Development of klystrons with ultimately high - 90% RF power production efficiency A. Baikov (MUFA), I. Syratchev (CERN), C. Lingwood, D. Constable (Lancaster University) LANCASTER UNIVERSITY POWERPOINT TEMPLATES These PowerPoint templates are for use by all University staff. Please see below for further information regarding the use of these templates. Should you have any further queries, please contact the marketing team via webmaster@lancaster.ac.uk Template slide 3: Insert a new slide If you need to insert a new slide, from the ‘home’ toolbar, click on ‘new slide’ and select from the templates the style you require from the dropdown box. Template slide 4: Typing new text and copying text from another document New text should be typed over the text in the appropriate template. Copy and pasting text from another document will result in changing the style of the typography and layout. This is unavoidable as it is part of the Microsoft software. We appreciate that in sometimes you will need to copy text from another document into this template. Once you have pasted the existing text into the template, you will need to change the formatting so that they typefaces, sizes, colour, line spacing and alignment are consistent with the rest of the template. Template slide 5: Inserting images There are three choices of templates with images already inserted. Please use the template with the relevant image size and positioning. To insert an image, please go to ‘insert’ then ‘picture’ and find your image, highlight it and ‘insert’. Resize the image and position as per the example template. Template slide 6: Text boxes If a text box is deleted, either insert a new slide (using the appropriate template) or go to another slide and copy a text box. To select a text box for copying, please click on the outer edge of the text box so that the line goes solid (not dashed). Right click your mouse and select ‘copy’, then go back and ‘paste’ it into the slide where the text box is missing which should paste into the correct position on the slide. Template slide 7: Other information Typefaces, sizes and colours All copy is Calibri. Slide title copy throughout: Size: 36 point Colour Lancaster University red: (RGB) R: 181 G: 18 B: 27 (recent colours on PowerPoint) Small copy on first and last slide: Size: 16 point Colour grey: (RGB) R: 102 G: 102 B: 102 (recent colours on PowerPoint) Sub-headings: Size: 24 point – italics Colour grey: (RGB) R: 102 G: 102 B: 102 (recent colours on PowerPoint) Bullets copy and body copy: Size: 24 point (see below for a further option) Colour grey: (RGB) R: 102 G: 102 B: 102 (recent colours on PowerPoint) It isn’t advisable to have too much text on a slide, however on rare occasions it may be necessary, therefore there is a slide using 20 point bullet pointed text. Line spacing and alignment Single line spacing (apart from the main headings which is ‘exactly 35 point’) All text is aligned left Slide title options There are two options for titles on the slides – one line or two lines for longer titles. Ideally, the one line title should be used, however on rare occasions a two line title maybe needed.

Introduction FCC has high power requirements due to high SR in FCC-ee. 100 MW CW Traditional Klystrons -> 70% Theoretical efficiencies up to 80% MBIOT -> 80% Simple minded calculation of wall plug power @ 70% = ~140 MW @ 90% = ~110 MW

State of art: L-band 10 MW MBK klystrons for ILC. In terms of achieved efficiency at 10 MW peak RF power level, the existing MBK klystrons provides values very close to 70%.

Scaling of the klystron parameters thanks to (C Marelli) Design values are in black To go higher in efficiency, the intrinsic limits of the bunching processes and deceleration in the output cavity need to be understood

Process in the traditional Klystron Bunching monotonic – electrons move to center of bunch Many electrons miss bunch. Significant energy left in bunch! Significant charge outside bunch. Velocities aligned…

Process in the traditional Klystron Bunching monotonic – electrons move to center of bunch Significant charge outside bunch. Velocities aligned… Many electrons miss bunch. Significant energy left in bunch!

Limitations For high efficiency traditionally we chase low perveance High voltages Low currents (many beams) For high power both become “unpleasant”. Limited by the slowest electrons (must avoid trapping or reflecting electrons) Ultimate theoretical efficiency limited to 80%

Methods to get high efficiency Space charge Debunching Bunching split into two distinct regimes: non-monotonic: core of the bunch periodically contract and expand (in time) around center of the bunch outsiders monotonically go to the center of the bunch Core experiences higher space charge forces which naturally debunch Outsiders have larger phase shift as space charge forces are small Very long very efficient tubes result. Phase Traditional bunching Core oscillations Cavity ё Space

90% Efficient Klystron Efficiency increases with number of core oscillations and reaches 88-90% for 4-5 oscillations

Process in the high efficiency klystron Final compression and bunch rotation prepare congregating FS bunch. After deceleration all the electrons have identical velocities. Mission accomplished Electron velocity/density The fully saturated (FS) bunch

Process in the high efficiency klystron Final compression and bunch rotation prepare congregating FS bunch. After deceleration all the electrons have identical velocities. Mission accomplished Electron velocity/density Output of traditional klystron The fully saturated (FS) bunch

Comparison of the two bunching methods. Core oscillations RF current harmonics For the ultimate high efficiency, there is a substantial increase of the bunching length. Efficiency degradation up to perveance as high as 110-6 appeared to be rather small (about 3%). Standard practice of reducing the klystron perveance is not the way to achieve very high, above 80%, efficiency.

