A NEW METHOD OF PARTICLE ACCELERATION

In linear accelerators, elementary particles and ions are accelerated directly by the electric field of a traveling or standing electromagnetic wave. Recently*) a new principle has been proposed for accelerating elementary particles and ions: the accelerating field is produced here not directly by the electromagnetic wave, but by a certain space charge moving together with the accelerated particles. The focusing and translational motion of this space charge is provided by a traveling electromagnetic wave specially created for this purpose. The principle of operation of such an accelerator is described below.

*) H. Alfvén and O. Wernholm, Arkiv for Fysik, 5, No. 12 (1952).

Fig. 1 shows a vertical section of the system; Fig. 2, a horizontal one.

The electrons that create the space charge move from cathode \(C\) to anode \(A\). A high-frequency magnetic field is applied between anode \(A\) and the additional electrode \(A'\). It is produced, for example, by two Lecher systems \(L_1L_2\) and \(L_3L_4\), along which a traveling wave propagates (in Fig. 2, in a direction perpendicular to the drawing). The alternating field of this wave and the electrostatic field between cathode \(C\) and anode \(A\) focus the electrons into an electron cloud of very small dimensions—\(F\)—which moves downward (Fig. 1) with a velocity equal to the phase velocity of the wave focusing it.

The particles to be accelerated, for example positive ions, are emitted by the corresponding source in the vertical direction and move downward together with the electron cloud \(F\). If the ions lag slightly behind the cloud \(F\), then the field of the cloud accelerates them.

The phase velocity of the wave in the Lecher systems \(L_1L_2\) and \(L_3L_4\) does not remain constant, but increases in the direction of motion of the cloud \(F\) and of the ions; the ions are thereby accelerated.

It is evidently not difficult to achieve a change in phase velocity in the vertical direction; it is sufficient, for example, to construct the systems \(L_1L_2\) and \(L_3L_4\) with electrical parameters varying along the length of the lines.

It is significant that, as the authors indicate, phase stability of the particles occurs in the accelerator—both longitudinal and transverse.

The maximum field strength that can be attained near the electron cloud \(F\) is of the order of

\[ \frac{E}{d} \]

—the ratio of the electron energy to the cloud diameter \(d\). For \(E = 50\ \text{keV}\) and \(d = 2\ \text{mm}\), this field is of the order of \(0.25\ \text{MV/cm}\). The high-frequency magnetic field required for focusing is \(\sim 100\) gauss. To create a traveling wave with such field strength, pulse powers of \(\sim 10^6\ \text{W}\) are needed.

Focusing may also be accomplished by combining a high-frequency electric field with static magnetic lenses.

M. P.