CEPC 650MHz High Efficiency Klystron R&D Zusheng ZHOU(周祖圣) RF Power Source Group Accelerator Division, IHEP May. 4, 2016

Contents ■Present situation ■Design proposal ■R&D Progress ■Summary

Present situation The RF relevant FCC and CEPC machine parameters have well converged – they clearly indicate the need for Higher Energy Efficiency. Tetrodes(四极管） IOTs Conventional klystrons Solid State PA Magnetrons （磁控管） DC – 400 MHz (200 – 1500) MHz 300 MHz – 1 GHz DC – 20 GHz GHz range 1 MW 1.2 MW 1.5 MW < 1MW 85% - 90% (class C) 70% 50% 60% 90% Remark Broadcast technology, widely discontinued Oscillator, not amplifier!

Present situation 2014 saw a breakthrough in klystron theory: The “congregated bunch” concept was re-introduced [V.A. Kochetova, 1981] (later electrons faster when entering the output cavity). The concept of “bunch core oscillations” was introduced [A. Yu. Baikov, et al.: “Simulation of conditions for the maximal efficiency of decimeter-wave klystrons”, Technical Physics, 2014] (controlled periodic velocity modulation) The “BAC” method was invented [I.A. Guzilov, O.Yu. Maslennikov, A.V. Konnov, “A way to increase the efficiency of klystrons”, IVEC 2013] (Bunch, Align velocities, Collect outsiders) These methods together promise a significant increase in klystron efficiency (approaching 90%) An international collaboration has started – prototypes are being designed. (SLAC plans to convert an existing 5045 klystron– simulations are encouraging)

“Bunch core oscillations” explained “Classical” bunching RF=78.0% Useful RF phase Normalised velocity RF period, rad particle phase 2 Output cavity RF=78.0% Useful RF phase New bunching method with core oscillations particle phase 2 Output cavity RF=89.6% Normalised velocity RF period, rad Normalised velocity

HEIKA collaboration HEIKA – “High Efficiency International Klystron Activity” is evaluating and implementing this “breakthrough”. HEIKA Members: Labs (CERN, ESS, SLAC, CEA), Universities (MFUA, Lancaster), Industry (Thales, L3, CPI, VDBT) It studies theoretically and experimentally high efficiency klystrons for both pulsed (e.g. CLIC, ESS) and CW applications (FCC). I. Syratchev (CERN), A. Baikov (MFUA), I. Guzilov (VDBT), J. Neilson, A. Jensen (SLAC), G. Burt, D. Constable, C. Lingwood (U Lancaster), A. Mollard (CEA), R. Marchesin (Thales), Q. Vuillemin (Thales/CERN), C. Marrelli (ESS), R. Kowalczyk (L-3com), (Toshiba), T. Grant (CPI)

Klystron key design parameters Centre frequency (MHz) Collider RF power source Klystron key design parameters Parameters mode Now Future Centre frequency (MHz) 650+/-0.5 Output power (kW) 800 Beam voltage (kV) 80 70 Beam current (A) 16 15 Efficiency (%) 65

实施途径 速调管效率提高主要通过以下途径： （1）低导流系数电子枪 （2）采用高次谐波腔 （3）采用多注速调管 （4）利用降压收集极 （5）束团内核振荡（COM）方案，比如BAC （Bunch Alignment Collector）方法 （6）绝热群聚方法 （7）Congregated bunch concept 目前低导流系数和高次谐波腔组合、低导流系数和多注的组合技术成熟，速调管效率65%以上。 要实现极高效率（80%以上）通常需要多种方法的组合，CEPC速调管将采用方案以下两种方案： （1）单注低导流系数和高次谐波腔组合，理论模拟效率72%以上。 （2）单注低导流系数和BAC组合，期望效率高于80%。 CEPC速调管为CW速调管，功率较低，即使在低导流系数下电压也不算太高，因此可先采用较为简单的单注速调管，多注速调管可同步进行模拟设计。

Short term Schedule

Klystron Schedule and strategy Because of klystron efficiency is more than 80%, in order to fulfill this program, there may have following problems: we (China) have not an experienced to manufacture the high power, UHF klystrons. There is not the big furnace infrastructure in China also. Design and simulation are not enough and matured, therefore we need to step up one by one. Let’s start from beam tester, classical design and currently progressed design such as HEIKA. In order to save the money and time, demountable structure is another way.

