SRF Capabilities | Applied Physics and Superconducting Technology Directorate

To run its leading scientific research program in superconducting RF and state-of-the-art cryomodule production activities, the Applied Physics and Superconducting Technology Directorate (APS-TD) maintains comprehensive SRF capabilities that include full life-cycle treatment of cavities and cryomodules, from design through to production. The world-class facilities and wide spectrum of expertise are described in the following sections:

Vertical Testing of Cavities
Horizontal Testing of Cavities
Cavity Surface Processing
Cavity Post-Processing
Cavity String and Cryomodule Assembly
RF Design and Test
Cryomodule Engineering
Optical Inspection and Eddy Current Scanning
Material Studies

Vertical Testing of Cavities

Fermilab’s SRF Vertical Test Stand Facility encompasses three large liquid helium dewars, each of which can test multiple production-style cavities simultaneously. The RF control system is capable of high power operation over a wide frequency range, and the cryogenic system can cool liquid helium baths as low as 1.4 K. Advanced diagnostic equipment includes second sound quench detection system, temperature mapping system for single cell cavities, and externally controlled magnetic field environment.

Three vertical test dewars with sliding radiation shielding block, cryogenic lines and preparation staging area.
Installation of a cavity to a top plate prior to installation in a dewar.
Dewar used for vertical testing of cavities prior to installation in pit
Fully instrumented top plate installed in dewar
Control room for vertical cavity testing.
Temperature mapping system being installed on a cavity to locate regions of heating.

Horizontal Testing of Cavities

After a cavity treatment has been qualified in vertical test, it may then be evaluated in the Horizontal Test Facility (HTF), to make sure that it works well with all auxiliary components to achieve the required performance. In preparation for hoizontal test, the cavity is equipped with additional components such as tuners which maintain the cavity at the desired operating frequency, high power couplers that are used to power the cavity, and other ancillary systems that are needed to ensure the cavity will perform well in the accelerator. Fermilab has established three test stands at the HTF designed to test different types of accelerating cavities.

The HTS-1 is designed to test multi-cell elliptical cavities operating at either 1.3GHz or at 3.9GHz. . The HTS-1 includes RF systems to power the cavities, system to control the RF power and cryogenic system, and diagnostics to allow full characterization of the cavity, coupler, tuner, and other associated components.
The Spoke Test Cryostat (STC) was designed to test lower frequency cavities (325MHz) used primarily for accelerating beams of heavy ions or protons.
A 325MH spoke cavity that will be part of a PIP-II prototype cryomodule is being installed in the STC cryostat .
The design and fabrication of the HTS-2 cryostat is a joint effort between Fermilab and the RRCAT laboratory in India. The HTS-2 cryostat will be able to accommodate up to two 650MHz (or 1.3GHz) cavities at a time, dramatically increasing test throughput.
A titanium helium vessel is welded to a 1.3 GHz cavity in the argon atmosphere of a glovebox, in preparation for horizontal testing.

Cavity Surface Processing

High quality polishing is essential to deliver maximum accelerating gradients and high quality factors in SRF cavities. Chemical processing of superconducting cavities is mainly used to remove contaminants from the Niobium surface. Fermilab capabilities include electropolishing (EP), buffered chemical polishing (BCP), and centrifugal barrel polishing (BCP) for a wide variety of cavity types. Surface processing facilities are located in the Cavity Processing Lab in IB4 and the joint cavity surface processing facility at Argonne National Laboratory.

An external water cooling system is used during electropolishing to minimize temperature gradients over the cavity surface.
The electropolishing tool can accommodate several cavity types. Here a dressed 9 cell cavity (i.e. with helium vessel) is shown.
The centrifugal barrel polishing tool repairs 1.3 GHz inner surfaces when defects appear. Two 9-cell 1.3 GHz cavities can be tumbled simultaneously.
Electropolishing R&D on 1-cell 1.3 GHz cavities occurs on a semi-automated tool. The tool is upgradable to 9-cell 1.3 GHz capacity.

Cavity Post-Processing

Heat Treatment Facilities
Two high temperature vacuum furnaces and two low temperature ovens are used to perform heat treatments on SRF cavities. A major focus is to develop and optimize recipes and create new Niobium (Nb) compounds aimed at reducing RF losses in the cavities which can dramatically reduce operating costs. Other important applications of heat treatment are hydrogen degasification, annealing, and mild baking to remove high field Q-slope.
Class 100 Clean Rooms
SRF cavity cleanroom assembly capability is essential to good cavity performance. Fermilab clean rooms contain automated high-pressure rinsing tools used for cleaning the interior surface of cavities. They are also used to assemble cavities and pump them down to ultra high vacuum. Cleanroom facilites are located in IB4, Lab 2, and the joint cavity surface processing facility at Argonne National Laboratory.

LCLS-II cavities are processed in ultra-high-vacuum vacuum furnaces at 800 C to degas hydrogen, followed by injection of low-pressure nitrogen, which enters the material and dopes it.
IB4 CPL Cleanrooms include Class 10 particle-free assembly space, ultra-pure water high pressure rinsing, and Class 1000 UHV part cleaning facilities. SRF cavities of many varieties can be cleaned, rinsed, and assembled in the IB4 Cavity Processing Lab cleanroom space.
CPL Class 10 cleanroom with HPR tool and SRF cavity assembly infrastructure.
Low temperature ovens, up to 300 C, are used to bake cavities and UHV devices. A separate UHV vacuum cart is used to evacuate cavities.

