20/3/2014 MEW weekly meeting Summary on high efficiency klystron design Chiara Marrelli

How a klystron works Continuous beam emitted from the thermionic cathode Input bunching cavity (RF input from the preamplifier) The beam is velocity modulated Velocity modulation become density modulation in the drift Due to bunching the beam current gets a strong RF component The RF component of the beam current excites a field in the output cavity. The field is extracted by the output waveguide and the beam looses its energy and reaches the collector (like waves on the shore, as the greek word κλυσ suggests). Idler cavities can enhance bunching

Klystron efficiency CLIC drive beam 3 TeV: 1638 accelerating structures with 1 GHz RF pulses of 15MW: total pulsed RF power of GW required Pulse length of 150 ms and repetition rate of 50 Hz average RF power at the accelerating structure input is 184 MW with challenging specifications in terms of stability: ± 0.05° in phase 0.2% in amplitude Very high efficiency sources needed to provide this power at affordable cost (efficiency>70%) low microperveance Another obstacle to high efficiency is velocity spread in the bunch, which prevents the particles from being decelerated to the minimum energy in the last cavity, as we are limited by the slowest electrons (to avoid reflected electrons). Perveance indicates how much beam current comes out of the cathode when the voltage V is applied between the cathode and the anode. Perveance can be considered as well as a measure of space charge forces. Lower perveance beam with weaker space- charge forces enables stronger bunching and thus consequently higher efficiency.

Klystron design AJDisk: 1-D code currently used at SLAC for the design of round and sheet beam klystrons; The beam is split into a series of disks of charge moving only in the longitudinal direction; Disks are acted by both the cavity fields and the space charge fields; 1-D code: doesn’t take into account focusing problems; Very reliable for small beam currents; Kinetic efficiency Electric efficiency Procedure: Simulations and optimization with AJDisk (1-D klystron code from SLAC) Cavity design (SUPERFISH, HFSS) PIC simulations

Klystron design: classical approach During optimisation, the tuning of all parameters is done to provide the highest bunched current harmonics at the entrance of the output cavity. The inner structure of the bunch must also be optimal in terms of change density and electron velocities distributions to get highest efficiency. The optimization of the parameters includes : Cavity impedances Drift lengths Input and output coupling Coupling coefficients M (transit time factors)

Cavity detuning If the cavity is detuned at f>f 0 (inductive detuning) β V (kV) t Accelerated particles (bunch tail) Decelerated particles (bunch head) If the cavity is detuned at f<f 0 Velocity spread is increased, and bunching is enhanced Accelerated particles (bunch head) Decelerated particles (bunch Tail) β V (kV) t Detuning at frequencies lower than the working frequency has the opposite effect. Velocity spread is reduced (good before the extraction cavity), but bunching is reduced.

Cavity detuning If the cavity is detuned at f ≈ 2f 0 (2 nd harm. cavity) Collecting outside particles (bunch tail) V (kV) β t (bunch head) If the cavity is detuned at f ≈ 3f 0 (3 rd harm. cavity) βV (kV) t No harmonic cavities used: Space charge forces accelerate the electrons in front of the bunch and decelerate those immediately behind the bunch center. This causes a second harmonic to be added to the velocity distribution. Two bunches are formed inside the bunch, leaving the area near = with a sparse electron population. Then, in the final drift, these two bunches converge towards the central electrons. This process require a long drift space. Klystron with 2 nd (and 3 rd ) harmonic: The harmonic component is added “artificially” by the cavity, producing the two bunches inside the bunch in a shorter space. This process is very efficient at low perveance (less space charge forces). For higher perveances the harmonic component produced by the space-charge is more effective.

The extension of existing technology above 20 MW RF power and 75% efficiency looks very challenging. One will need to increase substantially the number of beamlets in MBK and/or cathode voltage. All these might be rather expensive (complicated cathode unit and long tube length). High efficiency klystron design: new perspectives Which are the factors limiting the klystron efficiency? Perveance can be considered a measure of space charge forces. Lower perveance beam with weaker space-charge forces enables stronger bunching and thus consequently higher efficiency. Another obstacle to high efficiency is velocity spread in the bunch, which prevents the particles from being decelerated to the minimum energy in the last cavity, as we are limited by the slowest electrons (to avoid reflected electrons). High efficiency klystron design: new perspectives

Klystron design: optimum bunch structure for maximum efficiency Congregated bunch (Moscow Phys.Tech. Institute, 1978) Each consequent electron entering the cavity has higher velocity than preceding one. If then, at the cavity exit all the electrons have equal velocities – the ultimately high efficiency can be obtained. For the given beam power and output cavity impedance this solution is unique. Input velocity distribution along the RF phase for optimal bunch (electrons decelerated to the same final speed=to 0.3 of the initial speed – red line)

Klystron design: optimum bunch structure for maximum efficiency An 80% efficient 7-beam MBK was built in by S. Lebedinskiy, USSR, following this strategy in the design. Simulations (dash) Measured (solid) Pin (nominal) Pin (nominal/2) To approach efficiency in vicinity of 100%, it is necessary that all the particles, including those who experienced least modulation will reach the core of the bunch. How can this be achieved? “Simulation of Conditions for the Maximal Efficiency of Decimeter-Wave Klystrons”, A. Yu. Baikov, O. A. Grushina, M. N. Strikhanov It is necessary that the bunch contains all particles, including the ones at the periphery of the period (with phase close to ±π). Bunching has to proceed nonmonotonically; particles near the center first approach it and the move away, performing oscillations due to space charge forces, whereas peripheral particles approach the center all the time. 90% efficient klystron

Optimum bunch structure for maximum efficiency Benchmarking with different 1D codes (5 cavities tube) A.Baikov KlypWin Efficiency 86% I. Guzilov KLYS4.5 Efficiency 82.7% Efficiency 86%, with original parameters from A. Baikov but reflected electrons. 80% after further optimization to avoid reflected particles C. Marrelli AJDisk

Optimum bunch structure for maximum efficiency Interesting Phase space rotation at the end of every drift due to space charge forces

Optimum bunch structure for maximum efficiency: BAC method To increase the efficiency, one should increase the length of interaction space and wait while outsiders join the bunch successfully. As a result of this approach, the device could be very long, especially at low perveance and high power. New design methods proposed by JSC “Basic Technology of Vacuum Devices”, Moscow, Russia, to get very high efficiency (80%) MBK. Each oscillation in BAC method consists of 3 stages: - traditional bunching, which increase the space charge density of the core; - alignment velocity spread of electrons; - collecting the “particles-outsiders”, which reduce the space charge density of the core. In order to intensify the process of the core oscillations, you can use the external forces – BAC method: This method of spatial enhancing of the core oscillations frequency allows reducing at least by factor of 2 the length of the interaction space for high efficiency klystrons. Tube designed by I. Guzilov: length:1.2 m; 116 kV; efficiency: 80.3%; Output power: 0.67 MW/beam, 30 beams

BAC method study 10 cavities, total length 1.25 m, 75% efficiency (to be improved): Second harmonic cavities + Lower frequency cavities + Ind. detuned cavities

Between 3 rd and 4 th cav Input of 5 th cav Input of 6 th cav Between 6 th and 7 th cav Input of 8 th cav Before output BAC method study

Inside last cavity: Thanks to: -I. Syratchev -I. Guzilov -A. Baikov