Oscillator For A Cyclotron Having Two Dees

Abstract

An oscillator for a cyclotron having two accelerating electrodes which are not connected together. The oscillator comprises two electron tubes, of which the cathodes are connected together by a low-loss impedance (capacity or inductance). As a result of this the oscillator oscillates either with the two accelerating electrodes in the push-pull mode while excluding the parallel mode, or in the parallel mode while excluding the push-pull mode.

Description

The invention relates to an oscillator for a cyclotron comprising two accelerating electrodes, an envelope surrounding the two accelerating electrodes, two wave tubes each having an inner conductor and an outer conductor which are shortcircuited at one side, of each of which on the other side the inner conductor is connected to one of the accelerating electrodes and the outer conductors of which are connected to the envelope, two coupling impedances for the two wave-tubes and an electron tube.

Such an oscillator is known, for example, from the article "The Design of Cyclotron Oscillators" in "The Review of Scientific Instruments", vol. 22, nr. Feb. 2, 1951, pp. 84-92.

In a cyclotron having two accelerating electrodes, sometimes termed Dees, the high-frequency voltages at the Dees must have a phase difference of 180.degree. relative to each other if atomic particles in the cyclotron are accelerated by an electric field between the two Dees mutually. In that case the Dees cannot be connected together. If atomic particles are accelerated by an electric field between each of the two Dees and a so-called dummy Dee present between said two Dees, a phase difference of 0.degree. is sometimes necessary between the high-frequency voltages at the dees. It is often desirable not to connect the Dees together, although it would be possible in this case, so as to enable rapid commutation to a phase difference of 180.degree..

A Dee with its associated wave tube, may be considered as a parallel circuit having an inductance L and a capacitance C. It is to be noted that the term wave tube is to be understood to mean herein a piece of high-frequency conductor of the type which is sometimes termed coaxial, although this does not imply either an exact coaxial state or a constant or a circular cross-section. In a cyclotron having two substantially identical Dees which are not connected together, the system consisting of the two Dees with associated wave tubes may be considered as two identical parallel circuits which are directly connected on one side and are connected together on the other side with the interconnection of a capacitance C' which is small relative to the capacitance C. The capacitance C' is the capacity between the two Dees. In the cited article it is calculated that such a system which is consequently has the same equivalent circuit diagram as a band filter, has two resonant frequencies, namely angular frequencies of the values (LC).sup.-.sup.1/2 and [L(C + 2C')] .sup.-.sup.1/2. At the first-mentioned resonant frequency the voltages across the two circuits are in phase and there is no voltage across C', and at the last-mentioned resonant frequency, the voltages across the two circuits are 180.degree. out of phase and there is a voltage across C'. It is obvious that, because C' is much smaller than C, both resonant frequencies do not differ much and that for accelerating particles the system must be compelled to resonate with one of the two resonant frequencies while excluding the other.

This first way of the resonating is termed parallel mode and the second way is termed push-pull mode.

In order to compel a system to resonate in the desirable mode, it is incorporated, in the described known oscillator in which a phase difference of 180.degree. is necessary, in a circuit in which one wave tube is coupled to the cathode and the other wave tube is coupled to the anode of a triode. In a correctly proportioned circuit the voltages of the cathode and the anode are exactly 180.degree. out of phase. The known oscillator circuit is furthermore proportioned so that the voltages at the Dees are also exactly 180.degree. out of phase.

However, the correct proportioning of such a circuit is very difficult, in particular due to the large dimensions which cyclotron oscillators have in connection with the required power. An additional complication is that the cathode side and the anode side of the circuit must be constructed differently in connection with the different output impedances which a triode shows at the cathode and anode, while at the same time the phase shift in both parts of the circuit must be the same.

It has therefore been found in the known oscillator that difficulties often occur with respect to the maintenance of the correct mode of oscillation, in particular if the frequency must be adjustable over a certain range.

It is the object of the invention to provide an oscillator for a cyclotron which is compelled, by a special circuit, to oscillate in a distinct mode and in which the adjustment of the frequency within a certain range is possible.

According to the invention, an oscillator for a cyclotron comprising two accelerating electrodes, an envelope surrounding the two accelerating electrodes, two wave tubes each having an inner conductor and an outer conductor which are shortcircuited at one end, of each of which at the other end the inner conductor is connected to one of the accelerating electrodes, and the outer conductors of which are connected to the envelope, two coupling impedances for the two wave tubes and an electron tube, is constructed so that the oscillator comprises a second electron tube, each electron tube being connected as an oscillator with a wave tube, an accelerating electrode and a coupling impedance, a lowloss impedance being connected between the cathode of the first electron tube and the cathode of the second electron tube, and being proportioned so that the oscillator oscillates with the two accelerating electrodes either in the push-pull mode while excluding the parallel mode or in the parallel mode while excluding the push-pull mode.

