VUV monochromators for the Daresbury SRS

Nuclear Instruments and Methods 172 (1980) 137-141 © North-Holland Publishing Company

VUV MONOCHROMATORS FOR THE DARESBURY SRS K. CODLING J.J. Thomson Physical Laboratory, Reading R G6 2AF, England

]'he Daresbury SRS is expected to produce synchrotron radiation by the middle of 1980. At that time two beam lines will be completed, up to and including the monochromators. One beam line (VUV 6) will be for vacuum ultraviolet and soft X-ray solid state work, the second (X-ray 7) an exclusively X-ray beam line for EXAFS, topography etc. A third beam line is already funded (VUV 3) and should be completed up to the shield wall by then. It is hoped that a further three beam lines (a high aperture port for lifetimes work, an infra-red port and a second X-ray line) will be in a similar state of preparation. This brief review will concentrate on the VUV and soft X-ray monochromators that are either already built, under construction or in the design stage and which will be installed on beam lines VUV 6 and 3.

1. Introduction

port (12) and infra-red port (13) could also be available to experimenters in the early stages. The high aperture port, which is to be used for lifetime experiments requiring high flux, will have a 7 mrad (vertical) by 40 mrad (horizontal) aperture. The radiation will be reflected upwards and allow a considerably closer access to the storage ring than would otherwise be possible, The infra-red port is designed to have an aperture of about 100 mrad by 50 mrad. These large values are to be achieved by placing a mirror a distance o f only 1.5 m from the electron orbit tangent point. Indeed, the mirror is expected to be so close to the electron beam that it will have to be moved during the injection phase of operation of the storage ring when the electron beam becomes considerably larger than under normal stored conditions. Damage to the mirror can be avoided by splitting the mirror into two and allowing the potentially dangerous X-ray component to pass through the central portion. This central beam could then b e used for further X-ray experiments. A superconducting "wiggler" magnet will be installed in the ring within the first year of operation to allow beam port 9 to be used for hard X-ray experiments. The layout of the X-ray beam port 7 has yet to be finalised but has been designated for X-ray use because an 80 m long beam line can be accomodated for topography experiments. The requirement of such a path length is a direct consequence of the large electron beam cross section. The SR will pass through Be windows and the first 4 mrad of horizontal aperture used for protein crystallography. The central 10

The Daresbury 2 GeV storage ring (SRS) is expected to have a circulating electron beam in mid1980 and by early 1981 reach its first stage design current of approximately 350 mA with a lifetime of the order of 8 hours. Fig. 1 shows a plan of the storage ring. A 1 0 - 1 5 MeV linear accelerator feeds a 600 MeV booster synchrotron, which in turn feeds the storage ring. A number of the 13 possible beam ports indicated on the figure have been designated in relation to the region of the spectrum that will be covered by the monochromators initially installed. Before discussing some of these monochromators in detail, it is worth pointing out the relevance of the numbers involved in the labelling of the beam ports. These relate to the particular dipole magnet through which the electrons pass when they emit the synchrotron radiation (SR). The predicted electron beam dimensions in the odd-numbered dipoles, assuming 10% coupling, are 0.4 mm (vertical) by 13.0 mm (horizontal) fwhm. In the even-numbered dipoles, the cross section is 0.7 mm X 6.4 mm. Such a relatively large source size has implications for the design of almost all monochromators and associated optical components. Each beam port will provide an emergent beam of SR with a horizontal angular aperture of at least 20 mrad. Radiation from each port will be split, by beam-deflecting or focussing m i r r o r s where appropriate, into three or more beam lines. The beam ports which have been funded to date are VUV 3, VUV 6 and X-ray 7. The so-called "high aperture" 137

IV. XUV MONOCHROMATOR SYSTEMS

K. Codling /Monochromators for the Daresbury SRS

138

~,,~ r,t,Control Room El t - ~ I r I I I L ~

.

.

.

.

g~.D . . . . . . . . . . . . . . . . .

r~

'"

.......

