In May, 2018, the Cornell-University Brookhaven-National-Laboratory EnergyRecovery-Linac Test Accelerator (CBETA) fired its first electron beam tests using the first pass (S1) splitter line and first girder of the return loop (FA). At various locations on the S1 Line and FA section, the positions of the electron beam were recorded using ten Beam-Position-Monitors (BPMs) as the strength of the steering and focusing electromagnets were varied. Datasets from these tests contain measurements of beam position during magnet scans for the following beam momenta: 38, 42, 47, and 53MeV. An additional dataset was taken at the momentum 42MeV again, but this time after turning off the eight quadrupole magnets. Analyzing these datasets, this paper will characterize the steering electromagnets on CBETA’s S1 Splitter Line by using a subsection of the data recorded to calculate the magnet’s apparent calibration value and compare it to the design value. This subset of the available data, the beam position measurements from the closest-downstream BPM, is furthermore used to measure the relative roll angle between magnet-BPM pairs. After disregarding datasets obstructed by alignment errors (characterizations of magnets MD1DIP01, MS1DIP07, MS1DPB01/08 and MS1CRV04 are inconclusive in this study), we find the calibration accuracy for dipoles and vertical correctors are between 1% − 9% and 2% − 3% of their design values respectively. Many apparent dipole calibration values are in agreement with their design value; most measurements for the vertical correctors are as well. Furthermore, we find that our study can determine relative roll angles within uncertainties ranging between 5 − 20milliradian for dipoles and 8 − 30milliradian for vertical correctors. In many cases, the roll angle uncertainty is larger than the roll angle measurement; the calculations for these cases are consistent with zero relative roll angle between a magnet-BPM pair. In the other cases where the measured roll angle is larger than the uncertainty, more investigation is needed to make sense of the source. CBETA machine note #028

First tests of electron beam transport in the CBETA S1 splitter line

CBETA Beam Position Monitor System Design and Plan for Measurement of Different Energy Bunches (CBETA Technical Note 030)

