Development of an Optimized Quadrupole Resonator at HZB

Abstract Current superconducting cavities are generally made of solid Niobium. A possibility to reduce cost as well as increase the quality factor and or accelerating fields is to use thin film coated cavities. Apart from Niobium thin films, other substances such as Magnesium diboride, Niobium nitride and Niobium tin are promising candidates. Measuring the RF properties of superconducting thin films, specifically the surface resistance, with a high resolution at frequencies, magnetic field levels and operating temperature as realized in RF cavities, is needed to drive forward this development. Presently, only few setups exist capable of measuring the surface resistance of thin films samples with a resolution in the nano ohm range at RF frequencies below 3 GHz. A dedicated test stand consisting of a quadrupole resonator is therefore being constructed at the Helmholtz Zentrum Berlin. Starting with the 400 MHz quadrupole resonator developed by CERN, the design was adapted and optimized to 433 MHz making available the higher harmonic mode at 1.3 GHz for RF characterization of samples in the L band using simulation data obtained with CST Microwave Studio. A number of relevant figures of merit have been improved to provide a higher resolution, a lower peak electric field and less sensitivity to microphonics, enabling measurements with high resolution at high magnetic field level

High Q Operation of SRF Cavities The Impact of Thermocurrents on the RF Surface Resistance

We present a study concerning the operation of a superconducting RF cavity non doped niobium in horizontal testing with the focus on understanding the thermoelectrically induced contribution to the surface resistance. Starting in 2009, we suggested a means of reducing the residual resistance by warming up a cavity after initial cooldown to about 20 K and cooling it down again [1]. In subsequent studies we used this technique to manipulate the residual resistance by more than a factor of 2 [2]. We postulated that thermocurrents during cooldown generate additional trapped magnetic flux that impacts the cavity quality factor. Since several questions remained open, we present here a more extensive study including measurement of two additional passband modes of the 9 cell cavity that confirms the effect. We also discuss simulations that substantiate the claim. While the layout of the cavity LHe tank system is cylindrically symmetric, we show that the temperature dependence of the material parameters result in a non symmetric current distribution. Hence a significant amount of magnetic flux can be generated at the RF surface resulting in an increased surface resistance [3]

High Q0 research The dynamics of flux trapping in superconducting niobium

The quality factor Q0 that can be obtained in a superconducting cavity is known to depend on various factors like niobium material properties, treatment history and magnetic shielding. We believe that cooling conditions have an additional impact, as they appear to influence the amount of trapped flux and hence the residual resistance [1 3]. We constructed a test stand using a niobium rod shorted out by a titanium rod to mimic a cavity in its helium tank to study flux trapping. Here we can precisely control the temperature and measure the dynamics of flux trapping at the superconducting phase transition. We learned that magnetic flux can be generated when a temperature gradient exists along the rod and when the niobium transitions into the superconducting state it subsequently remains trapped. Furthermore, it was shown that the cooling rate during isothermal cooldown through the transition temperature can influence the amount of externally applied flux which remains trapped. The acquired knowledge may be used to modify the cooldown procedure of SRF cavities leading to a reduced level of trapped flux and hence operation closer to the BCS limit

Impact of Trapped Flux and Thermal Gradients on the SRF Cavity Quality Factor

The obtained Q0 value of a superconducting niobium cavity is known to depend on various factors like the RRR of the Niobium material, crystallinity, chemical treatment history, the high pressure rinsing process, or effectiveness of the magnetic shielding. We have observed that spatial thermal gradients over the cavity length during cool down appear to contribute to a degradation of Q0. Measurements were performed in the Horizontal Bi Cavity Test Facility HoBiCaT at HZB on TESLA type cavities as well as on disc and rod shaped niobium samples equipped with thermal, electrical and magnetic diagnostics. Possible explanations for the effect are discusse

Ecological Niche Modeling of Francisella tularensis Subspecies and Clades in the United States

Two subspecies of Francisella tularensis are recognized: F. tularensis subsp. tularensis (type A) and F. tularensis subsp. holartica (type B). Type A has been subdivided further into A1a, A1b, and A2, which differ geographically and clinically. The aim of this work was to determine whether or not differences among subspecies and clades translate into distinct ecological niches. We used 223 isolates from humans and wildlife representing all six genotypes (type A, B, A1, A2, A1a, or A1b). Ecological-niche models were built independently for each genotype, using the genetic algorithm for rule-set prediction. The resulting models were compared using a non-parametric multivariate analysis-of-variance method. A1 and A2 are ecologically distinct, supporting the previously observed geographic division, whereas ecological niches for types A and B overlapped notably but A1a and A1b displayed no appreciable differences in their ecological niches

RF Measurements of the 1.6 cell Lead Niobium Photoinjector in HoBiCat

The development of a simple and robust SRF photoinjector capable of delivering 1 mA average current in c.w. operation continues to advance with the horizontal RF testing of the 1.6 cell Pb Nb hybrid photoinjector. This injector utilizes a sputtered lead coating on a removable Nb cathode plug as the photoelectron source and has recently been tested in the horizontal test cryostat facility, HoBiCaT, at Helmholtz Zentrum Berlin. In this paper we will report on the status of these RF measurements and compare the performance to previous vertical RF tests performed at Jefferson Laboratory. We will also provide a summary of the cavity tuning range and microphonics measurements now that it has been installed into a helium vessel equipped with a Saclay style tuner

BERLinPro A Compact Demonstrator ERL for High Current and Low Emittance Beams

The HZB previously BESSY was the first institution in Germany to build and operate a dedicated synchrotron light source BESSY I . About 10 years ago BESSY II, a third generation synchrotron light source, was commissioned and is very successfully running since that time. Due to its expertise in development and operation of accelerator facilities HZB is ideally suited to realize new accelerator concepts. Therefore HZB is proposing to build a demonstrator ERL facility BERLinPro that will realize high current and low emittance operation at 100 MeV. BERLinPro is intented to bring ERL technology to maturity. This paper presents an overview of the project and the key components of the facilit

Results from Beam Commissioning of an SRF Plug Gun Cavity Photoinjector

Superconducting rf photo electron injectors SRF photoinjectors hold the promise to deliver high brightness, high average current electron beams for future light sources or other applications demanding continuous wave operation of an electron injector. This paper discusses results from beam commissioning of a hybrid SRF photoinjector based on a Pb coated plug and a Nb rf gun cavity for beam energies up to 2.5MeV at Helmholtz Zentrum Berlin HZB . Emittance measurements and transverse phase space characterization with solenoid scan and slitmask methods will be presente