Multipactor in Low Pressure Gas and in ... - of Richard Udiljak
Multipactor in Low Pressure Gas and in ... - of Richard Udiljak
Multipactor in Low Pressure Gas and in ... - of Richard Udiljak
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<strong>Multipactor</strong> <strong>in</strong> <strong>Low</strong> <strong>Pressure</strong> <strong>Gas</strong><br />
<strong>and</strong> <strong>in</strong> Nonuniform RF Field Structures<br />
RICHARD UDILJAK<br />
Department <strong>of</strong> Radio <strong>and</strong> Space Science<br />
Chalmers University <strong>of</strong> Technology<br />
Abstract<br />
Resonant electron multiplication <strong>in</strong> vacuum, multipactor, is analysed<br />
for several geometries where the RF electric field is nonuniform. In particular,<br />
it is shown that the multipactor behaviour <strong>in</strong> a coaxial l<strong>in</strong>e is<br />
both qualitatively <strong>and</strong> quantitatively different from that observed with<br />
the conventionally used simple parallel-plate model. Analytical estimates<br />
based on an approximate solution <strong>of</strong> the non-l<strong>in</strong>ear differential<br />
equation <strong>of</strong> motion for the multipact<strong>in</strong>g electrons are supported by extensive<br />
particle-<strong>in</strong>-cell simulations. Furthermore, <strong>in</strong> a microwave iris the<br />
electrons tend to perform a r<strong>and</strong>om walk <strong>in</strong> the axial direction <strong>of</strong> the<br />
waveguide due to the <strong>in</strong>itial velocity distribution. The effects <strong>of</strong> this phenomenon<br />
on the breakdown threshold are analysed. The study shows<br />
that the threshold is a function <strong>of</strong> the height-to-length ratio <strong>of</strong> the iris<br />
<strong>and</strong> for a fixed value <strong>of</strong> this ratio, the multipactor susceptibility charts<br />
can be generated <strong>in</strong> the classical eng<strong>in</strong>eer<strong>in</strong>g units. Us<strong>in</strong>g the parallelplate<br />
concept, the multipactor threshold <strong>in</strong> low pressure gases has been<br />
analysed us<strong>in</strong>g a model for the electron motion that takes <strong>in</strong>to account<br />
three important effects <strong>of</strong> electron-neutral collisions, viz. the friction<br />
force, electron thermalisation, <strong>and</strong> impact ionisation. It is found that<br />
all three effects play important roles, but the degree <strong>of</strong> <strong>in</strong>fluence depends<br />
on parameters such as order <strong>of</strong> resonance <strong>and</strong> secondary emission<br />
properties. In addition, a new method for detection <strong>of</strong> multipactor is<br />
presented. By apply<strong>in</strong>g a weak amplitude modulation to the <strong>in</strong>put signal<br />
<strong>and</strong> perform<strong>in</strong>g a fast Fourier transform on the detected signal, accurate<br />
<strong>and</strong> unambiguous measurement results can be obta<strong>in</strong>ed. It is demonstrated<br />
how the method can be used <strong>in</strong> both s<strong>in</strong>gle <strong>and</strong> multicarrier<br />
operation.<br />
Keywords: <strong>Multipactor</strong>, discharge, breakdown, microwave discharge,<br />
nonuniform fields, coax, iris, low pressure gas, detection methods.<br />
iii
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