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 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

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]

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

Commissioning results of the HZB Quadrupole Resonator

Recent cavity results with niobium have demonstrated the necessity of a good understanding of both the BCS and residual resistance. For a complete picture and comparison with theory, it is essential that one can measure the RF properties as a function of field, temperature, frequency and ambient magnetic field. Standard cavity measurements are limited in their ability to change all parameters freely and in a controlled manner. On the other hand, most sample measurement setups operate at fairly high frequency, where the surface resistance is always BCS dominated. The quadrupole resonator, originally developed at CERN, is ideally suited for characterization of samples at typical cavity RF frequencies. We report on a modified version of the QPR with improved RF figures of merit for high field operation. Experimental challenges in the commissioning run and alternate designs for simpler sample changes are shown alongside measurement results of a large grain niobium sampl

High Q Cavity Operation Study on the Thermoelectrically Induced Contribution to 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

RF input power couplers for high current SRF applications

present day accelerator science. The bERLinPro project is presently being built at HZB to address the challenges involved in high current SRF machines with the goal of generating and accelerating a 100 mA electron beam to 50 MeV in continuous wave cw mode at 1.3 GHz. One of the main challenges in this project is that of handling the high input RF power required for the photo injector as well as booster cavities where there is no energy recovery process. A high power co axial input power coupler is being developed to be used for the photo injector and booster cavities at the nominal beam current. The coupler is based on the KEK cERL design and has been modified to minimise the penetration of the coupler tip in the beam pipe without compromising on beam power coupling Qext 105 . Herein we report on the RF design of the high power 115 kW per coupler, dual couplers per cavity bERLinPro BP coupler along with initial results on thermal calculations. We summarise the RF conditioning of the TTF III couplers modified for cw operation performed in the past at BESSY HZB. A similar conditioning is envisaged in the near future for the low current SRF photo injector and the bERLinPro main linac cryomodule

Impact of geometry on flux trapping and the related surface resistance in a superconducting cavity

In order to minimize the surface resistance in superconducting cavities, a deeper understanding of residual resistance due to trapped magnetic flux is necessary. For that purpose, a combined temperature and magnetic field mapping system is employed to map magnetic flux trapped in a superconducting cavity, and the related increase in surface resistance. By cooling down a 1.3 GHz TESLA single cell cavity several times with externally applied static magnetic fields with different orientations with respect to the cavity, a statement can be made about how the angle between the applied magnetic field and the cavity s surface affects flux trapping, and surface resistance. For example, a significantly higher increase in surface resistance is observed when the applied magnetic field is perpendicular to the cavity s surface compared to when it is paralle