accelerating-news-arc.web.cern.ch - Crab Cavity

Wake field monitoring to improve FEL performance

Livia Lapadatescu — Fri, 19 Jun 2015 07:17:51 +0000

Wake field monitoring to improve FEL performance
by Micha Dehler (PSI), Nicoleta-Ionela Baboi (DESY)

Down stream end of X band structure with wake field monitor pickup. Image credit: PSI

Detecting structure tilts in the spectral density of a WFM signal. Image credit: DESY

Beam degradation due to RF structure misalignments poses a challenge in the operation of ultra high brightness FELs. Wake field monitors offer a direct way to diagnose and correct these effects.

Misalignment between the beam and RF accelerating structures lead to transverse wake field excitation which degrades the beam quality. Wake field monitors (WFMs) are devices which couple these fields and their components, the Higher Order Modes (HOM), and therefore enable their reduction. This is of special interest in modern free electron lasers where the beam quality is of extreme concern.

Two teams from EuCARD2 WP12 are exploring this field, one developing a front end for the WFMs of a 12 GHz RF structure used for the SwissFEL and the other dealing with 1.3 and 3.9 GHz superconducting cavities of the European X-ray Free Electron Laser (E-XFEL).

The RF front end for the 12 GHz structure employs an innovative electro-optical down conversion scheme of the HOM fields, which is attractive in terms of band width, radiation hardness and the capability of transporting signals over kilometers. During first beam tests, by using a spectral analysis method, the system could identify the beam to structure offset and tilt. With the resolution improving to micron range, the device is expected to show even finer details such as structure bends or kinks.

Until recently, the HOM based system at FLASH was only used for beam alignment, but showed drifts over time as a beam position monitor. A way to stabilize the signals has been found based on a frequency domain analysis, such that a resolution of few microns was preserved over several months. A new direct sampling approach for the E-XFEL 1.3 GHz cavities reduces the number of electronic components and therefore their drifts with time. At the same time these are the first electronics to offer the possibility of monitoring the beam phase with respect to the RF pulse, important for optimization of the longitudinal beam properties.

Read more >>

wakefield monitors

FEL

radiofrequency

Development and Testing of Crab Cavities for High Intensity Colliders

Sabrina El Yacoubi — Thu, 27 Nov 2014 14:21:39 +0000

Development and Testing of Crab Cavities for High Intensity Colliders
by Peter McIntosh (STFC)

Fig 1: CLIC ‘Undamped’ Crab Cavity Fabricated.Image credit: Shakespeare Engineering Ltd (UK)

Fig 2: LHC 4-rod Crab Cavity Fabricated. Image credit: Niowave Inc (USA)

The development of innovative crab cavity solutions for high intensity particle colliders is part of both the FP7 EUCARD and HiLumi framework programmes.

The activity has been led by Lancaster University and STFC in the UK, in direct collaboration with CERN. For LHC, a compact Superconducting RF (SRF) solution at 400 MHz, capable of fitting into the confined space available at the LHC interaction regions is proposed. A compact TEM type deflecting structure has been manufactured at Niowave from bulk Niobium, which has been tested in SM18 at CERN, reaching a surface magnetic field of 33 mT (limited by a serious LHe leak). This represents the world's first high field test of a compact SRF deflector and the cavity is currently undergoing additional BCP processing in order to reach the LHC design gradient of 6 MV/m (Bpk~70 mT).

For CLIC, a high gradient 12 GHz, normal-conducting travelling-wave structure, with a high group-velocity to minimise the effects of beam loading, has been developed. Two ‘undamped’ structures have been fabricated, one in the UK by Shakespeare Engineering Ltd and the other at CERN. Systematic high gradient tests are planned at SLAC and CERN, to study breakdown differences between deflecting and accelerating structures. A third ‘damped’ cavity is currently being developed to allow verification of the operational performance. A high phase stability control system, which will keep the phase from a klystron stable over long distances, is also in development to meet the stringent CLIC phase stability tolerances.

Crab Cavity RF System

Margarita Synanidi — Thu, 20 Nov 2014 11:10:54 +0000

Crab Cavity RF System
by Thomas Hind (CERN) and Rama Calaga (CERN)

Fig 1. 400 MHz Tetrode Amplifier. Image credit: Eric Montesinos (CERN)

As part of the HL-LHC upgrade, a conceptual RF system layout for a local crab crossing scheme has been presented.

Four cavities, grouped into pairs, on each side of the collision point (IP) per beam are required to produce the transverse kick to correct the geometric effects at the collision point. A two-cavity configuration also allows for good sectorization of the cavities, both for spare policy and maintenance.

Each cavity will have an independent powering system for precise control and reliable operation of the cavities. The input power coupler will use a single coaxial disk window to separate the cavity vacuum and the air side. These powering systems are assumed to use two 40kW LEP type Tetrodes modified to 400MHz to deliver the specified 80kW to cope with beam and cavity transients with some additional margin.

Fig 2. Schematic of the crab cavity layout on one side (not to scale) Image credit: Rama Calaga (CERN)

The cavity controls consists of a fast loop to ensure a rapid response time (around 1 microsecond) and a central (slow) control loop, which performs the task of field and phase control in the cavity.

A proof of principle test will be carried out in the SPS as a pre-requisite before an installation in the LHC.

Due to the limited space available in the interaction region, a detailed study is underway to determine the best option to fulfil the RF requirements in a cost effective manner.

Read more >>

Tags: