I SLAC - PUB - 3993 June 1986 (4 THE SECOND GENERATION SLAC MODULATOR* A. R. DONALDSON, J. C. CRON, AND R. R. HANSELMAN Stanjord Linear Accelerator Center, Stanjord University, Stanjord, Cdijornia QdSOS SUMMARY The Stanford Linear Accelerator Laboratory has undertaken the construction of a single pass electron-positron collider. In order to reach required beam energy 235 new klystrons needed upgraded modulator systems. The collider will use 50 GeV electrons and positrons. The increase in accelerator energy from the present 30 GeV neces- sitates the replacement of existing 35 MW klystrons with new 67 MW units. The doubling of klystron output power required a redesign of the modulator system. The 67 MW klystron needs a 350 kV beam voltage pulse with a 3.7 w pulse’width. A new pulse transformer was de- signed to deliver the increased voltage and pulse width. Pulse cable design was evaluated to obtain increased reliability of that critical element. The modulator, with the exception of its power supply, was rebuilt to produce the required power increase while enhancing reliability and improving maintainability. An investi- gation of present thyratron switch tube performance under the new operating conditions resulted in agitation and some war- ranted panic but these conditions were mitigated after several successful experiments and some evolutionary narrowing of the klystron pulse width. The discussion to follow will cover the up graded modulator system specifications and some details of the new pulse transformer tank,pulse cable, modulator, and modu- lator switch tube. BACKGROUND The klystron that made theStanford two mile linear accel- erator possible was the XK5.’ The 30 GeV electron linac used 245 XK5 stations. The XK5 was initially designed for a 250 kV beam voltage but was later successfully operated at 265 kV with a pulse current of 273 A and a 2.5 us flattop. The XK5 klystron specifications and modulator drive requirements are given in Ta- ble 1. The modulator was of the classic line type with a pair of 10 section pulse forming networks (PFN’s) in parallel which were subresonantly charged and thyratron switch discharged at up to 360 pps. The paralleled PFN’r resulted from the inability to procure 42 kV, 5900 amp, 3.5 us equivalent square wave (ESW) switches in 1963 and the consequent use of two smaller thyratrons discharging individual PFN’s. Single thyratrons be- came available in 1964 from Tung-Sol/Wagner (CH1191) and ITT/Kuthe (KU275A) at which time the PFN’s were paral- leled at the thyratron anode. For the past 20 years these two tube types were successfully used. The average life of the 245 operating tubes has depended upon the accelerator operating regime. Thyratron lifetimes as high as 70,006 hours have been recorded with the accelerator operating at a low duty factor of 60 pps. The lifetimes for operation at 360 pps have averaged 10,000 hours. A program of scheduled ranging of the reservoir voltage has been responsible for the long lifetime and excellent performance (less than one fault or overcurrent per eight hour period). TABLE 1. COMPARISON OF THE ORIGINAL AND SECOND GENERATION SPECIFICATIONS KLYSTRON SPECIFICATIONS F = 2856 MHz THE ORIGINAL SECOND GENERATION SLAC model designation XK5 5045 Beam voltage (kV) 265 350 Beam current (A) 273 414 Microperveance 2 2 RF peak output (MW) power 35 67 RF pulse width (Qs) 2.5 3.7 MODULATOR SPECIFICATIONS FOR MICROPERVEANCE=2 THE ORIGINAL SECOND GENERATION SLAC model designation 6575 150MW Output voltage pulse (kV) 22 23.5 Transformer ratio 1:12 1:15 Maximum peak (MW) power 76 152 Maximum average power (kW) 96 BD 36Opp.s 136 B 18Opps 91 @ 12opps Thyratron anode voltage (kV) 44.2 46.7 Thyratron current (A) 3280 6225 Power supply voltage (kV) 22.2 23.5 Output pulse ESW (us) 3.5 5.0 Repetion rates (pps) 60,120,180,360 60,120,180 Rise time (Qs) 0.7 0.8 Fall time (Qs) 1.2 1.8 PFN impedance (ohms) 7 4 PFN total capacitance (QF) 0.28 0.70 * Work supported by the Department of Energy, contract DE-AC03-76SF00515. Contributed to the 1986 Seventeenth Power Modulator Symposium, Seattle, Washington, June 23-25, 1986