What is C-band? C-band RF System Smart Modulator Klystron Pulse Compressor Accelerating Structure Back to Home

C-band is a name of RF frequency band. Various bands of the microwave frequency are named as the figure.

We use 5712 MHz as the drive frequency. It is twice higher than S-band (2856 MHz) frequency, which has been widely used in conventional electron accelerators.
The reason why we chose C-band is,
(1) Since the shunt-impedance (a measure of power efficiency in the accelerating structure) becomes higher at higher rf frequency, we can obtain a very high accelerating gradient such as 35 MV/m using 50 MW-class klystron.
(2) A  50 MW-class klystron can be developed by scaling from the existing S-band high power klystrons, and the power level is technically feasible. We have already developed 50 MW C-band klystrons.
(3) Since the wavelength becomes shorter, thus the size of various components in RF system becomes smaller as proportional to the wavelength, resulting in reducing size and cost of the accelerator system. This is important for the large scale Linear Collider project, as well as industrial applications including the medical oncology electron accelerator, and compact sterilization machine.

One unit of the C-band accelerator system is shown here, which is designed to generate 35 MV/m accelerating gradient for the multi-bunch beam of 200 nsec width, and 40 MV/m for single-bunch.
In the case of the Linear Collider project, to reach 500 GeV c.m. (center-of-mass energy), we need 2000 units of C-band accelerator unit in total for electron and positron beams.
In the case of SCSS X-ray FEL project, four units of the same system will be used to generate 1 GeV beam within 30 m long C-band accelerator.
Smart Modulator

Traditionally, the modulator is the most troublesome component, while it was expensive. In the Linear Collider project, it is one of the most important R&D items to develop a reliable and low-cost modulator. To do this, we C-band group is currently developing the "Smart Modulator", in which a high-voltage inverter-switching power supply is used to charge the PFN capacitors, which simplified the modulator circuit and reduced its size, drastically. It also contributed to reduce cost.

In order to reduce modulator size further, we are developing "oil-filled modulator". All of PFN unit, thyratron switch and protection circuit will be installed in one tank, and filled with the insulation oil. First model will be developed in 2001 and tested in the SCSS project at SPring-8.



C-band high-power pulse-klystron was developed by collaborating with TOSHIBA Co. The developed klystron is already on the product list on TOSHIBA klystron tube, the catalog number is E3746.
The developed C-band klystron is designed to generate 50 MW peak power for pulse width 2.5 micro-sec, and repetition rate 50 pps. In the R&D, we have developed three C-band klystrons, and tested life-time for 5000 hours on each tubes. Test results were acceptable for the Linear Collider project at 500 GeV energy stage.

The next generation high-power klystron will use PPM (Periodic Permanent Magnet) focusing, instead of solenoid focusing. We are currently developing C-band PPM klystron, and the first tube generated 40 MW peak power.
In order for generating very high accelerating gradient in the accelerating structure, the RF power from the klystron is compressed and its peak power is multiplied in the RF pulse compressor. It firstly stores the klystron output power in the high-Q energy storage cavity, then reversing the RF phase at the klystron input. By mixing the emission power from the high-Q storage cavity with klystron output power at the hybrid-combiner, 3.5 times higher peak power in generated at the hybrid-combiner and fed into the accelerating structures.
C-band group developed a special RF pulse compressor "Three-cell Coupled-Cavity Pulse Compressor", which generates a flat output pulse, which is very important for the multi-bunch beam acceleration in the Linear Collider.
C-band accelerating structure uses "Choke-Mode cavity" inside, which damps unwanted noise power (wakefield) from the beam and stalely accelerate multi-bunch beam.
The first C-band accelerating structure (left figure) was developed and its HOM (Higher Order Mode) damping performance was tested in the ASSET beam line at SLAC, in 1998. Test result proved the Choke-mode cavity damped the HOM power as designed.






What is C-band? Accelerator System Smart Modulator Klystron Pulse Compressor Accelerating Structure Back to Home