Amos Dexter

Frequency and phase modulation performance of an injection-locked CW magnetron

It is demonstrated that the output of a 2.45-GHz magnetron operated as a current-controlled oscillator through its pushing characteristic can lock to injection signals in times of the order of 100-500 ns depending on injection power, magnetron heater power, load impedance, and frequency offset of the injection frequency from the natural frequency of the magnetron. Accordingly, the magnetron can follow frequency and phase modulations of the injection signal, behaving as a narrow-band amplifier. The transmission of phase-shift-keyed data at 2 Mb/s has been achieved. Measurements of the frequency response and anode current after a switch of phase as a function of average anode current and heater power give new insight into the locking mechanisms and the noise characteristics of magnetrons

Design of the ILC crab cavity system.

The International Linear Collider (ILC) has a 14 mrad crossing angle in order to aid extraction of spent bunches. As a result of the bunch shape at the interaction point, this crossing angle at the collision causes a large luminosity loss which can be recovered by rotating the bunches prior to collision using a crab cavity. The ILC baseline crab cavity is a 9-cell superconducting dipole cavity operating at a frequency of 3.9 GHz. In this paper the design of the ILC crab cavity and its phase control system, as selected for the RDR 1 in February 2007 is described in fuller detail.

Rapid generation of multipactor charts by numerical solution of the phase equation

Multipactor charts for parallel plate geometries have been computed using a program that numerically solves a phase equation for the crossing times rather than by detailed computation of electronic paths. Motion in three-dimensional is tracked and detailed models for secondary emission and elastic collisions with the walls are employed. The speed advantage of the method allows the dependence of multipactor prediction on surface properties and multipactor criteria, such as the number of electrons within a shower that are followed, to be assessed over a broad parameter range.

Acceleration of electrons in the plasma wakefield of a proton bunch

High-energy particle accelerators have been crucial in providing a deeper understanding of fundamental particles and the forces that govern their interactions. To increase the energy of the particles or to reduce the size of the accelerator, new acceleration schemes need to be developed. Plasma wakefield acceleration1–5, in which the electrons in a plasma are excited, leading to strong electric fields (so called ‘wakefields’), is one such promising acceleration technique. Experiments have shown that an intense laser pulse6–9 or electron bunch10,11 traversing a plasma can drive electric fields of tens of gigavolts per metre and above—well beyond those achieved in conventional radio-frequency accelerators (about 0.1 gigavolt per metre). However, the low stored energy of laser pulses and electron bunches means that multiple acceleration stages are needed to reach very high particle energies5,12. The use of proton bunches is compelling because they have the potential to drive wakefields and to accelerate electrons to high energy in a single acceleration stage13. Long, thin proton bunches can be used because they undergo a process called self-modulation14–16, a particle–plasma interaction that splits the bunch longitudinally into a series of high-density microbunches, which then act resonantly to create large wakefields. The Advanced Wakefield (AWAKE) experiment at CERN17–19 uses high-intensity proton bunches—in which each proton has an energy of 400 gigaelectronvolts, resulting in a total bunch energy of 19 kilojoules—to drive a wakefield in a ten-metre-long plasma. Electron bunches are then injected into this wakefield. Here we present measurements of electrons accelerated up to two gigaelectronvolts at the AWAKE experiment, in a demonstration of proton-driven plasma wakefield acceleration. Measurements were conducted under various plasma conditions and the acceleration was found to be consistent and reliable. The potential for this scheme to produce very high-energy electron bunches in a single accelerating stage20 means that our results are an important step towards the development of future high-energy particle accelerators21,22.Electron acceleration to very high energies is achieved in a single step by injecting electrons into a ‘wake’ of charge created in a 10-metre-long plasma by speeding long proton bunches.

