Accurate Modelling of Monotron Oscillations in Small and Large Signal regimes

In Klystron amplifiers, monotron oscillations may cause unacceptable beam instabilities. To facilitate fast and accurate analysis of such processes, rather than performing timeconsuming Particle-in-Cell simulations, the theory of the monotron oscillations has been further developed for the small and large signal regimes and implemented into the klystron computer code KlyC. This development includes full considerations of the space charge effects, relativistic effects and can operate with arbitrary field distributions of the resonant mode. The effectiveness of these methods, have been demonstrated through benchmarking against PIC codes and has shown good (at 1% level) agreement, whilst KlyC computation time is significantly (at least 100 times) faster than in the PIC simulations

Beam Optics Study on a Two-Stage Multibeam Klystron for the Future Circular Collider

The two-stage (TS) multibeam klystron (MBK) technology has recently attracted significant research attention due to its compactness and high-efficiency (HE) performance. However, there is still a lack of scientific research on the beam optics for such microwave power sources integrated with a postacceleration (PA) gap. In this article, a comprehensive optics study based on the newly developed 2-D optics code CGUN is conducted for the first time to demonstrate the most critical steps in the optics design process, by adopting the 400-MHz TS MBK for the future circular collider (FCC) as an example. Two specific challenges arise in this TS MBK, which are studied in this article, and solutions are given. First, due to the combination of slow electrons, impedance change from individual beamtubes into common volume, and the mild decay of the magnetic field, there are possible reflected electrons at the collector entrance. This requires an increase in the beam voltage to 80 kV, beyond the requirements from considering the output gap alone, as well as tighter control on bouncing electrons. The beam scalloping is also found to be highly sensitive to the position of the PA gap and magnetic field, which later demonstrates that large gap length and magnetic field are required. Final all-in-one particle-in-cell (PIC) simulations of this klystron equipped with this special optics design demonstrate that the specification of 1.2-MW continuous wave (CW) power is practically attainable with an efficiency of 77.5% and without the presence of reflected electrons at any point in the whole circuit

High gradient testing of an X-band crab cavity at XBOX2

CERN’s Compact linear collider (CLIC) will require crab cavities to align the bunches to provide effective head-on collisions. An X-band quasi-TM11 deflecting cavity has been designed and manufactured for testing at CERN’s Xbox-2 high power standalone test stand. The cavity is currently under test and has reached an input power level in excess of 40MW, with a measured breakdown rate of better than 10-5 breakdowns per pulse. This paper also describes surface field quantities which are important in assessing the expected BDR when designing high gradient structures

Particle-in-cell simulation of second and third harmonic cavity klystron

This paper outlines the results obtained from Magic software for the CSM_23 (Core Stabilization Method) klystron. This klystron implements the use of a second and third harmonic klystron to increase the efficiency. From the PIC simulation an efficiency of 78.1% was achieved

MAGIC2-D simulations of high efficiency klystrons using the core oscillation method

Klystrons employing traditional monotonic electron bunching are capable of efficiencies up to ~70%. The use of the core oscillation method (COM) of electron bunching has predicted a significant improvement in efficiency towards 90%. Here, we document refinements on previously presented geometries, with PIC simulations predicting efficiencies up to 85%

A high-gradient test of a 30 GHz copper accelerating structure

The CLIC study is investigating a number of different materials at different frequencies in order to find ways to increase achievable accelerating gradient and to understand what are the important parameters for high-gradient operation. So far a series of rf tests have been made with a set of identical-geometry 30 GHz and X-band structures in copper, tungsten and molybdenum. A new test of a 30 GHz copper accelerating structure has been completed in CTF3 with pulse lengths up to 70 ns. The new results are presented and compared to the previous structures to determine dependencies of quantities such accelerating gradient, material, frequency, pulse length, conditioning rate, breakdown rate and surface damage

High-Power Test of Two Prototype X-band Accelerating Structures Based on SwissFEL Fabrication Technology

This article presents the design, construction, and high-power test of two $X$-band radio frequency (RF) accelerating structures built as part of a collaboration between CERN and the Paul Scherrer Institute (PSI) for the compact linear collider (CLIC) study. The structures are a modified 'tuning-free' variant of an existing CERN design and were assembled using Swiss free electron laser (SwissFEL) production methods. The purpose of the study is two-fold. The first objective is to validate the RF properties and high-power performance of the tuning-free, vacuum brazed PSI technology. The second objective is to study the structures' high-gradient behavior to provide insight into the breakdown and conditioning phenomena as they apply to high-field devices in general. Low-power RF measurements showed that the structure field profiles were close to the design values, and both structures were conditioned to accelerating gradients in excess of 100 MV/m in CERN's high-gradient test facility. Measurements performed during the second structure test suggest that the breakdown rate (BDR) scales strongly with the accelerating gradient, with the best fit being a power law relation with an exponent of 31.14. In both cases, the test results indicate that stable, high-gradient operation is possible with tuning-free, vacuum brazed structures of this kind

Prospects for developing new tubes

No abstract availabl