Jens Knobloch

Adapting TESLA technology for future cw light sources using HoBiCaT

The HoBiCaT facility has been set up and operated at the Helmholtz-Zentrum-Berlin and BESSY since 2005. Its purpose is testing superconducting cavities in cw mode of operation and it was successfully demonstrated that TESLA pulsed technology can be used for cw mode of operation with only minor changes. Issues that were addressed comprise of elevated dynamic thermal losses in the cavity walls, necessary modifications in the cryogenics and the cavity processing, the optimum choice of operational parameters such as cavity temperature or bandwidth, the characterization of higher order modes in the cavity, and the usability of existing tuners and couplers for cw.

Dark current measurements on a superconducting cavity using a cryogenic current comparator

This paper presents nondestructive dark current measurements of tera electron volt energy superconducting linear accelerator cavities. The measurements were carried out in an extremely noisy accelerator environment using a low temperature dc superconducting quantum interference device based cryogenic current comparator. The overall current sensitivity under these rough conditions was measured to be 0.2 nA/Hz(1/2), which enables the detection of dark currents of 5 nA.

Status and perspectives of superconducting radio-frequency gun development for BERLinPro

As part of the BERLinPro study, HZB is developing an SRF photoelectron injector. The R&D will be carried out in three stages, the first of which is currently being installed at HZBs HoBiCaT facility. It consists of an SRF cavity with SC solenoid, electron beam diagnosics and drivelaser systems.

Superconducting radio-frequency virtual cavity for control algorithms debugging

Superconducting radio-frequency (SRF) cavities are one of the most important elements in modern particle accelerators as they are used for beam acceleration, bunch manipulation, bunch focusing and defocusing, etc. Nevertheless, the availability of these complex structures prior to their installation in the accelerator is limited, either due to a lack of a real cavity or due to the time needed for the experiment setup (vacuum, cryogenics, cabling, etc.), and thus it can block or delay the development of new algorithms such as low level RF control, quench detection, etc. In this paper, we present a hardware virtual cavity to be used in hardware-in-the-loop simulations. The system implements a cavity electrical model for the transmitted and reflected voltages and more advanced features such as mechanical vibration modes driven by Lorentz-force detuning or external microphonics, hard quenches, and the Q-slope. As viewed from the RF input and output, this virtual cavity acts like a real SRF cavity and can replace such a system in early-stage debugging and operation of ancillary control systems.

Magnetometric mapping of superconducting RF cavities

A scalable mapping system for superconducting RF (SRF) cavities is presented. Currently, it combines local temperature measurement with 3D magnetic field mapping along the outer surface of the resonator. This allows for the observation of dynamic effects that have an impact on the superconducting properties of a cavity, such as the normal to superconducting phase transition or a quench. The system was developed for a single cell 1.3 GHz TESLA-type cavity, but can be easily adopted to arbitrary other cavity types. A data acquisition rate of 500 Hz for all channels simultaneously (i.e., 2 ms acquisition time for a complete map) and a magnetic field resolution of currently up to 14 mA/m/μ0 = 17 nT have been implemented. While temperature mapping is a well known technique in SRF research, the integration of magnetic field mapping opens the possibility of detailed studies of trapped magnetic flux and its impact on the surface resistance. It is shown that magnetic field sensors based on the anisotropic magnetoresistance effect can be used in the cryogenic environment with improved sensitivity compared to room temperature. Furthermore, examples of first successful combined temperature and magnetic-field maps are presented.

