components determine the frequency. Frequency
adjustment of about 0.1 MHz is possible by changing the temperature of the components by
adjusting the water-cooling set point. The normal water temperature is at 44.60 ± 0.05C.

The choice of operating frequency was due to the 50-year legacy of development and
manufacturing at this frequency and availability of standard components developed at Stanford
University and SLAC.

Pulsed power is available at up to 6 pulses per second with pulse widths from 2.5 to 10
µs.
Total peak power of 20 to 70 megawatts is available depending on the klystron(s) used.

The power is distributed as follows:
Electron gun 4-10 megawatts
Accelerating sections 5-30 megawatts (each)

The pulses of RF power are rectangular with rise and fall times of about 400
nanoseconds. Pulse amplitude should be constant to ±0.3% with a pulse-to-pulse repeatability of
better than ±0.05%. Noise and FM jitter should not exceed 0.5 degree during the pulse.

The source of RF is a temperature controlled crystal oscillator at 40.8 MHz. The
frequency drift is less than 1 part in 10
7

per day. The output of this oscillator is used to provide a
synchronizing signal to the laser system and as the input to a frequency multiplier.

The frequency multiplier is used to provide a signal at the 35th harmonic of 81.6 MHz for
the klystron system. This 2856 MHz output is also used to provide signals for master timing
reference and to power diagnostic equipment.


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2.
The Amplifier System

2.1
Introduction

This system operates with each stage operating in a non drive-saturated condition in order
that the output power of the klystron may be controlled by the input low power drive signal.
There is a separate amplifier system for each klystron, though in order to maintain synchronism
all systems are driven from the common oscillator

The klystron output power varies with anode voltage and with input drive power.
Because changes in anode voltage change the electron beam velocity, it also causes a change in
the phase shift across the klystron. In normal operation the klystron has about 1300 degrees of
phase shift across it. A 1% change in anode voltage will then result in about 13 degrees of phase
shift. For that reason it is desirable to control klystron power output by varying the drive power
which has only a small second order effect on phase. Changes in anode voltage are used to
control klystron efficiency and maximum power output.

At low anode voltages bunching occurs at frequencies above the cutoff mode of the
klystron bore tube and may allow the klystron to self oscillate at frequencies which will cause
klystron failure. This requires that anode voltage not be applied below 200 kV.

2.2
Amplitude Modulator

The output of the oscillator system is first coupled to a voltage controlled attenuator for
each klystron to servo its output and to allow control of the output pulse shape. This allows
compensation for energy changes due to beam loading and reduction of energy ripple.

2.3
Solid State Amplifier

The output of the attenuator is fed to a 30-db solid-state amplifier, which will deliver
about 1 watt CW at 2856 MHz. Whether this pre-amplifier is used or not depends on the gain of
the 1KW Amplifier used (see 2.4 below).

2.4
Cascade Triode Amplifier

The next stage is a 1KW pulsed RF Amplifier consisting of a cascade of 2, 3, or 4
identical vacuum tube tuned triode cavities (depends on amplifier), used to obtain a pulsed power
output of up to 1200 watts. These units are grid modulated and are capable of pulses of 15
microseconds at up to 60 pulses per second. The output of this amplifier drives the klystron
amplifier. The ATF owns three 1KW amplifiers, 2 used in operations and 1 spare unit. Their
gains and input power requirements vary from one unit to another.

2.5
Klystron Amplifier

The klystron is a fixed tuned 5-cavity solenoid magnet focused assembly. The klystron
was developed at Stanford University and then at SLAC and has been manufactured at SLAC,
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and by several U.S., European, and Asian manufacturers. Several versions of the tube exist with
power output capabilities of 20 to 65 megawatts. Some of the tubes have been tested at power
output of up to 180 megawatts.

The commercial version of the tube produced originally for SLAC had a rated power
output of 2 megawatts and was designated as EIA type 8568. This tube and several different
improved versions of it were designated as type XK5 and eventually upgraded to 35 megawatts
by increasing klystron efficiency.