The invention is based on the recognition of the fact that, if the circuit with the electron tubes oscillates in the parallel mode, the impedance connected between the cathodes passes no current and hence is inactive because in that case there is no voltage difference between the two cathodes. In the push-pull mode on the contrary there is a voltage difference between the cathode and hence the impedance is active. The circuit arrangement is furthermore proportioned so that either the operation of said impedance is necessary to satisfy the oscillation condition, or the inoperation of the impedance is necessary to satisfy the oscillation condition.

A favorable embodiment of an oscillator according to the invention which is to resonate in the push-pull mode is such that the low-loss impedance is capacitive and proportioned so that the operation of the oscillator in the parallel mode is excluded.

It is to be noted that from the U.S. Pat. No. 2,701,304 an oscillator is also known for a cyclotron having two Dees which comprises two electron tubes (triodes). In this case also a measure is taken to maintain the correct mode of oscillation. In this oscillator, however, a wave tube is used having one outer conductor and two inner conductors which are each connected to a Dee and coupling loops are used which only comprise magnetic field lines in the case of the correct mode of oscillation. This construction is restricted to the coupling by means of loops in the wave tube, whereas the oscillator according to the invention can also be constructed with the structurally very simple capacitive coupling.

In order that the invention may be readily carried into effect, one embodiment thereof will now be described in greater detail, by way of example, with reference to the accompanying drawing, the sole FIGURE of which shows an embodiment of an oscillator according to the invention with a capacitive coupling. For clarity, the Dees and the wave tubes are shown slightly diagrammatically. The oscillator shown as an example is to oscillate in the push-pull mode.

Referring now to the drawing, two Dees (accelerating electrodes) 100 and 200 are arranged in an envelope 2 and have an accelerating gap 1 between them. The dee 100 is connected to an inner conductor 101 of a wave tube 104. An outer conductor 102 of the wave tube 104 is connected to the envelope 2. A shortcircuit 103 which is slidable determines the operative length of the wave tube 104. The dee 200 is connected to a wave tube 204 in quite the same manner.

High-frequency energy is supplied to the dee 100 via a connection 105 in the wave tube 104 by a part 125 of the oscillator circuit. High-frequency energy is supplied in the same manner to the dee 200 via a connection 205 in the wave tube 204 by a part 225 of the oscillator circuit. The parts 125 and 225 of the oscillator circuit furthermore have a common connection 3 to the envelope 2 which is connected to earth at 5.

The part 125 of the oscillator circuit is accommodated in a housing 122 which is connected to earth and comprises as an electron tube a triode 110 having an anode 107, a grid 108 and a cathode 109, furthermore a coupling capacitor 124, a feedback capacitor 111, a cathode circuit 126 consisting of a coil 112 and a capacitor 113, a grid capacitor 118, a grid resistor 119, a choke coil 116 for an anode supply (not shown) which is connected between 120 and earth, a capacitor 114 and a connection 123 which is connected to the housing 122 and is connected to earth with said housing. A filament supply (not shown) for the cathode 109 is connected between 121 and earth.

The part 225 is connected in quite the same manner and is provided with reference numerals which are one hundred larger. The proportioning is also the same as that of the part 125. The capacitors 114 and 214 are connected together by means of a connection 4.

As shown in the drawing, the parts 125 and 225 of the oscillator circuit are each individually connected as an oscillator. An impedance between the anode 107 and the cathode 109 is constituted by the feedback capacitor 111. An impedance between the grid 108 and the cathode 109 is constituted by the cathode circuit 126; in this connection it is assumed that the grid capacitor 118 forms a shortcircuit for high-frequency currents. An impedance between the grid 108 and the anode 107 is constituted by the input impedance, between the connection 106 and earth, of the wave tube 104 connected in series with the coupling capacitor 124 and a coaxial transmission line between 105 and 106. It should be noted that the coil 112 is manufactured from hollow pipe in the interior of which the connection 115 is provided. In this case the connection 115 with the coil 112 constitutes a coaxial choke coil. If this is not done, the connection 115 must be constructed as a separate coil to prevent the cathode 109 from being shortcircuited to earth for high-frequency currents via the filament supply. In addition it is to be noted that the grid capacitor 118 and the grid resistor 119 together ensure a negative grid voltage and that the choke coil 116 should be proportioned so that the influence of the same can be neglected.

