Mingqi Ge

High-efficiency, High-current Optimized Main-linac ERL Cryomodule

The Main Linac Cryomodule (MLC) prototype is a key component of the Cornell-BNL ERL Test Accelerator (CBETA) project, which is a 4-turn FFAG ERL currently 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. After a successful initial MLC testing, the MLC has now been moved into its final location for the CBETA ring. For a first beam test of the MLC and CBETA, the Cornell ERL high voltage DC gun and SRF injector cryomodule were connected to MLC via an entry beam line; a beam stop assembly was also installed at the exit line. In this paper, we summarize the performance of this novel ERL cryomodule including the results of the first beam test and the additional tests focused on RF field stability and cavity microphonics.

Performance of the Cornell Main Linac Prototype Cryomodule for the CBETA Project

The Cornell Main Linac Cryomodule (MLC) is a key component in the Cornell-BNL ERL Test Accelerator (CBETA) project, which is a 4-turn FFAG ERL under construction at Cornell University. The MLC houses six 7cell SRF cavities with individual higher order-modes (HOMs) absorbers, cavity frequency tuners, and one magnet/BPM section. Here we present final results from the MLC cavity performance and report on the studies on the MLC HOMs, slow tuner, and microphonics.

Performance of the Novel Cornell ERL Main Linac Prototype Cryomodule

The main linac cryomodule (MLC) for a future energyrecovery linac (ERL) based X-ray light source at Cornell has been designed, fabricated, and tested. It houses six 7cell SRF cavities with individual higher order-modes (HOMs) absorbers, cavity frequency tuners, and one magnet/BPM section. Cavities have achieved the specification values of 16.2MV/m with high-Q of 2.0e10 in 1.8K in continuous wave (CW) mode. During initial MLC cavity testing, we encountered some field emission, reducing Q and lowering quench field. To overcome field emission and find optimal cool-down parameters, RF processing and thermal cycles with different cool-down conditions have been done. Here we report on these studies and present final results from the MLC cavity performance.

Measurements and Analysis of Cavity Microphonics and Frequency Control in the Cornell ERL Main Linac Prototype Cryomodule

The Cornell Main Linac cryomodule (MLC) is a key component in the CBETA project. The SRF cavities with high loaded-Q in the MLC are very sensitive to microphonics from mechanical vibrations. Poor frequency stability of the cavities would dramatically increase the input RF power required to maintain stable accelerating fields in the SRF cavities. In this paper, we present detailed results from microphonics measurement for the cavities in the MLC, discuss dominant vibration sources, and show vibration damping results. The current microphonics level meets the CBETA requirement of a 36MeV energy gain without applying fast tuner compensation.

Cool-Down Performance of the Cornell ERL Cryomodules

In the framework of the ERL prototyping, Cornell University has built two cryomodules, one injector module and one prototype Main Linac Cryomodule (MLC). In 2015, the MLC was successfully cooled down for the first time. We will report details on the cool-down as well as cycle tests we did in order to achieve slow and fast cooldown of the cavities. We will also report on the improvement we made on the injector cryomodule which also included a modification of the heat exchanger can that allows now a more controlled cool-down, too.

Advanced Vertical Electro-Polishing studies at Cornell with Faraday

Cornell’s SRF group and Faraday Technology, Inc. have started collaborations on two phase-II SBIR projects. Both projects are aiming for the development of advanced Vertical Electro-Polishing (VEP) for Nb SRF cavities, such as HF free or acid free VEP protocols. These could be eco-friendlier alternatives for the standard, HF-based EP electrolyte used, and could bring new breakthrough performance for Nb SRF cavities. Here we give a status update and report first results from these two projects.

Vertical Electropolishing Studies at Cornell with KEK and Marui

Cornells SRF group has been developing Vertical Electro-Polishing (VEP) which was applied on 1.3GHz Niobium SRF cavities as the primary surface treatment. The process was done in the vertical direction, the upper and the lower half cell had removal difference. Cavity need to be flipped over during the process to compensate this. Cornell has started collaboration with KEK and Marui Galvanizing Co. Ltd. (Marui) in 2014. The first step of collaboration focused on the demonstration of Marui’s original VEP cathode named “i-cathode Ninja®” at Cornell, which was developed to make more uniform VEP removal. The results of VEP using Ninja cathode at Cornell will be presented in this paper.

