Upgrade from SURF II to SURF III

The first stored beam and production of synchrotron radiation from SURF occurred at 11:10 AM 17 Dec 1998. The initial electron beam was an unknown current (probably on the order of a few mA) of 100 MeV electrons. Later in the day that beam was ramped up to 380 MeV. As the commissioning process continued, the maximum energy, beam current, and beam lifetime were improved.

A web page describing SURF III and the current science programs is available.

• See the construction of the SURF magnet.
• See the assembly of the SURF lower yoke.
• Take a pictorial tour of the disassembly of SURF II.

Project Overview

The Electron and Optical Physics Division (now part of the Sensor Science Division) of the Physical Measurement Laboratory has operated the Synchrotron Ultraviolet Radiation Facility at the National Institute of Standards and Technology (NIST) for over 30 years. Initially operated in a parasitic mode at the NIST electron synchrotron, the facility was converted into the SURF II electron storage ring in 1974. SURF II has been the United States national standard of irradiance in the vacuum ultraviolet spectral region as well as a world-class research facility for many diverse fields of investigation that require high-intensity continuum radiation from the soft x-ray to the infrared spectral regions.

The radiometric accuracy of SURF II as an irradiance standard was limited by the uniformity of the magnetic field encountered by the electrons as they traversed their orbit. Variations of up to 0.5% limited the knowledge of the local bending radius at any tangent point and restricted the accuracy of the irradiance calculations. Additionally, the performance of the magnet structure is no longer state-of-the-art. Modern magnetic materials have better performance characteristics than materials available in the 1940's, when the electron synchrotron magnet used by SURF II was built, and the conversion of SURF from a 60 Hz AC synchrotron to a DC storage ring changed the desired characteristics of the magnet material and the shape of the poles so that the original design choices are no longer the most appropriate.

Too improve the radiometric accuracy of SURF, an entirely new magnetic structure was designed and built. The general contractor for the project is PSL Engineering and Instrumentation. The azimuthal uniformity of the SURF  magnetic field will be improved by a factor of 50, allowing irradiance calculations to be made with much higher accuracy than SURF II could achieve. As an additional benefit, the use of improved magnet material, a smaller air gap between the poles, and a higher electrical current (new magnet windings will be installed) will increase the magnetic field strength at the electron orbit, allowing SURF to store electrons at energies as high as 400 MeV, compared to 300 MeV for SURF II. The higher electron energy will extend the usable range of radiation from SURF to shorter wavelengths, enabling experiments in the "water window" from 2.3 nm to 4.4 nm.

Improvements

The primary purpose of the upgrade from SURF II to SURF III is to improve the radiometric accuracy of SURF as an irradiance standard by improving the magnetic field uniformity. The upgrade project offers an excellent opportunity to make other improvements and modifications to the facility. The following list outlines the major changes being made and the improvements they offer.

New yokes and backlegs for magnet

The new magnet structure consists of several solid pieces of iron designed for the DC operation of a storage ring. The previous magnet consisted of thousands of layers of very thin iron laminations pressed together - a design appropriate for the old synchrotron which operated in a 60 Hz AC mode. The solid pieces of iron will support higher magnetic fluxes than the laminated structures.

New poles for the magnet

The new poles are designed to create a magnetic field with a three-dimensional structure appropriate for a storage ring. The previous pole pieces were designed for a betatron, requiring that the magnetic field be corrected by "trim coils" when converted to a storage ring. Additionally, the air gap between the upper and lower poles will be smaller in SURF, providing increased magnetic field strength at the electron orbit.

SURF  is designed so that there is no need for trim coils to correct the magnetic field structure at the principal energies to be used for radiometry. Two trim coils are being installed, however, to allow some tunability in the magnetic field to correct for possible eddy current effects during the ramp from injection energy (10 MeV) to the final operating energy and to allow stable operation at a wide selection of electron energies.

SURF III's magnetic field will be:

• more uniform (improves radiometric accuracy)
• stronger (allows higher electron energies)
• the correct structure for a storage ring

New electrical windings for magnet

The SURF II magnetic coils are over twenty years old and have become unreliable due to their age. The new coils will be designed to carry higher electrical currents - on the order of 850 A instead of the 700 A carried in SURF II, and the induction will be increased from 182 kA t (kiloampere turns) to 204 kA t. This higher induction will further increase the magnetic field strength, allowing higher stored electron energies.

Improved cooling system

SURF III will operate with higher electrical currents in the magnet windings, and will dissipated somewhat more power in the magnet structure. An improved cooling system is being installed to handle the additional heat load.

Two new beamlines

Two new photon beam ports are being installed in the vacuum chamber between the existing beamline 6 and beamline 7 ports. The new beamlines (BL-6a and BL-6b) will provide additional experimental capacity for a UV/Visible radiometry program and will allow the extension of SURF III as an irradiance standard into the visible and infrared spectral regions as well.

Two high acceptance beamlines

Two of the existing beamlines will be modified to accept over 80 mrad in both the horizontal and vertical directions, an increase of almost 25% over the present configuration. These high angular acceptance beamlines will provide users with high photon fluxes, particularly in the infrared spectral region where the intrinsic divergence of synchrotron radiation is high.

New accelerator control system

A modern control system with a graphical user interface and running on a desktop computer will be installed. This system will replace the present analog control system, which dates from 1974.

Possible: New RF accelerating cavity

The increase in stored electron energy in SURF III will require a higher RF voltage in the accelerating cavity. This voltage is near the design limitation of the existing cavity. If the cavity shows indications of breaking down at the high voltages required, a new RF cavity will be designed and installed.