POWER SUPPLY OF THE CUTOFF PLATES OF THE BEAM INJECTION SYSTEM OF A 300 MeV ELECTRON SYNCHROTRON

L. I. MINENKO

For injecting an electron beam with a narrow energy spectrum into the accelerator chamber over a certain range of orbits, a pair of cutoff plates [1] is installed in the beam injection system (plate length 100 mm, distance between plates 20 mm). The injection system makes it possible to completely eliminate the influence of electrons outside this range during adjustment and investigation of the accelerator. The cutoff plates are powered

Fig. 1. Block diagram of the power supply of the cutoff plates

from a stabilized high-voltage source. When an electron beam with an energy of 260 keV passes between the plates, the electrons do not enter the accelerator chamber when the voltage on the cutoff plates is 4 kV. At the required moment in time, the voltage on the plates is sharply reduced to zero level, which ensures passage of the electron beam into the chamber. The end of electron injection into the chamber is determined by the moment at which voltage is applied to the cutoff plates. The power supply of the cutoff plates is implemented according to the block diagram shown in Fig. 1.

The cutoff plates are connected in parallel to the load capacitor \(C_1\) (“output”). When thyratron \(L_2\) is fired, the capacitance of the cutoff plates is charged to the full voltage—4 kV. After the capacitance of the cutoff plates has been charged, the current through the thyratron ceases. Subsequently, the voltage on the cutoff plates is maintained at the level of 4 kV by coupling the plates to the power supply by means of tube \(L_5\) (Fig. 2). The reduction of the voltage on the cutoff plates to zero occurs when \(L_5\) is cut off and thyratron \(L_1\) is fired by a pulse formed by the first delay circuit (\(L_7 \div L_{10}\)). In our case this delay circuit made it possible to adjust the instant at which the voltage on the cutoff plates was reduced, relative to the beginning of the injection pulse, from 0.5 to 7 μsec. The delay circuit is triggered from a special unit for forming the synchronizing pulse of a permalloy pickup located in the gap of the synchrotron. A positive pulse with an amplitude of 40 V and a duration of 10 μsec is applied to the “Input” of the circuit, Fig. 2.

The injection time (the time for which the voltage is removed from the cutoff plates) is determined by the time shift between the firing instants of thyratrons \(L_1\) and \(L_2\). In our case it was adjustable from 0.1 to 6 μsec by means of the second delay circuit (\(L_{11} + L_{15}\)).

Fig. 3. Oscillogram of the reduction of the voltage to zero on the cutoff plates. Pulse amplitude 4 kV. Pulse duration 0.7 μsec (time-marker frequency 11 MHz)

The high voltage for supplying the cutoff plates, with an instability of 0.5%, was supplied from a standard source of the “Orekh” type, equipped with rectifiers for 2.5 and 5 kV. The filaments of thyratrons \(L_1\) and \(L_2\) are supplied from separate transformers, which were fed by an SNE-0.5 ferroresonant stabilizer. The use of two transformers to supply the thyratron filaments was necessitated by the need to reduce the coupling between the thyratron cathodes and, consequently, the parasitic oscillations during formation of the pulse on the cutoff plates.

The reduction of the voltage on the cutoff plates to zero occurs when the capacitance of the plates is discharged through the TGI-90/8 thyratron, provided that the neutralization condition is fulfilled [2]. In addition, the residual voltage is determined by oscillations that arise when the discharge current of the plate capacitance flows through the parasitic inductance in the discharge circuit. To reduce the amplitude of these oscillations, the discharge circuit was made with minimal parasitic inductance. For this purpose the discharge unit was placed in the immediate vicinity of the plates, and appropriate measures were taken to reduce the parasitic inductance inside the unit. During adjustment of the system, a specially selected resistance \(R_2\) was introduced into the discharge circuit, providing optimum damping without a substantial increase in the voltage fall time. The residual voltage on the cutoff

Fig. 2. Schematic diagram.

plates, which is applied to the plates at the moment of injection, is regulated by changing the voltage on capacitor \(C_2\). Regulation of this voltage makes it possible to vary smoothly the angle at which the electrons enter the synchrotron chamber.

The considerable capacitance of the cutoff plates, together with the parasitic capacitance and the capacitance of capacitor \(C_1\) (900 pF), eliminates the effect of the injection-current pulse (one ampere) on the value of the residual voltage of the cutoff plates.

Figure 3 shows an oscillogram of the decrease of the voltage on the cutoff plates to zero. The pulse duration is \(0.7\ \mu\text{s}\), and the pulse amplitude is \(4\ \text{kV}\) (time-marker frequency \(11\ \text{MHz}\)).

In conclusion, it should be noted that the results of the experimental investigation of the beam-injection system proved satisfactory. A bunch of electrons was admitted into the accelerator chamber at the required time and with the required duration, with fronts of \(0.05\ \mu\text{s}\) [3].

REFERENCES

1. L. G. Kositsyn, “Injector and injection channel of a 300 MeV electron synchrotron,” in the present collection.

2. L. I. Minenko, V. P. Anokhin, PTE, 3, 88, 1962.

3. V. P. Anokhin, V. P. Eponeshnikov, L. G. Kositsyn, “Correction of the magnetic field by the beam and quasi-betatron regime in a 300 MeV synchrotron,” in the present collection.