Double Crystal Monochromator System (DCM)

ADC delivers standard synchrotron equipment, including a double crystal monochromator (DCM) that is cryogenically or water-cooled. The DCM is suitable for use on bending magnet and insertion device beamlines at second and third generation sources and is capable of being used in fixed exit or pseudo channel-cut.

General Description

ADC proposes a concept for a medium-high energy single bounce, double crystal monochromator based on working designs used in the synchrotron community at such facilities as Argonne National Laboratories. These designs have been modified to allow for the exchange of multiple crystal sets by incorporating a lateral translation before the Bragg angle rotation in the crystal kinematic support chain.

One unique feature of ADC's concept is that it is possible to decouple the vacuum chamber from the support structure for the crystals. This will reduce the effect of vibrations transmitted from other beamline components on the crystals' position and ultimately increase beam stability. If other methods of damping such as frequency canceling active dampers are employed, their effectiveness will be bettered by this isolation.

Primary Drive and Support System

The foundation of the monochromator is the synthetic granite base that provides a highly rigid, self-damping, exceptionally flat surface to which the primary translation and rotation are fixed. This base will be custom designed concurrently with the monochromator to optimize the system frequency response and provide the most mechanically and thermally stable optical platform.

The crystal exchanging translation was chosen as the first component of the kinematic support chain because, once aligned, the crystal swap should have no dependency on the position of the main Bragg angle rotation position. This motion, though not necessarily requiring high positioning accuracy, requires premium travel straightness such that the angular deviation between crystal positions is less than the specified 1µrad. In addition, the stiffness of the system must be kept at a maximum to ensure relatively high fundamental vibration frequencies.

To this end, ADC has chosen to use a pair of high quality, recirculating roller type, highly preloaded linear bearings made by Bosch Rexroth. These bearings use two rows of recirculating rollers arranged in an "O" configuration to provide the stiffest mounting with good running characteristics. ADC uses the same configuration to provide the exceptional straightness required for positioning components of our elliptically polarizing undulators.

The Bragg angle rotation is accomplished using a custom designed rotation stage equipped with the highest quality bearings capable of meeting the required 5µm runout with 25µrad axis wobble. ADC uses a pair of duplexed, preloaded angular contact bearings arranged in an "X" configuration to ensure both axial and radial rigidity while allowing the necessary rotations for alignment. This axis is encoded with a modular rotary encoder from Heidenhain.

Because the crystal cage is cantilevered, though with a large diameter axle, a secondary high-class radial bearing is used for additional support. In addition, because of the "X" configuration of the angular contact bearings, this bearing is strongly influence the axis wobble. The position of this bearing, both in the beam direction and vertically, is used to align the axis of the Bragg rotation with the axis of the crystal swap motion. This alignment can be achieved within 5µm over the length of the stroke. The stroke, of course, is dependent on the physical width of the crystals and necessary liquid nitrogen supply and return lines.

Crystal Cage

The first and second crystal cages is assembled on a plate where they are preliminarily aligned and tested before being bolted to the monochromator Bragg rotation axle. They will then go through a secondary alignment as a system to fully align the crystal faces with the axis of rotation.

The first crystals have a manual coarse perpendicular adjustment and a motorized coarse roll adjustment. The crystals are mounted accurately in place using a layer of either indium foil or indium gallium eutectic as a thermal transfer facilitator.

The second crystal translations, both perpendicular and parallel, is provided by crossed roller bearing guided, stepper motor driven vacuum stages. A vacuum compatible linear encoder, from Heidenhain, is encode each axis' position as well as provide the limit signals and reference position marks. Each axis is backed by appropriate hard stops as well.

The remaining second crystal steering motions is accomplished with by number of methods with a flexure guided stepper motor driven coarse adjustment and a piezo stack coaxial with the linear actuator to provide the very fine resolutions required for optimum beam positioning and stability.

Crystals, Mounts, and Cooling

In order to prevent the Bragg rotation from inducing undue stress on the crystals, mounting, and stages, all water cooling feed and return lines, thermocouple wires, motor wires, and encoder wires are entered the chamber at the axis of rotation. The rotary motion of these components is dealt with by routing outside of the vacuum envelope.

A complete thermo-mechanical FEA is performed on the monochromator crystal cage to determine the steady state temperatures at various energies, thermal strains on each relevant component, and, of course, the result on the figure of the first crystal.

Vacuum System

The fundamental design of this chamber is minimal contact with the Bragg rotation through the differentially pumped rotary seal. This is fabricated from a 12" CF flange with two viton quad ring seals to allow for both rotation and linear translation. The linear translation is aligned to the brag rotation axis which is collinear with the axis of the rotary seal such that there is no additional force on the rotary joint as the linear joint is actuated.

Another key concept is easy access to the internals of the chamber including a large viton sealed door. There are a large number of viewports to allow inspection of the first and second crystals throughout their travels as well as a number of other ports.

The DCM provides a tuneable energy range of 4.0 to 22 keV or modified to meet customer requirments. A water-cooled entrance mask is mounted at the entrance of the DCM to protect the drive mechanism of the goniometer from miss-steered SR beam. A water-cooled white beam stop is mounted at the exit port to prevent damage to downstream components. Both crystals of the DCM is fitted with temperature monitoring sensors.

The first crystal reflects the beam upwards and is indirectly water-cooled from a reduced vibration cooling water supply system. The cooling scheme is sufficient to provide temperature stabilization within 1 degree.

The second crystal is long enough to intercept the reflected beam from the first crystal of the DCM without meridional translation. The Bragg motion of the DCM has a high resolution encoder, and all motions of the primary drive of the DCM are backlash-free. All motions are encoded, and fitted with limit switches and end-stops.

The DCM includes a fine adjustment actuator on the pitch of the second crystal and the associated Monochromator Stabilization (MOSTAB) control unit. Other motions include Roll for both Crystals 1 and 2, Pitch adjustment for Crystal 2, yaw a justment on Crystal 2, height adjustment for the whole goniometer, vertical translation for Crystal 2 and Lateral adjustement of the whole assembly for alignment purposes.

The DCM vacuum is standard UHV. All components downstream of the DCM see only monochromatic beam and do not require water cooling. Details of the motions and the technical specifications for the DCM are summarized in following table: