The electron accelerator Desy

Abstract

Before giving a short report of the present status of the electron accelerator at Hamburg, let me describe very briefly the principal features of the design. The electron accelerator is a strong focusing synchrotron for accelerating electrons up to about 6 GeV. Such a machine differs in two essential features from a proton synchrotron: (i) At an injection energy of 40 MeV, the velocity of the electrons is practically equal to that of light, so no frequency modulation of the R. F. accelerating units is necessary. (ii) A very intense synchrotron radiation is emitted when an electron moves in a transverse magnetic field. Since the radiated power is inversely proportional to the fourth power of the rest mass of the accelerated particles, the radiation from electrons is about 10 13 times more intense than that from protons with the same energy. The radiation loss at 6 GeV amounts to 3.6 MeV per turn; so the radio-frequency power requirements are greatly increased. In addition, the motion of the particles is strongly influenced by radiation forces, and while the vertical betatron oscillations are damped, the radial betatron oscillations are rather strongly antidamped. Figure 102 demonstrates this effect. Further, particles can be lost from the stable region of the synchrotron oscillations by too small a radio-frequency amplitude of the accelerator units. Since the radiation loss per turn is inversely proportional to the radius, it was necessary to choose a rather large radius of curvature for the accelerator; namely 31.7 m. One accelerates 50 times per second to keep small the excursions of the particles from the mean orbit. The acceleration time is therefore about 10 ms. Figure 103 shows the dependence with energy of the oscillation amplitudes. One sees that the amplitudes reach a minimum in the region of 2 to 3 GeV, due to the adiabatic damping of the synchro­tron and betatron oscillations, and afterwards they increase due to the radiation effects.