It is known from elementary oscillator theory that a circuit having a triode -- if between anode and cathode a capacitive impedance is present which is the case here in connection with the feedback capacitor 111-(211)-- can oscillate only if between the cathode and grid a capacitive impedance is also present and between the grid and the anode an inductive impedance is present.

The parts 125 and 225 of the oscillator circuit shown are proportioned in accordance with the invention so that the cathode circuits 126 and 226 each have a resonant frequency which lies slightly above a frequency at which the overall oscillator circuit is to oscillate. In addition the proportioning is so that circuits which are constituted by the circuits 126 and 226 with capacitors 114 and 214, respectively, connected parallel thereto, (so if the connection 4 would be connected to earth) each have a resonant frequency which lies slightly below the frequency at which the total oscillator circuit is to oscillate.

Both resonant frequencies of the system of the dees 100 and 200 in the envelope 2 and the wave tubes 104 and 204, namely in the push-pull mode and in the parallel mode, lie very close together and are adjusted by means of the slidable shortcircuits 103 and 203, such that the resonant frequency of the push-pull mode lies slightly above the frequency at which the total oscillator circuit is to oscillate. Actually, the impedance between the anode and the grid (earth) must be inductive. Furthermore, said two resonant frequencies lie between the two resonant frequencies mentioned in the preceding paragraph.

With this proportioning the total oscillator circuit can oscillate only in the push-pull mode. In that case, actually, the connection 4 has earth potential, the capacitor 114 is parallel to the capacitor 113, and a capacitive impedance is present between the cathode and the grid of the triodes 110 and 210, which is necessary for oscillation. In the parallel mode, the cathodes 109 and 209 have the same voltage and the capacitors 114 and 214 are inoperative, as a result of which an inductive impedance is present between the cathode and the grid of the triodes 110 and 210 and no oscillation is possible.

It is to be noted that the capacitors 114 and 214 could be combined to form one capacitor. From a point of view of manufacture and equal construction of the housings 122 and 222, however, it is convenient to use two capacitors. It is to be noted in addition that in the circuit described the triodes 110 and 210 operate with a grid which is earthed for high-frequency currents so that no neutrodynisation is necessary. Of course, the invention is not restricted to this type of circuit.

All kinds of variations of the circuit are possible, for example, by using inductive impedances instead of the feedback capacitors 111 and 211, or by using inductive coupling with coupling loops in the wave tubes 104 and 204.

Inductive coupling also makes it possible to cause the dees 100 and 200 to resonate in push-pull, while the triodes 110 and 210 operate in the parallel mode. Instead of the capacitors 114 and 214 an inductive impedance would have to be used in this case. All these variations fall within the characterizing feature of the invention that the cathodes 109 and 209 are connected by an impedance which is not operative in the parallel mode (of the electron tubes) but is operative in the push-pull mode.

In the circuit chosen as an example, two transmitter tubes (triodes) are used which have an amplification factor of approximately 30. The circuit arrangement oscillates at a frequency of approximately 16 MHz. The values of the most important components are as follows: feedback capacitor 111 (211): approximately 100 pF. coil 112 (212): approximately 70 nH. Capacitor 113 (213): approximately 900 pF. Coupling capacitor 124 (224): approximately 200 pF. Capacitor 114 (214): approximately 1000 pF.

the transmitter tubes convey an anode voltage (relative to the grid earthed for high-frequency currents) with an effective value of approximately 7 kV and an anode current with an effective value of approximately 10 A.

The voltage between the connection 4 and earth can be used as a signalling and, possibly, safety of the correct mode of oscillation. This voltage is zero in the push-pull mode of the triode and equal to the cathode voltage in the parallel mode.

Claims

1. An oscillator for a cyclotron comprising two accelerating electrodes, an envelope surrounding the two accelerating electrodes, two wave tubes each having an inner conductor and an outer conductor which are shortcircuited at one side, of each of which on the other side the inner conductor is connected to one of the accelerating electrodes and the outer conductors of which are connected to the envelope, two coupling impedances for the two wave tubes and an electron tube, characterized in that the oscillator comprises a second electron tube, each electron tube being connected as an oscillator with a wave tube, an accelerating electrode and a coupling impedance, a low-loss impedance being connected between the cathode of the first electron tube and the cathode of the second electron tube, and being proportioned so that the oscillator oscillates with the two accelerating electrodes either in the push-pull mode while excluding the parallel mode

2. As oscillator as claimed in claim 1, characterized in that the low-loss impedance is capacitive and proportioned so that the oscillator oscillates with the two accelerating electrodes in the push-pull mode while excluding the parallel mode.