From Current Literature

The First Operating Synchrotron at 8 Million Electron Volts

F. K. Goward and D. E. Barnes1 have built a small synchrotron for 8 million electron volts. The principle of the synchronous acceleration of particles was proposed by V. I. Veksler23 and E. McMillan4 and consists in the fact that, in an accelerator of the cyclotron type, the magnetic field is slowly varied.5678 The increase of the magnetic field makes it possible, on average, to keep the angular velocity of rotation of a particle constant, even though its energy is increasing. As is known, the period of revolution of an electron is equal to \(T = \frac{2\pi \varepsilon}{ecH}\), where \(\varepsilon\) is the total energy and \(H\) the magnetic-field strength. In a synchrotron, on average, \(\frac{\varepsilon}{H}\) is a constant quantity. Thus resonant acceleration of particles takes place as long as the magnetic field is increasing. V. I. Veksler showed that, in order to maintain the constancy of \(\frac{\varepsilon}{H}\), there is no need to select a law for the variation of \(H\); it is only necessary to increase its value adiabatically and continuously.

For a rapid test of the basic principles of the synchronous acceleration of particles, F. K. Goward and D. E. Barnes decided to use a 4-million-electron-volt betatron at their disposal. This betatron had a number of special features and was one of Kerst’s early models.3

The radius of the equilibrium betatron orbit was 7.5 cm. But the electrons moved at this radius only for a short time, since at a magnetic-field phase of \(24^\circ\) relative to the minimum value (this means that at that instant the magnetic field was equal to \(H_{\max}\sin 24^\circ\)) the magnet core begins to saturate. The electrons, not receiving energy from the magnetic field, move along a spiral of decreasing radius, and when the phase of the magnetic field is \(90^\circ\) they strike a target located at a radius of 4.6 cm. At this instant the field at the orbit \((r = 7.5\ \text{cm})\) reaches 2000 gauss. The effective 50-period current in the magnetizing coils was 35 amperes. Increasing the current to 70 amperes, i.e., to the maximum value attainable without breakdown, did not increase the final energy of the electrons, since in this case the core already saturated at a phase of \(12^\circ\) and the electrons struck the target at a phase of \(30^\circ\), i.e., when the field had reached only half of its new maximum value.

F. K. Goward and D. E. Barnes decided to make full use of the possibilities of the betatron magnet by converting the betatron into a synchrotron. For this purpose the high-frequency system was made by the simplest means. A porcelain toroidal vacuum chamber coated with Aquadag was placed in the magnet field. The high frequency was excited in a quarter-wave resonator of the coaxial-line type by means of a small loop. The frequency was 610 megacycles/sec. The electrons gained energy in the gap through which the resonator passed. The resonator could not be made continuous, since in that case Foucault currents would have led to a perturbation of the electron orbit. It was therefore made

of 26 wires spaced \(1/16\) inch apart and mounted on Deestrol spacers. The wires were connected together in current bundles. The r.f. system was bent so as to encompass the accelerating chamber. Such a system possessed few shortcomings, since the field was very nonuniform and was weakened by the porcelain walls of the chamber. But its simplicity compensated for all its deficiencies. The mean power delivered was 1 watt. The voltage amplitude did not exceed 100 volts and was established in 10 microseconds. It was possible to regulate the duration of the electric field and the moment of switching on.

Control of the operation of the accelerator was carried out in the following way. The current feeding the electromagnet was applied to the horizontal plates of the oscillograph. The generator and Geiger counters, which recorded the presence of radiation, were connected to the vertical plates of the oscillograph. During the entire time that the high frequency was applied to the resonator, the sinusoid was displaced by a constant amount. Curve 1 in the figure shows the oscillogram when the accelerator operated as a betatron (without a high-frequency electric field); curve 2 shows the oscillogram for the synchrotron. The letter \(b\) indicates the moment of injection, \(c\) the signal from the counters, and \(f—f\) the time interval during which the high frequency is switched on.

1 — schematic oscillogram of the betatron; 2 — schematic oscillogram of the synchrotron.

On curve 1 (betatron) the signal from the counters is obtained at a phase of \(30^\circ\). On curve 2 the signal from the counters arrives at a phase of \(80^\circ\). However, the counters also operate at \(130^\circ\). This shows that not all the particles were captured in the synchronous mode of operation and were accelerated to the same energies as before. Thus, by means of insignificant changes the authors doubled the maximum attainable energy and obtained electrons with an energy of 8 million electron-volts.

The X-ray beam was directed into an ionization chamber. When the high frequency was switched on, the ionization increased by a factor of 4. It was calculated that if all electrons had been accelerated to 8 million electron-volts, the ionization would have increased by a factor of 6. The authors note that the ionization increases when the amplitude of the high frequency is raised, and also when the rate of rise of the amplitude is increased.

CITED LITERATURE

1. F. K. Loward and D. E. Barnes, Nature 158, 413.
2. V. I. Veksler, DAN 44, No. 9, 393 (1944).
3. V. I. Veksler, Journ. of Phys. 9, 153 (1945).
4. E. M. Millan, Phys. Rev. 68, 143 (1945).
5. M. Rabinovich, Journ. of Phys. 10, 523 (1946).
6. D. M. Dennison and T. H. Berlin, Phys. Rev. 70, 58 (1946).
7. N. N. Frank, Phys. Rev. 70, 177 (1946).
8. Z. Foldy and D. Bohn, Phys. Rev. 70, 249 (1946).
9. D. W. Kerst, Phys. Rev. 60, 47 (1941).

M. S. Rabinovich