Proceedings of EPAC 2002, Paris, France DEVELOPMENT OF AN 805-MHZ, 550 KW PULSED KLYSTRON FOR THE SPALLATION NEUTRON SOURCE* S. Lenci, E. Eisen, and B. Stockwell, CPI, Palo Alto, CA, USA Abstract The Spallation Neutron Source (SNS) is an accelerator-based neutron source being built in Oak Ridge, Tennessee, by the U.S. Department of Energy. The SNS will provide the most intense pulsed neutron beams in the world for scientific research and industrial development. CPI has supported the effort by developing 2.5 MW and 550 kW pulsed klystrons. The 2.5 MW tube met all performance requirements, but a 5 MW version was chosen as the project evolved. Los Alamos National Laboratory (LANL) has placed an order with CPI for 65 of the 550 kW klystrons for the super-conducting portion of the accelerator. The primary output power requirements are 550 kW peak, 49.5 kW average at 805 MHz, with an electron beam-to-rf conversion efficiency of 65%. The prototype unit is schedule to be in test in July. Performance specifications, computer model predictions, and prototype operating results are presented. long pulse device. Great care is taken to ensure a wellbehaved beam is obtained. The rf-circuits contain six cavities, including one tuned slightly below the second harmonic of the operating frequency. The designs are optimized to provide the required efficiency and gain without compromising bandwidth. The first two cavities are staggered around the operating frequency to provide the bandwidth. Next is the second-harmonic cavity followed by two inductively tuned cavities to optimize the electron bunching. The output cavity then extracts energy from the beam. The rf-circuit is designed using 1-D and 2-D particlein-cell codes developed at CPI. Many years of benchmarking the codes to measured results has lead to high confidence in the results. SUPERFISH is used for cavity design, while HFSS and MAFIA are used for the output cavity, coupling loop, and output window design. 2.2 Mechanical Design 1 INTRODUCTION CPI, formerly the Electron Device Group of Varian Associates, has a long history of building high-power pulsed UHF klystrons for many applications. In the early 1970’s, approximately 70 VA-862A klystrons were built for the Los Alamos Meson Physics Facility (LAMPF, now known as the Clinton P. Anderson Meson Physics Facility). These tubes were rated for 1.25 MW peak, 150 kW average at 805 MHz. CPI was awarded the contract by LANL to build one 2.5 MW klystron, VKP-8290A, in May 1998, with the unit delivered in September 1999. An order for 65 of the 550-kW tube, VKP-8291A, was place in February 2001. The prototype unit had a performance issue and the second unit is being assembled with modifications. The contractual delivery rate is 2 per month 2 DESIGN 2.1 Electrical Design Figure 1: Comparison of SNS Klystrons The electron gun design is primarily performed using XGUN, starting with the electrostatic beam optics. Once the performance is satisfactory, the design is refined with magnetic field is applied. Care is taken to evaluate and minimize the beam scallop down the drift tunnel. Analyses are performed at various operating conditions. The voltage gradients of the gun electrodes are analyzed with a goal of a maximum gradient of 60 kV/cm for this _________________________________________ *Work supported by US Department of Energy Both klystrons were required to operate in a vertically with the gun down. Figure 1 shows the tubes on the same scale. The two buncher cavities and the two inductively tuned cavities have stainless steel walls with copper endwalls, with cavities 4 and 5 copper plated to reduce resistive loss. The second harmonic and output cavities have OFE copper walls. All cavities on the 2.5 MW klystron, except the output, have one adjustable drift-tube tip and an adjacent flexible cavity endwall to allow for 2323 Proceedings of EPAC 2002, Paris, France beam voltage of 113 kV and a beam current of 41.5 amperes. Additionally the klystron had to demonstrate stable performance and achieve 85% of its rated power at six equally spaced positions of a 1.5:1 mismatch. Figure 5 plots a transfer curve at different mismatch positions. 