Emittance Growth Caused by Current Variation in a Beam-Transport Channel
Time variation of space-charge forces in a beam-transport channel will lead to a time-averaged emittance growth of the beam. The Kapchinskii-Vladimirskii (K-V) equations have been used to follow the envelope of a round beam with effective beam current fluctuation i through a transport channel. The area of the ellipse that encloses the varying ellipses at the end has been used as the criterion for emittance growth. Simple formulae give the relation between i, initial emittance ¿, allowable fractional emittance growth ¿¿, effective average current I, average beam radius R, and transport length. For example, for a long transport channel a nominally compensated beam must have i <(ß¿¿2I0/R2)¿¿, where I0 = 4¿¿0Mc3/e, and ß¿ is the usual relativistic factor. Results for other conditions are presented. A comparison with a numerical calculation from the TRACE code for transport of a 100-keV, 100-mA beam is made.
Theoretical and experimental study of space charge in intense ion beams
The highly collimated beams of energetic neutral atoms used in controlled thermonuclear fusion research (CTR) require the virtual elimination of space-charge forces in the primary ion beam in order to minimize the angular divergence. A model is presented which describes the behavior of an intense ion beam passing through a gas cell. This theory is used to derive the space-charge field produced by such a beam and shows how its effect can be minimized. This model agrees well with experimental measurements and enables emittance-dominated beams of very high brightness to be obtained which could find applications in fields other than CTR.
Berkeley Proton Linear Accelerator
A linear accelerator is now in use which increases the energy of protons from a 4‐Mev Van de Graaff injector to a final energy of 31.5 Mev. The accelerator consists of a cavity 40 feet long and 39 inches in diameter, excited at resonance in a longitudinal electric mode with a radio‐frequency power of about 2.1×106 watts peak at 202.5 Mc. Acceleration is made possible by the introduction of 46 axial ``drift tubes'' into the cavity, which is designed so that the particles traverse the distance between the centers of successive tubes in one cycle of the rf power. The proton bunches are longitudinally stable as in the synchrotron, and are stabilized transversely by the action of converging fields produced by focusing grids. The electrical cavity is constructed like an inverted airplane fuselage and is supported in a vacuum tank. Power is supplied by 9 high‐powered oscillators fed from a pulse generator of the artificial transmission line type. Output currents are 3×10−7 ampere, average, and 60 μa, peak. The beam has a diameter of 1 cm and an angular divergence of 10−3 radian.