Jacek Sekutowicz

Preliminary Results From Single Crystal and Very Large Crystal Niobium Cavities

We have fabricated and tested several single cell cavities using material from very large grain niobium ingots. In one case the central grain exceeded 7“in diameter and this was used to fabricate two 2.2 GHz cavities. This activity had a dual purpose: to investigate the influence of grain boundaries on the often observed Q-drop at gradients Eacc> 20 MV/m in the absence of field emission, and to study the possibility of using ingot material for cavity fabrication without going through the expensive rolling process. The sheets for these cavities were cut from the ingot by wire electro-discharge machining (EDM) and subsequently formed into half–cells by deep drawing. The following fabrication steps were standard: machining of weld recesses, electron beam welding of beam pipes onto the half cells and final equator weld to join both half cell/beam pipe subunits. The cavities showed heavy Q–disease caused by the EDM. After hydrogen degassing at 800 ° C for 3 hrs in UHV and about 200 μm total removals from the inner surface by BCP 1: 1: 1, the cavities showed promising results, however, the Q-drop was still present. In the two cavities made from large grain material accelerating gradients of 30 MV/m have been reached. After “in-situ” baking the Q-drop disappeared. The smaller cavities made from single crystal material showed very low residual resistances and accelerating gradients up to Eacc= 45 MV/m were reached (one of the highest ever achieved), corresponding to a peak surface magnetic fields (Bp) of 160 mT.

Design of a Low Loss SRF Cavity for the ILC

An international team comprising SLAC, KEK, FNAL, JLAB and DESY is collaborating on the design, fabrication and test of a low loss, 1.3 GHz 9-cell SRF structure as a potential improvement for the ILC main linac. The advantages of this structure over the TESLA structure include lower cryogenic loss, shorter rise time, and less stored energy. Among the issues to be addressed in this design are HOM damping, Lorentz force detuning and multipacting. We will report on HOM damping calculations using the parallel finite element eigenmode solver Omega3P and the progress made towards an optimized design. Studies on multipacting and estimates of the Lorentz force detuning will also be presented.

Status of Nb-Pb superconducting RF-GUN cavities

We report on the progress and status of an electron RF* gun made of two superconductors: niobium and lead [1]. The presented design combines the advantages of the RF performance of bulk niobium superconducting cavities and the reasonably high quantum efficiency of lead. The design of RF-gun and performance of 3 test cavities without and with the emitting lead spot are reported in this contribution. Measured quantum efficiency for lead at 2 K is presented briefly. More details are reported in [9].

Electron Cooling of RHIC

We report progress on the R&D program for electron-cooling of the Relativistic Heavy Ion Collider (RHIC). This electron cooler is designed to cool 100 GeV/nucleon at storage energy using 54 MeV electrons. The electron source will be a superconducting RF photocathode gun. The accelerator will be a superconducting energy recovery linac. The frequency of the accelerator is set at 703.75 MHz. The maximum electron bunch frequency is 9.38 MHz, with bunch charge of 20 nC. The R&D program has the following components: The photoinjector and its photocathode, the superconducting linac cavity, start-to-end beam dynamics with magnetized electrons, electron cooling calculations including benchmarking experiments and development of a large superconducting solenoid. The photoinjector and linac cavity are being incorporated into an energy recovery linac aimed at demonstrating ampere class current at about 20 MeV.

Progress on Lead Photocathodes for Superconducting Injectors

We present the results of our investigation of bulk lead, along with various types of lead films, as suitable photocathode materials for superconducting RF injectors. The quantum efficiency of each sample is presented as a function of the photon energy of the incident light, from 3.9 eV to 6.5 eV. Quantum efficiencies of 0.5% have been obtained. Production of a niobium cavity with a lead-plated cathode is underway.

Nb-Pb Superconducting RF Gun

We report on the status of an electron RF-gun made of two superconductors: niobium and lead. The presented design combines the advantages of the RF performance of bulk niobium superconducting cavities and the reasonably high quantum efficiency of lead, as compared to other superconducting metals. The concept, mentioned in a previous paper, follows the attractive approach of all niobium superconducting RF-gun as it has been proposed by the BNL group. Measured values of quantum efficiency for lead at various photon energies, analysis of recombination time of photon-broken Cooper pairs for lead and niobium, and preliminary cold test results are discussed in this paper.

Testing of HOM Coupler Designs on a Single Cell Niobium Cavity

Coaxial higher order mode (HOM) couplers were developed initially for HERA cavities and subsequently for TESLA cavities. They were adopted later for SNS and Jlab upgrade cavities. The principle of operation is the rejection of the fundamental mode by the tunable filter and the transmission of the HOMs. It has been recognized recently that for continuous wave or high duty factor applications of the TESLA coupler the output pick-up probe must stay superconducting in order to avoid its heating by the fundamental mode residual magnetic field leading to deterioration of the cavity quality factor. In addition, the thermal conduction of existing rf feedthrough designs is only marginally sufficient to keep even the niobium probe tip superconducting in cw operation. We have equipped a single-cell niobium cavity with the modified HOM couplers and tested the new designs by measuring Q vs Eaccbehavior at 2 K for different feedthroughs and probe tip materials.

Design and Fabrication of an FEL Injector Cryomodule

Advanced Energy Systems has recently completed the design of a four cavity cryomodule for use as an FEL injector accelerator on the JLAB Injector Test Stand. Fabrication is nearing completion. Four 748.5 MHz single cell superconducting cavities have been completed and are currently at Jefferson Lab for final processing and test prior to integration in the module. This paper will review the design and fabrication of the cavities and cryomodule.

Photoemission tests of a Pb/Nb superconducting photoinjector

We report recent progress in the development of a hybrid lead/niobium superconducting RF (SRF) photoinjector. The goal of this effort is to produce an injector with the SRF properties of a niobium cavity along with the superior quantum efficiency (QE) of a lead photocathode. Quantum efficiency measurements have been performed on an RF cavity with an electroplated lead plug as the photocathode. The effect of the laser on the RF performance of the cavity was characterized. Further measurements of the cavity RF performance are reported in these proceedings [6].

Purity of Nb and Pb Films Deposited by an Ultrahigh Vacuum Cathodic Arc

This paper reports on recent progress in the application of ultrahigh vacuum arc technology, which was proposed as an alternative solution for the deposition of thin superconducting films of pure niobium upon the inner surfaces of RF cavities designed for particle accelerators. New experiments were conducted to deposit superconducting films of pure niobium and lead needed for the modern accelerator technology. Presented scanning electron microscopy, scattered ion mass spectroscopy technique, and glow discharge-optical emission spectroscopy studies of such produced Nb and Pb films showed that the concentration level of impurities is lower than 0.2% and 1%, respectively. Achieved cleanliness goes together with outstanding superconducting properties. The main experimental results and characteristics of arc-deposited thin superconducting films are discussed, and the progress achieved recently in the formation of such films is presented.