Injectors for 15–25 MeV Betatrons

A. G. Vlasov

(Presented by the scientific seminar of the Physico-Technical Faculty)

For injecting electrons into the accelerating chamber of betatrons with accelerated-electron energies up to 25–30 MeV, injectors in the form of a three-electrode system are most often used [1–5].

Such an injector system was first used by Kerst in his first 2.3 MeV betatron, and then, in a somewhat modified design, in a 20 MeV betatron [2].

The injectors used by us for 5–25 MeV betatrons constitute the above-mentioned three-electrode system, similar to Kerst’s injector for a 20 MeV betatron [3]. The injector consists of an anode, a cathode, and a focusing cylinder.

The anode is made in the form of a small box from tantalum sheet 0.1 mm thick on a special mandrel (Fig. 1). The blank for the anode is cut in such a way as to make it possible to obtain a longitudinal cut in the anode, necessary for reducing eddy currents induced by the magnetic field, and a slot of specified dimensions for ejecting electrons into the chamber. In this connection the anode box is made of three parts, welded by spot electric welding. To reduce the potential gradient between the anode and the focusing cylinder, the edges of the slot are made with roundings of radius 0.2–0.3 mm.

For fastening on the base, lugs are welded to the anode, by means of which it is placed onto the holders of the base. On the rear side of the anode, with the aid of tabs made of tantalum sheet welded to the anode box, a tungsten target 0.4–0.5 mm thick is secured.

The focusing cylinder is likewise made from tantalum sheet 0.1 mm thick on a special mandrel. The cylinder slot is made with rounded edges of radius 0.2–0.3 mm.

The width of the focusing-cylinder slot is the same as the width of the anode slot, or else 0.1–0.2 mm larger, which somewhat improves the focusing properties of the injector. The focusing cylinder is fastened to the base by means of a small molybdenum rod, welded by electric welding to the cylinder, and a nickel tube placed over the rod and the molybdenum lead-in. This makes it possible to move the cylinder in the radial direction during assembly of the injector (Fig. 2).

The cathode is made of tungsten grade VA, or of thoriated tungsten grade VT-10, 0.15 mm in diameter, in the form of a spiral wound on a mandrel 0.3 mm in diameter with a pitch of 0.22 mm, with 21–23 turns. The filament voltage is measured at about 3–5 V at a current of 2.5–3.5 A, depending on the type of cathode used. The optimum emission from a cathode made of tungsten grade VA, of the order of 100–200 μA, can be obtained only

at increased heating, which sharply reduces its service life. The advantage of such a cathode is its mechanical stability during manufacture and operation.

Focusing cylinder  Cathode  Anode

Fig. 1. Injector electrodes

Cathodes made of thoriated tungsten are mechanically unstable and often sag. To prevent sagging they must be formed in a hydrogen flame. Their second drawback is the instability of the emission. When the gun breaks down, the emission drops sharply, although subsequently it is restored again.

The only advantage of this cathode is its good emission capability at low heating.

The cathode is fastened by spot electric welding to the molybdenum leads of the base. Since tungsten welds poorly to molybdenum, the ends of the filament are wrapped with a sheet of nickel and only then welded.

The injector base is made of grade 3S-5 glass, into which three central molybdenum leads are sealed for fastening the cathode and the focusing cylinder, two stems for fastening the anode, and a lead for grounding the anode, which is connected to the anode by a nickel strip (Fig. 2).

The anode holders are sealed into the walls of the tube, which makes it possible to avoid breakdown between the anode and cathode along the glass surface,

The injector base is fastened to the chamber branch pipe by means of a putty of the picein type.

When tuning the betatron, the injector is mounted on a Sylphon bellows, which makes it possible to move the injector in the radial direction and to determine the most rational position of the injector in the chamber.

Figure labels: molybdenum $\varnothing 10$; nickel $2 \times 0.1$; glass KhS-5; molybdenum $\varnothing 10$; Kempten tube; copper twist.

Fig. 2. Electron injector of a 15 MeV betatron

After adjustment, an injector of the selected dimensions is made and installed without a Sylphon bellows. Its position in the chamber is established visually, since glass chambers coated with a sufficiently transparent layer of silver are usually used.

The use of an injector of the design described above is justified by its simplicity of manufacture, although it gives a beam with insufficient focusing.

The focusing of the beam in this injector is further worsened by the fact that, in order to simplify the injector, the cathode and the focusing cylinder are at the same potential.

