[en] We performed in-situ cryogenic testing of four silicon diodes as possible candidates for field emission (FE) monitors of superconducting radio frequency (SRF) cavities during qualification testing and in accelerator cryo-modules. We evaluated diodes from 2 companies - from Hamamatsu corporation model S1223-01; and from OSI Optoelectronics models OSD35-LR-A, XUV-50C, and FIL-UV20. The measurements were done by placing the diodes in superfluid liquid helium near the top of a field emitting 9-cell cavity during its vertical test. For each diode, we will discuss their viability as a 2K cryogenic detector for FE mapping of SRF cavities and the directionality of S1223-01 in such environments. We will also present calibration curves between the diodes and JLab's standard radiation detector placed above the Dewar's top plate.

[en] The great majority of experience in niobium SRF cavity processing at Jefferson Lab is with BCP etching. This has been used on CEBAF cavities and others totalling over 600 in number. With improved process quality control, field emission is now largely controlled and other factors limit performance. All of the prototype cavities developed for the 12 GeV upgrade, although meeting minimum requirements, have demonstrated a Q-drop in the 17-23 MV/m range that is not remedied by 120 C bake. Most of these cavities received >250 micron removal by BCP etch. Two of these cavities have been electropolished using the protocol under development within ILC R and D activities. The first such cavity was transformed from Q = 3 x 10⁹ at 17 MV/m to quench from 1 x 10¹⁰ at 35 MV/m. The details of this and subsequent electropolished JLab 7-cell cavities will be reported.

[en] We performed Centrifugal Barrel Polishing (CBP) on a 1.3 GHz fine grain TESLA single cell cavity and 1.5 GHz fine grain CEBAF high gradient superconducting radio frequency (SRF) single cell cavity following a modified recipe originally developed at Fermi National Accelerator Lab (FNAL). We were able to obtain a mirror like surface similar to that obtained at FNAL, while reducing the number of CBP steps and total processing time. This paper will discuss the change in surface and subsequent cavity performance post CBP, after a 800 C bake (no pre-bake chemistry) and minimal controlled electro-polishing (10 micron). In addition to Q vs. E_ACC thermometry mapping with preheating characteristics and optical inspection of the cavity after CBP will also be shown.

[en] We have studied thermal breakdown in several multicell superconducting radiofrequency cavity by simultaneous excitation of two TM₀₁₀ passband modes. Unlike measurements done in the past, which indicated a clear thermal nature of the breakdown, our measurements present a more complex picture with interplay of both thermal and magnetic effects. JLab LG-1 that we studied was limited at 40.5 MV/m, corresponding to B_peak = 173 mT, in 89 mode. Dual mode measurements on this quench indicate that this quench is not purely magnetic, and so we conclude that this field is not the fundamental limit in SRF cavities

[en] Quench limits accelerating gradient in SRF cavities to a gradient lower than theoretically expected for superconducting niobium. Identification of the quenching site with thermometry and OST, optical inspection, and replica of the culprit is an ongoing effort at Jefferson Lab aimed at better understanding of this limiting phenomenon. In this contribution we present our finding with several SRF cavities that were limited by quench

[en] A fundamental limitation towards achieving high quality factors in superconducting radio-frequency cavities is the so-called residual resistance. Understanding and controlling the residual resistance has important implications towards improving the efficiency and reduce the operating cost of continuous wave superconducting linear accelerators. In this contribution we will report on the residual resistance values obtained from measurements of the quality factor of a large set of cavities, with resonant frequency between 805 MHz and 1.5 GHz, all of them processed and tested at Jefferson Lab. Surface treatments included both buffered chemical polishing and electropolishing. The results indicate an approximate value of the residual resistance of about 7-10 n Omega.

[en] PKU-4C-1206 is a 4-cell 1.3GHz cavity manufactured by Peking University. It was processed and cryogenically RF tested at Jefferson Lab under cooperative research and development agreement (CRADA, No. JSA2013S011).

[en] A DC high-brightness laser driven by photo emissive electron gun is being developed at Beijing University, in order to produce 50∼100 ps electron bunches of high quality. The gun consists of a photocathode preparation chamber and a DC acceleration cavity. Different ways of fabricating photocathode, such as chemical vapor deposition, ion beam implantation and ion beam enhanced deposition, can be adopted. The acceleration gap is designed with the aid of simulation codes EGUN and POISSON. 100 kV DC high voltage is fed to the anode through a careful designed ceramic insulator. The laser system is a mode locked Nd-YAG oscillator proceeded by an amplifier at 10 Hz repetition rate, which can deliver three different wavelength (1064/532/266 nm). The combination of a superconducting cavity with the photocathode preparation chamber is discussed

[en] Jefferson Lab plays an active role in high-gradient SRF R and D in the frame work of the internationally coordinated ILC S0 program. The S0 aim is to push the yield at 35 MV/m in 9-cell cavities. So far, twelve cavities have been electropolishing (EP) processed and RF tested by using the state-of-the-art recipes at JLab, in close collaboration with FNAL and KEK. Seven of them reached a best gradient of over 31.5 MV/m. Understanding gradient limiting mechanisms in real 9-cell cavities is an important component of our studies. Thermometry and high-resolution optical inspection are used to locate and understand the source of gradient limits. Experimenting with selective cavities is still a necessary method for process optimization. One example is the first demonstration of 35 MV/m without detectable Bremsstrahlung X-ray after a light EP is applied to a previously heavy BCP etched 7-cell cavity. Some new understanding has been gained with regard to quench behaviors, field emission behaviors as

[en] A laser driven photoemissive high-brightness electron source at Beijing University is reported. Through a DC accelerating gap of 100 kV voltage, the device is capable of delivering high-brightness electron beam of 35-100 ps pulse duration when irradiated with a mode-locked YAG laser. The geometry of the gun is optimized with the aid of simulation codes EGUN and POISSON. The results of experimental studies on ion implanted photocathode and cesium telluride photocathode are given. The proposed laser driven superconducting RF gun is also discussed