Depressed collector option for HE 50 MW Xband

Depressed collector option for HE 50 MW X-band klystron Jeff Neilson, Mark Kemp – SLAC CLIC Workshop January 23, 2019

Overview • Pulsed depressed collector technology • Test results • Application to CERN HEX klystron • Conclusions 2

What is a Depressed Collector? Spent Beam for CERN HEX Klystron 3

Conventional Depressed Collector Power Supply In the conventional approach to depressed collectors, the power supply must be replaced with one appropriate for multiple collector stages Power Supply Upgrading existing systems is very expensive Power Supply 4

Feed-forward energy recovery scheme Power Supply Energy Recovery Ideal Characteristics for “drop-in” retrofit for use of depressed collectors for increased efficiency : • Existing infrastructure is re-used so power supply remains the same • Energy is recovered by a separate component, and returned to the DC supply • Energy recovery is completely passive 5

An “Inverse” Marx Energy Recovery Modulator • Capacitors charge in series, and discharge in parallel • Each cell pre-charged to a voltage • The initial voltage at a tap is the number of cells below that tap times the pre-charge voltage During pulse • Voltage rises in response to current to stage, and capacitance In-between pulses • CLC resonant structure transfers energy in capacitors to modulator • Voltages re-settle to pre-charge value 6

Ability to Recover Rise and Fall of Pulse Amplitude RF is applied when the cathode current is flat • RF Cathode current Time A key advantages of a depressed collector for pulsed klystrons is the ability to recover energy during the rise and fall time • Even for well designed modulators with short rise and Energy wasted in collector during rise and fall times fall, large fractions of the energy supplied to the beam can be wasted • With proper energy recovery, could relax modulator rise and fall constraints, potential cost savings 7

Test Results and Current Programs

Sub-booster Demonstration (S band, 65 k. W, 25 k. V tube) 9

Sub-booster Demonstration Comparison of Simulation to Experiment Without pre-bias applied: During Pulse Voltage (V) Capacitors pre-charged: During Pulse Cathode Potential (/3) Collector Stages Time (µs) 10

Pulsed Depressed Collector Programs at SLAC Two programs to upgrade efficiency of existing moderate efficiency (~40%) Sband klystrons with pulsed depressed collectors (DC) • 5. 5 MW, 6 us, 180 Hz, 120 k. V, VKS 8262 tube in collaboration with CPI. Target efficiency of 57% with DC. • 65 MW, 3 us, 120 Hz, 350 k. V, 5045 tubes used in SLAC LCLS. Target efficiency of 65% with DC. 11

Recent Demonstration at SLAC • Four stage collector on modified VKS-8262 tube • First test of multi-stage depressed collector with energy recovery between pulses has been completed • Measured power recovery close to predicted values Depressed Collector on VKS-8262 “Inverse” Marx power recovery module 12

Application to HEX Klystron

DC Efficiency Gain Estimate for CLIC HEX Klystron 14

Improvement in Total System Efficiency for DC 15

Power Savings Estimate • For recent 380 Ge. V CLIC concept, with operation time of 5000 hours/year, 50 MW rf power, duty cycle 0. 01%, 60 €/MW hour - 3 K €/year/tube for a system efficiency of 50% - 2. 2 K €/year/tube for a system efficiency of 68% • Savings around 800 Euro/year/tube • ROM cost for single stage energy recovery 18 K – 26 K € per tube • Calculation doesn’t include reduction in cooling costs and capital cost savings from power plant size reduction 16

Capital Cost Reduction • Average power draw reduction with depressed collector would be around 2. 6 k. W/tube • Assuming cost of depressed collector modification is 22 K €/tube capital cost of power plant would have to be >8 K €/ k. W to break even immediately, 5 K €/ k. W for 10 years • Capital cost of nuclear power plants on order of 5 K €/ k. W 17

Conclusions • Pulsed depressed collector technology particularly useful for retrofitting low efficiency (<50%) existing systems where energy costs are high • Because of high tube efficiency of HEX klystron, largest gain is from recovery of pulse rise/fall time power loss • Use of depressed collector for HEX klystron could potentially decrease power requirement by 25% • Cost recovery case for use of DC not obvious 18

Backup Slides

The Effect of Collector Efficiency on Overall Efficiency “If I have a 50% efficient klystron, and a 60% efficient collector, 60% of the wasted energy is recovered, so do I have a 80% efficient system? ” With 10 W beam power in and 5 W RF out, 3 W (0. 6 * 5 W) would be recovered and 2 W would be wasted. To produce that 5 W of RF power, how much net power is input to the system? • 5 W (for RF out) plus 2 W (wasted in collector) is 7 W total input to the system • System efficiency = RF Pout/Pin = 5 W/7 W = 71. 4% not 80%! Correct relation: