Evaporation Model of The Near Surface Microwave Discharge
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EVAPURATION MODEL OF THE NEAR SURFACE MICROWAVE DISCHARGE Yu.V. Institute Street
BYKUV, A.U. ERIEIAEEV, A.A.SORUKIN of Applied Physics uf (Auademy 46, 60,3600 Gorky, USSR
of
Sciences,
*Uljanov
of on the use applications based Many technological intensive microwave radiation are connected with high the microwave power temperature material processing. (As a rule the material must provide heating of the being absorbed in processed zone up to rather high temperatures ( .> 1000 C ) within fairly small time intervals. "hese requirements are dictated either by the nature of the process ( surface joining ceramics and metals, cutting transformation hardening, provide sufficiently large ) or by necessity to materials process output ( coating of materials, joining of dielectrics, melting, etc.). The upper value uf the exploited radiation intensity is limited by effects of gas breakdown near the treated surface. The considerable part of microwave rad.ation can be absorbed and dispersed by the discharge plasma, hence microwave energy supplied to the material significantly decreases. The experimental values of the Lhreshold intensity of microwave radiation that corresponded to the breakdown near the heated surface at the atmospheric air pressure are smaller by a factor of about 1000 than the threshold intensity of the air breakdown in free space. Such considerable lowering of the threshold intensities is well known in studies on laser interaction with solids and for the laser pulse duration above I msec is fairy well described by so called evaporation model model is this of experimental study show that LI1. The results suitable for the breakdown near the surface of dielectric materials which are exposed to the intensive radiation of a millimeter wave range. As the radiation is absorbed the surface is heated up to the temperatures at which its significant evaporation takes place and in the case the intensity is above a certain threshold value the discharge starts up in the mixture of ambient gas and the material vapor containing the elements with low ionisation potential. In this mode.l the time td of discharge arising is a sum of
the duration
th
of
the
surface
heating
and
the
duration
te
of electron concentrations increase up to the threshold value. In the case of neglecting the heat losses the surface temperature growth Al in the one-dimensional approximation is linked with the radiation intensity I by the A) - It h =1 at
P1
material
> I
(
p
-
absorptivity
of
the
1/2
the radiation
wavelength,
I -- 2(et
h)
-
depth of
heat penetration into the material, M temperature conductivity ). The experimental results indicate the fulfillment of the condition t
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