Plasmas.
Breakdown condition
Breakdown voltages w.r.t. pd (pressure times gap distance) shows similar trend with Paschen’s curve in conventional DC discharge.
Discharge mode
Discharge current increase with voltage like the abnormal glow regime in a conventional DC plasma.
Due to limited electrode area, current density increase with the discharge voltage.

7. New Operation Mode characterized by High Discharge Current Occurs in Hydrogen Discharge.
1. Low current Mode
Like helium discharge, currents increases as increasing voltage.
High resistance(V/I) mode: 9 k (10 sccm)
Conventional mode in constricted DC plasma.
2. Mode transition
At certain threshold currents, discharge voltages drops steeply.
The transition occurs more easily at higher discharge pressure 1 2 3
3. High current mode
Discharge can be sustained at over 2 times lower voltage than in low current mode.
Low resistance mode: 150  (10 sccm), plasma impedance decreases abruptly.
At fixed voltage, plasma can be sustained at higher current.
This new operation regime has not be reported yet.

8. What Happen in High Current Mode? Multi-Layered Glows are generated.
Conventional Mode
Conventional DC Plasmas consist of an anode glow (plasma ball) and cathode glow (negative glows).
Conventional mode in operating constricted DC plasma.
Layered-Glows Mode
Several layered glows are generated between the plasma ball and negative glows corresponding to the mode transition.
The glows are concave toward the plasma ball and divided by dark space.
The structure of the stacked layers looks like the striation known as a transverse instability in DC glow.
Number of the layered glows increases with discharge current.
Brightness of plasma ball is higher.

9. Structure in Layered-Glows Mode: Plasma Ball, Cathode Glow, Layered Glows, Double Layers.
Dark Space - Double Layer(DL)
high electric field exists.
electrons gain energy efficiently in the DL.
DLs enhance efficiency and performance of the plasma source
Anode
Dark Space

10. Lower Operating Pressure is preferable in Layered-Glows Mode.
Discharge currents are decreased as the operating pressure increases in contradiction to the conventional mode.
Electrons gain energy efficiently in narrow gap between layers in which electric field is very high.
Increment in population of neutral particles results in frequent collisions between electrons and neutrals and remove energy from electrons
Conventional mode
Discharge current increases with increasing operating pressure at fixed discharge voltages.
In conventional mode, neutral particles contribute to enhance possibility of ionization as conventional DC plasma does.
Layered-Glows
Conventional Glow
Note: In layered-glows mode, discharge can be sustained more efficiently at lower pressure, which is desirable in ion source in the view of differential pumping.

11. Drastic Enhancement in Hydrogen Ions Beam Current Density occurs in Layered-Glows Mode.
At fixed power, 30 W
Layered glows are sustained at lower voltage of 375 V and higher current of 80 mA than in conventional mode (760 V, 40 mA)
Ion beam current density is about two times higher in layered-glows mode than in conventional mode.
At fixed discharge voltage, 400 V
Sustaining current in layered-glows mode is about 40 times than in conventional mode at fixed voltage, 400V.
Difference of Current density between two mode is almost two orders of magnitude.
Note: Discharge current, rather than discharge voltage or power, seems as a key to rule ion beam current.

12. Discharge Current has a linear Dependency with Ion beam Current Density.
Discharge current rules ion beam current
Ion beam current increases monotonically with discharge current at fixed extraction voltages of 2.5~4.5 kV.
Extracted ions mainly comes from plasma ball near the extraction hole.
Ion density of plasma, plasma ball precisely, would be increased with discharge current. (At higher discharge current, the plasma ball has higher luminosity.)
Discharge current represents plasma density roughly.
Note: In the plasma ion sources utilizing localized plasma near extraction hole, it has been reported that the ion beam currents are proportional to the currents into electrode in which the extraction hole is located.
Y.J. Kim et al., Rev. Sci. Instrum. 77, 03B507 (2006)

13. Beam Extraction Result at Various Hole Thicknesses Indicates that the Plasma Ball Does Not Intrude into the Aperture
Effect of hole thickness on beam current density indicates that plasma ball does not intrude into anode hole.
Beam current density decreases with increasing aperture thickness at both fixed discharge current and fixed discharge current density (considering sidewall of hole). In this sense, Plasma ball does not intrude into aperture.
Therefore, main discharge mechanism of this plasma has a relation with constricted anode structure not with hollow anode effect.
Hole Thickness
Discharge Condition
1 mm
2 mm
3 mm
At Fixed Discharge Current (400 mA)
306 mA/cm2
29 mA/cm2
6 mA/cm2
At Fixed Discharge Current Density (32 mA/mm2)
125 mA/cm2
18 mA/cm2
5 mA/cm2

14. Summary & Conclusion Summary Conclusion
An mode transition has been observed in constricted DC plasma.
Before the mode transition, the plasma is operated in conventional mode. - consist of anode and cathode glows → conventional glow - high impedance (several k), low discharge current
Multi-layered glows are generated after the mode transition. (layered-glows mode) - low impedance (a few of hundred ), high discharge current - The layered-glows mode can be sustained by lower voltages at lower pressure.
Beam current density is two orders of magnitude (or two times) higher in the layered- glows mode than in the conventional mode at fixed discharge voltage (or fixed power).
Discharge current controls the extracted ion beam current and indicates plasma density.
Plasma ball does not intrude into hollow anode.
Conclusion
Utilizing layered-glows mode, constricted DC plasma can be sustained at higher discharge current with higher power efficiency in lower operating pressure.
Higher current density ion beam can be achieved with this layered-glows in constricted DC discharge.

15. Enhancement of Ion Beam Current with Layered-Glows in Constricted DC Plasma Ion Source
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