IV. RF OSCILLATOR. MOPA – Master Oscillator Power Amplifier ideology was decided upon as it is known to be the most stable as well as the simplest to invoke. The MOPA system oscillates at the driving frequency with great stability, even under glow discharge conditions. However, because the cyclotron tank circuit possess a high Q, very careful tuning becomes necessary when ensuring maximum power delivery. Other oscillator systems were considered, such as an SEO – Self Excited Oscillator, where active feed back from a pickup loop in the chamber allows for the natural frequency of the tank circuit to be sought out and oscillate automatically. Another advantage of SEO systems is their characteristic to have a very high efficiency. However, self excited systems are very complicated and require utmost care from an experienced radio engineer to prevent unwanted modes of oscillations, known as parasitic oscillations.
Initially a commercial RF generator was intended to be used as the primary RF power source. It is capable of supplying 2.5 kW to a 50W load. The generator operates at a fixed frequency of 13.56 MHz – a standard commercial processing frequency. Initial tuning and tank circuit preparations were conducted with this generator. Thus, as mentioned earlier, adopting the frequency of 13.56 MHz as the operating frequency. However, it became evident that the RF power requirement was not so great, allowing a smaller RF source to be utilized.
Fig.3 General cyclotron RF layout.
The transmatch may be recorded as the most home-brewed system of this cyclotron. It utilizes the lumped capacitance of the DEE, which is approximately 70pF, to create a tank circuit out of the chamber itself. Using the resonance equation for an inductor in parallel with a capacitor:
fr=1/2p Ö (LC)
L, the inductance, was chosen to bring the fr to resonance at 13.56 MHz. Initially, coarse tuning was to create an 8 turn coil of length 5 inches, with a cross sectional area of 2.14 inches2, out of ¼-inch copper refrigeration tubing. The coil was then mounted
Plate.1 Transmatch in RF cabinet.
in an RF tight enclosure known as the RF cabinet. One end of the coil was connected to the protruding DEE stem, while the other end of the coil is connected to chamber ground. Another coil of copper tubing larger in cross sectional area, of only three turns, is mounted co-axially about the first coil. The outer coil constitutes the primary while the inner coil makes up the secondary of the transformer. Adjustable taps were then placed on the primary to locate the 50W loading point. Second order tuning was accomplished empirically. Either expanding or compressing the coil changed the inductance thereby altering fr. Fine tuning was completed at the driving oscillator. Cooling became a necessity when the RF power began to heat the secondary coil such that thermal expansion changed the tank fr. General Electric Dielectrol transformer oil is pumped through the ¼ inch tubing of the secondary. The oil was then passed through a small heat exchanger that is cooled by flowing water. The oil is then returned to the pump reservoir of approximately two gallons volume. No effort was made to measure the cooling rate of the oil. There are future plans to remove the oil-water heat exchanger from the cooling circuit in order to perform calorimetry roughly estimating the tank circuit efficiency.
The Q of an oscillator follows several definitions which are interchangeable. 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 Estored /D Elost
however there are several other definitions:
Q= w L/R = fr/D f
Q of the tank circuit expressed in terms of w L/R assumes no loading. Thereby w L/R is the theoretical maximum Q. For this cyclotron the non-loaded Q was about 1600. The measured Q of the tank circuit is somewhat less due to loading, denoted as QL. Looking at the voltage developed on a capacitve pickup very loosely coupled to the DEE, a sweeping RF signal was injected into the transmatch. The voltage response was then plotted against frequency, shown in Fig.3. Using the Q formula, fr/D f the QL was measured. fr was found to be 13.60 MHz. Because the response is measured in voltage rather than power, D f is measured at 70.7% of the maximum height, which was found to be 90kHz. Thus the QL of the tank circuit was measured to be 150, a very reasonable QL for a tank circuit of this type.
Fig.4 Q measurement of loaded tank circuit.
DEE voltage measurement and pickup calibration were completed with the use of the simple rectifier and voltage divider network illustrated in Fig.5. The high voltage capacitor, denoted as C1, was charged to the peak RF voltage through the rectifier and bled off by the high impedance resistor network. A DMM with a high input impedance was placed across R2 to
Fig.5 DEE voltage measurement circuit.
measure the developed voltage. The ratio of R2 to R1 is 1:9.1E+5, thus the peak DEE voltage is:
VD-peak= 9.1E+5 x Vr2
Voltage measurements were taken as a function of incident RF power, as well as noting the induced peak voltage on the capacitive pickup to gain a simple DEE voltage reference. As expected, the peak DEE voltage rises as the square root of the applied RF power, Fig.6, and the peak induced voltage is linearly proportional to the peak DEE voltage, Fig.7.
Fig.6 DEE voltage as a function of RF power.
Fig.7 DEE capacitive pickup calibration.
When beginning RF testing it was thought that the chamber must be precisely centered in the magnet gap. To ensure centering, shims of equal thickness wedged the chamber in between the poles, snuggly securing the chamber from top and bottom. When testing the RF system and the magnet together it was noted that at high magnetic fields the fr moved down, as plotted in Fig.8. After an investigation it was decided that
Fig.8 fr shift due to chamber compression.
the magnet poles must be attracting one another under the tremendous force, thereby squeezing the lids on the vacuum chamber. The inward movement of the lids would decrease the distance between the DEE and the lids creating an increase in chamber capacitance, thereby bringing down fr. The distance of movement was calculated from the change in frequency. Just using the approximation for a parallel plate capacitor the distance the gap decreased was on the order of 7 nanometers. The attractive force between the two poles was also estimated, at 1 Tesla the attractive force is approximately 16,000 N which the equivalent of placing a 3,500 pound mass on the top yoke. Under such forces it is reasonable to imagine deflection on the order of 70 Angstroms.
It is worth noting that even under maximum ion current conditions of 50 nanoamps no beam loading was noticed.