HOM Power Levels in the BESSY VSR Cold String

The BESSY VSR upgrade of the BESSY II light source represents a novel approach to simultaneously store long and short bunches in the storage ring. This challenging goal requires installation of four new SRF cavities (2x1.5GHz and 2x1.75GHz) in one module for installation in a single straight. These cavities are equipped with strong waveguide HOM dampers necessary for stable operation. The expected HOM power and spectrum has been analyzed for the complete cold string. The cold string is a combination of various elements such as SRF cavities, bellows with and without shielding, warm HOM beam-pipe absorbers and UHV pumping domes. The presented study is performed for various BESSY VSR bunch filling patterns with 300 mA beam current. The contribution of each component to the total HOM power is presented. INTRODUCTION The BESSY Variable pulse length Storage Ring (VSR) project [1, 2] is a future upgrade of the 3rd generation BESSY II light source. The key feature of the project is the simultaneous storage of long (ca. 15ps) and short (ca. 1.7ps) electron bunches under “standard” user optics. This challenging goal requires installation of SRF higher harmonic cavities of the fundamental 500MHz at two different frequencies. Therefore four new SRF cavities (2x1.5GHz and 2x1.75GHz) are designed [3, 4]. These cavities will operate in CW mode at high gradients of 20MV/m. The combination of these factors with a high beam current (Ib=300 mA) make the cavity design a challenging goal, since stable operation must be ensured. Thus special attention was paid to the damping of HOMs excited by the beam that may otherwise lead to coupled bunch instabilities. The HOM power levels for different cavity arrangements in the SRF module are presented. A dedicated spectral weighting technique for calculation of RF power propagation due to the HOMs excited by the circulating beam in SRF cavities is used [5, 6]. The method makes use of wakefield simulations using the CST software [7] and an external post-processing of the port signals. Calculations were performed for different bunch filling patterns of the BESSY VSR project. The propagated HOM RF power is obtained by spectral weighting of port signals (calculated with a single bunch excitation) with the bunch train spectrum. In this manner the resonances of the cold string component excited by the periodic bunch pattern will be detected. The evaluation procedure is used for the calculation of the expected HOM powers (broadband) to be absorbed in the RF loads and of the efficiency of HOM dampers in terms of power flow balance between FPC, HOM waveguides and beampipes. The HOM power levels for complete cold string with warm elements outside the SRF module are presented as well. THE BESSY-VSR FILLING PATTERN The realisation of the BESSY VSR project implies installation of a single superconducting module with four cavities in one of the straight sections of the existing BESSY II ring (Fig.1). The module integration is a challenging engineering task because of strict space limitations of the ring-straight to ~4.5m. The complete module design is currently in the development stage. Figure 1: Schematic view of BESSY VSR cavities in ring straight. The nominal BESSY VSR filling pattern of the 240m circumference ring is shown in Figure 2 where the short and long bunches will be stored simultaneously. In total 400 RF buckets with 2ns bunch spacing are available. Figure 2: BESSY VSR filling pattern including short (blue) and long (red) bunches. Two type of bunch filling patterns are considered, so-called “extended” shown in Figure 2 and “baseline” with omission of 150 short-pulse, low-charge bunches. The repetition rates of 500MHz and 250MHz are defined by the bunch spacing in each pattern, respectively. In the estimated HOM power levels given in this paper we present results for “baseline” pattern as the highest contributor. Note that in case of single bunch operation the bunch repetition rate will be 1.25MHz, defined by ring circumference of 240m corresponding to 800ns revolution time. ____________________________________________ † email address andranik.tsakanian@helmholtz-berlin.de 9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW Publishing ISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-WEPML048

First LLRF Tests of BERLinPro Gun Cavity Prototype

The goal of Berlin Energy Recovery Linac Project (BERLinPro) is the generation of a 50 MeV, 100-mA low emittance (below 1 mm mrad) CW electron beam at 2 ps rms bunch duration or below. Three different types of 1.3 GHz SRF modules will be employed: the electron gun, the booster and the main linac. Precise RF amplitude and phase control are needed due to the beam recovery process. In this paper we describe the first tests of the Low Level RF control of the first injector prototype at the HoBiCaT facility, implemented in the digital VME-based LLRF controller developed by Cornell University. Tuner movement control by an mTCA.4 system, together with further plans of using this technology will be also presented. INTRODUCTION The bERLinPro Energy Recovery Linac is a single pass, high average current and all superconducting CW driven ERL currently in construction by Helmholtz Zentrum Berlin (HZB). Its purpose is to serve as a prototype to demonstrate low normalized beam emittance of 1 mm·mrad at 100 mA and short pulses of about 2 ps [1]. bERLinPro will be formed by three 1.3 GHz modules with different characteristics and parameters [2]. The first module is a 1.4-cell gun cavity using a high quantum efficiency (QE) normal conducting multi-alkali cathode, which will deliver 2.3 MeV. The gun module is then followed by the booster module formed by three high power 2-cell booster cavities of Cornell type, where two of them deliver 2.1 MeV each and the third one is operated in zero crossing for bunch compression. The beam is merged into the main linac module consisting in three 7-cell cavities where it is accelerated to 50 MeV in a first pass and decelerated again to 6.5 MeV in a second pass. The beam is finally dumped in a 650 KW beam dump. The gun is one of the most critical components and in order to mitigate risk, it is being developed in several stages. The first one, the so-called Gun0, was a fully superconducting system with a super conducting lead deposited on the back. It allowed beam studies without a complex insert of a high QE normal conducting cathode in a SC environment, [3]. The prototype presented here, called Gun1.0, is a medium power version of the final high power structure and utilizes CW modified TTF-III couplers. It is a beam dynamic optimized design with high QE cathode insert system allowing the generation of a beam up to 4 mA, [4]. It will be used to study bERLinPro bunch parameters and the usage of high QE NC cathode within a SC environment. The last step in the gun development is the Gun2.0, which will feature two modified KEK c-ERL high power couplers [5] to allow 100 mA average current operation. Figure 1: Gun1.0 cavity’s cold mass with fundamental power couplers (left), blade tuner and cathode insert (right). GUN1.0 CAVITY After several vertical and horizontal tests at JLab and HZB where the Q0 specifications were met [2], cold mass assembly and first horizontal tests under module conditions in the horizontal bi-cavity testing facility (HoBICaT) at HZB have been carried out [6]. Table 1: Main Parameters of Gun1.0 Max E0 Max Pf QL 30 (MV/m) 20 KW 3·106 3·107 The cold mass consisting of the magnetic shielding, a blade tuner with a stepper motor and four piezo actuators, and the cathode insertion system, which includes a Petrov filter and a Helium gas cooler, was installed in HZB’s clean room together with the fundamental power couplers. Figure 1 depicts the gun cavity’s cold mass next to the HoBiCaT module. The installed coupler can stand an average input power up to 2 KW, but it is foreseen to equip later with modified warm part to allow 10 kW per coupler [7]. Unfortunately the penetration depth is lower than expected, which led to a higher QL and narrower bandwidth than expected. The last step in the cold mass assembly was to install the blade tuner including the motor and the piezo-actuators, whose pre-stress was adjusted by capacitance measurement. Table 1 shows the expected main parameters for the Gun1.0 cavity. The forward power will be delivered by two power couplers. ___________________________________________ * Work supported by German Bundesministerium für Bildung und Forschung, Land Berlin, and grants of Helmholtz Association † pablo.echevarria_fernandez@helmholtz-berlin.de Proceedings of IPAC2016, Busan, Korea TUPOW035 02 Photon Sources and Electron Accelerators A18 Energy Recovery Linacs (ERLs) ISBN 978-3-95450-147-2 1831 C op yr ig ht

Influence of oxides on the field emission of molybdenum

Molybdenum is widely used in fundamental research and industry, for example as substrate for photocathodes or electrode material in DC photoelectron guns. Usually, Mo is heated in situ to several hundred degrees to achieve an oxygen free surface for the photocathode deposition or to reduce the surface outgassing rate in the electron gun. Since enhanced field emission (EFE) is often observed there, we have investigated the influence of oxides on the EFE of Mo by means of a field emission scanning microscope (FESM) and x-ray photoelectron spectroscopy (XPS).