1. Department of Nuclear Engineering Seoul National University
Characterization
of a Microwave Plasma Source
using Cavity Resonator
Department of Nuclear Engineering Seoul National University
Kim, Hyun Tae
Park, Yong shin
Sung, Choong Ki
Yi, Jae Ryung
Hwang, Yong Seok

2. Abstract
The purpose of this research is to make an optimized rectangular type microwave cavity resonator for effective plasma breakdown and sustaining.  Plasma diagnostic along the cavity size, microwave power and operation pressure was also performed.
Cavity size in ideal conditions was calculated by using HFSS(High Frequency Structure Simulator), because cavity design which is specific to the microwave frequency is important for high efficiency of energy delivery to the cavity from Magnetron. The lengths of each axes are 155, 100, 25mm to make 2,1,0 mode.
However, in the real situation, the cavity condition would be affected by the permittivity of background gas or plasma. The effective permittivities of gas and plasma are different, So the optimum cavity size becomes also different after breakdown. Therefore the cavity size was made to be changeable. By observation of the input power at which the breakdown begins, it is possible to find optimum cavity size before breakdown. And measurement of sustaining plasma density which varies along the cavity size enable us to get the optimum value.
Before breakdown, we could find that, observing the breakdown power along pressure, the results of which shape is like Paschen-curve and the optimum length is 145mm. After breakdown, the plasma density increased along the input power, and also the optimum length was less than 135mm.

3. Motivation of microwave cavity resonator
It is designed for microwave ion beam source.
To accommodate to the existing device, it was made by rectangular type cavity.
For compact ion source, antenna connected to coaxial cable was used.
Coaxial cable
Resonator
Quartz
Microwave in
Plasma Electrode
Bias Electrode
Ground Electrode
Ceramic
Microwave power generator
AX 2115
Power output : 0.125~1.5kW
Frequency :2.45 GHz

4. Rectangular Cavity resonatorb d a
Effective permittivity
<Rectangular cavity>
6mm
m, n, l : mode numbers
By refering to MDC feedthrough antenna
Hole diameter : Antenna diameter
=3:1
Antenna length in cavity : Cavity depth
=2:1
12mm
18mm
<Antenna>

5. Rectangular cavity design by HFSS
d =135mm
d =145mm
d=165mm
d=155mm

6. d =135mm
d =145mm
d =150mm
d=155mm
d=165mm

7. Rectangular cavity design by HFSS
Diameter : 6mm,
center : x=30, y=50
60mm
100mm
Center
X=120mm,
Y=50mm
18mm
XY plane,Z=12mm
25mm
155mm
High E-field is
concentrated in quartz.
ZX plane,
Center of quartz
YZ plane,
Center of quartz

8. Antenna design by HFSS By sweeping in HFSS, it is possible to validate
optimized antenna position.
-> center of Z axis
To enhance microwave coupling,
metal disk was attached.

9. Rectangular cavity resonator design by inventor
assembly

10. Experimental background
Assuming -> perfect E boundary
vacuum
But -> quartz in cavity
various gases or plasma
Cavity optimizing is possible
by varying length d.
How to find the optimum cavity size
1. Before breakdown : breakdown E field
-> Breakdown Power
2. After breakdown : plasma density
-> Langmuir Probe d
Movable cavity door

11. Experimental Results of Breakdown
Breakdown pressure : 0.1~1 [Torr]
Breakdown power : 100~550[W]
After breakdown,
plasma is sustained by 50W
in far lower pressure (10-3 Torr )
Gas : Ar
Pressure measurements
: thermocouple gauge
: ion gauge

12. Microwave plasma discharge mechasinism
Low pressure
Diffusion losses impede the breakdown!
(Elastic collision X)
High pressure
Elastic collisions impede the breakdown!
(diffusion loss X)
Breakdown critrion
:electron energy gain between collisions by field, :Ar ionization energy, :effective collision frequency
:system size, :microwave frequency

13. Langmuir Probe & I-V curve
Surface area of probe(tungsten)

14. Experimental Results of Plasma Sustaining
Plasma density ne increases
along the input power
and is saturated to 1018[m-3]
when the power density <P> is
2.89[W/cm-3].
In the case of Xe
with <P> 1[W/cm-3],
ne [m-3] is possible.1
Xe ionization energy : 1170[kJ/mol]
Ar ionization energy : 1520[kj/mol]
1<Joseph Root and Jes Asmussen, Rev Sci. instrum. 56(8), August 1985>

15. Optimization of microwave plasma cavity
Optimum Cavity length d
: 140~145[mm] < 155[mm]
Effective permittivity
(Ar, air and quartz)
is higher than
ideal permittivity(vacuum).
Optimum Cavity length d
< 135[mm]
After breakdown in the quartz,
Effective permittivity increases.
So, d length of cavity in sustaining plasma is smaller.

16. conclusion
It was possible to make microwave rectangular cavity resonator, and to optimize by varying d length.
Breakdown field is drawn like Paschen-curve in the pressure range, 0.1~1Torr.
Plasma density increases along the input power.
Optimum cavity sizies are different,
because of effective permittivities.
simulation in ideal condition : d = 155mm
experiment before breakdown : d = 140~145mm
experiment after breakdown : d < 135mm

17. Department of Nuclear Engineering Seoul National University
Thank you!
Do you have any questions?