Experimental Design to Study RF Pulsed Heating Page: 3 of 4
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2.2 Diagnostic Setup
Damage to the metal due to surface fatigue will manifest
itself in degradation of the unloaded Q of the cavity. We
wish to measure this degradation as well as the local
pulsed temperature rise of the surface while high power
is applied to the cavity. Exciting and measuring the
properties of a low-power steady-state TE012 mode in the
cavity allows us to perform such a measurement.
The TE012 mode is excited through a circular aperture
by a waveguide coupler with a width of a WR-42
waveguide and a height of a WR-62 waveguide.
Including the effects of the coupling aperture the
resonant frequency of this mode is 17.811 GHz. The so-
called diagnostic coupler is mounted similarly to the
fundamental mode coupler except it is placed one-fourth
of the cavity length away from the center where the
maximum of the magnetic field for the TE012 mode
occurs. The width of the diagnostic coupler is tapered to
the width of a WR-62 waveguide after a length of 10 cm
to allow the use of available vacuum windows. The
diagnostic coupler is cutoff to the fundamental mode
frequency of 11.424 GHz. After 10 cm, this signal is
attenuated by over 150 dB to ensure no damage occurs to
the diagnostic equipment.
The TE012 mode is designed to be critically coupled
assuming a 10% degradation in the theoretical Q of
21906. As with the fundamental mode, MAFIA was
used to model the endcap groove and radial gap in the
cavity. The resonant frequency of the degenerate TM112
mode is reduced by 200 MHz and all other spurious
modes are at least 150 MHz away. The unloaded Q's of
all these modes are at least 8000.
The measurement of the pulsed temperature rise will
be performed as follows. Before the application of a
high-power RF pulse, the TE012 mode will be set up in
steady state using a frequency generator. The surface of
the cavity will heat up as high power is applied. Since
the resistivity of the surface of the metal will increase
with temperature, the unloaded Q of the mode will
decrease. This Q degradation will result in a change of
coupling over the time of a RF pulse causing a change in
the reflected power seen from the diagnostic port. The
phase of the reflected signal will also change because of
the change in resonant frequency due to thermal
expansions and a changing Q. The local temperature rise
of the surface of the cavity can be inferred from the
knowledge of the amplitude and phase of the reflected
power over time and the variation of the surface
magnetic field.
The diagnostic apparatus used to perform these
measurements is shown in Figure 2. A quadrature IF
mixer is used to measure the amplitude and phase of the
reflected signal over time. A RF switch is also utilized
to allow the measurement of Q between RF pulses when
the effects of pulsed heating have disappeared. This
measurement allows us to determine when the Q getspermanently degraded from surface fatigue. Not shown
in Figure 2 is the ability to use an event-counter to count
the number of RF pulses applied to the cavity and bin
them by power level. The counter will allow us to
determine the number of RF cycles it takes to cause a
certain amount of Q degradation.
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Figure 2: Diagram of diagnostic setup
3 COLD-TEST RESULTS
Some problems have arisen during the cold-test phase of
the experiment. Broadband resonances are noticed while
measuring the reflected signal of the TE012 mode. A
representative spectrum is shown in Figure 3. These
broadband resonances change the apparent coupling to
the TE012 mode as well as its unloaded Q. Accurate
values for these quantities cannot be determined at the
time of this writing.
There are two possible explanations for these
problems. One is the coupling aperture for the
fundamental mode greatly affects the coupling of the
diagnostic coupler to the TE012 mode and perhaps
introduces spurious modes. High transmission (-7 dB) is
measured from the diagnostic coupler to the fundamental
mode coupler and it is known that the signal propagates
in the fundamental TE10 mode in the WR-90 waveguide
at 17.8 GHz.
Figure 3: Frequency spectrum of TE12 mode. Resonant
frequency is approximately 17.83 Gllz.
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Pritzkau, David P. Experimental Design to Study RF Pulsed Heating, report, April 9, 1999; Menlo Park, California. (https://digital.library.unt.edu/ark:/67531/metadc624381/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.