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               Pansak Kerdtongmee, Mudtorlep Nisoa, Thammanoon Srinum,
                       Praiwan Kerdtongmee and Jirapong Galakarn
Experimental Physics Research Unit, School of science, Walailak University
222 Taiburi, Tasala, Nakhon-si-thammarat, 80160, Thailand


Cylindrical concentric plasma is produced by microwave power system, developed from
microwave oven. High voltage power supply is modified so that magnetron will generate
continuous microwave and output power can be adjusted from 0-700 watt. The microwave
field is transmitted through waveguide and stub tuner into cylindrical concentric cavity to
produce plasma. The concentric volume is formed by two cylindrical chambers of difference
diameter. The large one is a stainless steel where as the smaller is pyrex tube. Impedance
matching is controlled by 3-stub tuner and plunger. Plasma production at various microwave
power, gas pressure and plasma column length were studied by single Langmuir probe.

        Ion beam in an important technology for advance material processing such as surface
modification, semiconductor fabrication, solar cell fabrication and genetic engineering.
Microwave induced plasmas generally have a high electron kinetic temperature and can be
operated over a wide range of gas pressure, ranging from atmospheric pressure, down to 10-6
Torr for some ECR microwave discharge. Because of their higher electron kinetic
temperature and lower pressure, microwave discharges are capable of providing a higher
fraction of ionization than DC or low frequency RF discharge [1]. Microwave plasmas do not
have a high voltage sheath, which can induce ion sputtering of the wall. These advantages
have been neglected because of difficulty in microwave cavity and high cost of the system
[2]. Thereby, considerable knowledge of microwave techniques are required to procure and
assemble an adequate system.
        In this paper, we have developed simple, safe and low cost of microwave system
modified from microwave oven. The plasma will provide uniform distribution surrounding
cylindrical concentric cavity effectively.

       For study of plasma production in laboratory, microwave power supply have modified
from that of microwave oven which consist of magnetron tube and high voltage power
supply. The microwave radiation of 2.45 GHz is generated by magnetron, where electrons are
emitted from heated filament and accelerated by high voltage DC electric field through
permanent magnetic field in the vacuum resonant cavity of cathode and anode, typically over
4000 volt. The magnetron will begin radiated power out at threshold voltage. Consequently,
the voltage when higher than that threshold seem become into current for increasing
microwave power. However, the microwave power from microwave oven is discontinuous
and fixed at rated due to voltage-double power supply features. The high voltage source
which able to supply continuous and variable voltage over 4000 volt required of two
transformers, high voltage bridge rectifier, high voltage capacitor and phase control device.
Circuit diagram of modified power supply that is shown in Fig.1

                                                         Fig.1 Circuit diagram of high voltage bridge
                                                         power supply for magnetron tube

        One of the materials that is easily to made on transmission system is copper. The field
radiated from magnetron which is direct coupling at centering-end side of waveguide is
automatically TM mode. The waveguide configurations is characterized to support the wave
propagation of TM01 mode

                                                        Fig.2. TM01 field pattern
                                                        inside the square waveguide

The field is propagated uniformly inside symmetrical 9 cm square cross section guide shown
in Fig.2. Major advantage of that is formed with a fixed as required dimension easier than an
expensive cylindrical copper tube.TM01 mode field is radiated to TM011 mode cavity directly
because pattern field of both are same features.
        Matching device configurations shown in Fig.3 is the 3-stub tuner. It used to change
the waveguide characteristic impedance by vary feeding length of stub as changing LC
equivalent of waveguide. Maximum power is transferred to concentric resonant chamber.

                                                               Fig3. 3-stub tuner
                                                               (a) schematic configuration
                                                               (b) device setup

                 (a)                             (b)
        The concentric plasma required uniform field distribution inside the reactor wall thus
field pattern which appropriated to work on cylindrical geometric chamber due to the
direction of propagation is TM011 mode. The field pattern that illustrated in Fig.4 is obtained
the dimension of resonant cavity.
                                                       Fig.4. TM011 field pattern
                                                       inside the cylindrical cavity

Direct coupling magnetron resulting microwave power is sent into concentric plasma load

        The experimental apparatus shown in Fig.5 includes of microwave generator,
waveguide, stub tuner and concentric chamber. The concentric chamber constructed of two 18
cm length coaxial cylinders with difference diameter. The outer is 10 cm diameter stainless
steel and another one is 8 cm diameter pyrex tube. The concentric cavity have space of 1 cm
and 2x10-2 Torr base pressure by single rotary pump. Microwave power which radiated from
air cooled magnetron is transmitted pass waveguide and stub tuner into concentric cavity to
produce plasma from outside of pyrex tube until inside of outer shell. Moreover, plunger at
inside of the pyrex tube usually roughing slide for staring plasma glows at various argon
pressure. This generated plasma can be fine tune for the best absorbing power by 3-stub tuner

                      Fig.5. Experimental apparatus consists of microwave
                      generator, waveguide, 3-stub tuner and resonant cavity

Single Langmuir probe with outer diameter of 0.2 mm (Tungsten wire) is attached on the end
plate and the center of the chamber for studying production of plasma.

       The plasma density(np) have studied on various argon pressures(PAr), plasma column
length(L) and microwave power(Pmw). The result of plasma density investigation on various
argon pressure with 40 watt of microwave power shown in Fig.6(a). The dependence of np on
PAr is opposite direction that is the density is decreased when the pressure is increased.
Fig.6(b) illustrated variation of plasma density on plasma column length, the graph show that
np is vary on L and np is highest at around 6-cm of L.



                                                     Fig.6 Dependence of np on
                                                     (a) PAr with 40 W of Pmw
                                                     (b) L with 40W of Pmw and
                                                     (c) Pmw at various PAr

The last experimental result of plasma density, np is depend on Pmw where variation as same
way shown in Fig.6(c).From that, the np is greater than 1011 cm-3 with 30 watt of Pmw

       Microwave generator which developed from microwave oven is capable for plasma
generation without directional coupler and circulator. However, the delivered power is
displayed on calibrated input volt-meter. The generator is effective to glows plasma for np >
1011cm-3 with Pmw less than 50 watt. The system is easy to control on working pressure and
low power losses at copper waveguide because it has room temperature consequently, the
system have no required to used of quartz window. However, to produce high density ion
beam, more detail study on cylindrical concentric microwave plasma is need for optimum the
plasma source continuously.

1. J. R. Roth, Industrial Plasma Engineering, IOP Publishing Ltd, (1995), 240 pages.
2. L.G. Meiners and D.B. Alford, Rev.Sci.Instrum., Vol.57(1986), pp.164-166
3. D.M. Pozar, Microwave Engineering, (John Weiley & Sons, Inc.,1998), 345 pages.

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