US2778937A - Cyclotron square wave rf system - Google Patents

Description

Jan. 22, 1957 G. B. ROSSI 2,778,937

CYCLOTRON SQUARE WAVE RF SYSTEM Filed April 22, 1954 MAIN 5/ OSCILLATOR SECONDARY OSCI LL ATOR INVENTOR. GUIDO B. ROSS! BY /M4 4% ATTORNEY.

CYCLOTRON SQUARE WAVE RF SYSTEM Guido B. Rossi, Berkeley, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Application April 22, 1954, Serial No. 425,061

10 Claims. (Cl. 250-27) The present invention relates to a particle accelerator and more particularly to an improved radio frequency system for a cyclotron.

The principles of operation of cyclotrons are well known in the art, including those of fixed frequency cyclotrons. Several problems exist which limit the stability 'of particles being accelerated, as Well as the final operating conditions. Most of such problems are related to the phasing between the radio frequency accelerating voltage, the limitations imposed by design parameters, and the angular velocity of the particles.

It has been determined that, by replacing the sine wave radio frequency accelerating voltage with a square wave voltage of the same maximum value, improvements in accelerating conditions result from (a) an average dee voltage increase of and '(b) a change in time distribution of the peak accelerating voltage. Under the former condition (a) the operation improvements resulting from higher dee voltages include an increased ion catching phase, a decreased number of revolutions to the extraction radius,

and a decreased phase defocusing. Under the latter condition (b) the peak accelerating voltage appears through substantially 180 degrees of the radio frequency cycle and additional operation improvements result which cannot be gained simply by increasing the maximum value of a sine wave of acceleration voltage. Such advantages are (l) greater compensation for variations in the angular velocity of the particles, (2) better initial bunching of ions at the start of the acceleration cycle, (3) fewer particle revolutions to the extraction energy, and (4) a possible compensation for relativistic loss of phase when such losses are not large. Also, where dee voltage limitations exist (set by dee to ground discharges), such effects are especially advantageous, since the result is an action to increase maximum beam levels for the available peak values of accelerating voltage.

In spite of the foregoing desirable advantages to be gained, it is not feasible to directly excite the high Q resonant circuits, as associated with the cyclotron, with a square wave. However, to achieve the advantages of a square wave acceleration voltage, the present invention provides for a mixing of harmonics with a fundamental sine wave to produce an approximation of a square wave voltage at the dees. That such mixing of voltages is feasible to achieve the desired waveform will be readily apparent by considering well-known principles of the electrical art. A sine wave used for excitation of conventional cyclotron accelerating electrodes may be represented by the equation:

V=A cos Where V is the value of the instantaneous voltage, A is the value of the peak voltage, and 0 is the ,phase angle. It is well known that the Fourier series gives the equatlon "nited States Patent 0 'paratus for the Acceleration of Ions.

2,778,937 Patented Jan. 22, 1957 of a square wave and so, in similar terms, the equation may be written:

V=A1 cos 0-A cos 30+A3 cos 56 +(1)" An cos (2n--1).0

It has been found that a wave containing the .first two terms of the latter formula approximates a square wave where A2 is made substantially equal to 1/ 9 A1. The first term (A1 cos 0) represents the fundamental sine wave and the second term (-A2 cos 30) represents the third harmonic of such fundamental sine wave. While not affording the maximum benefits of a full square wave, the mixing of the fundamental and third harmonics provides a close approximation to extend the period of the peak voltage by a factor of two or more.

It is therefore an object of the invention to provide an improved radio frequency system for a cyclotron.

Another object of the invention is to provide a square wave accelerating voltage at the does of a cyclotron.

Still another object of the invention is to provide a constant frequency cyclotron characterized by increased average dee voltage, beam intensity, and beam stability.

A further object of the invention is to provide an improved radio frequency accelerating system for a cyclotron wherein harmonics and a fundamental sine wave are mixed to produce a square Wave accelerating voltage.

Other objects and advantages of the invention will be apparent in the following description and claims considered together with the accompanying drawing which is a semi-schematic and sectional View of the invention.

Referring to the drawing in detail there is illustrated a portion of a cyclotron 11 which is similar to that described and claimed in U. S. Patent 1,948,384 issued to E. 0. Lawrence, February 20, 1934, for Method and Ap- Such cyclotron 11 comprises, in general, a vacuum tank 12 disposed between pole faces 13 (one of which is shown) of an electromagnet, which is excited by windings 14, and a pair of dee electrodes 16 and 17 disposed 'within the tank. Various circuits and other elements are necessary for a detailed description of the operation of the cyclotron; however, such elements are conventional and a detailed account thereof is omitted to provide a better understanding of the present invention.

A main oscillator 21 and a secondary oscillator 22, both of conventional design, are provided to furnish radio frequency accelerating voltages for the cyclotron 11. The frequency of the main oscillator 21 is established at 'a fundamental frequency in accordance with the type of particles to be accelerated and the frequency of the secsynchronizing purposes to maintain the proper frequency difierence and phase relationship therebetween.

