High Power International Workshop on Breakdown Science and High Gradient Technology Peter Kolda, Galen Aymar, Adam Balkcum, Thomas Habermann, CPI Mark Kemp, SLAC High Power Klystrons

2 Startups – 130 Years of Innovation

1885 1909 1934 1939 1948 1954 1956 1957 1962 1968 1970 1976 1998 2003

Poulsen Wireless

3 Silicon Valley Began Here

Silicon Valley

4 Early Days of the Valley • Mission Dolores, 1776 • Mission Santa Clara, 1777 • Discovery of Gold, Sutter’s Mill, 1848 • Gold Rush in 1949 (49ers) • Leland Stanford, Palo Alto, 1851 • , established 1884 • Poulsen Wireless, First Tube Company in Silicon Valley, 1909

5 How Vacuum Tubes Grew in Silicon Valley

Bill Eitel & Jack McCullough Charles Litton Russel and Sigurd Varian, Litton Industries Varian Associates San Carlos, CA San Carlos, CA Palo Alto, CA

Ham Radio Market Vacuum Equipment Klystrons for Radar Landing System

6 CPI High Power Klystrons

• CPI was formed from Varian Associates Electron Device Business in 1995

• Varian brothers’ effort to develop airplane landing radar system • Leads to idea and Stanford University grant to develop, 1937 • Varian product was used in superheterodyne radar receivers • Varian developed linear accelerators with William Hansen of Stanford • High power klystrons followed

HG2018 7 Design Tools for First Pass Success

• Advancement in Software Results in First Design Pass Success • Equivalent Circuit Codes – Interaction Structure Design • Thermomechanical Finite Element Analysis Tools – Product Cooling • Electromagnetic Field Solvers – Cavities, Waveguide and Window Structures • Magnetic Field Solvers – Focusing Magnet • Particle in Cell Codes – Beam Optics, RF Circuit Design, Gun and Collector Design • Solid Modeling – Hardware Design and Drafting of Parts • Resulting in Fewer Hardware Experiments than in the Past • Faster Design Cycle Time • Lower Development Costs • More New Products Developed in Shorter Timeframe

• CPI Designs at Least One New Klystron Each Year

HG2018 8 High Power Klystrons Model Frequency (GHz) Peak Power (MW) VKS-8252 2.856 5-10 VKL-8301 1.3 10 VKX-8311A/B 11.424/11.994 50 VKX-8253A 9.3 GHz 5 VKS-8265A 2.856 65 VKP-9170A 0.704 1.2 VKP-8292A 0.704 1.2 VKS-8245B 2.9985 20 HG2018 9 VKS-8252

• Designed for Varian Clinac linear accelerator for X-ray cancer therapy • 2.856 GHz • 5 MW peak output power

HG2018 10 VKL-8301B Multibeam Klystron

• Designed for DESY European XFEL • 1.3 GHz • 10 MW

HG2018 11 VKX-8253A

• Designed for Compact X-ray Accelerator for Cancer Therapy • 9.3 GHz • 6 MW peak power • 20 kW average power

HG2018 12 VKS-8265A (SLAC 5045)

• 2.856 GHz, 65 MW peak power

• EK = 350 kV IK = 410 A • Solenoid focused • Water cooled • Commercialized version of 5045

13 VKX-8311A/B (SLAC XL4 & XL5)

• Collaboration with SLAC at Stanford University • Commercialized version of XL4 & XL5, 11.424 GHz and 11.994 GHz • Customers include • CERN • Tsinghua University • SINAP • PSI • Applications are • Accelerator beam linearization • Transverse deflection of beams • Component testing

HG2018 14 VKX-8311 Product Improvement

• CERN observation of oscillation superimposed on beam pulse • RF circuit ruled out as source of instability • Observed frequency between 600 MHz and 1.2 GHz • Appeared to change with modulator and klystron pairing • Attributed to resonance between electron gun and modulator oil tank • External loading successfully created to dampen and eradicate mode • Internal solution to disrupt and eradicate oscillation successful and implemented

