Mirror Systems

ADC designs and manufactures high performance UHV-compatible x-ray mirror systems suitable for bending magnet or insertion device beamlines. Mirror systems are supplied to suit specific customer requirements.

The mirror positioner provides five fully motorized degrees of freedom (mirror height, pitch, roll, yaw and horizontal translation). The motion is independent and isolated from the vessel. Different bending mechanisms can be implemented depending on customers' specific needs. Other options include mirror cooling, choice of substrate coatings, mirror bending, multiple or multi-stripe mirrors with translation system, integral cooled mask, control system, pumps and choice of fiducial mounting position.

Mirror positioner & vacuum tank:

• Full UHV compatibility (better than 2 x 10^-10 mbar).
• Metal wire sealed chamber allowing easy access to the in-vacuum components.
• Mirror lengths up to 1500 mm.
• 3-points kinematic-mount positioner.
• Choice between in-vacuum or externally driven horizontal translation.
• Optional mirror decoupling from the vacuum tank in order to minimize vibration transmission.
• Possibility of accommodating several different types of commercially available benders, depending on customers' needs.

Mirrors

• Mirrors are available in all major substrates: silicon, fused silica, Zerodur, ULE, silicon carbide, Glidcop
• Mirrors are available in flat, cylindrical, ellipsoidal and toroidal fixed radius or meridional bendable configurations.
• Options for the metallic coatings includes, Pt, Rh, Au, Cr, Ni …

Chamber

The following is an example of a custom chamber fabricated for a previous design for Brookhaven National Laboratories (BNL) and Advanced Photon Source (APS). Key features include an aluminum foil seal, multiple view ports, water and vacuum guard feedthrus, as well as pump and auxiliary ports. ADC can design the chamber to open on the side or use a top cover, depending on customer preference.

Bending Mechanism

Like most bending schemes, the mirrors are bent into a circular profile by applying an equal moment to both ends of the mirror. The method used by ADC to create this bending moment employs four arms, one clamped to each of the four corners of the mirror. The four arms are connected to a single actuator through a wiffle tree linkage (2) that distributes force equally among the arms.

For large bending radii, < 8 [km], the four arms are themselves leaf springs. The bending actuator displaces the free end of the spring bar (1), the support structure supplies the reactionary vertical force, and the mirror provides the moment reaction. Leaf springs are used in this case because they show less hysteresis than a conventional coil spring.

Small bending radii, 1 [km] < r < 8 [km], require the use of rigid bending arms (4) and a coil spring pack (5) in conjunction with the Belleville spring stacks in the clamps to translate the applied displacement from the actuator into a moment supported by the mirror. The largest bending moment we have applied in the past is approximately 120 [N*m].

The actuator itself can be either manually driven with a micrometer head adjuster (for light loads), or motorized using the ADC standard crossed roller jack. Crossed roller jacks typically use stepper motors in the drive train, but DC servos with rotary encoders can be substituted.

The image below shows the key features of this system including the mirror, bending arms, and liquid level adjuster. This cylindrical cut mirror, bent torroidal, is an example of previous work.

For mirrors in the bounce down configuration, ADC uses a discrete spring based gravity compensation mechanism. Finite element analysis determines the optical number, spacing and force of each spring.

In a design completed for BESSRC CAT at APS, ADC used the following configuration for the monochromatic bounce down mirror.

Each spring was pre-stressed to prevent long-term relaxation and then set to provide the necessary force within +/- 2%. The discrete spring arrangement added approximately .25 µrad RMS slope error.

The mirror is supported through a system of flexure style bearings that significantly reduce the residual stresses in the mirror due to machining tolerances in the mounting scheme. Clamping on the ends of the mirror and applying a force to each of four arms accomplishes the bending. The force is equal on each arm due to the wiffle tree linkage shown in the following image.

The mirror position and figure are controlled by an out-of-vacuum translation system. Five stepper motor driven linear actuators, with the option of linear encoders with resolutions of up to .1 [um], provide 5 degrees of freedom to the mirror.

The extremes of our current work are listed below.

Gravity Compensation and Cooling System

When the sag of the mirror due to gravity induces unacceptable slope errors in the system, some form of gravity compensation must be used. ADC currently uses two different methods: discrete spring approximation, and exact buoyant support.

The traditional method of using a spring system to apply a force at discrete points along the length of the mirror works by approximating the effects of gravity. This compensation method provides adequate reduction in slope error due to the multiple support points and requires an exacting spring set force.

When water-cooling of the optic is required, the gravity compensation arises from the dual use of Indalloy as both a heat transfer medium and buoyant support. The force due to liquid beneath mirror is equal and opposite that of gravity. To further correct the buoyant force when the mirror is pitched, the bottom of the mirror is lined with Macor pucks with thickness varying along the length.

By varying the depth of the liquid, a parabolic shape may be superimposed on the circular profile of the mirror. This allows for additional freedom in compensation for figure errors in the mirror either inherent in the optic or resulting from bending. In addition, an ellipse may be approximated using this method.

The unique feature of our mirror system is the dual use of Indalloy tm- as a mechanism for heat transfer and as a buoyant support for the mirror. Although the specification does not require active cooling of the mirror since it is only used with a monochromatic beam, the Indalloy tm, in conjunction with a series of Macor pucks, allows an exact compensation for gravitational effects. In addition, the shape of the mirror can further be controlled by either manual or motorized adjustment of the liquid height. The height adjustment allows the superposition of a parabolic deformation on the circular caused by the bender. It may be possible to approximate an ellipse with this method.

Clamping and Mirror Support

The various links in the mirror support system ensure that it was not over-constrained in any direction that affects the moment applied and subsequent shape. They also provided a kinematicly determinate mounting system (three points) using four clamps that eliminate the effects of initial surface misalignment when clamping the mirror. Uneven heating and the bending itself cause strains in the mirror that, if over-constrained, would have affected the required curvature.

Finite Element Analysis (FEA)

ADC conducts detail finite element analysis of the system before manufacturing. A report will be provided for customer review covering; heat load thermal distortions, convective cooling, gravity, and buoyancy to model the effectiveness of the support and cooling mechanism.

Publication: The Ninth International Conference on Synchrotron Radiation Instrumentation; May 28 – June 2, 2006, DAEGU, EXCO, KOREA; White Light Focusing Mirror

Eric Johnson, Aaron Lyndaker, Alex Deyhim, Michael Sullivan*, Mark Chance*, Don Abel*, John Toomey *, Steven Hulbert**

Advanced Design Consulting USA, Inc., 126 Ridge Road, P.O. Box 187, Lansing, NY 14882, USA * Case Western Reserve University, Center for Synchrotron Biosciences,

Building 725A, Upton, NY 11973 **National Synchrotron Light Source, Upton, NY 11973