Haipeng Wang

Elliptical Cavity Shape Optimization for Acceleration and HOM Damping

We report a survey of center cell shapes developed for Superconducting Radio Frequency (SRF) multi-cell cavities for different projects. Using a set of normalized parameters, we compare the designs for different frequencies and particle velocities for the fundamental mode. Using dispersion curves of High Order Modes (HOM) (frequency verse phase advance) calculated by MAFIA for a single cell, we further optimize the cavity shape to avoid a light cone line crossing at the dangerous resonance frequencies determined by the beam bunch structure and eliminate the trapped (or high R/Q) modes with a low group velocity. We developed this formulation to optimize a 5-cell, 750MHz cavity shape, with good real-estate accelerating gradient and a strong HOM damping waveguide structure for the JLab 1MW ERL-FEL project.

SNS cryomodule performance

Thomas Jefferson National Accelerating Facility, Jefferson Lab, is producing 24 Superconducting Radio Frequency (SRF) cryomodules for the Spallation Neutron Source (SNS) cold linac. This includes one medium-/spl beta/ (0.61) prototype, 11 medium-/spl beta/ production, and 12 high beta (0.81) production cryomodules. After testing, the medium-/spl beta/ prototype cryomodule was shipped to Oak Ridge National Laboratory (ORNL) and acceptance check out has been completed. All production orders for cavities and cryomodule components are being received at this time and the medium-/spl beta/ cryomodule production run has started. Each of the medium-/spl beta/ cryomodules is scheduled to undergo complete operational performance testing at Jefferson Laboratory before shipment to ORNL. The performance results of cryomodules to date will be discussed.

Status of the R&D towards electron cooling of RHIC

The physics interest in a luminosity upgrade of RHIC requires the development of a cooling-frontier facility. Detailed calculations were made of electron cooling of the stored RHIC beams. This has been followed by beam dynamics simulations to establish the feasibility of creating the necessary electron beam. The electron beam accelerator will be a superconducting Energy Recovery Linac (ERL). An intensive experimental R&D program engages the various elements of the accelerator, as described by 24 contributions to the 2007 PAC.

Overview of SNS Cryomodule Performance

Thomas Jefferson National Accelerating Facility (Jefferson Lab) has completed production of 24 Superconducting Radio Frequency (SRF) cryomodules for the Spallation Neutron Source (SNS) superconducting linac. This includes one medium-β (0.61) prototype, eleven medium-β and twelve high-β (0.81) production cryomodules. Nine medium-β cryomodules as well as two high-β cryomodules have undergone complete operational performance testing in the Cryomodule Test Facility at Jefferson Lab. The set of tests includes measurements of maximum gradient, unloaded Q (Q0), microphonics, and response to Lorentz forces. The Qext’s of the various couplers are measured and the behavior of the higher order mode couplers is examined. The mechanical and piezo tuners are also characterized. The results of these performance tests will be discussed in this paper.

JLAB high-current CW cryomodules for ERL and FEL applications

We describe the activities underway at JLab to develop new CW cryomodules capable of transporting up to Ampere-levels of beam currents for use in ERLs and FELs. Goals include an efficient cell shape, high packing factor for efficient real-estate gradient and very strong HOM damping to push BBU thresholds up by two or more orders of magnitude compared to existing designs. Cavity shape, HOM damping and ancillary components are optimized for this application. Designs are being developed for low-frequency (750 MHz), Ampere-class compact FELs and for high-frequency (1.5 GHz), 100 mA configurations. These designs and concepts can easily be scaled to other frequencies. We present the results of conceptual design studies, simulations and prototype measurements. These modules are being developed for the next generation ERL based high power FELs but may be useful for other applications such as high energy light sources, electron cooling, electron-ion colliders, industrial processing etc.

Performance of the first refurbished CEBAF cryomodule

The Thomas Jefferson national accelerator facility has begun a cryomodule refurbishment project. The goal of this project is robust 6 GeV, 5 pass operation of the continuous electron beam accelerator facility (CEBAF). The scope of the project includes removing, refurbishing and replacing 10 CEBAF cryomodules at a rate of three per year. Refurbishment includes reprocessing of SRF cavities to eliminate field emission and increase the nominal gradient from the original 5 MV/m to 12.5 MV/m. New dogleg couplers between the cavity and helium vessel flanges will intercept secondary electrons that produce arcing on the 2 K ceramic window in the fundamental power coupler (FPC). Modification of the Qext of the FPC will allow higher gradient operations. Other changes include new ceramic RF windows for the air to vacuum interface of the FPC and improvements to the mechanical tuners. Any damaged or worn components will be replaced as well. Currently, the first of the refurbished cryomodules has been installed and tested both in the cryomodule test facility and in place in the North Linac of CEBAF. This paper will summarize the results of these tests.

Preliminary results from prototype niobium cavities for the JLAB Ampere-class FEL

In a previous paper the cavity [1] design for an Ampere-class cryomodule was introduced. We have since fabricated a 1500 MHz version of a single cell cavity with waveguide couplers for HOM and fundamental power, attached to one end of the cavity, a 5-cell cavity made from large grain niobium without couplers and. a 750 MHz single cell cavity without endgroups to get some information about obtainable Q-values, gradients and multipacting behavior at lower frequency. This contribution reports on the various tests of these cavities.

RF-thermal-structural analysis of a waveguide higher order mode absorber

For an ongoing high current cryomodule project, a total of 5 higher order mode (HOM) absorbers are required per cavity. The load is designed to absorb Radio Frequency (RF) heat induced by HOMs in a 748.5MHz cavity. Each load is targeted at a 4 kW dissipation capability. Water cooling is employed to remove the heat generated in ceramic tiles and by surface losses on the waveguide walls. A sequentially coupled RF-thermal-structural analysis was developed in ANSYS to optimize the HOM load design. Frequency-dependent dielectric material properties measured from samples and RF power spectrum calculated by the beam-cavity interaction codes were considered. The coupled field analysis capability of ANSYS avoided mapping of results between separate RF and thermal/structural simulation codes. For verification purposes, RF results obtained from ANSYS were compared to those from MAFIA, HFSS, and Microwave Studio. Good agreement was reached and this confirms that multiple-field coupled analysis is a desirable choice in analysis of HOM loads. Similar analysis could be performed on other particle accelerator components where distributed RF heating and surface current induced losses are inevitable.

Simulations and measurements of a heavily hom-damped multi-cell SRF cavity

After an initial cavity shape optimization [1] and cryomodule development [2] for an Ampere-class FEL ERL, we have simulated a complete 5-cell high-current (HC) cavity structure with six waveguide (WG) couplers for Higher Order Mode (HOM) damping and fundamental power coupling. MAFIA time-domain wakefield simulations have been used to calculate the cavities broadband HOM impedance spectra. Microwave Studio (MWS) has been used in addition to evaluate the external Q of the fundamental power coupler (FPC) and the R/Qs of the HOMs. A half scale 1497 MHz single-cell model cavity and a 5-cell copper cavity including dummy HOM WG loads were fabricated to bench measure and confirm the design performance. Details of the multi-beam wakefield simulations, the HOM damping measurements and multi-peak data fitting analysis techniques are presented.

System study using injection phase locked magnetron as an alternative source for superconducting radio frequency accelerator

As a drop-in replacement of Continuous Electron Beam Accelerator Facility (CEBAF) 5kW CW klystron system, a 1497MHz, high efficiency magnetron using injection phase lock [1] and slow amplitude variation using magnetic field trimming and anode voltage modulation has been studied systematically using MatLab/Simulink simulations. The magnetron model is based the characteristics of experiment and manufacture chart on a 2.45GHz cooker type CW magnetron. To achieve high performance of a superconducting radio frequency (SRF) acceleration cavity with an electron beam loading, the magnetrons low level radio frequency (LLRF) control has been studied in two lock loops. In the frequency lock loop, the characterized anode V-I curve, output power (the tube electronic efficiency) and frequency dependence to the anode current (pushing by Vaughan model) and the Rieke diagram (frequency pulling by the reactive load) are simulated. The magnetic field B and anode voltage V in Hartree condition are satisfied and the effect of filament heater power to the frequency lock is also included. In the phase lock loop, the Adler equation governing injection phase stability is included in this study. The control of the magnet trim-coil power-supply and of the anode voltage modulation-switching power-supply has been also simulated to achieve the amplitude modulation. The result of linear responses to the amplitude and phase of SRF cavity will be presented in this paper. The requirement of LLRF control will be given by this result.