Synchrotron radiation based beam diagnostics on CAT-TGM beamline

K J S Sawhney, A K Sinha and R V Nandedkar

Synchrotron Utilisation Division

Introduction

Certain parameters of the synchrotron radiation (SR) beam like height and angle of emission, beam divergence, beam stability, etc. are desirable to be known for proper and efficient installation of the beamlines. Though theoretical values of some of these SR parameters are available from accelerator physics calculations [1], no experimental data was available during the installation of the first beamlines on Indus-1. Therefore, in conjunction with installing the CAT-TGM beamline, we undertook certain SR based beam diagnostics (BD) on this beamline [2]. BD was done using the visible part as well as the soft X-ray part of the SR spectrum. For visible light BD two type of detectors were employed : (i) a CCD camera with frame grabber card and Promise software, and (ii) a Schottky photodiode (Hamamatsu). The visible part of the SR was taken out through a glass viewport at the end of the frontend of the beamline.

For measurements with soft x-rays, an XUV Si photodiode with Ti filter of ~ 1000 Å was used to detect soft x-ray component of SR. This detector detects soft x-rays in the 40 – 150 Å wavelength range. The detector was mounted in a chamber just outside the shielding wall after gate valve GV3 and evacuated to 3 x 10^-9 mbar. The Y-motion of the detector was motorised with a total travel of 50 mm in UHV and a resolution of 10 mm.

Measurements and Discussion

The first detection of soft x-ray radiation from Indus-1 was done on 10^th July, 2000 (Fig. 1). This figure shows that soft x-rays form a well-defined narrow peak. Measured data shows an excellent fit to a Gaussian shape with a r.m.s divergence of 0.56 mrad. This divergence corresponds to SR of wavelength ~44 Å. Moreover, our measurements show that the mean electron beam position is at 1251.2 mm above ground level which matches quite well with the theoretically expected value of 1250 mm.

To determine the stability of the electron beam orbit, the vertical peak position was observed over several days, over several injections and different electron-beam currents.

Fig.2:   Stability of Indus-1 electron-beam orbit.

 Fig 2 shows that the electron beam orbit is stable within ±25 mm over a period of 10 days. The stability of the electron-beam in the same injection, over a period of 2 hours – from 100 mA to 10 mA ring currents, was observed to be better than ±12 mm.

In Indus-1 storage ring there are several magnetic steering coils, which when energised, steer the electron beam to a new mean electron orbit. These steering coils are identified by power supplies PS34, PS35, etc. PS34 and PS35 power supplies are the most suited to effect an electron beam shift in the vertical direction at the dipole of our beamline. To calibrate the steering coils, electron beam was systematically steered by passing –3A to +3A current, to PS34 and PS35 power supplies respectively and the effect of the electron beam steering was observed by measuring the vertical beam profile of soft x-rays for each case. The results are shown in Fig. 3. PS34  resulted in 0.784 mm of vertical beam steering per ampere of steering current through the coil against theoretically expected value of 0.873 mm/A [3]. We believe that the measured values are very reliable as well as reproducible; and the difference from theoretical values may be due to practical non realisation of the conditions assumed for the calculations.

Several measurements were taken to monitor the stability of the peak position (vertical) as well as its spread. Fig.4 shows that the peak width of the soft x-rays detected by XUV

higher currents indicate instabilities in the electron beam dynamics that can be either due to spatial spread of the electron beam or spread in the angular emission. Most probably, it is a combination of the two effects. In any case, this will lead to loss of photon flux and/or spectral resolution available at the sample position at the end of the beamline and should be improved in due course.

The co-operation of Indus-1 operations group during the measurements is acknowledged. HS Vora is acknowledged for kindly giving the Promise software. The measurements reported here were carried out on the CAT-TGM beamline that has been designed, fabricated and commissioned by the Synchrotron Utilisation Division and all members of this team are gratefully acknowledged.

References

1. S S Ramamurthi and G Singh “Status of the Indus-1 SR source”, Nucl. Instr. and Meth., A359, 15-19(1995)
2. K J S Sawhney, A K Sinha and R V Nandedkar “Synchrotron radiation based beam diagnostics on CAT-TGM beamline” (unpublished ) (July, 2000)
3. A D Ghodke et al (personal communication)