Performance Studies of a NOvA 53 MHz RF Cavity Frederic Jones 1 1 Stony Brook University, Stony Brook, NY Fermilab National Accelerator Laboratory, Batavia, IL SIST Final Talks August 9, 2011, Fermilab Supervised by David Wildman 2

NOvA Project Fermilab Recycler Ring will be upgraded to Proton Ring. Three new RF cavities will be required to perform multi- batch slip stacking. Two of the cavities will operate at different frequencies; one will be used as a spare. We present results from low power measurements of one these cavities.

Characteristics of the Cavity

Measurement Set-up Printer

Dependence of Resonant Frequency on the Gap Distance We adjusted the bottom fixture to change the gap distance A Wider gap produces a lower capacitance and as result a higher resonant frequency. Narrower gap produces lower frequency

The Effect of Higher Order Modes (HOMs)

Frequency Spectrum of the Cavity With HOMs Identified The fundamental TEM mode, which is used to accelerate the beam was measured to be MHz. This is used to accelerate the beam

Damping of HOMs

Before After Before and After Damping of 151MHz and 221 MHz Modes 151 MHz Ring mode 221 MHz TEM mode

Tuning of the Resonant Frequency An aluminum-doped YIG tuner will be used to tune the frequency. The tuner is inductively coupled to the cavity and contains ferrite stack at the other. By changing the current in the bias field, we can change the frequency. A prototype tuner was developed - this tuner is a rigid half- wavelength coaxial waveguide.

Results from Cavity Tuning Prototype Garnet Tuner A 20kHz frequency tuning range was measured with the prototype tuner which is sufficient to reliably tune the cavity during operation. We will operate in this region

A mechanical tuner will be also used the tune the resonant frequency. Unlike garnet tuner, this changes the frequency slowly over a long period of time. Conducting copper block Frequency change is proportional to the square of the field strengths at the location of displaced volume.(Slater’s perturbation theorem). We can vary the location and depth of the tuner to find optimal tuning range. Copper block FlangeAdjuster

Results from Cavity Tuning Mechanical Tuner

Gap Voltage Monitor Side monitor produce tolerable voltage on the wire when 150 kV is across gap Gap voltage adjusts depth inside cavity to optimize voltage reading on wire The Effective length is distance from flange to circular plate. This creates a capacitance between the plate and cavity wall By reducing the length we reduce the voltage on the plate, and thus the voltage on the wire

Conclusion The resonant Frequency of the cavity was adjusted close to desired operational value of MHz. All HOM modes were identified. The Prototype HOM dampers and Tuners developed in this study proved to be successful in optimizing the cavity’s performance. Namely, the damper reduce the strengths of the 155 and 221 MHz modes, which verifies the effectiveness of the proposed damping scheme. The fast cavity-tuner system demonstrated the necessary 10kHz tuning range that is required to reliably adjust the resonant frequency.

Acknowledgements Supervisor: David Wildman Fermilab SIST committee Dr. James Davenport Dianne Engram Jamieson Olsen Linda Diepholz Robyn Madrak

References [ 1] K. Seiya, et al, “Progress in Multi-batch Slip stacking in the Fermilab Main Injector and Future Plans,” Proceedings of the 23 rd Particle Accelerator Conference, May 4 th – 8 th, 2009, Vancouver, British Columbia, Canada. [2] J.E. Griffin, A Numerical Example of an RF Accelerating System", in R.A. Carrigan, F.R.Huson, and M. Month, ed., Physics of High Energy Particle Accelerators (Fermilab Summer School, 1981), American Institute of Physics, New York, 1982, p [3] H. Elnaiem, “Prototype NOvA RF Cavity for the Fermilab Recycler Ring,” May 18th-August 7th, 2009, internal publication.

Equivalent circuit of the cavity-tuner system