HG2018 15 VKP-9170 European Spallation Source

• Multibeam Inductive Output Tube (IOT) • Not really a klystron but has klystron features • Potential Source for ESS, European Spallation Source • Collaboration between CPI and Thales • Prototype built and tested • Achieved over 1 MW • Window assembly arcing caused rework of unit • Rebuilt unit in 2017 • In queue for retest at CERN • Testing resumes in July

16 VKP-8292A

• European Spallation Source sponsored development • 704 MHz • 1.2 MW peak power output • 36 units procured between 2 suppliers for first phase • CPI awarded 18 units • RF station total is 120 • Next klystron request for tender anticipated to be in Fall 2018

HG2018 17 Most Recent Development - VKS-8245B

Developed in 2017-18 for STFC at Daresbury for CLARA TDC

An example of first design pass Success

2.9985 GHz Parameter Value Units

EK 280 kV

IK 230 A

PD 500 W

tP(RF) 6 µs PRF 150 Hz

PO (peak) 20 MW

PO (avg.) 20 kW HG2018 18 VKS-8245 Design Summary

• 5–cavity design • Cavities 1 through 4 factory-tunable 1D RF Simulation (LSCEX) • Grounded collector Eb = 243 kV Ib = 230 A

Design process utilizing • HFSS, Superfish (RF Cavities, OP Window) • LSCEX, TESLA (RF Design) • MagNet (Magnetic Field) • XGun, MICHELLE (Beam Optics) • ElecNet (Electrostatics) • ANSYS (Mechanical/Thermal) • CFX (Cooling) Electron Gun Field Analysis

Mesh geometry engineered to give Objective is to detail to areas of interest (high 200 kV/cm analyze electric field intensity gradient regions) 197 kV/cm in the electron gun

182 kV/cm Reduce electric field stress to below threshold of breakdown for high reliability Focus Electrode

Electrostatic Field Profile In Gun Region

HG2018 20 Focusing Electromagnet Design

Values in Teslas Analyze magnetic flux Magnetic Field Profile at Bore in return path for saturation of iron.

Designed to provide Magnetic Field Profile shown on right.

9 Coil Design

Solenoid outer shell peak magnetic field < 6,000G IP PP peak <2,000G OP PP peak <5,000G Collector Analysis

Analyze the electron trajectories including Primaries secondary, tertiary, etc., electrons

Design to prevent electrons from being Gen 1 reaccelerated toward the cathode

Analyze the thermal loading of the internal Gen 2 collector structure for thermal management plan

Gen 3 HG2018 22 VKS-8245B First Test Results – 20 MW

Drive Power Green Output Power Blue First Beam Voltage Yellow delivery Beam Current Pink scheduled in June

HG2018 23 Boosting Klystron Efficiency

• DOE Accelerator Stewardship GREEN-RF Project • Dr. Mark Kemp, SLAC Lead Investigator • CPI VKS-8262 retrofitted with multistage depressed collector for Inverse Marx Generator • First pass success • Planning next steps

Compliments of Dr. Mark Kemp, SLAC

24 Inverse Marx Generator Topology

Simplified equivalent circuit model of energy recovery circuit (left) during the klystron pulse and (right) during energy recovery. The load circuit is an approximation of the cathode modulator of the klystron. The energy recovered to this load capacitance is equivalent to the energy recovery of a DC filter capacitor in a modulator. Compliments of Dr. Mark Kemp, SLAC

HG2018 25 GREEN-RF Collector Test Results

• Measured klystron beam voltage and Current • Current measured in the secondary of the modulator transformer • Data taken March 2018

Compliments of Dr. Mark Compliments of Dr. Mark Kemp, SLAC Kemp, SLAC HG2018 26 Inverse Marx Generator Performance

Top: Measured collector stage voltage compared to beam voltage divided by 4 to match scale.

Bottom: Measured collector stage current.

Compliments of Dr. Mark Kemp, SLAC HG2018 27 Preliminary GREEN-RF Conclusions

• The SLAC team has determined: 1. The approach does allow for control of the collector depression voltages 2. Energy is recovered 3. Energy in the rise and fall time are recovered 4. Approach works on a bunched beam

Next step: Compare design models prediction to results

Compliments of Dr. Mark Kemp, SLAC

HG2018 28 Thank you

HG2018 29