12 INCH CYCLOTRON RF TEST AND MEASUREMENT PAGE
Initial tests on the RF system outlined on the 12-Inch RF System page were of resonance and power loss mechanisms.
Previous experience with the 9-inch prototype put us in the ball park of 15 Mhz without any strenuous calculations or measuremnt. Resonance was very first inadvertantly precisely measured by the many iterations of empirically locating the 50-Ohm
loading points of the primary (outter coil).
Fig.1 Close up of tank cicuit inductor and coupling loop (outter coil).
Once the coil was within 20% of the desired frequency, "fine" tuning was also completed by empirical iterations
of adjustment and measurement. Not much more effort was expended once within a few percent of desired operating
frequency as the frequency source has precision frequency adjustment.
The measurement of the tank circuit performance, Q for quality factor, was characterized without using high levels of RF power through the use of a network analyzer. Q is a unit less number describing the quality of an oscillator. Perhaps the best way to think of Q is the ratio of stored energy over lost energy per cycle:
Q = D
Fig.2 S-21 transmission measurement of tank. Q-loaded > 150.
The test arrangement was straight forward, the 50-Ohm source of the network analyzer was connected to the 50-Ohm input of
RF matching circuit, the same port that the power amplifier would normally be connected to. The input to network
analyzer was connected to the capacitive pickup located on the cyclotron chamber wall. A 50-Ohm termination resistor
was placed in parallel with the pickup in an attempt to properly match. The cyclotron chamber was evacuated to ensure the most precise measurement. Because the mutual inductance coupling constant is unkown, as well as the capacitive
coupling constant being unkown, this measurement can only yield the Q of the entire system, or Q-loaded.
Fig.3 Close up of capacitive pickup located behind the DEE.
The measured Q-loaded was on the order of 150, which is quite high. The theoretical Q of the tank circuit excedes 1000.
A high Q has an interesting implication: as mentioned earlier Q can be thought of as energy lost per cycle, thus
it leads to the inverse situation of how much energy can be put into the resonant circuit per cycle. This is a measurement
of the "filling time."
This measurement does not yield any more information than the transmission measurement
with the network analyzer, however, it may help the reader to visualize the meaning of Q.
Fig.4 Tank circuit "ringing" after driving signal is removed.
Using the "burst" option of an HP8165A, only 20 cycles of the resonant frequency were applied to build up
the RF power to some non-plateaued level, then the tank cicuit was allowed to "empty out." Another plot of
the same measurement, only of longer duration, shows that the tank circuit "rings" for a long time after the
driving frequency is removed.
Fig.5 Same as Fig.4 with expanded time base.
(Note the approximate 2-cycle phase delay in the tank circuit response, this is from cable length)
Fig.6 DEE Voltage (peak-to-peak) as a function of
forward RF power.
We used a Tektronixs P6015 High Frequency, High Voltage resistor divider probe attached directly to
the DEE stem to make a direct measurement of the peak voltage on the DEE. The peak voltage above
ground potential is just half the value. In other words, the maximum DEE voltage above ground is
approx 4,500 V with 400 watts of input power, but the ion recieves a 9,000 V "kick" per revolution (this is
because it crosses the gap twice per revolution, 180 degrees apart of course, utilizing both polarities of
the cycle. The capacitance that the probe introduced was 3 pF in parallel with the DEE's 70.1pF,
in principle, the resonant frequency should shift down by 2%, and indeed the resonant frequency shifted from
15.061 MHz to 14.777 MHz. This provided a nice check that the probes capacitance was indeed in parallel
with the DEE and not located "inside" the lumped inductance, thus ensuring a true division of the peak DEE
voltage. In addition to the measurement of the DEE voltage, a corresponding measurement of the peak-to-peak
voltage of the capacitively couple chamber RF pickup was made and recored to serve as a routine reference
of the DEE voltage. However, this calibration is sensitive to the geometery and proximity of the DEE. If
any movement or change of the DEE occurs, the calibration is no longer valid.
Fig.7 RF Pickup Calibration for quick
DEE voltage Reference.
Further measurements, such as optimization, will be added as the measurements are made. If you have any questions or comments please e-mail us.
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