High-Efficiency Klystron Design for the CLIC Project Antoine Mollard1, Claude Marchand1, Franck Peauger1, Juliette Plouin1 Armel Beunas2, Rodolphe Marchesin2 1DRF/Irfu/SACM/Lisah CEA Saclay 91191 Gif-sur-Yvette Cedex 2Thales Electron Devices 2 Rue Marcel Dassault 78140 Vélizy-Villacoublay DDays 2016| Mollard Antoine antoine.mollard@cea.fr Antoine Mollard – DDays 2016

Journées des Doctorants Antoine Mollard Education Engineer Arts et Métiers ParisTech (AM2010) Mechanical and industrial engineering MSc Université d’Aix-Marseille Fusion Science and Technologies PhD contact PhD project proposed by Juliette Plouin (Irfu/SACM) Motivation The diversity of the topics and activities: RF engineering, material science, electromagnetism, modelling and simulation, industrial process monitoring, tests. CEA and research surroundings. Partnership with Thales. Contribution of this project to a bigger one (CLIC). Antoine Mollard – DDays 2016

Journées des Doctorants High-Efficiency Klystron Design for the CLIC Project CLIC is a electron-positron collider project with a 120000 cavities beam line. These cavities need to be conditioned and tested with high power RF signals provided by 12GHz-klystrons. We are developing a high-efficiency klystron in order to save power. Collider : particle accelerator involving directed beams of particles; collisions byproducts give evidences of subatomic structures and high-energy particles. Klystron : 100MHz – 100GHz radio-frequency amplifier used for radar, telecommunication, radiation therapy, particle accelerator or nuclear fusion power plant. Efficiency : klystron’s efficiency is the ratio between its RF output power and its electrical input power. Antoine Mollard – DDays 2016

High-Efficiency Klystron Design for the CLIC Project Klystron principle CLIC and the kladistron project How to make a kladistron? Our work so far Antoine Mollard – DDays 2016

Klystron principle Antoine Mollard – DDays 2016

Working at 100MHz- 100GHz with a narrow bandwidth. What is a klystron? An amplifier Working at 100MHz- 100GHz with a narrow bandwidth. Used in: Radar Telecommunication Radiation therapy Plasma heating Particle accelerator Antoine Mollard – DDays 2016

Klystron principle: how does it work? Cold water Hot water Linear-beam vacuum device which is used as a narrow bandwidth amplifier for high radio frequencies (100 MHz- 100GHz) Creation of a stream of electrons by the electron gun (thermionic emission) Confinement by an external magnetic field Triggering of the beam modulation by the wave in the input cavity Modulation of the beam into a set of electron bunches in the gain cavity/ies Drifting of the electron bunches Energy transfer between the bunches and the output antenna Absorption of the lower energy electron beam by the collector Collector Ionic pump 7 RF output 6 Ps Middle cavity 5 Solenoid 2 4 RF input 3 Pe Control anode Electron beam 1 Vk Ib Klystron efficiency is the ratio of the output power to the electron gun supply power. The microperveance characterizes the space-charge effect on the electron beam motion. 𝜂= 𝑃 𝑜𝑢𝑡 𝑉 𝑘 ∗ 𝐼 𝑏 µ𝑃= 𝐼 𝑏 ∗ 10 6 𝑉 𝑘 3 2 Antoine Mollard – DDays 2016

CLIC and the Kladistron project Antoine Mollard – DDays 2016

CLIC accelerator RF cell Decelerated Drive Beam 100 A The CLIC project CLIC : Compact LInear Collider 3TeV 50km-long electron/positon collider CLIC accelerator is designed with several cavities (about 120000) These cavities need conditioning on specific benches with high-power RF source (klystron). D R I V E B A M L O P S N J C T Y U e - G + F W Z H ( n o t s c a l ) d g r v i m 1 k CLIC accelerator RF cell RF Power (120 MW) Decelerated Drive Beam 100 A A high-efficiency klystron would lead us to a cheaper solution. Accelerated Main Beam 1 A Antoine Mollard – DDays 2016

Kladistron 𝑷 𝒊𝒏 𝑷 𝒐𝒖𝒕 A Kladistron (Kl-adi(adiabatic)-stron) is a high-efficiency klystron with a large number of cavities (at least twice as many as in a classical klystron). 𝑽 𝒌 N cavities ↗ 𝑹 𝑸 ↘ => Efficiency (η= 𝑷 𝒐𝒖𝒕 𝑽 𝒌 ∗ 𝑰 𝒃𝒆𝒂𝒎 ) ↗ 𝑷 𝒊𝒏 𝑷′ 𝒐𝒖𝒕 𝑽 𝒌 Antoine Mollard – DDays 2016

AJDisk 1D-simulations of a 12GHz-Kladistron 10 cavities – 12GHz µP = 1.5 A.V^-3/2 Efficiency 67.2 % Length 197 mm 20 cavities – 12GHz µP = 1.5 A.V^-3/2 Efficiency 78 % Length 285 mm Our preliminary AJDisk results show a higher efficiency than what we can expect from a klystron with such a high microperveance.

