BEAM AND RADIATION INDICATION SYSTEM IN A 300 MeV SYNCHROTRON

V. P. ANOKHIN, L. G. KOSITSYN

To start up the accelerator and tune it to the optimum operating mode, it is necessary to have devices for indicating the position and intensity of the beam both on the first revolutions in the vacuum chamber and throughout the entire acceleration cycle. The indication system of a 300 MeV electron synchrotron must provide, at the moment of injection:

1. The possibility of adjusting the moment and angle of injection of the injected electron beam and of visually observing the position and shape of the beam in the accelerator chamber.

2. Measurement of the current of the electron beam introduced into the synchrotron chamber.

3. The possibility of investigating the instantaneous orbit of the electrons.

4. Measurement of the electron-beam current in the injection path.

All elements of the indication system are made movable and can be retracted into the pipes of the vacuum chamber. The arrangement of the elements of the indication system in the synchrotron chamber is shown in Fig. 1.

To carry out the tasks listed above, the following indication devices are used:

1. Transparent fluorescent grids made of brass with dimensions \((36 \cdot 102)\ \mathrm{mm}^2\), coated with phosphor B-3 with a long afterglow time. The transparency of the grids is \(\sim 50\%\). Selection of the moment and angle of electron injection by the method of suppressing free oscillations is described in [1]. Setting the injection moment was carried out with grids I, II, and the injection angle with grid V. The duration of the injection pulse, \(0.1\text{–}0.3\ \mu\mathrm{s}\), was set by the cutoff generator [2], which made it possible to work with a beam of size \(\sim 1\ \mathrm{cm}\) in radius.

2. Current flags with a radial dimension of \(80\ \mathrm{mm}\), a height of \(36\ \mathrm{mm}\), made of copper (brass) \(1\ \mathrm{mm}\) thick. These flags were also used for tracing the electron orbits.

3. Shadow flags measuring \(36\ \mathrm{mm}\) vertically and \(30\ \mathrm{mm}\) radially, made of brass \(1\ \mathrm{mm}\) thick. The flags are electrically connected to the chamber layer.

The grids should be regarded as the most convenient in operation; with their help one can not only adjust the moment and angle of injection, but also rapidly determine the trajectories of electron motion on the first revolutions. Using the indicated devices, it was possible, from the beam, to select the parameters of the injection system and correction of the magnetic field of the synchrotron

and to achieve a sufficiently stable quasi-betatron acceleration regime.

1. Measurement of the electron beam current in the injection channel was carried out with a Faraday cylinder and a magnetic current sensor with a ferrite core. The principles of operation and designs of magnetic current sensors are known. As the core of the magnetic current sensor we use ferrite F-250 with the following dimensions: inner ring diameter 50 mm,

Fig. 1. Placement of elements of the indication system in the synchrotron chamber

outer diameter 70 mm and thickness 10 mm. The aperture of the sensor is \(\varnothing = 44\) mm. To prevent the electron beam from striking the sensor, it is enclosed in a brass shield. Sensor parameters: number of winding turns \(w = 210\), shunt resistance \(R_{\text{sh}} = 1100\ \Omega\), characteristic slope \(S = 3.85\ \text{mV/mA}\). The drop of the flat part of the sensor current pulse, for a pulse duration of 7 μsec, is 25%. The sensor parameters make it possible to measure the beam current from 10 mA to 10 A. The correctness of the readings of the magnetic current sensor was checked with a Faraday cylinder.

Indication of the Presence of Acceleration in the Synchrotron

The need for this indication (we shall call it the “crystal circuit”) was due to difficulties that arose during startup of the accelerator. The main purpose of this device was to study acceleration at the beginning of the cycle, but it also made it possible to observe radiation during the entire acceleration cycle. The main element of the indicator is an NaI(Tl) crystal (or stilbene), the duration of whose light flash is \(5 \cdot 10^{-3}\) sec. At the energy of the injected electrons \(W_i = 260\) keV, the time for one revolution of the electrons is \(3.78 \cdot 10^{-8}\) sec. Thus, the crystal makes it possible to observe separately the light flashes from electrons that have struck the crystal ...

became distinguishable, with a time difference of \(5 \cdot 10^{-8}\) sec, i.e., the electron turns in the accelerator can be separated.