Phase space analysis of multipactor saturation in rectangular waveguide

In certain high power RF systems multipactor cannot be avoided for all operating points, but its existence places limits on performance, efficiency, lifetime, and reliability. As an example multipactor in the input couplers of superconducting RF cavities can be a major limitation to the maximum RF power. Several studies have concentrated on rectangular waveguide input couplers which are used in many light sources. Most of these studies neglect space charge assuming that the effect of space charge is simply to defocus the electron bunches. Modelling multipactor to saturation is of interest in determining the performance of waveguide under a range of conditions. Particle-in-cell modelling including space charge has been performed for 500 MHz half-height rectangular waveguide. Phase plots of electron trajectories can aid understanding the processes taking place in the multipactor. Results strongly suggest that the multipacting trajectories are strongly perturbed by space charge causing the electrons to transition from two-surface to single-surface trajectories as the multipactor approaches saturation.

Power coupler for the ILC crab cavity

The ILC crab cavity will require the design of an appropriate power coupler. The beam-loading in dipole - mode cavities is considerably more variable than accelerating cavities, hence simulations have been performed to establish the required external Q. Simulations of a suitable coupler were then performed and were verified using a normal conducting prototype with variable coupler tips.

System study using injection phase locked magnetron as an alternative source for superconducting radio frequency accelerator

As a drop-in replacement of Continuous Electron Beam Accelerator Facility (CEBAF) 5kW CW klystron system, a 1497MHz, high efficiency magnetron using injection phase lock [1] and slow amplitude variation using magnetic field trimming and anode voltage modulation has been studied systematically using MatLab/Simulink simulations. The magnetron model is based the characteristics of experiment and manufacture chart on a 2.45GHz cooker type CW magnetron. To achieve high performance of a superconducting radio frequency (SRF) acceleration cavity with an electron beam loading, the magnetrons low level radio frequency (LLRF) control has been studied in two lock loops. In the frequency lock loop, the characterized anode V-I curve, output power (the tube electronic efficiency) and frequency dependence to the anode current (pushing by Vaughan model) and the Rieke diagram (frequency pulling by the reactive load) are simulated. The magnetic field B and anode voltage V in Hartree condition are satisfied and the effect of filament heater power to the frequency lock is also included. In the phase lock loop, the Adler equation governing injection phase stability is included in this study. The control of the magnet trim-coil power-supply and of the anode voltage modulation-switching power-supply has been also simulated to achieve the amplitude modulation. The result of linear responses to the amplitude and phase of SRF cavity will be presented in this paper. The requirement of LLRF control will be given by this result.

Impedance measurements on a test bench model of the ILC crab cavity

In order to verify detailed impedance simulations, the resonant modes in an aluminium model of the ILC crab cavity were investigated using a bead-pulling technique as well as a stretched-wire frequency domain measurement. The combination of these techniques allow for a comprehensive study of the modes of interest. For the wire measurement, a transverse alignment system was fabricated and RF components were carefully designed to minimize any potential impedance mismatches. The measurements are compared with direct simulations of the stretched-wire experiments using numerical electromagnetic field codes. High impedance modes of particular relevance to the ILC crab cavity are identified and characterized.

Design and Simulation Studies of the Novel Beam Arrival Monitor Pickup at Daresbury Laboratory

We present the novel beam arrival monitor pickup design currently under construction at Daresbury Laboratory, Warrington, UK. The pickup consists of four flat electrodes in a transverse gap. CST Particle Studio simulations have been undertaken for the new pickup design as well as a pickup design from DESY, which is used as a reference for comparison. Simulation results have highlighted two advantages of the new pickup design over the DESY design; the signal bandwidth is 25 GHZ, which is half that of the DESY design and the response slope is a factor of 1.6 greater. We discuss optimisation studies of the design parameters in order to maximise the response slope for bandwidths up to 50 GHz and present the final design of the pickup.

Saturation of multipactor in rectangular waveguide

Summary form only given. Multipactor is a limiting factor in a many RF systems, restricting performance, efficiency, lifetime and reliability. In many situations it is possible to avoid multipactor by various suppression methods or the avoidance of specific operating points. However this cannot always be achieved. When multipactor cannot be avoided, the saturation level becomes critical. In for instance, superconducting applications, multipactor which does not saturate at a low level can limit peak power.