Cornell has designed, fabricated, and completed initial cool down test of a high current (100 mA) CW SRF main linac prototype cryomodule for the Cornell ERL. This paper will report on the design and performance of this very high Q0 CW cryomodule including design issues and mitigation strategies. INTRODUCTION Cornell University has proposed to build Energy Recovery Linac (ERL) as drivers for hard x-ray sources because of their ability to produce electron bunches with small, flexible cross sections and short lengths at high repetition rates. The proposed Cornell ERL is designed to operate in CW at 1.3GHz, 2ps bunch length, 100mA average current in each of the accelerating and decelerating beams, normalized emittance of 0.3mmmrad, and energy ranging from 5GeV down to 10MeV, at which point the spent beam is directed to a beam stop [1, 2]. The design of main linac prototype cryomodule (MLC) for Cornell ERL had been completed in 2012. The fabrication and testing of MLC components (cavity, high power input coupler, HOM dampers, tuners, etc.,) and assembly of MLC cold mass had been completed in 2014. MLC installation and cooldown preparations began in this summer. We will describe about MLC and initial cool down results in this proceeding. MLC GENERAL LAYOUT The general layout of an ERL main linac cryomodule (MLC) is shown in Fig. 1. It is 9.8 m long and houses six 1.3 GHz 7-cell superconducting cavities with Individual HOM absorbers and one magnet/BPM section. Each cavity has a single coaxial RF input coupler which transfers power from an RF power source to the beam loaded cavity. The specification values of 7-cell cavities are Qo of 2.0e10 at 16.2MV/m, 1.8K. Due to the high beam current combined with the short bunch operation, a careful control and efficient damping of higher order modes (HOMs) is essential. So HOMs are installed next to each cavity. To minimize ambient magnetic field of high-Q 7-cell cavities, MLC has three layers of magnetic shielding; 1) Vacuum Vessel (carbon steel), 2) 80/40 K magnetic shield enclosing the cold mass, and 3) 2 K magnetic shield enclosing individual cavities. All components within the cryomodule are suspended from the Helium Gas Return Pipe (HGRP). This large diameter (280mm) titanium pipe will return the gaseous helium boiled off the cavity vessel to the liquefier and act as a central support girder. The HGRP will be supported by 3 support post. The middle one is fixed; the other side posts are not and will slide by 7-9mm respectively during the cooldown from room temperature to cold. 7-CELL CAVITIES FOR MLC Vertical Test Results All 7-cell cavities for MLC were fabricated in house. Three of six cavities were stiffened cavity and the other three were un-stiffened cavity. Cavity surface preparation recipe consists of bulk Buffered Chemical Polishing (BCP, 140um), degassing (650degC*4days), frequency and field flatness tuning, light BCP (10um), low temperature baking (120degC*48hrs), and HF rinse [3]. Figure 2 shows best Q(E)curve of MLC 7-cell cavities during vertical test (VT) at 1.8K. All 7-cell cavities had surpassed the specification values of Qo=2.0e10 at 16.2MV/m, 1.8K. In fact, average Qo=(3.0±0.3)*1e10 had been achieved during VT at 16.2MV/m, 1.8K. All VT was limited by administrative limit, no radiation or no quench were detected during VT. ____________________________________________ * Work is supported by NSF Grants NSF DMR-0807731 and NSF #ff97@cornell.edu PHY-1002467 Figure 1: Cornell ERL Main Linac Prototype Cryomodule Proceedings of SRF2015, Whistler, BC, Canada FRAA04 SRF Technology Cavity E06-Elliptical performance ISBN 978-3-95450-178-6 1437 C op yr ig ht © 20 15 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s Horizontal Test Results In parallel with six 7-cell cavities fabrication and VT for MLC, the first prototype 7-cell cavity, ERL7-1, was tested and qualified in one cavity Horizontal Test Cryomodule (HTC) through three stages of hardware implementation [4]. Figure 3 shows Q(E) comparison between VT and HTC-1 at 1.8K. Higher Qo of 3.5e10 at 16.2MV/m, 1.8K than that of VT (Qo of 1.9e10) was achieved in HTC-1. HTC was special cryomodule designed for high-Q cavity operation and had much better magnetic shielding than VT pit, this better magnetic shielding was one of the essences which provided higher Qo in HTC. Estimated residual resistance (Rres) of ERL71 during cryogenic testing was reduced from 11nOhm (VT) to 3.2nOhm (HTC-1). The other essence was suppression of thermocurrent effect. During the cooldown through Tc, cavity usually had some temperature gradient over cavity. This could provide thermocurrent and more chance of flux trapping into cavity. During the 7cell VT, we clearly saw the temperature gradient affected on cavity Qo [5]. In case of 7-cell VT processed with BCP and 120C bake, the smaller temperature gradient between cavity top and bottom provided higher Qo (lower Rres). This means smaller temperature gradient was effective to suppress induced flux by thermocurrent, and minimize Rres of 7-cell. In HTC case, applying several thermal cycles to above Tc could be effective to reduce temperature gradient and suppress flux trapping induced by thermocurrent. After applying thermal cycles on HTC, the world record Qo of 6.0e10 at 16.2MV/m, 1.8K was obtained in HTC. The conditions when cavity had achieved highest Qo were 10K thermal cycle with small temperature gradient over cavity (~0.2K) and slow cool down rate near Tc (~0.4K/h). This result was very encouraging, because it suggested that the highest Qo in cryomodule could be achievable by flux control with temperature gradient control during the cooldown. On the other hand, we applied the rapid cooldown with larger temperature gradient at the last thermal cycle of HTC, but this did not make any negative impact on Qo [4]. So once thermocurrent effect was well suppressed by repeated thermal cycles, temperature gradient over cavity might not become issues. Figure 4 and 5 shows Q(E) plot and surface resistance (Rs) plot before/after thermal cycles as comparison. HIGH ORDER MODES DAMPERS Figure 6 shows a cross section view of the production version of the Cornell HOM Absorber. For these production pieces, the absorbing material is Silicon Carbide, SC-35® from Coorstek [6]. For the HTC cavity testing, 3 prototypes of the HOM absorber have been built with a slightly different design [7]. These prototypes where built using the same absorbing ceramics. Within the HTC, a 7-cell ERL cavity is placed between two HOM absorbers. We achieved an excellent higher order mode damping (Figure 7) with preserving the high quality factor of the fundamental mode. More details and beam test results were seen on reference [8] Figure 2: VT results of ERL 7-cells at 1.8K 1.0E+10 0 2 4 6 8 10 12 14 16 18 20 Qo Eacc [MV/m] ERL7-2a ERL7-3 ERL7-4 ERL7-5 ERL7-6 ERL7-7

Performance of the Cornell ERL Main Linac Prototype Cryomodule

The main linac cryomodule (MLC) prototype is a key component for the Cornell-BNL ERL Test Accelerator (CBETA) project, which is a 4-turn FFAG ERL under construction at Cornell University. This novel cryomodule is the first SRF module ever to be fully optimized simultaneously for high efficient SRF cavity operation and for supporting very high CW beam currents. Initial MLC testing has demonstrated that cavity performance and HOMs damping meet specification values. Recent, additional tests have focused on RF field stability, and cavity microphonics. In this paper, we summarize the performance of this novel ERL cryomodule and evaluate its beam capabilities based on the measured performance.

ERL Cryomodule Testing and Beam Capabilities

Contributors: N. Banerjee, J. Barley, A. Bartnik, I. Bazarov, I. Ben-Zvi, J. S. Berg, S. Brooks, D. Burke, J. Crittenden, J. Dobbins, R. Gallagher, B. Heltsley, R. Hulsart, J. Jones, D. Jusic, R. Kaplan, D. Kelliher, V. Kostroun, B. Kuske, Y. Li, M. Liepe, W. Lou, G. Mahler, M. McAteer, F. Méot, R. Michnoff, M. Minty, R. Patterson, S. Peggs, V. Ptitsyn, P. Quigley, T. Roser, D. Sabol, D. Sagan, J. Sears, K. Smolenski, E. Smith, S. Trabocchi, J. Tuozzolo, N. Tsoupas, V. Veshcherevich, J. Vöelker, D. Widger

The CBETA Fractional Arc Test

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The CBETA Fractional Arc Test