A cw klystron operating at 476 MHz has been developed jointly by SLAC and Varian Associates. The unique set of characteristics of this tube were strongly guided by requirements of the fast feedback necessary to prevent oscillations of the storage ring beams caused by the detuned accelerating cavity. This requires a combination of bandwidth and short group delay within the klystron. The RF feedback stabilization scheme also requires amplitude modulation making it necessary to operate the klystron about 10% below saturation. Performance specifications and initial operating results are presented

Caryotakis, G.

Doyle, E.

Fowkes, W. R.

UNT Digital Library

1.2 MW KLYSTRON FOR ASYMMETRIC STORAGE RING B FACTORY W. R. Fowkes, G. Caryotakis, E. Doyle, E. Jongewaard, C. Pearson, R. Phillips, J. Sackett, E. Wright Stanford Linear Accelerator Center, Stanford University, CA 94309 USA and H. Bohlen, G. Huffman, S. Lenci, E. Lien, E. McCune and G. Miram Varian Associates, Palo Alto, CA 94304 USA Fj E p F-j 1 i-p. & r p 4  v z : ,  & 9.-b Abstract A cw klystron operating at 476 MHz has been developed jointly by SLAC and Varian Associates. The unique set of characteristics of this tube were strongly guided by requirements of the fast feedback necessary to prevent oscillations of the storage ring beams caused by the detuned accelerating cavity. This requires a combination of bandwidth and short group delay within the klystron. The RF feedback stabilization scheme also requires amplitude modulation making it necessary to operate the klystron about 10% below saturation. Performance specifications and initial operating results are presented. I. INTRODUCTION SLAC and Varian have joined efforts under a Cooperative Research and Development Agreement (CRADA) to develop and test a new UHF super power klystron as a prototype 476 MHz RF source for the Asymmetric Storage Ring B Factory under construction at SLAC. This klystron was originally designed to produce 1.6 MW CW saturated to be operated at 90 KV, 27 amperes. After the klystron development was well underway, some of the requirements were relaxed as a result of further study in the accelerator physics aspects of the machine. As a result, the klystron will be operated conservatively at 83.5 KV and would deliver 1.2 MW if saturated. The klystron will be operated 10% below saturation and must be able to respond to fast feedback correction in both amplitude and phase in accelerating cavity oscillations induced storage ring beams. This feedback scheme requires the klystron to have very short group delay (d$/do) and wide bandwidth. When this development effort began neither of these features were available off the shelf in commercial tubes of similar power and frequency such as those used at LEP and TRISTAN storage rings. A multistage depressed collector, designed by Varian was very seriously pursued because much of the efficiency that is given up running underdriven is recovered. The operating cost savings would have been substantial but the manufacturing risks and additional upfront costs were considered too great to continue along the MSDC path. The "headroom" needed for amplitude modulation was reduced from 25% to 10%. The design parameters for this tube are listed in the table below. DESIGN PARAMETERS Operating Frequency (MHz) Output Power at Saturation (KW) Operating Point Below Saturation (KW) Beam Voltage (KV) Beam Current (A) Efficiency (%) Saturated Gain (dB) 1 dB Bandwidth (MHz) Group Delay at k0.5 MHz (nanosec.) 476 1200 1100 83.5 24.1 >60 >43 f 3  .O 100 *Work supported by Department of Energy contract DE-AC03-76SF005 15 ~~~~~~~~~~ OF WlS D O C U M M  0s ~L~~~ BS Presented at the 16th IEEE Particle Accelerator Conference (PAC 95) and International Conference on High Energy Accelerators, Dallas, TX, May 1-5, 1995 -- 2 100- - n 11. ELECTRICAL DESIGN - - - - - - - - The Varian electron gun design has a peak cathode loading of 0.31 Ncm2 and a maximum surface gradient of 50 KVlcm. Computer simulation predicts a beam diameter of 4.5 cm with a 5.8% scallop in a 7 cm tunnel diameter. The mechanical design was borrowed from the SLAC 5045 gun which allows adjustment of the radial and axial position of the cathode and focus electrode with respect to the anode. The alignment must be preserved while operating horizontally. It is primarily radiatively cooled to the anode housing. An internal copper web conducts heat from the region of the focus electrode adjacent to the cathode out to the large outside diameter where it is radiated away to the anode housing. 55 1 1 1 l ( 1 1 1 1 ( 1 1 1 1  - - - 3 50- z. - - - - - - C .