Methods to get high efficiency BAC Method (I. Guzilov) Again based on core oscillations Interaction space is wasted “waiting” for space charge forces to debunch. A cavity can achieve the same thing in a shorter space by aligning electron velocities Structure half the length while maintaining efficiency. ё 13

Potential FCC Klystron Specification Using core oscillation method Frequency: 0.8 GHz RF power: 1.5 MW Beam voltage: 40 kV Number of beams: 16 Total current: 42A Per beam 2.6 A Efficiency: 90% Perveance: 0.326 microPerv Duty 100% ~90% @ ~0.3

Tube configuration Cathode loading: 2 A/cm^2 Beam radius: 3 mm Filling factor 8 mm Length: 2.3 m Beam circle radius: 75 mm Solenoid field (2x): 600 G Solenoid radius: 150 mm Collector: common Nominal load: 170 kW Peak load: 1.5 MW (no RF)

MBK Structure: Cavity Coaxial cavity Best trade off between compactness, R/Q and manufacturability R/Q 22 Ohms @ ~800 MHz Single beam equivalent 352 Ohms Ez

Proposed Interaction Structure 3 Oscillations of core. 90.42% efficient

AJDisk comparison A dubious 92.2% but some validation Some reflected electrons Greens function method limited at high efficiency

PIC

Outlook 90% is theoretical but: No new materials needed No new manufacturing techniques needed No additional complexity Simply existing technology reconfigured Would normally expect to lose 5% points (so 85%) from simulation to reality Tube also well suited to ESS, potential for prototype? Prototypes planned for proof of concept

HEIKA Collaboration ( as of January 2015) CEA PEAUGER Franck PLOUIN Juliette DALENA Barbara Thales MARCHESIN Rodolphe VUILLEMIN Quentin Lancaster LINGWOOD Christopher CONSTABLE Dave HILL Victoria ESS MARRELLI Chiara CCR Inc. READ Michael CERN SYRATCHEV Igor MUFA, Moscow BAIKOV Andrey JSC ‘VDBT’ GUZILOV Igor

HEIKA’s tubes selection L-band: CLIC: Frequency 1.0 GHz, pulse length 150 microsecond, 20 MW Multi-beam klystron with 40-60 beams. Microperveance per beam 0.3-0.5, operating voltage below 60 kV. Expected efficiency above 85%. FCC (ESS): Frequency 0.8 GHz, continuous wave, 1.5 MW Multi-beam klystron with 10-16 beams. Microperveance per beam ~0.2, operating voltage 40-50 kV. Expected efficiency above 90%. S-band: 3 GHz technology demonstrator. 6 microsecond, 6 MW Multi-beam klystron with 40 beams. Microperveance per beam <0.3, operating voltage 52 kV. Expected efficiency >70% (with PPM focusing). X-band: 12 GHz klystron with adiabatic bunching. 5 microsecond, 12 MW. Microperveance per beam ~1.5, operating voltage 170 kV. Expected efficiency >75%. Low perveance MBK High perveance single beam

Prototype (KIU-147A) I Guzilov (JSC) F 2.856 GHz Power 6 MW Voltage 52 kV Efficiency > 76 % F 2.856 GHz Power 6 MW Voltage 52 kV Efficiency 45 % Beams 40

Roadmap for high-efficiency high RF power klystron development L-band, CW/long pulse FCC, ESS <20 beams; <50 kV Optionally – gun with controlled electrode (2.5 kV) 3 years 3 years L-band. CLIC. 40 beams; 60 kV 1.0 year X-band Kladistron S-band Demonstrator 40 beams; <60 kV L-band CLIC 6-10 beams; ~160 kV L-band ILC 6 beams; 116 kV 1.0 year Exists 2 6 20 40

Conclusion Using new bunching theory 90% (at least in simulation) looks possible for FCC/CLIC/ESS klystrons Low voltages achievable No new technology, simply a design breakthrough Prototypes and further validation required International collaboration at work