The crystal, installed in a branch pipe of the accelerator vacuum chamber (Fig. 1), is placed in a duralumin shield 2 mm thick. Only its end face is open. Electrons falling out of acceleration strike the chamber walls and, consequently, also strike the crystal. The light flash arising in the crystal when electrons hit it passes through a light guide to an FEU-11B photomultiplier. To increase the signal amplification, we use the forced power-supply mode of the photomultiplier [3].

At the moment of injection the signal from the crystal and from the photomultiplier is large. To avoid this, we use pulsed power supply of the photomultiplier modulator, by means of which the photomultiplier is blanked for the duration of injection. With the selected photomultiplier power-supply mode, a blanking-voltage pulse with an amplitude of 200 V and adjustable duration switched off the photomultiplier quite reliably at the moment of injection. When a large injection current is admitted into the accelerator chamber, in order to switch off the photomultiplier completely it is necessary to increase the blanking voltage.

For indicating the presence of acceleration in the synchrotron, in addition, a betatron target (\(\beta\)-target) was used. The \(\beta\)-target is installed in a branch pipe of the synchrotron chamber on the inner side of the orbit. The optimal radius for its installation is at the boundary of the working region of the synchrotron magnetic field. The target is a copper flag 1 mm thick with a radial dimension of 40 mm and a vertical dimension of 33 mm. The inner end of the flag (\(\sim 10\) mm) is bent at an angle of \(45^\circ\) relative to its radial axis and is coated with a phosphor (\(\mathrm{CaWO_4}\)). This makes it possible to observe from the \(\beta\)-target both the current signal and the signal of the light flash on the \(\beta\)-target by means of the “light guide–photomultiplier” system (Fig. 1). The current signal from the \(\beta\)-target is small; it is observed when the quasi-betatron acceleration mode is satisfactorily tuned and serves to monitor the normal operation of all accelerator systems. A fluorescent \(\beta\)-target with a photomultiplier is a more sensitive device. As a signal amplifier, an FEU-19M with a cathode follower was initially used. At present a more powerful amplifier, an FEU-11B, has been installed.

Fig. 2. Oscillograms of pulses of the quasi-betatron acceleration mode (upper pulse—from the current \(\beta\)-target, lower pulse—from the circuit with the crystal)

Figure 2 shows oscillograms of the pulses of the quasi-betatron acceleration mode from the current \(\beta\)-target and from the circuit with the crystal (lower pulse).

The photomultiplier in the circuit with the crystal opens at the moment when the injection pulse ends (on the oscillogram the trailing edge of the injection pulse and the quasi-betatron pulse are visible). The duration of the quasi-

the betatron acceleration mode is $\sim 3.5$ μsec. The current β-target gives a current pulse of electrons captured into the quasi-betatron acceleration mode. Figure 3 shows oscillograms of the pulse from the β-target with the FEU (upper pulse) and from the circuit with the crystal (the first peak is the injection moment, the last is the quasi-betatron pulse; the FEU operates with locking).

Fig. 3. Oscillograms of pulses of the quasi-betatron acceleration mode (upper pulse—from the β-target with FEU-11B, lower pulse—from the circuit with the crystal)

In conclusion, we note that the circuit with the crystal and the β-target with FEU-11B provided reliable indication of the presence of acceleration during startup of the synchrotron in the synchrophasotron mode (up to 4 MeV). When starting the synchrotron up to the nominal energy of 300 MeV, indication of the presence of acceleration was carried out by the circuit with the crystal and an MS-4 Geiger counter.

REFERENCES

1. Collection of articles Electron Accelerator at 680 MeV. Gosatomizdat, Moscow, 1962.
2. L. I. Minenko, V. P. Anokhin. PTE, No. 3, 88, 1962.
3. O. M. Kovrizhnykh, O. B. Likin, P. A. Yampolsky. PTE, 6, 69, 1960.