To suitably couple the outputs of the oscillators 21 and 22 to the dee electrodes 16 and 17 of the cyclotron 11, there are provided a pair of similar radio frequency systerns 26, only one of which will be described in detail. Such a system 26 comprises, in general, a coaxial transmission :line having an inner conductor 27 and an outer conductor 28. At one end the system 26 is connected with the inner conductor 27 tied to the dee electrode 16 through an opening 29 in the tank 12 and the outer condoctor 28 tied to the tank about such opening in a vacuum-tight manner, as by welding. The other endo'f the system 26 is extended, externally of the tank 12, for a distance substantially equal to an electrical length of an odd number of quater-wavelengths at the frequency of the main oscillator 21. The outer conductor 28 at such extremity is sealed to maintain the vacuum-tight condition of the cyclotron 11 while the inner conductor 27 is terminated short of the outer conductor seal and is suitably supported by a disc 31 of conducting material.

A stub 36 is included in the system 26 by electrically connecting a length of conductor 37 at one end to the inner conductor 27 and extending the same transversely through an opening 38 in the outer conductor 28. A conducting sheath 39 having a length greater than the extended portion of the stub conductor 37 is electrically connected about the opening 38 as a vacuum seal and as the outer conductor of the coaxial stub 36. The electrical length of the stub 36 is selected toprovide a capacitive reactance (one quarter-wavelength or less) at the frequency of the secondary oscillator 22 and the point of connection to the inner conductor 27 is selected along the inner conductor from the dee electrode 16 so that the capacitance and a portion of the inductance of the system form a resonant circuit at such frequency.

The two radio frequency systems 26 extend in a substantially parallel manner from the cyclotron tank 12 and are suitably excited by the main and secondary oscillators 21 and 22. To provide such excitation an output terminal 41 of the main oscillator 21 is connected by a radio frequency lead 42 to a first coupling loop 43 extending through the outer conductor 28 of the systom 26 which is connected to the dee electrode 16 be tween the dee electrode and the stub 36. Also an output terminal 46 of the secondary oscillator 22 is connected by a lead 47 to a second coupling loop 48 extending through the outer conductor 28 between the stub 36 and the disc 31. Similarly, a second output terminal 51 of the main oscillator 21 and a second output terminal 52 of the secondary oscillator 22 are connected to coupling loops 53 and 54 of the other system 26 by leads 56 and 57, respectively.

It is to be noted that, while the secondary oscillator 22 has been specified in the foregoing as an essential element as an aid to understanding the invention, such secondary oscillator and auxiliary elements are not required where the inherently occurring third harmonic frequency component of the fundamental frequency exists in the systems 26 with a sufficiently strong peak value. In such instance the presence of the stubs 36 provide a peaking effect at the third harmonic frequency and reflect a voltage at such frequency having a value which mixes with the voltage of the fundamental frequency at the dee electrodes 16 and 17 to provide the desired waveform. Thus it is only where the value of the voltage at the third harmonic frequency is to be controllable that the secondary oscillator 22 is required.

Now with the conditions for operation of the cyclotron 11 established, the main oscillator 21 and the secondary oscillator 22 are energized to respectively establish output voltages at a fundamental frequency and at three times the fundamental frequency with the latter having a peak value sufiicient to impress a voltage at the dee electrodes 16 and 17 which has a peak value one-ninth that of the former. Such voltages separately excite the two radio frequency systems 26 with voltages 180 electrical degrees apart for both the fundamental and third harmonic frequencies.

Considering the fundamental frequency excitation first, it will be noted that the electrical length of the systems 26 and does 16, 17 is established to be an odd number of quarter-wavelengths and that the extremity away from the dee electrodes 16 and 17 is short circuited by the disc 31. The result of such conditions is that a voltage maxima is impressed at the dee electrodes 16 and 17, respectively.

Now, considering the voltage having a frequency three times that of the fundamental frequency, it will be noted that a resonant circuit at the third harmonic frequency has been provided across the coaxial system 26 (the stubs 36) to maximize the voltage at such harmonic frequency in the systems 26. With voltages at both frequencies impressed at the dee electrodes 16 and 17 the resultant voltage between such electrodes is the sum of the two. As has been stated previously, the two oscillators 21 and 22 are interconnected so that the proper frequency and phase relationship is maintained to provide an approximation of a square waveform of voltage between the dee electrodes 16 and 17 with the attendant advantages.

While the salient features of the present invention have been described in detail with respect to one embodiment it will be apparent that numerous modifications may be made within the spirit and scope of the invention and it is therefore not desired to limit the inven tion to the exact details shown except insofar as they may be defined in the following claims.

