EXPERIMENTAL SETUP FOR ACCELERATION OF CHARGED PARTICLES IN SMOOTH WAVEGUIDES BY FAST ELECTROMAGNETIC WAVES IN AN INCREASING MAGNETIC FIELD

Yu. V. Panichkin, E. T. Protasevich, Yu. G. Shtein

(Presented by the seminar of the microwave and current-carrying sector of the Nuclear Physics Research Institute)

Existing methods for accelerating charged particles by a traveling electromagnetic wave assume the presence of a longitudinal component of the electric field and synchronism between the particle velocity and the phase velocity of the wave. The desire to reduce the phase velocity of an electromagnetic wave to a velocity less than or equal to the speed of light leads to the creation of complex slowing structures. Therefore it is of interest to investigate the acceleration of charged particles in smooth waveguide systems, when no slowing of the electromagnetic wave occurs.

In [1—3] an autoresonance method of acceleration was considered, in which the interaction between a charged particle and an electromagnetic wave is effected by imposing an external constant longitudinal magnetic field. Experimental investigations of this method showed that acceleration of particles in the field of an \(H\)-type wave in a smooth waveguide occurs over a limited section along the waveguide, after which deceleration of the particles is observed [4]. In order to obtain a regime of continuous acceleration of particles, it was proposed in [5] to create a spatially nonuniform magnetic field. Theoretical investigations of such an acceleration method were carried out in [6—7].

This paper gives a description of an experimental setup that makes it possible to study the acceleration of electrons by an \(H_{10}\)-type wave in a rectangular smooth waveguide of the ten-centimeter wavelength range, with an external nonuniform magnetic field imposed.

Figs. 1 and 2 present calculated curves, obtained from the formulas of [7], for determining the values of the magnetic field at the beginning \(H_{zн}\) and at the end \(H_{zs}\) of acceleration and the length of the accelerating section \(z\) as a function of the phase velocity of the wave \(v_{\phi}=\beta_{\phi} c\) for various values of the initial \(\gamma_{н}\) and final \(\gamma_s\) particle energies \(\left(\gamma_{н}=\dfrac{\varepsilon_{н}}{m_0c^2},\ \gamma_s=\dfrac{\varepsilon_s}{m_0c^2}\right.\), where \(\varepsilon_{н}\) is the injection energy, \(\varepsilon_s\) is the energy at the accelerator exit, \(m_0\) is the rest mass of the particle, and \(c\) is the speed of light).

From Fig. 2 it is seen that for \(z=\mathrm{const}\) the particle energy increases with increasing electric-field strength, where \(e_0=\dfrac{eE}{m_0c^2}\). Therefore, in order to create a high microwave field strength, the power was directed from the output of the accelerating waveguide to its input [8]. Such a system, consisting of

from the accelerating waveguide 3 with feedback loop 5 (Fig. 3), may be regarded as a traveling-wave resonator (TWR) [8].

The TWR made it possible to increase the power of the traveling wave in the accelerating waveguide by a factor of 8. With the magnetron generator power of 1400 kW, the electric-field strength reached 20–23 kV/cm.

Fig. 1. Dependences of the external magnetic field on the phase velocity of the wave for different values of the initial particle energy and \(\gamma_s = 6\): 1—2—\(\gamma_n = 1.13\); 3—4—\(\gamma_n = 1.33\).

Fig. 2. Dependences of the variation in the length of the accelerating section on the phase velocity of the wave for different values of the energy of the injected electrons and \(\gamma_s = 6\): 1—\(\gamma_n = 1.13\); 2—\(\gamma_n = 1.2\); 3—\(\gamma_n = 1.33\).

In the acceleration regime, the electron beam loaded the accelerating waveguide and changed the resonator parameters, which led to a mismatch between the resonator and the feed line. Therefore, for more optimal matching, an adjustable directional coupler 7 was installed.

Fig. 3. Schematic of the experimental setup: 1—vacuum pump, 2—injector, 3—accelerating waveguide, 4—solenoid sections, 5—directional coupler, 6—generator, 7—phase shifter.

Fig. 4. Distribution of the external magnetic field along the accelerating waveguide: 1—required, 2—experimental.

The magnetic field was created by a solenoid consisting of 15 sections connected in series. The number of turns in each section varied in proportion to the change in the magnetic-field strength along the accelerating waveguide. The distribution of the magnetic field along the solenoid axis is shown in Fig. 4. Two dips are explained by the presence of gaps between sections, which are necessary for accommodating the TWR. The solenoid was powered by a direct-current source. The magnitude of the magnetic-field strength could be smoothly regulated within the range from 0.5 to 2.0 kOe at the beginning of the accelerating section, but in this case the ratio of the ...

...of the field strengths at the beginning and end of the accelerating waveguide remained unchanged,

\[ \frac{H_{zs}}{H_{zh}}=3.125=\text{const}. \]

Electron injection was carried out from an electron gun with a voltage from 20 to 120 kV.

The entire high-frequency line was evacuated by diffusion pumps to a pressure of \(7\cdot10^{-6}\div2\cdot10^{-5}\) mm Hg. The accelerated electrons, striking the walls of the waveguide at the end of the accelerating section, produced x radiation directed along the length of the accelerator; this radiation was monitored with an SP-1M x-ray meter. The maximum radiation was observed at a definite value of the magnetic field and at the maximum microwave power introduced into the traveling-wave resonator. When the solenoid sections were successively switched off from the end of the accelerator, a decrease in the x radiation was observed. The dependence of the x-radiation power on the length of the magnetic-field region is shown in Fig. 5. It follows from this that electron acceleration occurs over the entire length of the accelerating waveguide.

Fig. 5. Dependence of the radiation magnitude on the length of the external magnetic field.

At present, work is under way on extracting the accelerated electron beam and determining its parameters.

REFERENCES

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