Fumio Furuta

Study on the 1.3 GHz low loss shape superconducting cavities at IHEP

As part of the international research program on the superconducting cavity for the International Linear Collider (ILC) R&D on the 1.3 GHz low loss superconducting cavities has been carried out at the Institute of High Energy Physics (IHEP) since 2005. A design of 1.3 GHz low loss cavity shape was proposed and six single-cell cavities of different niobium material were successfully fabricated with standard technology. In this study our priority was on large grain (LG) cavities. The two LG cavities were treated with complete procedures of surface treatments based on chemical polishing (CP) without electro polishing (EP) at IHEP. The two LG cavities and a fine grain cavity were sent to KEK for vertical testing. All the three cavities reached accelerating gradients higher than 35 MV/m and the maximum gradient of 40.27 MV/m was achieved in the LG cavity. This paper presents the process of the vertical RF tests and the comparison of the LG and fine grain cavitiess performance.

Test operation of ball-screw-type tuner for low-los high-gradient superconducting cavity at 77 K

Super-conducting Radio Frequency (SRF) 9-cell cavities have been developed in Low-Loss (LL) ICHIRO shape at KEK aiming at high-gradient operation for the International Linear Collider (ILC). One of the most important issues to realize high-gradient linac with SRF cavitiest in pulsed-mode operation is the compensation of the Lorentz detuning of cavities which amounts to 3 kHz at our goal of 45 MV/m acceleration field. None of tuners to date have achieved this specification. A coaxial ball- screw tuner was designed, fabricated and proven to reach this specification in room temperature. The performance was studied also at liquid-nitrogen temperature and the needed dynamic-range for 45 MV/m operation was proven at this temperature. The microphonic vibration was measured to be 100 Hz order in a test setup. In this paper, we describe these studies and evaluate the feasibility of ball-screw tuner operation at 2 K.

Large Grain Cavity R&D in KEK

We have started high gradient SRF cavity R&D with the combination of ICHIRO shape, sliced large grain niobium (LG), and buffered chemical polishing (BCP). We fabricated and tested large grain single and 9‐cell cavities with this combination. LG single achieved 42 MV/m, LG 9‐cell without end groups achieved 27 MV/m so far. The current status of fine grain (FG) cavities processed by electro polishing (EP) also will be reported in addition to the details of LG R&D.

Experimental study on SC RF cavities by using China large grain niobium for ILC

Large grain niobium has the potential of simplifying the production sequence and consequently reducing the cost of the superconducting RF cavities for ILC. To investigate the feasibility of fabrication and the possibility to achieve high gradient by large grain cavities, two 1.3 GHz cavities were made of China large grain niobium and a series of vertical tests were carried out following several different surfaces treatment procedures. Two cavities have both reached the high gradient of more than 43 MV/m repeatedly and the maximum accelerating field of 47.9 MV/m has been achieved by China large grain niobium. This paper introduces the features of the fabrication and surface treatments on the large grain cavities and presents the preliminary results of the research.

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.

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

Revision of accelerating and damping structures for kek STF 45 MV/m accelerator modules

KEK is constructing its superconducting RF test facility and installing 1.3 GHz superconducting accelerator structures. Learning from experience with our first 45 MV/m 9-cell accelerating structures, we have redesigned the structures to improve the characteristics and the performances. Problems found in the earlier structures are resolved in the new structures.

Electro-polished cavities using china ningxia large grain niobium material

For the International Linear Collider (ILC), superconducting RF cavity technology was chosen. The superconducting cavity is made of polycrystalline niobium material so far. However, the material cost is high and the cavity performance has a rather scatter now. Large grain (LG) niobium cavity has the potential of simplifying the production and reducing the cost of the superconducting RF cavities for the ILC. To investigate the feasibility of fabrication and the possibility to achieve high gradient by LG cavities, three single-cell cavities were made of China Ningxia LG niobium. A series of vertical tests has been carried out on several different surfaces treatment procedures by electro polishing. One cavity has reached the high gradient of more than 43 MV/m repeatedly. The maximum accelerating field of 47.9 MV/m has been achieved. This paper describes the features of electro polishing on China Ningxia LG niobium and presents the preliminary results of the research.