From microscopic to macroscopic dynamics of super-conducting accelerating cavities

by Anirban Krishna Bhattacharyya (Uppsala University)

Thursday 19 Oct 2017, 15:15 → 16:15 Europe/Stockholm

Å12167

Abstract

The super-conducting cavities used in particle accelerators are essentially resonant structures with very high intrinsic quality factor (Q₀). The high Q₀ of the cavities leads to increased reflection during charging of the cavities to nominal voltage, which leads to high energy loses in case of pulsed machines. In my thesis and this seminar I explore and present a novel technique to optimally charge the super-conducting cavities with the particular example of the spoke cavities to be used for the European Spallation Source (ESS) project in Lund, Sweden. The analysis reveals that slow charging with hyperbolic sine (sinh) cavity voltage profile leads to energy efficient filling.

However, a filling rate lower than some particular value is counterproductive as then the cryogenic losses start to dominate the wasted energy. Such cryogenic losses are dependent on cavity Q₀, which for super-conducting cavities is a function of cavity operating electromagnetic field gradient. So methods for accurate estimation of cavity Q₀ as function of cavity field gradient are essential for effective characterisation of the super-conducting cavities. In the thesis I propose a novel method to accurately estimate Q₀ as a function of cavity field gradient and present experimental results performed on the prototype spoke cavity at FREIA, in Uppsala University.

The cavity quality factor is also dependent on the purity and the resultant magnetic flux trapping property of the super-conducting material. Recent studies have revealed that the rate of cooling of materials through the critical temperature has an effect on the residual flux trapped in the material. In the thesis I study the use of the Time-dependent Ginzburg Landau equations to model the process of state transition from normal to super-conducting state. This study may allow to theoretically explain experimentally observed results from the first principles.

Slides The super-conducting cavities used in particle accelerators are essentially resonant structures with very high intrinsic quality factor (Q0). The high Q0 of the cavities leads to increased reflection during charging of the cavities to nominal voltage, which leads to high energy loses in case of pulsed machines. In my thesis and this seminar I explore and present a novel technique to optimally charge the super-conducting cavities with the particular example of the spoke cavities to be used for the European Spallation Source (ESS) project in Lund, Sweden. The analysis reveals that slow charging with hyperbolic sine (sinh) cavity voltage profile leads to energy efficient filling. However, a filling rate lower than some particular value is counterproductive as then the cryogenic losses start to dominate the wasted energy. Such cryogenic losses are dependent on cavity Q0, which for super-conducting cavities is a function of cavity operating electromagnetic field gradient. So methods for accurate estimation of cavity Q0 as function of cavity field gradient are essential for effective characterisation of the super-conducting cavities. In the thesis I propose a novel method to accurately estimate Q0 as a function of cavity field gradient and present experimental results performed on the prototype spoke cavity at FREIA, in Uppsala University. The cavity quality factor is also dependent on the purity and the resultant magnetic flux trapping property of the super-conducting material. Recent studies have revealed that the rate of cooling of materials through the critical temperature has an effect on the residual flux trapped in the material. In the thesis I study the use of the Time-dependent Ginzburg Landau equations to model the process of state transition from normal to super-conducting state. This study may allow to theoretically explain experimentally observed results from the first principles.