Cavity String and Cryomodule Assembly

APS-TD has two independent cavity string assembly facilities, located in MP9 (the largest cleanroom at Fermilab, ~250 m²) and Lab 2. Each contain cleanroom areas with several levels of cleanliness:

Class 1000 (ISO6) ante cleanroom area: Preparation of the dressed cavities for transportation into the assembly cleanroom.
Class 100 (ISO5) sluice area: Parts and Fixtures final preparation to enter the Class 10 assembly area.
Class 10 (ISO4) assembly cleanroom area: Where the cavity vacuum is vented to interconnect them with bellows.

Production floor areas for cryomodule assembly are located in ICB and Lab 2.

Large assembly area of MP9 cleanroom, where clean assembly of the LCLS-II cavity strings is performed
Clean assembly requires highly skilled workers, state-of-the-art facilities, and careful procedures.
Preparation area for cleaning cavities prior to entry in the main assembly area of MP9.
Completed cavity string in the MP9 cleanroom ready for leak-checking.
LCLS-II cavity string after removal from MP9 cleanroom.
Coldmass insertion to a vacuum vessel in ICB.
Cryomodule assembly floor of ICB.
Cleanroom assembly area of Lab 2 showing assembly rail.
The production floor area of Lab 2.

RF Design and Test

The central part of any particle accelerator is accelerating structure or RF cavity. Successful operation of cavities requires optimized design, precise production, and low power RF tuning. We have tools for providing superior design, precise measurements and tuning of accelerating cavities both for prototyping and production. The simulations codes HFSS, COMSOL, CST and ANSYS are installed in specialized TD servers to allow us to increase design efficiency, performing advanced analysis for optimized cavity design. Optimization includes reducing peak surface fields, multipactor mitigation, higher order modes extraction and damping, microphonics sensitivity, Lorentz force detuning minimization, structural and thermal stability analysis, and study of multipole effects of the beam.

Our sophisticated RF measurement equipment and tuning tools allow us to work with different type of RF cavities in wide frequency range. We are responsible of RF testing and quality control on all received cavities from vendors along the process of qualifying them for cryomodule operation. This includes monitoring key cavity performance parameters such as frequency spectrum, field flatness, eccentricity and physical length.

The automatic tuning machine (CTM) at the IB4 RF clean room is being used for RF QC and tuning the 1.3GHz Tesla style 9-cell cavities.
Process for evaluating frequency change of cavity due to a pressure fluctuation in the liquid helium bath.
The RF clean room is equipped with various RF QC and manual tuning fixtures for 650 MHz, 1.3 GHz, and 3.9 GHz half-cells, dumbbells, and cavities.
Electromagnetic design of SSR2 cavity (quarter section).
A button Beam Position Monitor (BPM) developed for the low beta section of the Project X Injector Experiment (PXIE) at Fermilab.
Simulations of higher order modes and wakefields in the 1.3 GHz and 3.9 GHz cavities for LCLS II.

Cryomodule Engineering

Cryomodules are the vacuum-insulated assemblies of superconducting RF cavities and associated hardware, including instrumentation and often also a superconducting magnet package. Thus, a cryomodule is the fundamental building block of a superconducting RF accelerator. These complex assemblies provide numerous challenges to engineers and designers. The cryomodules provide support, precise alignment, and thermal and magnetic shielding for the dressed cavities, as well as feed-throughs from the outside environment to low temperatures for the RF power and instrumentation. The cryomodule must provide the required insulating and beam vacuum and provide robust support with minimal cavity vibration and good cavity alignment reliably with thermal cycling. The design provides for cooling of the RF cavities to superconducting temperatures and protects the helium and vacuum spaces including the RF cavity from exceeding allowable pressures. Design of cryomodules calls upon a wide range of mechanical, thermal, and electrical engineering skills and experience.

Optical Inspection & Eddy Current Scanning

The optical inspection system consists of a camera and lighting system mounted to a rod that is inserted into the cavity. It is used for routine inspection of cavities during the processing cycle as well as computer-aided close inspection of particular regions of interest. Eddy current scanning is used to examine niobium sheets prior to cavity fabrication to check for impurities.

A 650 MHz 5 cell cavity on the inspection test stand.
Schematic illustration of the optical inspection system.
The optical inspection head consists of a camera mounted to a rigid rod with lighting and a mirror.
Typical optical inspection image showing the weld bead of the equator on the left.
Eddy current scanning of niobium sheets to check for the presence of impurities before cavity production.

Material Studies

A wide range of materials studies tools are used, including scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) capabilities, physical property measurement system (PPMS), Instron tensile testing, Keyence laser confocal microscopy, and surface topological replicas. Chemical treatment of samples is also a critical capability.

The Jeol JSM 5900LV Scanning Electron Microscope has composition analysis capabilities with EDS and crystal structure analysis with EBSD.
The Physical Property Measurement System (PPMS) is a versatile tool used to measure samples at low temperatures and under high magnetic fields using various diagnostics. Measurements include AC transport measurement, thermal transport measurement, AC susceptibility and DC magnetization, vibrating sample magnetometer, atomic force microscopy, and magnetic force microscopy.
The Instron testing apparatus is used to measure critical parameters such as modulus and yield strength.
The Keyence laser confocal microscope is capable of measuring high resolution 3D images of surfaces.
Using rapid-setting resin to create topological replicas, non-destructive measurements are performed of the interior surfaces of cavities.
Chemical capabilities at MDTL include electropolishing and acid etching of samples.