RRCAT: Raja Ramanna Centre for Advanced Technology, Indore

A K Sinha, K J S Sawhney, A Verma and R V Nandedkar
Synchrotron Utilization Division

Optical design of CAT - TGM beamline was done by minimizing the aberrations [1]. This design freezes the distances of the focusing and dispersion elements. Effect of misorientation of the optical elements on the performance of the beamline, has been found to be large for TGM beamlines [2]. Therefore, equally important task is to place the optical components at precisely the same configurational positions (positions x, y, z and angles roll, yaw and pitch). Some provisions have been made in the beamline hardware for viewing the optical components (view ports), beam diagnostics (beam viewer and VUV diode). Hardware like bellows, window gate valves, mechanical feed-throughs for manipulation and also mechanism for precision manipulations of the mirror [3], has been provided for the adjustments of the optical elements.

Co-ordinate axes for the text are defined as follows. Z-axis is the beam direction and the orthogonal horizontal direction is X-axis. Y-axis is the vertical axis orthogonal to X and Z. Rotations pitch, yaw and roll are the rotations along the X, Y and Z-axes respectively.

The desired position of an optical component is manifested in terms of position co-ordinates and the three rotations. Zero order alignment was done with the help of a dummy mirror (plane) of the same size as the real one. The requirements for the mirror were x = z = 0, y = 1250 mm, Roll = Yaw = 0 and Pitch = 4.5⁰. Pitch of 4.5⁰ is achieved by having a precisely machined structure subtending an angle 4.5⁰ with the horizontal as the base for the mirror. Y = 1250mm is achieved by putting the mechanism on a stand. In the mechanism, manipulation of all the degrees of freedom is available. The grating monochromator, on the other hand, was mostly factory set (a commercial TGM from Jobin Yvon). Offline we had adjusted it for zero order reflection, using He – Ne laser. Similar to the pre-mirror, the post mirror system was also aligned.

2.2 Initial alignment of the beamline in air (optical axis and dispersion plane)

(shadow method ) [4]: Shadow method is used to mark the beam direction (Z axis) on the floor. This is done using visible light from the storage ring. The front end (FE) was evacuated. A plumb was hung such that the thread of the plumb passes through the center of the window (where the FE terminates). Another plumb was hung on a trolley, which could be moved along the beam direction. On the background there was a screen. Shadows of the two threads were visible on the screen. The shadow of the fixed plumb shows the direction tangent to the ring. The thread length of the plumb on trolley is adjusted so that it is just touching the ground level. The trolley with the plumb is moved along the beam path, the shadow of the two threads are matched on the screen and with the help of the marker the position of this plumb is marked on the floor. A line joining all these marks shows the Z-axis.

The height of the orbit (of electron circulation) is marked in the ring. This was taken out of the ring area and marked at several places on the shielding wall using a leveling telescope. This fixes the Y-axis. Z distances for various components are taken form the optical design of the beamline and can be marked using a 10m scale. This process fixes the position co-ordinates of all components. Approximate roll of 4.5⁰ and pitch of 0⁰ were achieved by making the mirror box horizontal with the help of a spirit level. The process is discussed in section 2.1. Yaw is also adjusted at approximately 0⁰ with respect to the beam direction by inspection. The feedback for final adjustment of yaw is taken by seeing the focused beam shape through the beam viewer. The grating orientations are factory set, only roll angle can be changed using a precision stepper motor.

When all the optical and other components of the beamline are placed they are integrated and evaluated for a base pressure of 10^-9mbar. The beamline was realigned because the linear positions of all the components are coupled and adjustment of linear positions can disturb the other degrees of freedom. In fact several such alignment steps were needed to fully align the beamline. A beam viewer with a removable mirror at 45⁰ to the beam direction has been placed in the beamline after the mirror for alignment purpose. This mirror reflects the visible beam and the imaging characteristics of the toroidal pre mirror can be seen on the screen. Yaw of 0⁰ can be achieved by seeing the shape of the reflected visible. It may be noted that correct pitch (4.5⁰) of the mirror gives the focus at the entrance slit of the monochromator. Our experience is that the yaw and pitch of the pre mirror had to be adjusted several times to get correct focus. Now the beam viewer is removed from the path of the beam and the beam is visualized of the entrance slit of the monochromator. Adjustment of the beam through pitch of the pre mirror is needed if the height of the beam at the entrance slit is not right. Also the monochromator needs adjustment along X direction. A photodiode has been kept in the beamline after the monochromator. The grating of the monochromator is adjusted for zero order by steeper motor and maximizing the photodiode output. Now, fine adjustments of pitch and yaw of the pre mirror are done by maximizing the diode signal. Same process is used for the post mirror by maximizing the photodiode signal in the experimental station at the sample position.

Spectrum (Fig. 1) and Resolution (Fig. 2) of the beamline were measured using XUV photodiode. We find a resolution of 450 for a slit width of 500 mm, against a design value of 500.


Fig. 1: Photon flux as a function of wavelength for the three gratings to cover the wavelength range 40Å to 1000Å.		Fig. 2: Diode photocurrent plotted as a function of Photon wavelength for Si filter. A resolution of 450 was obtained for a slit width of 500mm.

All the alignment with the beam was done at low ring currents; under the supervision of safety officer. Special hot radiation spots near the pre mirror and monochromator were taken care of.