__~G . . . . . . .

;

:_I 'J......

--~_]_:_J_.' "

Booster Synchrotron [] o o 0 o

10 X-ray

~ @ ~ A ]Linear ccelera

]Wiggler

12 High ....

I

13 Infr~

~//

~7~

C ;~

7 X.ray

Fig. 1. Layout of the Daresbury 2 GeV storage ring. Taken from the document The synchrotron source as Daresbury Laboratory, by K.R. Lea and I.H. Munro.

mrad will be used for experiments in small or large angle scattering using energy dispersive techniques (52 m), topography (with a large aperture at 80 m and a small aperture at 75 m) and X-ray interferometry. The final 4 mrad will be available for extended X-ray absorption fine structure (EXAFS) experiments at relatively long wavelengths.

2. The first VUV beam port (6) This port will initially be used exclusively for experiments in solid state physics. That is, no atomic or molecular beam experiments will be allowed which might contaminate surfaces. It has been decided to maintain the pressures in the monochromators at the

K. Codling / Monochromators for the Daresbury SRS I

Y

I

I~' I

"I I

I

Entrance aperture i

I

\~o/

!

/x

Is Exit

slit

1 I',=

1:t/2 Cos A 0

~I

I \, ~ " Spherical mirror ( radius R )

Fig. 2. Basic layout of the Miyake-type grazing incidence monochromator.

beam line pressure of about 10 -9 Torr. Diffusion pumps will not be allowed to evacuate experimental chambers. The reasons for this are to avoid catastrophic failure affecting the storage ring vacuum and to help to avoid oil coating of optical components and thus maintain high reflectivities in the VUV. The port will accommodate three VUV monochromators. The first is a straightforward plane grating monochromator of the Miyake type [1]. The Daresbury prototype instrument [2] is presently in use at the Bonn Synchrotron. The principle, whereby higher orders are removed by selective reflection, is wellknown. The layout of the basic monochromator at zero order is shown in fig. 2, with a plane grating, G, preceding a focussing mirror, M. In principle, therefore, only two reflections are required. However, the SRS general philosophy in the early stages will be that the first optical component should be an easily replaceable plane mirror used at a small grazing angle. This will be bent to a cylindrical shape in order to produce a vertical line focus at the site of the experiment (behind the exit slit). It can be shown that for given values of x, y and R (fig. 2) there are two values of Ao (and hence two positions of M) which give focussing at the exit slit, S. In this monochromator the problems created by having two physically separated exit slits [2] have been eliminated [3]. Here two mirrors of different radius of curvature (3.25 m and 4.5 m) can be placed alternately in the line of the SR and exit slit, giving a total of four focussing conditions. The monochromator has used laminar gratings and by suitable choice of square-wave depth and angle of incidence, an effective blaze wavelength is created. Such gratings have an in-built order-sorting capability, since even orders are supposed to be missing. This, and the selective reflection principle, combine to produce a mono-