What is claimed is:

1. In a radio-frequency system for a cyclotron having a pair of dee electrodes, the combination comprising an oscillator developing a voltage with a fundamental frequency component and a third harmonic frequency component having a peak value substantially one-ninth that of the fundamental component, two quarter-wavelength transmission line systems respectively connected to said dee electrodes, and coupling means connected between said oscillator and said systems, each of said systems including means resonant at the third harmonic frequency disposed to maximize the third harmonic component at said dee electrodes and provide a substantially square wave of voltage.

2. In a radio-frequency system for a cyclotron having a pair of dee electrodes, the combination compris ing an oscillator developing a voltage with a fundamental frequency component and a third harmonic frequency component having a peak value substantially one-ninth that of the fundamental component, two quarter wavelength coaxial transmission line systems respectively connected to said dee electrodes, and coupling means connected between said oscillator and said systems, each of said systems including an open-ended coaxial stub having an electrical length establishing a resonant circuit at the third harmonic frequency disposed to maximize the third harmonic component at said dee electrodes and provide a substantially square wave of voltage is impressed between said dee electrodes.

3. In a radio-frequency system for a cyclotron having a pair of dee electrodes, the combination comprising a first oscillator developing a voltage at a fundamental frequency, a second oscillator developing a voltage at three times the fundamental frequency, two transmission line systems singly connected to said dee electrodes, and means connected between said oscillators and systems for coupling the voltages of each oscillator to each of said systems, each of said systems including means for combining both voltages at said dee electrodes to provide a substantially square wave accelerating voltage.

4. In a radio-frequency system for a cyclotron having a first and a second dee electrode, the combination comprising a first oscillator developing a voltage at a fundamental frequency, a second oscillator developing a voltage at three times the fundamental frequency, two short-circuited transmission line systems singly connected to said dee electrodes, and means connected between said oscillators and systems for coupling the voltages of each oscillator to each system, each of said systems being substantially an odd number of electrical quartenwavelengths at said fundamental frequency in length and including means providing a resonant circuit at the frequency of said second oscillator for impressing the voltage of such oscillators at said dee electrodes.

5. In a radio-frequency system for a cyclotron hav ing a pair of dee electrodes, the combination comp-rising a first oscillator developing a voltage at a fundamental frequency, a Second oscillator developing a voltage at three times said fundamental frequency, two coaxial transmission line systems singly connected to said dee electrodes and being short-circuited at a point located an odd number of quarter-wavelengths at said fundamental frequency from such connection, a coaxial stub connected to each of said systems to provide a resonant circuit at the frequency of said second oscillator, and means connected between said oscillators and each of said systems for coupling the voltages of the former to the latter.

6. In a radio-frequency system for a cyclotron having a pair of dee electrodes, the combination comprising a first oscillator developing a voltage at a fundamental frequency, a second oscillator developing a voltage at three times said fundamental frequency, two coaxial transmission line systems having outer and inner conductors singly connected to said dee electrodes, a connecting conductor disposed between outer and inner conductors of said systems at a point located an odd number of quarter-Wavelengths from said dee electrodes at the frequency of said first oscillator, an open-ended coaxial stub having inner and outer conductors respectively connected to the inner and outer conductors of each of said systems, said stub being less than one quarter-Wavelength at the frequency of said second oscillator and being connected to said system to provide a resonant circuit at the frequency of said second oscillator, and coupling means connected between said oscillators and each of said systems.

7. The combination of claim 3 wherein the voltage of said second oscillator is characterized as having a peak value established to provide a voltage of substantially one-ninth of the peak value of the voltage of said first oscillator at said dee electrodes.

8. In a cyclotron, the combination comprising a pair of similar first transmission lines having an outer co-ndoctor and a coaxial inner conductor, said first transmission lines being resonant at the fundamental frequency of the cyclotron and having an open extremity, a dee electrode joined with said inner conductor at said open extremity of each of said first transmission lines, a stub transmission line secured to each of said first transmission lines and electrically connected therewith, said stub transmission line being resonant at the third harmonic of said fundamental frequency and being disposed at such point on said first transmission line as to establish a voltage maxima of said third harmonic frequency at said dee electrode, a first oscillator adapted to excite said first transmission lines at said fundamental frequency, and a second oscillator adapted to excite said stub transmission lines at said third harmonic frequency, said second oscillator being keyed to said first oscillator in such a manner as to maintain the proper phase and amplitude relationships to establish a substantially square wave alternating potential between said dee electrodes.

9. The combination of claim 5 wherein said second oscillator is characterized by providing an excitation having -a peak value substantially one-ninth that of said first oscillator at said dee electrodes.

10. In a radio frequency system for a cyclotron having a pair of dee electrodes, the combination comprising two trans-mission line systems respectively connected to said dee electrodes, means coupled to said systems to provide an excitation voltage having a fundamental frequency component and a third harmonic frequency component, and means included in each of said systems to maximize both frequency components at said dee electrodes to provide a substantially square wave voltage.

22, No. 2, The design of cyclotron oscillators, Backus, pages 84-92.

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