Transverse Deflecting Cavity for Longitudinal Beam Diagnostics at BERLinPro

The Berlin Energy Recovery Linac Prototype (bERLinPro) at Helmholtz Zentrum Berlin (HZB) aims to deliver a continuous-wave electron beam of high average current (100 mA) and brilliance (normalized emittance below 1 mm·mrad). The achievement of these ambitious goals necessitates a thorough determination of the bunch parameters after the first acceleration stages, namely the photoinjector and the succeeding booster module. For the measurement of primarily the bunch duration and subsequently the longitudinal phase space and transverse slice emittance, a single-cell 1.3-GHz TM110-like mode vertically deflecting cavity was manufactured by RI Research Instruments GmbH, following the respective design developed for the Cornell ERL injector. This article summarizes the design parameters, manufacturing procedure and testing of this pulsed RF resonator, together with the expected temporal measurement resolution for the nominal beam energies at the initial acceleration stages of bERLinPro. MOTIVATION AND DESIGN A new Energy Recovery Linac (ERL) is being constructed at Helmholtz Zentrum Berlin (HZB) under the name Berlin Energy Recovery Linac Prototype (bERLinPro). The goal of this project is to develop the ERL operation principle towards user-oriented applications. The design of bERLinPro is that of a single-pass electron ERL which utilizes superconducting radiofrequency (SRF) technology to deliver a continuous-wave operation of high average current (100 mA) and brilliance (normalized emittance below 1 mm·mrad) [1]. The longitudinal beam properties at the initial acceleration stages (up to 6.5 MeV kinetic energy) will be diagnosed by a transverse deflecting cavity (TCAV). A TCAV offers a straightforward way of measuring the electron bunch duration, or even the complete longitudinal phase space when combined with a spectrometer dipole, as well as the transverse slice emittance with the help of a quadrupole magnet [2]. It consists of an RF resonator operated in the TM110 mode (Fig. 1a), which when phased properly, transversely deflects the head and tail of the bunch in opposite directions. As a result, the longitudinal plane is projected into the transverse after some drift. Due to the damage threshold of the projection viewscreens, TCAV will be operated only in a special diagnostics mode of bERLinPro with reduced repetition rate which allows up to 0.5 μA average current. When passive, the cavity will not interfere with high beam currents as long as it is detuned from its ∗ georgios.kourkafas@helmholtz-berlin.de resonant frequency (up to 75 mA demonstrated at Cornell ERL [3]). The design of the cavity followed closely the one developed for the Cornell ERL injector, due to the resemblance of requirements for both applications [4]. In short, this design features a single-cell copper RF resonator with a transverse impedance of more than 5 MΩ and a resonance frequency of 1.3 GHz, tunable by a motorized plunger. Necessary adjustments were made to accommodate the available interfaces of HZB (Fig. 2): a) coaxial power feedthrough with 50Ω matched impedance, 100 W average power rating and 2.7 kVrms tolerance, b) coaxial 50Ω RF pickup coupled at -51 dB to the input power, providing a mW signal level after cable attenuation (-13dB) at nominal operation, c) UHVcompatible linear stage for the adjustment of the plunger position up to ±25 mm, positioned at the bottom of the structure for improved particulate-free conditions, d) ion getter pump at the top of the structure. (a) Electric field map. (b) Pot of TCAV after machining. Figure 1: Inner contour of TCAV. Protrusion cones improve the vertical impedance and orientation of the dipole mode. Two cavities were manufactured by RI Research Instruments GmbH according to these specifications. The manufacturing procedure, the results of the RF and vacuum testing and conditioning as well as the estimated resolution of the e-beam measurements at bERLinPro are described below. CAVITY MANUFACTURING The most significant adjustment from the original Cornell configuration was building the cavity out of two pieces, namely pot and lid, instead of two lids and a center ring. This increased the manufacturing precision, while minimizing the process time and risk. Each cavity part was milled from 29th Linear Accelerator Conf. LINAC2018, Beijing, China JACoW Publishing ISBN: 978-3-95450-194-6 ISSN: 2226-0366 doi:10.18429/JACoW-LINAC2018-THPO083 Technology Beam diagnostics THPO083 875 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 18 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I.