High Frequency Linear Colliders

In a linear collider, low frequency favours efficiency of power transfer to the beam and high frequency favours a short overall length. There are two reasons why an increase of the operating frequency tends to permit an increased accelerating gradient. The first one is related to field emission phenomena; it is in particular the observation that the onset-gradient of dark current is proportional to frequency. The second reason is rclatcd to the total stored electromagnetic energy which is proportional to gradient times wavelength squared; in a normalconducting linac it sets the minimum average RF power for given repetition rate. The reduced wakefields associated with a lower frequency facilitate acceleration of many bunches of par titles by the same RF pulse and this multihunching greatly enhances efficiency. Nevertheless, the proposed parameters for linear colliders show a clear tendency towards increased gradient with increasing frequency. Two types of high-frequency linear collider are being actively developed. The first type, in the 11 to 14 GHz range, is to be powered by a large number of discrete sources of pulsed RF power. The second type, at 30 GHa, is a two-beam device in which the power is derived from a relativistic drive beam running parallel to the main beam. Progress on both lines of development will be reported.