General Information

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Linac System

The NSLS linac operates at 2856 Mhz. The frequency is determined by the internal dimensions of the accelerator cavities, klystron and the waveguide components. Frequency adjustment of about 0.1 Mhz. is possible by changing the temperature of the accelerator cavities by adjusting the water system setpoint.

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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 2 pulses per second with pulse widths up to 3 microseconds. Total peak power of 20 megawatts is available. The pulses of RF power are rectangular with rise and fall times of about 400 nanoseconds. Pulse amplitude should be constant to within 1%.

The source of RF is a temperature controlled crystal oscillator 40.8 Mhz. The frequency drift is less than 1 part in 10 million /day. The output of this oscillator is the input to the 2856 Mhz. frequency multiplier. The frequency multiplier is used to provide a signal at the 70th harmonic of 40.8 Mhz. for the klystron system.

The RF Amplifier

The klystron output power varies with anode voltage and with input drive power. Since a change in anode voltage changes the electron beam velocity, it also causes a change in phase across the klystron. In normal operation the klystron has about 1300 degrees of phase shift across the klystron. A one- percent 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 voltage, 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 the anode voltage not be applied below 60% of the full rated value, even if only low RF output is required.

The output of the oscillator system is coupled to a mechanical attenuator to control the klystron output power and to allow control of the output pulse shape. This will allow compensation for energy changes due to beam loading and reduction of energy ripple.

The output of the attenuator is fed to a 30-dB solid-state amplifier, which will deliver about 1 watt of continuous wave (CW) at 2856 Mhz.

The next section consist of a cascade of 3 identical vacuum tube-tuned triode amplifiers, which is used to obtain a pulse 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 seconds.

The klystron is a fixed tuned five-cavity solenoid magnet focused assembly. The klystron was first developed at Stanford University, and then at SLAC, where they began manufacturing it for their own use. Presently there are several versions of the tube that 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 21 megawatts and was designated as EIA type 8568. This tube and several different improved versions of it were designated as type XK5. It was eventually upgraded to 35 megawatts by increasing the 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 megawatts average power. Most present versions of the tube are now capable of greater than 65 megawatts. The tubes presently used at the NSLS are of the XK5 type.

The klystron requires RF drive power of 120 to 400 watts depending on the type of tube used and operating conditions.

The RF power is coupled into the tube via 50-ohm coaxial cable. Because of the potentially high attenuation in the coaxial cable at this frequency (2856 MHz.) the losses may dissipate as much as 75% of the drive power.

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. The tube requires about 250 kilovolts at 250 amperes as anode voltage to obtain full power output. The 62.5 megawatts will result in 20 to 35 megawatts of RF 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 purposes. A negative voltage pulse of 250 kilovolts is then applied to the cathode.

The output of the tube is in a high vacuum waveguide of EIA type WR284 (2.84" x 1.35" cross section).

The Modulator

In order to produce 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 just 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 approximately 20 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 approximately 300 watts of filament power.

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

The following items make up the bulk of the modulator cabinet:

Klystron Filament Power Supply and Regulator:The 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.

Core Bias Power Supply: The core bias power supply provides 0-15 amperes DC to the primary winding of the pulse transformer to back bias the core to saturation. Its setting is not critical.

Energy Storage Pulse Forming Network (PFN): 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 30 kilovolts. If the total capacitance and inductance of the line is used, you get:
T = L*C1/2 & Z = L/C1/2

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

C is the total capacitance and L is the total inductance of the PFN.

The pulse transformer in the klystron tank requires 20 kilovolts at 3000 amperes. Setting Z equal to the transformer input impedance and T=2.5 microseconds, you get the following:

C = T/Z = 0.4uF & L = T*Z = 16.7uH

Since the complete PFN discharge requires the wave to travel down and back on the system, the actual discharge time is 2T. The PFN capacitors must be to charged twice the required discharge voltage. This requires a stored energy per pulse of:

E = CV2 /2 = (4.0x10-7*30K2) / 2 = 180 joules

About 30 to 45 percent of this shows up as RF output energy.

High Voltage Power Supply: Designed for capacitor discharge application, the power supply operates between 30 and 35 kV, depending on the operating conditions of the linac. The supply utilizes a high frequency series H-Bridge resonant inverter that operates in the 40 to 60 KHz range. A multiple secondary high voltage transformer is used to step up the inverter voltage to the required output. The servo system will regulate the PFN voltage until the discharge pulse takes place.

Pulse Monitoring System: The power supply current and voltage, the PFN charge voltage, the PFN discharge current, as well as the klystron cathode voltage are all monitored via a diagnostic panel.

Hydrogen Thyratron Switch Tube Assembly: The PFN is discharged by effectively grounding the high voltage side of the end capacitor of the network. The low voltage side of the network is connected to the primary of the pulse transformer and delivers the resulting 20-kilovolt pulse to the transformer.

A multi-grid hydrogen thyratron is used as the switch and requires a 1.5-kilovolt trigger pulses to its second grid. The first grid is held at a small voltage, which continually supplies hydrogen plasma in the tube. The third grid is used to control the voltage gradient in the tube.

Interlock and Overload Protection Circuits: Interlocks are provided to prevent application of high voltage if access doors are open or if appropriate auxiliary voltages and time are not correct. Further protection is provided in chase of overcharging or capacitor failure. Radiation interlocks are also used to prohibit access to the linac while the klystrons are operating.

Computer Interface: A computer interface system is install to allow remote on /off control to the high voltage. Two channels of 12 bit DAC and ADC are also used to control the modulator pulsed output. The entire interface is optically isolated to reduce coupling of the modulator noise into other systems.