Efforts of the Improvement of Cavity Q-Value by Plasma Cleaning Technology: Plan and Results From Cornell University

We reported the plasma works at Cornell University. The plasma has been generated for 1) surface cleaning to reduce field emission; 2) the cavity quality factor improvement. The experiment design, including RF design, the gas type and pressure selection, the external DC magnetic field calculation, had been discussed. The plasma experiment set-up by using a 1.3GHz single-cell cavity is shown. Argon and helium plasma was successfully ignited in the cavity; the results of the plasma processing will be displayed. INTRODUCTION Experience with larger SRF installations shows that occasionally SRF cavities have substandard performance, and an in-situ cleaning mechanism would be highly desirable to recover the performance. Plasma cleaning was successful in reducing field emission in cavities with poor performance [1]. Potentially, it might also be able to improve the quality factor Q0 not only by reducing field emission, but also by removing bad oxides or other surface contamination. Within this proposal we would study if in-situ plasma cleaning can be effective in recovering or even improving the medium field Q0 of SRF cavities. DESIGN OF THE PLASMA EXPERIMENT RF Design As the plasma project has two goals: 1) reducing the field emission; 2) The Q-value improvement. The selection of RF modes has been considered, because the plasma only concentrates in E-field region but not in Bfield region. For the Q-improvement purpose, the plasma should treat the surface on the cavity equator region; hence the RF modes should have E-field distribute on the equator region. The fundamental mode of a 1.3GHz cavity is TM010 which has E-field concentrating on the iris region but not the equator. Therefore the TM010 can be only used for the cleaning purpose. Several higherorder-modes have been considered as candidates for the Q-improvement: TM011, TE111, and TE211modes. They have E-field components on the equator. The couplers of those modes have been designed to transfer the RF power into the cavity without causing RF break-down in the transmission line. Sine we have several modes to excite, both the hook antenna and straight antenna have been designed, shown in Fig. 1. (a) The hook antenna for TE111 and TM011 mode (b) The hood antenna for TE211 mode (c) The straight antenna for TM010 mode Figure 1: Hook antenna and straight antenna designed for TE111, TE211, TM010, and TM011 modes. The external Q versus antenna length curves for three types of couplers is displayed in Fig. 2. Since we treated the cavities at room temperature, the Q0 of the cavity was around 5000. The plasma will consume RF power as well; hence the Q-value of the whole system could be even low to 1000. The external Q should match the system Q keeping the reflection power minimum. For our works, we selected the Qe is about 7e3 marked by the red rectangular in Fig. 2, because the antenna should not be too close to the cell. ___________________________________________ #mg574@cornell.edu Proceedings of SRF2015, Whistler, BC, Canada MOPB085 SRF Technology Processing F02-Surface treatments ISBN 978-3-95450-178-6 333 C op yr ig ht

Performance of the Cornell ERL Main Linac Prototype Cryomodule

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

Niobium Impurity-Doping Studies at Cornell and CM Cool-Down Dynamic Effect on Q0

As part of a multi-laboratory research initiative on high Q0 niobium cavities for LCLS-II and other future CW SRF accelerators, Cornell has conducted an extensive research program during the last two years on impurity-doping of niobium cavities and related material characterization. Here we give an overview of these activities, and present results from single-cell studies, from vertical performance testing of nitrogen-doped nine-cell cavities, and from cryomodule testing of nitrogen-doped nine-cell cavities. We show that 2K quality factors at 16 MV/m well above the nominal LCLS-II specification of 2.7 × 10 can be reached reliably by nitrogen doping of the RF penetration layer. We demonstrate that the nitrogen furnace pressure is not a critical parameter in the doping process. We show that higher nitrogen doping levels generally result in reduced quench fields, with substantial variations in the quench field between cavities treated similarly. We propose that this can be explained by the reduced lower critical field Hc1 in N-doped cavities and the typical variation in the occurrence of defects on a cavity surface. We report on the results from five cryomodule tests of nitrogen-doped 9-cell cavities, and show that fast cooldown with helium mass flow rates above 2 g/s is reliable in expelling ambient magnetic fields, and that no significant change in performance occurs when a nitrogen-doped cavity is installed in a cryomodule with auxiliary components.