- - - - - d 45- - - - - - - 1 1 1 l 1 1 1 1 1 1 1 1 1  40 - - - a I l l  I 1  I 1  I 0 -  I 0 10 20 30 40 50 4-95 Input Power (W) 7949A2 Figure 3. Predicted Output Power versus Drive at 90 KV III. MECHANICAL DESIGN 2 DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. the tube is operating. Each cavity except the output cavity has its own diagnostic RF monitor loop. The noses of all cavities except the prepenultimate and the second harmonic were coated with TiN to help suppress multipactor. In addition the input cavity drive loop, the output coupling loop and the vacuum side of the output window were also coated for the same reason. The output coupling loop and all cavities are water cooled to stabilize their operating temperature and hence frequency. All drift tubes are also water cooled as protection against electron beam interception. The limited bakeout station clearance required optimization of the collector design to minimize the tube length. The interior is contoured to have nearly constant flux for highest surface cooling efficiency. The large inside diameter of the collector necessitated a departure from standard collector designs where there is an inner collector with milled cooling channels and an outer, separable water jacket. This is to avoid excessively high stresses in the collector wall due to the pressure of the cooling water. This construction technique allows optimal design of the cooling channels and wall thickness for best cooling performance. In addition, one-piece construction allows the monitoring of beam deposition by measuring temperatures on the outer jacket. The coaxial output window has matching choke hubs in 50 ohm 6.125 " diameter coax line. Air cooling of the inner conductor and window face is accomplished by ducting air (approximately 50 scfm dry air) through the T- bar coupler into the inner conductor and across the window through jets at the bottom of the inner conductor matching choke. The air exits to the atmosphere via holes at the bottom of the outer conductor choke hub. The inner and outer conductors are water cooled on the vacuum side only. The T-bar coax to WR2100 waveguide transition was preferred over the doorknob type because of its good bandwidth and the configuration allows access to the inner conductor for blowing air and placing window temperature monitoring thermocouples. It also allows compliant coupling between the inner conductor and the T-bar to prevent excessive mechanical loading on the output window. A compliant membrane is also designed into the vacuum side output coax center conductor. The magnet consists of twelve individual convection cooled coils that are airspaced with approximately a 45% fill factor to allow access to the tube body for tuning. The magnet return path is formed by four symmetrically placed four inch diameter steel pipes that run the length of the klystron between the gun end and collector end polepieces. The magnet return path also serves to support and space the coils as well as to support the klystron in the horizontal position. Rails attached to the klystron body engage support shoes on each magnet coil support allowing transverse support and longitudinal freedom to allow differential thermal expansion between the klystron body and the magnet support structure. The support structure under the magnet frame has both heavy duty casters and jack screws so that the entire assembly including gun oil tank can be rolled into place in the final operating position in the B-Factory klystron housing. IV. TEST RESULTS The klystron went into test in mid-February and processed cleanly up to 60 KV. At this point the 2.6 MW dc power supply failed, putting an end to publishing any test results in time for this conference. The measured RF output power the lower voltages showed good agreement with simulation predictions where the efficiency is expected to be low. As of this printing the klystron is awaiting the repair of the power supply so that testing may continue. There is every indication that the klystron will meet all of the performance predictions. V. ACKNOWLEDGMENTS The authors wish to thank the many members of both Varian and SLAC for their valuable contributions to this development and manufacturing effort. G. Hu, F. Friedlander, L. Ives, R. Vranas of Varian provided much of the computer simulation work on the gun, output window collector and RF interaction region. Many valuable engineering contributions, advice and technical support were provided by several departments and shops at SLAC. These efforts will continue through the testing and installation of this tube. VI. REFERENCES [ 11 E. L. Lien, "High Efficiency Klystron Amplifier", Publication OF Eight Int-1 Conf. on M/W and Optical Generation and Amplification, Sept. 1970 [2] E. L. Lien, US Patent #3,811,065 May 14, 1974 [3] W. R. Fowkes et al., "PEP-I1 Prototype Klystron", Proceedings of the 1993 Particle Accelerator Conference, Wash. D. C. Vol. 2, p. 1259 3 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. 

1.2 MW klystron for Asymmetric Storage Ring B Factory

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1.2 MW klystron for Asymmetric Storage Ring B Factory

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