Kladistron and classical klystrons 12GHz « KLADISTRON » 4.9GHz « KLADISTRON » (technical demonstration) Antoine Mollard – DDays 2016

How to make a kladistron? Antoine Mollard – DDays 2016

Kladistron and classical klystrons 12GHz « KLADISTRON » 4.9GHz « KLADISTRON » (technical demonstration) Antoine Mollard – DDays 2016

TH2166 Klystron TH2166 front view TH2166 klystron was designed by Thales electron devices (TED) for Mainz Microtron. Features Frequency 4.9 GHz Output power 56 kW Efficiency 50% Ibeam 4.3 A Vk 26 kV µP 1.066 Number of cavities 6 Interaction line length 233mm 233mm This klystron will be modified to verify the kladistron principle. Antoine Mollard – DDays 2016

klystron TH2166 enhancement Cavities preliminary design The design we are looking for would let us : Use the TH2166 klystron test and conditioning bench  Total interaction line length of 233mm, same input and output cavities, same solenoid Use the TH2166 klystron electron gun and collector  Same microperveance of 1µA.V^(-3/2) Check the kladistron principle  More than 6 cavities Avoid cavities coupling  Drift space between cavities larger than 9mm Avoid gain peaks  Low coupling cavities (low R/Q and Q0 values) Thales-provided elements Antoine Mollard – DDays 2016

Our work so far Antoine Mollard – DDays 2016

Thales-provided elements Steel rods and Ionic pump Wave guide, Output cavity and Output antenna Collector These elements have already been delivered. Antoine Mollard – DDays 2016

Th2166 Klystron and Kladistron Comparison MAGIC2D Simulations Our kladistron simulation results reach an efficiency of six points above TH2166 simulation results. The electron bunching and the beam current growth are also smoother. TH2166 beam current along the interaction line TH2166 electron beam Kladistron beam current along the interaction line Kladistron electron beam Antoine Mollard – DDays 2016

klystron TH2166 enhancement Cavities preliminary design Type 1 Type 2 Type 3 Type 4 16 cavities interaction line TH2166 cavity shape Low-coupling cavity shapes Type 1 Type 2 Type 3 Type 4 According to our COMSOL simulations, these low-coupling cavities are fit for smooth electron bunching. Antoine Mollard – DDays 2016

Frequency shift : a reliable tuning system is required Zgap Zgap Type 2 Type 3 Δf/Δzgap ≈ 2.5MHz/µm Antoine Mollard – DDays 2016

Frequency shift : a reliable tuning system is required →Δzgap ≈ -4µm For each computation we shift the frequency of one of the cavities. Kladistron efficiency is sensitive to its cavities frequency shifts, especially at the end of the interaction line. Antoine Mollard – DDays 2016

Cavities and tuning system preliminary design Cooling system channels (4) Beam drift tube Copper This preliminary design is under study and it is considering the surrounding of the cavities (cooling system, solenoid,…). The tuning system is inspired by CLIC accelerating cavities design; a thigh copper membrane is misshaped to adjust cavities frequencies. This strain is controlled by an accurate screw thread. Thread steel pin brazed on the membrane Cavity Bronze bearing Beam drift tube Misshaped membrane Screw thread Tuning system (x4 at 60°) Antoine Mollard – DDays 2016

Tuning system preliminary design COMSOL simulations Displacement (mm) Δf (for each tuning system) (MHz) Δf (for 4 tuning systems) (MHz) 0.26 4.02 16.03 Antoine Mollard – DDays 2016

Prototype cavities to test Prototype cavities to be tested to validate tuning system (and tuning range & resolution) and brazing process. Prototype test bench Antoine Mollard – DDays 2016

The multipactor phenomenon Antoine Mollard – DDays 2016

Problem: the multipactor phenomenon Electric field Electric field Copper Copper ETC… Antoine Mollard – DDays 2016

Solution: titanium layer Electric field Titanium Copper Copper Antoine Mollard – DDays 2016

Solution: titanium Titanium layer: Titanium hydride deposition with a brush Titanium sheet brazing These two technologies will be tried out in a second testing phase. Antoine Mollard – DDays 2016

Conclusion and outlook TH2166 klystron study and codes validation TH2166 kladistron preliminary design TH2166 kladistron’s cavities design finished We will soon test two first prototype cavities (tuning system) Invitation to tender Second testing phase (titanium) TH2166 Kladistron assembled and tested by the end of the year. Antoine Mollard – DDays 2016

Thank you for your attention Antoine Mollard – DDays 2016