A UHV compatible precision mirror movement mechanism for Reflectivity beamline

Adu Verma, K.J.S. Sawhney, S. Chatterjee, R.V. Nandedkar
Synchrotron Utilization Division

Reflectivity beamline on Indus-1, is based on a toroidal grating monochromator.  In this beamline the pre and post mirrors are important optical components for precisely focusing the synchrotron radiation (SR)  on the entrance slit of the monochromator and on the target in the experimental chamber, respectively.

The performance of a beamline depends, among other parameters, crucially on the manufacturing accuracies of the optical elements and good alignment of the optical elements. To develop a beamline, misalignment effects have to be minimized and the mirror movement mechanisms, that hold toroidal mirrors, have to be fabricated within the specified tolerance limits. As per the estimation based on the effect of misalignment on beam spot, the design specifications of the mirror movement are as follows:

(a)    The mirror should be aligned angularly about three co-ordinate axes in the range of  0 to ±1º with an inherent position reproducibility of 10 arc sec, and

(b)   The position of the mirror should be adjusted linearly in three orthogonal directions within the range of 0 to ±10 mm and with repeatability of 10 µm.

(c)    The whole mechanism including the chamber should be UHV compatible.

The mirror movement mechanism reported here is based on a constrained kinematic chain. The kinematic system used is shown in Figure 1. The SR beam enters the mirror box from a 151 mm diameter port (1) and after specularly  reflecting at an angle of 85.5º by the toroidal mirror(2), comes out through the 63 mm diameter port(3). This exit port (3) is inclined at an angle of 9º with respect to the incoming SR beam. The mirror is inclined at 4.5º with SR beam and assembled in a mirror holder (4) of stainless steel AISI 304L. The mirror holder is fixed on a stainless-steel pipe (5), which is connected to a constrain kinematics chain (6). This chain provides inherent angular position adjustment to the focusing mirror with a reproducible angular resolution of 10 arc sec about X- and Y – axis (X axis being the SR beam). The kinematical chain consists of  three precision micrometer heads(7) of resolution 10 µm and three precision plain spherical angular contact thrust bushing(8). The micrometer heads are assembled on a mild steel plate (9) in a right angle triangle spacing. The plain spherical angular contact thrust bushing are assembled with a bracket (10) in the same right angled triangle spacing, as are the three micrometers. Thus, coupling three micrometer heads with three plain angular contact thrust bushings form a constrained kinematical chain. Micrometers provide rotational motions about X- and Y-axes with fine adjustment of 10 arc sec and within the range of 0 to ± 1. The third angular adjustment about Z direction to the mirror has been provided by a precision linear motion UHV feed through(11) with a fine adjustment of 10 arc sec and within the range of 0 to ±1º. The linear adjustments along X- and Y- axes are performed through other micrometer heads (12) which move the complete UHV chamber with a fine adjustment of 10 µm and within the range of 0 to ±10 mm. The UHV chamber is assembled with the bracket (10) in such a way that the center of the mirror coincides with the center of the kinematical chain. A hydroform bellow (13) is assembled between mirror holder and kinematical chain to enable transfer of rotational motions from air to vacuum. For this purpose the mirror holder has been mounted on one end of the bellow and a conflat flange has been mounted on the other end in such a way that the outer surface of the bellow is in vacuum while the inside portion of the bellow is in air. The fully assembled mirror chamber along with the kinemetic chain is kept on yet another alignment system(14) by which the centre of the mirror chamber can be aligned in the beam path.

The mirror chamber is initially evacuated to >1x 10^-6 mbar by a turbo molecular pump connected through gate valve to the 203 mm conflate flange(16). The whole system including the mirror chamber and the mirror-movement mechanism, but without the mirror, is backed at 250ºC for several hours. After baking, the gate valve is closed and
 

the Figure:. Assembly drawing of the mirror movement mechanism. (1) 151 mm diameter port,  (2) toroidal mirror, (3) diameter 63 mm port, (4) mirror holder, (5) pipe, (6) constrained kinematic chain, (7) micrometer heads, (8) plain spherical angular contact thrust bushing, (9) mild steel plate, (10)bracket,(11) linear motion Uhv feed through, (12) micrometer heads, (13) hydroform bellow, (14) alignment system, (15) BA gauge, (16) pre pumping port and (17) sputter ion pump.

chamber is pumped by a sputter ion pump(17) of 240 l/s. to achieve vacuum of better than 1x10^-9 mbar. Thermal expansion of the system during baking has no effect because the kinemetic coupling gives it freedom to move and keeps mechanical strains on the other parts of the instrument to a minimum. The mirrors used in the beamline have a highly polished and clean gold-coated surface. To avoid the coating of mirror surface by the titanium vapors of the sputter-ion pump, the latter is assembled through an elbow to avoid line –of-sight configuration. Before assembly all the parts of the mirror movement mechanism and UHV chamber were electropolished, ultrasonic cleaned and vacuum degassed at 600ºC in vacuum degassed furnace. The vacuum is measured by a BA gauge (15). The present kinematic chain has been designed for a load of 25 kg and it can be easily augmented for use with higher loads.

The rotational movements of the mirror mount mechanism has been tested using an indigenously built Fizeau interferometer. The Interferometer data show that the accuracy in the angle is within ±1.5 arc sec for a 5 arc sec rotation. These angular accuracies were checked for different angular position between 0 and 1º.