Mirror boxes and mirror mounts for photophysics beamline

 B N Raja Sekhar, S Padmanabhan, Aparna Shastry, P Meenakshi Raja Rao,

A S Raja Rao* and N C Das

 Spectroscopy Division, BARC, Trombay, Mumbai 400 085, India
*Centre for Advanced Technology, Indore, India 452 013

1.      Introduction:

One of the beamlines commissioned recently by Spectroscopy Division, Bhabha Atomic Research Centre, Mumbai i.e. Photophysics (1) beamline, makes use of a toroidal mirror to focus synchrotron light  (40 mrad (H) X 6 mrad (V)) from Indus-1 on to the entrance slit of a one meter Seya- Namioka monochromator. The monchromatic light coming out of the exit slit of the monochromator is focussed at a distance of one meter to illuminate a sample under study with the help of a second toroidal mirror. For connecting the beamline to the Indus-1 SRS, to protect the surface of the optical components used in the beamline and to transmit the Synchrotron light of energy in 6-30eV at the sample, an ultimate pressure of 10^-9 mbar has to be maintained in the beamline. Installation of the beamline, steering of synchrotron light on to the sample and maximization of the flux at the sample position require precise and reproducible rotational and transnational movements of the mirrors about X, Y and Z axes under ultra high vacuum conditions.  Various details of design, fabrication and testing of the mirror mounts required for achieving the above objectives and mirror chambers to hold the mirror mounts containing the mirrors are described in this article.

2. Description:

2.1. Design considerations for mirror mounts:

1. Independent rotational and transnational motions in three mutually perpendicular directions (X,Y and Z) without disturbing prevailing UHV conditions in the beamline.
2. Translational and rotational movements accuracies of the order of 1 ± 0.1 mm and 1 ± 0.2 mrad respectively.

                   If the specified design considerations for the movements are not followed, the image at the sample can shift by a minimum of 0.3 mm from the mean position resulting in a loss of flux of the order of 25%.

2.2.Design considerations for the Mirror boxes:

                  The mirror boxes need several ports not only for connecting them to the beamline but also to house mirror mounts, pressure gauges, pumping ports, beam viewers etc. Keeping all these things in mind as many as twelve openings with different sizes of conflat type flange connections are provided with.

3.Design Details and fabrication

                   Based on the design details described earlier and guidelines from the beamlines of the UVSOR, Japan (2), two mirror chambers and mirror mounts were designed and fabricated. Fig 1 and Fig.2 show photographs of a mirror box and a mirror mount. In a typical mirror mount, the mirror is enclosed in a frame provided with adjustable screws on the top and side directions. This arrangement facilitates the rotation of the mirror around an axis perpendicular to the plane of the mirror. The mirror frame is mounted on a copper block having an angle equal to the mirror tilt. The mirror frame has an arrangement for locating the mirror accurately by using fine pitch positioning screws. Two pivot joints provided on the frame to impart the rotation necessary for angle correction. The rotation of the mirror is controlled from outside by a bellows-sealed mechanism using a micrometer head and a spring-loaded plunger. The tube, which imparts this rotation also, acts as a carrier for coolant to the mirror frame. Using three long bellows sealed shafts located in a right-angled triangular geometry provides multiplane adjustment of the mirror. The mirror mount is welded to a 203CF type of flange. Mirror chambers are provided with twelve ports of different sizes of CF flanges.

Fig.1.  Photograph of a mirror Box 

Fig.2. Photograph of a mirror mount

                   SS304L is used for fabrication of the most components of the mirror mounts and mirror boxes. The bellows and coolant channel used are made of SS316. The supporting block of the mirror holder is made of copper for better thermal conductivity. The chambers and mounts are TIG welded from inside, cleaned, electro-polished and degassed in a vacuum furnace at a temperature of 800⁰ C for eight hours to reduce the time that can take for achieving ultimate vacuum.

4. Testing:

                   After degassing the mirror mounts are connected to the mirror boxes and were tested for ultimate vacuum. This was performed with the help of turbomolecular and sputter ion pumps.  For achieving this, the chambers and mounts were baked to a temperature of 250⁰ C after leak testing for leaks of the order of 10^-10 Std.cc/s. To estimate the constituents of the residual gases, mass spectra were recorded with the help of a quadruple mass spectrometer. Fig.3. shows one such residual gas spectrum. Subsequently reproducibility and minimum attainable transnational and rotational displacements under UHV conditions possible with the help of mirror mounts are tested. These tests are:

 

performed using He-Ne laser source and measuring the image displacement.
Fig.3. Residual gas spectrum at Ultimate pressure

corresponding to minimum rotational /transnational displacements. The results obtained i.e. minimum rotation being 0.0033⁰ and corresponding an image displacement of 75m, for both mirror mounts are good and are less than the minimum slit sizes of 100m for the Seya-Namioka monochromator under use.

                   At present these mirror boxes and mounts are part of the photophysics beamline and their performance is satisfactory in the operation and optimization of the beamline.

5. References:

1.       P.Meenakshi Raja Rao, B.N. Raja Sekhar, N.C.Das, H.A.Khan, S.S.Bhattacharya, A.S.Raja Rao and A.P.Roy, “Mirror chambers and mirror mounts for photophysics beamline”, Sadhana Journal, India, Vol. 22, 1-10 (1997) and references therein.