A new series of high power klystrons was developed at SLAC designated as type 5045
with an initial operating goal of 50 megawatts peak and 45 kilowatts average power. Most
present versions of the tube are now capable of greater than 65 megawatts peak power. The tubes
presently used at ATF are of the XK5 type. The klystron requires RF drive power of 120 to 400
watts depending on tube type and operating conditions. The drive power may be applied
independent of anode voltage.

The RF power is coupled into the tube via 50
Heliax cable. Because of high attenuation
in the coaxial cable at 2.856GHz the losses in this cable may dissipate as much as 75% of the
drive power depending on the distance between the 1KW Amplifier and Klystron. The output of
the Klystron tube is carried to the load via a high vacuum waveguide of EIA type WR284 (2.84
by 1.35 inches cross section).

An external solenoid magnet focuses the electron beam in the klystron. On the type 8568
and XK5 tubes this is normally done with a permanent magnet for circuit simplicity and
reliability. An electromagnet may be used on these tubes for slightly higher output power if
required and is necessary for the type 5045. The XK5 tube requires an anode voltage of about
250 kilovolts at 250 amperes to obtain full power output. These 62.6 megawatts will result in 20
to 35 megawatts of power output depending on tube efficiency. The tube is normally operated
with the anode at ground potential and the cathode assembly immersed in a tank of oil for
insulation and cooling. A negative voltage pulse of 250 kilovolts is then applied to the cathode.

3.
The Modulator

3.1 Introduction

In order to make a practical pulse modulator it is necessary to generate the pulse at a
lower voltage and use a step up transformer to transform the voltage to the correct level. The
transformer is located in the oil tank below the klystron cathode assembly. It has a step-up turns
ratio of 1:12. This requires the modulator to deliver a primary voltage of 21 kV at 3000 amperes.

The cathode is heated by passing filament power up two parallel secondary windings of
the step-up transformer. The tube requires about 300 watts of filament power.

To reduce the size and rise time of the pulse transformer it is designed with a minimum
amount of magnetic core material. This requires that it be supplied with a D.C. priming current
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to preset the operating point on the core saturation curve and allow sufficient pulse length
without core saturation.

The modulator cabinet contains the following items:

3.2
Klystron Filament Power Supply and Regulator

The klystron filament power supply provides a current regulated source of power to a
filament transformer located on the cathode/filament terminals in the oil tank. This source must
be regulated to ensure long cathode lifetime and to prevent destructive inrush current during
power turn on.

3.3
Core Bias Supply

The core bias power supply provides 9 amperes D.C. to the primary winding of the pulse
transformer to back bias the core to saturation. Its setting is not critical.

3.4 A.C.
Power
Distribution

The modulator uses 208VAC 3 phase power which is supplied to the modulator as two
separate systems. The first is used to power the filaments and all low voltage auxiliary supplies.
Power to these is interlocked by the sub-systems that they serve. For instance, a loss of cooling
water to the klystron filaments will turn off all filament-related equipment in the modulator.
The second system provides power to the high voltage power supply that charges the pulse
forming network (PFN). Power supplied by the second system is controlled in one of three
ways: by a Safety Switch by means of which power to the modulator can be locked out, by an
external security logic fault that prevents operation of the klystron if electrical or radiation safety
systems are not satisfied, and last by an interlock fault in the first power system. Both power
systems also contain primary over current protection and transient protectors.

3.5
Energy storage pulse forming network

The pulse-forming network consists of an array of capacitors and inductors connected as
an artificial delay line. The line is normally charged to about 42 kilovolts. If we use the total
capacitance and inductance of the line we get.

______
______
T =
(LC)
and
Z =
(L/C)

where T is the one way propagation delay time of the line and Z is the line impedance. Adding
more selections to the line increases T without changing Z.

The pulse transformer in the klystron tank requires 21 kilovolts at 3000 amperes. Setting
Z equal to this transformer input impedance