SRF Technology

• The JLab Ampere-Class Cryomodule
• SRF Cavities from Single-Crystal Niobium

Energy-Recovering Linacs

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

Electron Source

• Injector Advance: The Superlattice Photocathode

Accelerator

The JLab Ampere-Class Cryomodule

A new cryomodule is required for the next-generation compact FEL being developed at JLab. This cryomodule must be capable of accelerating up to ampere levels of beam current. Jefferson Lab's Institute for Superconducting Radiofrequency Science & Technology (ISRFST) has completed the conceptual design for such a cryomodule. High-current accelerator operation is an important R&D concern for ISRFST not only for the next generation of FELs and ERL light sources, but for electron cooling and electron-ion colliders.

To meet performance requirements cost-effectively for the JLab ampere-class cryomodule, ISRFST researchers have proposed to use a compact waveguide-damped multi-cell cavity packaged in a cryomodule of the style JLab developed for the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. The researchers chose a cavity shape that balances considerations of efficiency, HOM (higher-order mode) damping, and the suppression of multipacting. They have put the full five-cell cavity structure through simulation checks and have made measurements on a five-cell copper model that agree well with the calculations. For deep drawing of the end-group structures, they have also developed a method that has been successfully tested with niobium. The researchers have packaged these components into a compact high-gradient cryomodule design, including concepts for HOM loads, a tuner, fundamental power coupler, spaceframe and heat shield. Although the cavities will be the largest ever made at JLab, all dimensions are compatible with existing production facilities. Concepts for the cryomodule layout and ancillary components are closely based on existing proven designs and production methods and processes in use at JLab and elsewhere.

For more information

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

 

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)

For more information

 
 

 

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)