139

chromator with low contributions from higher spectral orders. This monochromator, which was installed for a period of time at the Wisconsin storage ring to allow experiments on angular distribution of photoelectrons from solid surfaces, will be used for similar experiments at the SRS. The wavelength range of the instrument is approximately 50-400 A. (30-250 eV) and the source size-limited resolution achieved at Wisconsin (typically 0.7 A) ought to be substantially improved at the SRS. A second monochromator to be installed is a grating-crystal monochromator which has been designed at Daresbury in conjunction with Bird and Tole (High Wycombe). The vacuum vessel will effectively house four monochromators. The first will be a conventional plane grating-concave mirror combination, as shown in the previous figure. The point here is that the angles of incidence, Ao, will be 83.6 ° and 88 ° for the two positions of the focussing mirror, M. It is hoped that a wavelength range of 10-100 A will be covered by this part of the instrument, but higher orders may be in evidence at the longer wavelengths. A lateral translation of the grating and mirror will bring a combination of two crystals onto the axis of the incident SR. The situation is shown schematically in fig. 3. The same drive that previously rotated the plane grating will now be used to rotate crystal 1. Crystal 2 must be rotated at the same angular rate as crystal 1 and at the same time be translated so that the SR remains central on crystal 2. The rotation of crystal 2 is accomplished via a worm and wheel, the worm gear having a rectangular shaft. Translation of the trolley holding crystal 2 (and also the mirror of the grating-mirror combination) is achieved by a tape drive not shown in the figure. Due to the large range of angular rotation in the crystal mode, the required rotational accuracy could not be achieved by use of a long arm and linear drive. Instead, a worm and wheel are used and the reproducibility is expected to be 5 second of arc at best. It may prove essential to couple accurate shaft encoders directly to the axles of both crystals to ascertain the actual movement. Moreover, these encoders would have to be compatible with the UHV environment of the monochromator. The crystals to be tested include Si, Ge, KAP, fi-alumina and InSb. These together should cover a wavelength range up to 20 A. The third monochromator for this VUV port has yet to be decided. It is likely to be a normal incidence IV. XUV MONOCHROMATORSYSTEMS

K. Codling / Monochromators for the Daresbury SRS

140

Drive for Crystal~__1

~!'

•

Beam in From~SRS

ii Entrance Crystal 1 ~ ~S[it " ~ ~ Crystal 2 f ~ II I\

~

~

k

-

~

Rotation Drive \

~orCrystal2 / ~

~

~ '

'~ E ~ ,' Beam Out to Experimen~

\

Trandation Dri'~e for Crystal 2 ~ Fig. 3. S c h e m a t i c

diagram

of the grating-crystM

monockro-

mator used in the crystal mode. Provided by D. Norman, Daresbury Laboratory.

monochromator of the modified-Wadsworth [4] type. Two such UHV compatible monochromators are available, one with vertical dispersion, the other with horizontal dispersion.

3. The second VUV beam port (3) The second VUV beam port will initially accomodate experiments using atomic and molecular gases. Because of the particular interests of the experimenters, it appears that two normal incidence and one grazing incidence monochromator will be required. Both a normal incidence and a grazing incidence monochromator have recently been funded and are in the design stages. Once again the initial philosophy is that the first optical component will be

"

an easily replaceable plane mirror, which could be bent to a cylindrical or indeed elliptical shape [5]. Ultimately, however, metal mirrors may be incorporated. The monochromators will be modular in concept with standard, fully adjustable, bilateral slit modules, for example. One possible layout of the normal incidence monochromator and associated optics is shown in fig. 4. Radiation from the storage ring is incident on the plane mirror, PM, at a grazing angle of 7½°. The toroidal mirror, TM, focusses the radiation on to the entrance slit of a 5 m monochromator. The grating will be scanned by a McPherson-type drive mechanism (external to the vacuum system) along the bisector of the entrance and exit slit arms. The angle of 8 ° (or 10 °) between the arms and a post-exit slit toroidal mirror combine to produce a horizontal beam of monochromatic radiation, focussed at the experiment, E. The first toroidal mirror could in principle demagnify the 0.4 mm object to a 0.1 mm image at the entrance slit. Assuming a 0.l mm entrance slit, a 1200 line per mm grating would give a slit-width-limited resolution of approximately 0.17 A at all wavelengths. The angular aperture of SR collected in this configuration is limited fundamentally not only by the f-number of the monochromator but also by the size of the first toroidal mirror. Assuming a grating ruled area of 100 mm X 100 ram, the angular acceptance could be as much as 5 m r a d X 5 mrad if a toroid of approximately 300 mm X 40 mm can be manufactured. The region between the exit slit and experiment will be used as a stage of differential pumping. There will almost certainly be a further stage of differential pumping between the grating and exit slit, close to the exit slit, where the beam size is small. Window valves will be incorporated for alignment purposes. It is not decided yet whether it will

r

..~PM

l'

,,

1"2m

/

./

~ / /

/ /

/

/

/

o I

/ / . / / /

/

/

/ - / / /

/

/-/-///

Fig. 4. Schematic diagram of the 5 m normal incidence monochromator.

