SRF Technology

• SRF Cavities from Single-Crystal Niobium
• Low-Temperature RadioFrequency Feedthrough For CW Applications
• Cavity Processing and Procedure Improvements

Energy-Recovering Linacs

• SRF-Based Energy-Recovering Linear Accelerators (ERLs)

Electron Source

• Injector Advance: The Superlattice Photocathode
• Fiber-Based Drive Lasers

Accelerator

SRF Cavities from Single-Crystal Niobium

A potentially cost-saving and performance-enhancing new approach to fabricating superconducting radiofrequency (SRF) accelerating cavities has been demonstrated by the Institute for Superconducting Radiofrequency Science & Technology (ISRFST) at Jefferson Lab. Several single-cell niobium cavities were made from material sliced from large-grain niobium ingots, rather than fine-grain material melted from ingots and formed into sheets by the traditional process of forging, annealing, rolling and chemical etching. In tests carried out by ISRFST, these cavities performed extremely well. If multi-cell cavities are also successful, the method could have a substantial impact on the economics of high-performance SRF—notably including for the ILC, the International Linear Collider, a 500 GeV machine that will need some 17,000 SRF cavities performing above 28 MV/m. This proof-of-principle work could lead to more reliable production and reduced costs.

Using a scaled version of a low-loss design proposed for the ILC, a test cavity supported an accelerating gradient of 45 MV/m. This figure is very close to both Cornell's current world record and the theoretical limit.

The work aimed to provide a deeper understanding of the influence of grain boundaries on the often-observed drop in Q (the cavity-performance quality factor) at accelerating gradients above 20 MV/m. "Q-drop" is not well understood, but it may be linked to contaminants and grain boundaries in the niobium.

The researchers used single-crystal niobium sheets for forming into half-cells, omitting expensive processing steps and producing cavities with few or no grain boundaries. Reference Metals Company Inc of Bridgeville, Pennsylvania, provided the niobium in a research collaboration with JLab.

Reference
P. Kneisel et al., "Preliminary Results from Single Crystal and Very Large Crystal Niobium Cavities" 2005 Particle Accelerator Conference

Low-Temperature RadioFrequency Feedthrough For CW Applications

Jefferson Lab continues to develop innovative solutions to problems shared by the accelerator community. A common challenge in accelerators based on superconducting radiofrequency technology (SRF) is the presence of additional radiofrequency waves inside the accelerator cavities, in addition to the primary frequency needed for accelerating particles. These so-called higher-order modes (HOMs) can seriously degrade beam quality.

For optimum particle acceleration, these HOMs must be extracted without significantly affecting the desirable accelerating functions. To address this challenge, several institutions, including Jefferson Lab, have adopted and scaled a particular design of HOM coupler that was developed at DESY for use on the Tesla cavities.

Originally intended to serve the pulsed-RF conditions of a collider, the couplers encountered difficulties when employed in continuous wave (CW) applications, such as Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF), and early tests showed the technology to be unusable for the CEBAF 12 GeV upgrade. The problem was excessive heating of the coupler when employed as originally designed.

Jefferson Lab staff developed a novel solution. The modified design allows heat generated in the coupler to bleed away from the accelerator cavity, using a superconducting niobium RF probe that is kept thermally anchored via a single-crystal sapphire dielectric that is brazed between the niobium probe and an externally accessible copper collar.

The solution has been demonstrated to be effective for the CEBAF 12 GeV upgrade cavities and also in Tesla-style cavities tested at BESSY and Daresbury. SURA has filed a patent application for this invention (co-inventors G. Wu and L. Phillips), and a non-exclusive license has been issued to ACCEL Instruments GmbH to produce and market the version for Tesla-style cavities. The units successfully tested at BESSY and Daresbury were produced under this license.

Cavity Processing and Procedure Improvements

Jefferson Lab continues to integrate the fruit of superconducting radiofrequency (SRF) R&D into the production of higher-performing accelerator components. One dimension of this is a program to leverage recent technological developments in the design and implementation of higher quality standards and more efficient techniques for the chemical processing, clean handling and assembly of accelerator components.

To secure the 6 GeV base of the Lab’s Continuous Electron Beam Accelerator Facility (CEBAF), a program (C50) is underway to rework weaker accelerator components. CEBAF originally contained 42 sections of accelerator components, called cryomodules. Each full cryomodule contains a string of eight niobium accelerator cavities. In this program, a cryomodule is removed from CEBAF, fully disassembled, the cavities reprocessed, and the full unit reassembled, tested, and returned to service.

The specification for cavities in the C50 cryomodules in CEBAF is 12.5 MV/m, creating a 50 MV cryomodule from hardware originally designed to produce 20 MV. This success is made possible by applying state-of-the-art procedures and techniques developed in the years since CEBAF was originally built.

Capital improvements made possible by work-for-other projects such as the construction of cryomodules for the Spallation Neutron Source (SNS) and an enhanced level of rigor and specificity in the assembly procedures have yielded a dramatic improvement in particulate contamination control. This new, automated equipment made possible clean acid etching of cavities and rinsing them with high-pressure, ultrapure water. The contamination responsible for the performance-limiting field emission phenomenon was thus eliminated.

Before this program, field-emission loading interior to the cavities was the principle performance-limiting phenomenon, causing an excessive heat load for the cryogenic system and also rf window arc-trips that interrupt beam delivery. The cavities are now limited by quenching attributed to original 1991 material or fabrication defects. The data for the early production cavities from the C50 effort are presented in the figure, together with the field-emission-limited performance from 1992 for the same cavities.

This evolution of the technology provides a foundation for the successful implementation of the 12 GeV upgrade and all other high-performance SRF accelerator applications.

SRF-Based Energy-Recovering Linear Accelerators (ERLs)

Jefferson Lab has become the acknowledged world leader in the development of an innovative use of electron linear accelerators (linacs) in light sources and, potentially, particle colliders: the energy-recovering linac, or ERL. Jefferson Lab built the first ERL with high average current to drive the first kilowatt-scale free-electron laser.

Unlike the recycling of electrons in a synchrotron or a storage ring, an ERL uses a conceptually simple phasing technique to recycle the electrons' energy. On a path measuring exactly an integer multiple of the linac radiofrequency (RF) wavelength plus a half-wavelength, an ERL’s accelerated beam travels through an experiment and re-enters the linac to yield back its energy, via the RF field, to the beam being accelerated. The decelerated beam is then dumped at low energy.

An obvious advantage of ERLs is economic. Consider, for example, the ERL-driven 4th Generation Light Source (4GLS) facility planned for Daresbury in the U.K., where a prototype ERL is under construction. In May 2003, Physics World reported that without energy recovery, "4GLS would consume roughly the output of a large commercial power station." Energy recovery also simplifies spent-beam disposal. But the overall promise of ERLs is far more than economic, and has been distilled in the paper cited below.

Reference
Lia Merminga, David R. Douglas and Geoffrey A. Krafft, Annual Review of Nuclear and Particle Science 53 387 (2003)

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Injector Advance: The Superlattice Photocathode

Improved technology for originating CEBAF's electron beam enables nuclear physics experimenters to gather better data more efficiently. CEBAF's injector originates the electron beam, establishes its special characteristics, and then injects it into the accelerator. Over time, as CEBAF experiments generate new knowledge, nuclear physics researchers need to interrogate nature itself ever more deeply — which means they need increasingly exacting characteristics in the beam. That makes injector advances important. JLab has advanced injector technology in lots of ways, but one research-and-development achievement meriting particular attention is the superlattice photocathode.

An injector's photocathode is the ultimate source of the beam's electrons. The CEBAF beam originates in either of a pair of electron guns, each driven by laser light. The gun uses photons — particles of light — to strip electrons from a photocathode. A few years ago, CEBAF's photocathodes were dime-sized crystals of a material called bulk gallium arsenide, but superlattice photocathodes use thin layers of gallium arsenide grown atop layers of gallium arsenide phosphide to alter the energy states of the material and provide much higher electron beam polarization.

Polarization can be thought of as precisely orienting the electrons' spin. This beam characteristic is crucial for more and more CEBAF experiments because it lets experimenters draw ever more precise inferences from their data. The new kind of photocathode allows for very high polarization — and the higher the polarization, the more efficient the data collection. That, in turn, makes CEBAF itself more efficient, because it lets a given experiment use less of the beam time that is in such great demand.

For some time before 2005, experimenters could expect beam polarization at the level of 75%. During 2005, CEBAF crossed the 80% threshold and reached beyond 85%. The recent Hall A Proton Parity Experiments, HAPPEx and HAPPExII, required high polarization, and profited greatly from the superlattice photocathode.

Reference
M. Baylac et al. Physical Review Special Topics – Accelerators and Beams 8, 123501 (2005)

Fiber-Based Drive Lasers

The new superlattice photocathode, adapted by Jefferson Lab scientists for use in CEBAF,
allows the use of readily available, fiber-based drive lasers, which require significantly less maintenance than laser types previously used in CEBAF. Introduction of these new lasers has reduced photo-injector downtime by more than 50% (from 2% total downtime to less than 1%).

A fiber-based drive laser was constructed using a gain-switched diode seed laser and ErYb fiber amplifier. Light from these components is converted to the 780 nm wavelength needed for the superlattice photocathode using a non-linear frequency doubling process to obtain more than four times the power of previous drive lasers. 

The result of these improvements (superlattice photocathode and fiber-based drive laser) has been to produce a highly reliable photo-injector capable of routinely producing up to 200 µA of electrons with a polarization level of 85%.