Klystron Schedule and strategy(2) Merits Saving the money Saving the time Possible to try 2 or 3 designs (1) Beam tester (2) #1 prototype (3) #2 prototype

Klystron Concept Design Power perveance efficiency Cathode current density frequency 800kW 0.68μP 65% 0.5A/cm^2 650000000Hz Rough Parameters Parameters for Electron optics Voltage Current Depressed Beam Potential Cathode radius Beam Radius Tunnel Radius b /a 阴极面积 电子枪面压缩比 Beam Current Density 80kV 15.385A 78.844kV 34mm 16mm 25mm 0.64 3674cm^2 4.57 1.91A/cm2 Relativistic Mass Factor σ v/c βe γ γb γa cut-off freq of tube Brillouin Flux (Gauss) 1.154 0.499 27.275 23.629 0.378 0.591 5.4 7.07 119.251 3.52E+09 4.60E+09 Plasma Frequency plasma propagation constant plasma wavelength Plasma Frequency Reduction Factor Mbar √Mbar Reduced Plasma Frequency Reduced Plasma propagation constant βq reduced plasma wavelength 1.29E+09 8.58 0.732 0.222 0.874 0.935 2.85E+08 1.905 3.298 Parameters for interaction region calculation

Electron Gun Design and Simulation Egun simulation result: Area compression ratio : 4.4 Perveance: 0.647μP Cathode current density：0.39A/cm2~0.43A/cm2 Max. electric field : 2.520kV/mm Cathode current density -57kV 2.52E+03V/mm -81.5kV 1.57E+03V/mm 0kV Gun region beam trajectory simulation with Egun Electrode E fied density optimization with Possion

Electron Optics Design and Simulation 200mm-1800mm MAX 17.815 200mm-1000mm 1000mm-1800mm 17.7848 MIN 16.7286 16.947 AVE 17.305 17.315 17.295 MAX-MIN 1.0864 0.8378 ripple 6.28% 6.27% 4.84% Laminar flow，Beam ripple <5.2% whole tube beam trajectory simulation with EGUN Magnetic field density of Brillouin Flux ：107.5 Gauss Magnetic field density at cathode: 41 Gauss Magnetic field density at tube body： 200Gauss K=0.75

Gun Assembly Design Basic drawing of gun assembly using Auto Inventor. Geometry is come from the POISSON calculation. Purpose is determine the ceramic sealing structure. And size to order to the company. Based on this first drawing, this will be revised by IHEP Mechanical Group to more complete drawing Current stage of drawing

Gun Assembly Design(2)

Gun Assembly Design(3) In order to study gun assembly problems, several problems occurred at the HV ceramic due to the multipactor are planed to be analyzed. Those are simulated by POISSON and CST, and in order to make clear the geometry, basic drawings.

Collector Design and Simulation Collector shape and beam dissipation optimization J collector optimization using universal beam spread curve collector optimization using EGUN code Analytic design Numerical design Collector Length 2147mm 1912mm Collector radius 210mm Total Beam power 1231kW 1230kW Capability of power density in collector 150W/cm2 Max power density in collector 197W/cm2 207W/cm2 Crosscheck of beam trajectory with EGUN and MAGIC

Collector Design and Simulation(2) Groove dimensions optimization Groove number Groove dimensions(a:b) Total water flow rate Water pressure loss for ideal smooth surface 180 1:2 1400kg/min 2.34E+4 Pa Contour of temperature and water pressure loss

Collector Design and Simulation(3) Collector Thermal analysis Groove structure for 2 meter tapered collector in Ansys-CFX contour of temperature on inner surface of copper domain contour of temperature of water domain

Collector Design and Simulation(4) Collector geometry of tapered part 2030mm A B C D E Geometry of collector tapered part Section A B C D E groove number 180 150 120 60 40

Cavities design and optimization Optimize cavity geometry to get the desired characteristic parameters and high R/Q. TM010 electromagnetic field pattern Table 1 Cavity geometry and characteristic parameters Parameters r_tube L_right_cavity r_nose h_nose Angle_nose r_cavity L_right_gap f MHz R/Q Q M Knife edge Output cavity 27.03 70.71 3 10 45deg 110 18.475 650 125 16984 0.8452 Knife edge input cavity 88.08 27.7 153.5 17399 0.7932 Knife edge 2nd harmonic cavity 36.463 60 9.25 1300 74 9146 0.616

Cavities design and optimization(2) cavity1 cavity2 cavity3 cavity4 cavity5 cavity6 R/Q Qe Qo fo Gap-Gap L Gap Length d 146.8 175.5 4000 648 0.0889 0.4 143.4 1.00E+308 654 0.45 125.6 644 0.05 0.82 111.1 658 0.55 110.3 660 0.3 92.91 38.9 20000 650 M Gb Qb Lmn Qu Qt βq*L*360/(2*PI) βe*d 0.622 2.922E-05 233.154 168.124 97.68 2.425 43.661 238.682 0.4 4000 225.24 49.119 0.829 1.780E-05 447.291 0.85 402.30 1.364 89.505 505.668 1.67 448.91 60.034 509.335 2.22 451.80 32.746 604.668 2.52 38.824 36.48 Total power gain57dB -3dB bandwith:643MHz~658.4MHZ

Current Status -Beam tester Concept Simulation Gun HV Seal Collector Drawing Gun HV Seal Drawing Collector drawing will be proceeded

Current Status -Beam tester Latest drawing of Beam test stand

Summary Complete gun design Order the cathode and ceramic Complete the collector design Complete the designing / drawing and start manufacturing In order to do the quick manufacturing, parallel schedule for manufacturing and cathode manufacturing and processing Furnace problem There is no big furnace in China, we are looking for appropriate company to build it.

Thanks for your attention!