Derun Li

Design and fabrication of an 805 MHz RF cavity with Be windows for a high RF power testing for a muon cooling experiment

We report the design and fabrication of an 805 MHz RF cavity with beryllium (Be) windows for a muon cooling experiment. The cavity resembles closely to a cylindrical pillbox cavity with the conventional beam iris covered by thin Be foils to enhance on-axis accelerating fields. The Be windows are bolted to the cavity body to allow for easy replacement and testing for different windows. The cavity is to be installed within a super-conducting solenoid with magnetic field up to 5 Tesla, and tested at high RF power.

Deflecting RF cavity design for a Recirculating Linac based facility for Ultrafast X-ray Science (LUX)

We report on superconducting deflecting RF cavity designs for a Recirculating Linac Based Facility for Ultrafast X-ray Science (LUX) at Lawrence Berkeley National Laboratory. The deflecting cavities operate in the lowest dipole mode and are required to produce a temporal correlation within flat electron bunches, as needed for X-ray compression in crystal optics. Deflecting voltage of up to 8.5 MV is required at 3.9 GHz. We present a 7-cell cavity design in this paper. Seven such cavities are required to generate the 8.5 MV deflecting voltage. Longitudinal and transverse impedance from LOMs (lower order mode) and HOMs (higher order mode) are simulated using the MAFIA code. Short-range and long-range wakefields excited through these impedances are calculated. Beam loading effects of the deflecting mode and parasitic modes are estimated. Q values of the LOM monopole modes in the cavity need to be damped to be below 10/sup 4/-10/sup 5/ levels in order to maintain the required energy spread.

Dark current and X ray measurements of an 805 MHz pillbox cavity

The muon cooling systems proposed for neutrino factories require low frequency (201 MHz) RF cavities with Be windows, at high gradient (E/sub acc/ /spl sim/ 16 MV/m), in strong solenoidal magnetic field (/spl sim/ 5 T). For the proposed Muon Ionization Cooling Experiment (MICE), an experimental demonstration of cooling, we have an additional constraint that we must operate sensitive particle detectors very close to the RF cavities, which produce backgrounds from dark currents and X rays. To understand the processes involved in cavity conditioning and operation near particle detectors, we have constructed a test facility at Lab G of Fermilab, where a 5 Tesla superconducting solenoid, a 14 MW peak power klystron and a pillbox test cavity at 805 MHz are available. We present measurements of dark currents, X rays and surface structure from the pillbox cavity, with both copper and beryllium endplates, and discuss the interaction between surface structure and radiation backgrounds produced.

Beam impedance calculation and analysis of higher order modes (HOMs) damped RF cavities using MAFIA in time domain

A time domain method using MAFIA code has been developed to calculate narrow band beam impedance in RF cavities over a wide range of frequency spectrum. The impedance is obtained through Fast Fourier Transformation (FFT) of computed wakefields by the MAFIA. Analysis of the calculated impedance spectrum will be presented. Application of the method to a known RF cavity design (PEP-II cavity) has shown good agreements with bench and beam measurements. The method has been applied to the RF cavity design of of Damping Rings for Next Linear Collider (NLC).

805 MHz and 201 MHz RF cavity development for MUCOOL

A muon cooling channel calls for very high accelerating gradient RF structures to restore the energy lost by muons in the absorbers. The RF structures have to be operated in a strong magnetic field and thus the use of superconducting RF cavities is excluded. To achieve a high shunt impedance while maintaining a large enough aperture to accommodate a large transverse emittance muon beam, the cavity design adopted is a pillbox-like geometry with thin Be foils to terminate the electromagnetic field at the cavity iris. The possibility of using grids of thin-walled metallic tubes for the termination is also being explored. Many of the RF-related issues for muon cooling channels are being studied both theoretically and experimentally using an 805 MHz cavity that has a pillbox-like geometry with thin Be windows to terminate the cavity aperture. The design and performance of this cavity are reported here. High-power RF tests of the 805 MHz cavity are in progress at Lab G in Fermilab. The cavity has exceeded its design gradient of 30 MV m−1, reaching 34 MV m−1 without external magnetic field. No surface damage was observed at this gradient. The cavity is currently under conditioning at Lab G with an external magnetic field of 2.5 T. We also present here a 201 MHz cavity design for muon cooling channels. The proposed cavity design is also suitable for use in a proof-of-principle muon ionization cooling experiment.