3000 Beam Voltage = 113 kV, Beam Current = 41.5 A 2500 -1 dB BW 2000 Output Power, kW adjusting the tuning. The 550-kW tube incorporates diaphragm tuners in the cavities. The rf energy is extracted through a single window with an alumina ceramic. The pillbox window is designed around WR-975 waveguide. The collector for both tubes is designed to dissipate the entire beam energy. On the VKP-8290A, it is isolated from the body to allow the monitoring of body current. It is made with thick-walled OFE copper with drilled-holes for the coolant to pass. The end-cap bolts on with an o-ring seal. On the VKP-8291A, the collector is made from thick-walled copper with grooves milled into the outer wall for the coolant to pass. The water-jacket is part of the brazed collector assembly. Both collectors were proof tested at 200 psi (13.6 bar). 1500 Beam Voltage = 118 kV, Beam Current = 39 A Measured Output Power, kW (9/23/99) 1000 Predicted Output Power, kW (Presented at CDR) 500 0 802.5 803 803.5 804 804.5 805 805.5 806 806.5 807 807.5 Frequency, MHz Figure 3: VKP-8290A Bandpass Curve 3 Beam Voltage = 113 kV, Beam Current = 41.5 A 2.5 Beam Voltage = 118 kV, Beam Current = 39 A Ou 2 tp ut Po we 1.5 r, k W 1 Measured Output Power (9/23/99) Predicted Output Power presented at CDR (11/3/98) 0.5 0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 Drive Power, Watts Figure 4: VKP-8290A Transfer Curve 3.000 #5 #6 #4 #1 2.500 #3 #2 O 2.000 ut pu t 1.500 Po we r, k 1.000 W Figure 2: VKP-8290A Klystron 3 TEST RESULTS 3.1 805-MHz, 2.5-MW Peak Klystron 0.500 The peak power (2.5 MW), efficiency (55%), gain (45 dB), and bandwidth (± .7 MHz) specification were all achieved. The bandpass and transfer curves can be seen in figures 3 and 4. The comparison to the predicted performance is quite close. This data was taken at a 0.000 0.0 2324 Cathode Voltage = 113 kV Mod Anode Voltage = 95.8 kV Beam Current = 41.2 A Frequency = 805 MHz PRF = 60 Hz RF Duty = 10% 20.0 40.0 60.0 80.0 100.0 Drive Power, Watts Figure 5: VKP-8290A Transfer Curves at Equally Spaced Mismatch Positions 120.0 Proceedings of EPAC 2002, Paris, France 3.2 805-MHz, 550-kW Peak Klystron 600 Peak Output Power (kw) The prototype unit had a performance issue and the second unit is being assembled with modifications. The following picture shows the klystron. Figures 7 and 8 present the predicted performance of the VKP-8291A. Actual test data is expected in July. 500 Max. Output Power Variation = 400 300 BW = +/- 1.3 MHz 200 Ek = 75 kV Ik = 11.3 a pd = 4.6 w 100 0 0.803 0.804 0.805 0.806 0.807 Frequency (GHz) Figure 8: VKP-8291A Predicted Bandpass Curve 4 CONCLUSIONS The measured results of the 2.5 MW klystron instill high confidence in our simulation codes. It also demonstrated a high degree of stability under various operating conditions. Although a minor set-back was experienced on the prototype 550-kW klystron, all aspects of the specification are expected to be met. Frequency Peak Cathode Voltage Peak Mod Anode Voltage Peak Beam Current Perveance Peak Output Power Efficiency RF Duty Cycle RF Pulse Length Peak Drive Power Gain VKP-8290A Measured 805 MHz 113 kV 95.8 kV 41.5 Amps 1.1 2,605 kW 55.5 % 10 % 1.67 msec 67 Watts 45.9 dB VKP-8291A Specification 805 MHz 75 kV N/A 11.3 Amps .55 550 kW > 65 % 9% 1.5 msec 5.5 Watts > 50 dB Figure 6: Prototype VKP-8291A Table 1: Performance Comparison Peak RF Output Power 60 500 58 Sat. Point @ 805 MHz po = 565.6 kw η = 66.7 % pd = 4.6 w Gain = 51.22 dB 400 300 56 Gain (dB) Peak Output Power (kw) 5 ACKNOWLEDGEMENT Gain 600 54 200 52 Ek = 75 kV Ik = 11.3 a 100 50 0 The authors would like to thank their co-workers at CPI for their contributions throughout the development of these products. They would also like to thank the CPI management team for their support. Finally many thanks go to the technical leaders and their colleagues at Los Alamos National Lab, in particular Dan Rees and Paul Tallerico. 48 0 1 2 3 4 5 6 7 8 RF Input Drive Power (w) Figure 7: VKP-8291A Predicted Transfer Curve 2325