The Next Linear Collider: NLC2001 Page: 45 of 159

                                
Chapter 4
Rf System Design
4.1 Introduction
4.1.1 Historical Perspective
The design of the NLC main linacs is based on the extensive experience gained from the design, construc-
tion, and 35 years of operation of the 3-km SLAC linac, which is powered at a frequency of 2.856 GHz.
Since its initial operation in 1966, the SLAC linac has been continuously upgraded for higher energy,
higher intensity, and lower emittance.
The initial gradient of the SLAC linac was 7 MV/m. The original design included an upgrade path in
which the number of klystrons would be quadrupled. The upgrades that were eventually implemented
involved replacing each of the initial 24-MW klystrons with a single higher-power klystron (first with 35-
MW, XK-5 klystrons and, later on, with 65-MW, 5045 klystrons), and adding a SLED pulse compressor
after each klystron to more than double the peak power. The SLAC linac is currently energized by 240
high-power S-band klystrons. The klystron peak power and pulse duration are, respectively, 65 MW and
3.5  s. After pulse compression, the power from one klystron feeds four 3-m constant-gradient S-band
accelerator structures operating in the 27/3 mode. The accelerator gradient has been tripled since 1966, to
21 MV/m, and the maximum beam energy is now 50 GeV.
4.1.2 NLC Rf System Overview
The NLC has two main linacs that accelerate electron and positron beams from 8 to 250 GeV in the initial
configuration, and to 500 GeV or more after full installation. The 11.424-GHz radio frequency (rf) system
used for this purpose is similar in character to that in the SLAC linac. The NLC system is illustrated in Fig.
4.1 and includes all the hardware through which energy flows, from the AC input to the beam-line
accelerator structures. Electrical energy is transformed in several stages: the induction modulators convert
AC power to high-voltage pulsed DC; the klystrons transform the pulsed DC to high-power rf, the Delay
Line Distribution System (DLDS) combines the power from eight klystrons and routes it up-beam se-
quentially to eight sets of accelerator structures; and finally, the six structures in each set convert it to
beam power. Because the power required to drive the accelerator structures is high, it is important that the
conversion and transmission of energy at every stage of the rf system be efficient. Every effort must be
made to maximize the pulse energy generation and handling capabilities of the subsystems to reduce the
number of components, and thus the cost.
The primary technical choice for the rf system is the 11.424-GHz frequency (2.62-cm wavelength).
This frequency, high in the X-band range (8.2 to 12.4 GHz), is exactly four times that of the SLAC 50-
GeV linac. The choice of such a high frequency, relative to existing linacs, allows the same rf-to-beam
efficiency for a given beam current to be achieved at a higher gradient (thus a shorter linac) with less rf
energy per pulse (thus fewer rf components). On the downside, stronger transverse wakefields are gener-
ated by off-axis beams in higher frequency accelerator structures, which act to increase the beam emit-
tances. The choice of 11.424 GHz gains the major cost benefits of a higher-frequency rf system while
allowing achievable alignment tolerances associated with the stronger wakefields.

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