1.4.1.2 Main Linac

The acceleration scheme of the main linac is a conventional one: The electric power from commercial line is converted to a high-voltage (a few hundred kilo-volts), short (a few microseconds) pulse by klystron modulators. This pulse is converted into microwave by high-power klystrons and is led to normal-conducting accelerating structures. What is not conventional is that a very high accelerating gradient is required to make the whole system reasonably short. Generally speaking, a higher accelerating frequency of microwave is better for higher gradient but is more difficult technologically. There are at present two possible choices of the main accelerating frequency, X-band (11.424 GHz) and C-band (5.712 GHz), both being higher than conventional frequencies for linacs. The latter is considered to be a backup scheme in case the X-band R&D would delay or fail.

The development of high-power klystrons is going well but the technological limit of the klystron peak power is far below the value needed to reach the desired accelerating gradient (over 50 MV/m for X-band and over 30 MV/m for C-band). On the other hand it is relatively easy to obtain a long klystron pulse. Therefore, one should compress the pulse to obtain a higher peak power with shorter length. The pulse compression scheme is different between X- and C-band designs. The X-band design adopts the DLDS (Delay Line Distributed System) as the effective pulse compression method. The output microwaves (1.5 $\mu$sec long) from 8 klystrons are combined and cut into four in time. Each time slice is delivered to different accelerating structures upstream. The C-band design adopts a disk-loaded structure made of 3-cell coupled-cavity. The power efficiency is lower than the DLDS but the system is much more compact. In the following we shall describe the X-band design. The parameters related to the X-band RF system are summarized in Table 1.2.