Improvement of the focusing in injectors of this system is possible by applying different potentials to the cylinder and the cathode, which leads to complication of the power-supply circuit.

When the same potential is applied to the cathode and to the focusing cylinder, improvement of the focusing is possible only by selecting the most rational position of the cathode in the focusing cylinder.

Investigations were carried out on the depth of immersion of the cathode in the cylinder; these confirmed studies performed by B. N. Gulko, a staff member of the Department of Electrovacuum Engineering, in an electrolytic bath.

The focusing of the beam can be judged approximately from the distribution of the cathode emission current between the anode and the layer.

When the cathode is immersed by 0.5 mm from the edge of the focusing cylinder, the current to the layer is 30–35% of the total emission current, while the remaining

…part of the electrons falls on the anode, which is highly undesirable, since the cathode emission is used inefficiently, the anode is heated unnecessarily, and this leads to a decrease in the electrical strength of the gun.

Such a distribution of currents also indicates poor focusing of the beam.

Immersion of the cathode by \(0.8\ \mathrm{mm}\) already gives a current distribution equal to \(1:1\), and with immersion by \(1.2\text{–}1.3\ \mathrm{mm}\) the current to the layer amounts to \(70\text{–}80\%\) of the emission current. Further immersion leads to a sharp decrease in the total emission current, since the electron cloud at the cathode is not dispersed by the extracting voltage.

The investigations described above were carried out directly in the chamber, without switching on the magnetic field. The injector was supplied from the injection circuit. The data on current distribution are given for the same voltage on the injector, equal to \(30\ \mathrm{kV}\).

These investigations were checked on a test installation with a luminescent screen, but since the observations were made visually, and measurements of the current-density distribution in the beam were not carried out, the results can be only approximate.

It was not possible to obtain a sharply focused image of the beam on the screen, which is explained by the insufficient focusing of this injector system.

Along the height of the slit, at a screen distance of \(30\ \mathrm{cm}\) from the gun, the spot had a size of \(3\ \mathrm{cm}\) and did not change when the immersion of the cathode in the cylinder was varied.

Across the width of the slit the spot did not fit on the screen; therefore the angle of divergence was measured by rotating the injector in a plane perpendicular to the slit, and the following data were obtained.

With immersion of the cathode by \(0.5\ \mathrm{mm}\), the angle of divergence is \(50\text{–}60^\circ\); with immersion of the cathode by \(0.8\ \mathrm{mm}\), the angle of divergence is \(40\text{–}45^\circ\); with immersion of the cathode by \(1.2\text{–}1.3\ \mathrm{mm}\), the angle of divergence is \(30^\circ\). Such focusing is clearly insufficient, and to improve it it is necessary, as already indicated above, either to supply voltage separately to the cathode and the focusing cylinder, or to change the configuration of the electrodes.

The most important requirement imposed on the injector is the requirement of increasing the injection voltage, since as the voltage is increased the radiation intensity increases.

With sufficiently good treatment of the electrode surfaces, the injectors described above, with an anode size of \(11 \times 5 \times 20\ \mathrm{mm}\), allow a voltage of up to \(30\ \mathrm{kV}\) to be applied (Fig. 1).

To increase the breakdown strength of the injector, we used anodes of larger dimensions.

An injector with an anode of dimensions \(13 \times 6 \times 20\ \mathrm{mm}\) already withstands up to \(40\text{–}45\ \mathrm{kV}\), and with dimensions \(15 \times 8 \times 25\ \mathrm{mm}\), up to \(60\text{–}65\ \mathrm{kV}\).

With an increase in the anode dimensions, the distance between the focusing cylinder and the anode was correspondingly increased. The focusing properties of the injector remain the same.

Raising the injection voltage above \(50\text{–}60\ \mathrm{kV}\) requires changing the design of the base, since the base of the design used by us begins to break down at \(50\text{–}55\ \mathrm{kV}\).

LITERATURE

1. Granberg A. I. Methods of Accelerating Charged Particles, GITTL, 1950.
2. Kerst D. W. Phys. Rev., 1941, 60, 47–52.
3. Kerst D. W. Rev. Sci. Instr., 1942, 13, 387–394.
4. Kerst D. W., Adams G. D. and Jr., Phys. Rev., 1950, 70, 297.
5. Kaiser H. F. Journ. Appl. Phys., 1947, 18, 1–13.