K. Codling /Monoehromators for the Daresbury SRS

141 TM

S

PM

E

i-2m

/

/

/

/

/

/

/

/

/

/

/

/

/

/

./

/

/

/

,/

/

/

~'~f

7"

~/"

J J . /

7

Fig. 5. Schematic diagram of the toroidai grazing incidence monochromator.

be essential to couple slits and grating together with a rigid framework. The grazing incidence m o n o c h r o m a t o r is shown schematically in fig. 5. Again a plane mirror, PM, and toroidal mirror, TM, combine to produce a horizontal beam of SR focussed on to the entrance slit of the toroidal grating monochromator [6] in both directions. The radiation from the toroidal grating, TG, is focussed on the exit slit and the second toroidal mirror focusses the diverging monochromatic radiation on to the experiment, E. Distances are such that the 0.4 mm X 13.0 mm object reduces to a 0.05 m m × 1.6 mm at the experiment. To utilise an angular aperture of 5 mrad in both vertical and horizontal directions, the first toroidal mirror and the toroidal grating would need to have a size 380 mm × 40 mm. In fact the vertical aperture need not be so large and so the length of the toroidal surfaces could be somewhat less. The resolution of the monochromator, based on a 0.2 m m entrance slit (0.1 m m exit slit) which accepts, in principle, all of the available SR within the 5mrad aperture is approximately 0.03 A as best focus, using a 1500 line per mm grating. An alternative to the plane and toroidal mirror combination depicted in figs. 4 and 5 would be a pair of orthogonal cylindrical mirrors, the first being produced by bending a plane mirror. Independent control of the focussing in the two directions should lead to a somewhat improved aperture of collection of SR. A ray tracing programme will be required to calculate the resolution expected as a function of wavelength and to test whether a "classical" grating is appropriate or whether a more exotic grating (where the aberrations are minimised) is required. It will also determine the o p t i m u m slit-width for maximum throughput times resolution. The m o n o c h r o m a t o r will incorporate variable apertures in order to be able

to mask down the grating and hence improve resolution at shorter wavelengths. Gratings will be interchangeable in the UHV environment. The third m o n o c h r o m a t o r has not been chosen but will be a normal incidence type. A Seya monochromator, built to UHV standards, is available. At present it has cylindrical mirrors ahead of the entrance slit and behind the exit slit to reduce the loss o f flux due to astigmatism and at the same time improve the resolution [7]. It is a pleasure to acknowledge Drs. D. Norman, J.B. West and I.H. Munro, for their constructive comments on the manuscript. However, the preceding article is essentially a personal view.

Note added in proof: The rotation drive mechanism depicted in fig. 3 is now much improved. The worm and wheel has been replaced b y separate linear drives to crystal 2 and the concave mirror and an accuracy o f 1 arcsec is expected, UHV-compatible shaft encoders will no longer be required.

References [1] K.P. Miyake, R. Kato and H. Yamashita, Sci. Light 18 (1969) 39. [2] J.B. West, K. Codling and G.V. Marr, J. Phys. E7 (1974) 137. [3] M.R. Howells, D. Norman, G.P. Williams and J.B. Wes*, J. Phys. E l l (1978) 199. [41 M. Skibowskii and W. Steinmann, J. Opt. Soc. Am. 57 (1967) 112. [5] J.H. Underwood mad D. Turner, Soc. Photo-Opt. Instr. Eng. 106 (1977) 125. [6] C. Depautex, P. Thiry, R. Pinehaux, Y. Petroff, D. Lepere, G. Passereau and J. Flamand, Nucl. Instr. and Meth. 152 (1978) 101. [7] N. Rehfeld, U. Gerhardt and E. Dietz, Appl. Phys. 1 (1973) 229. IV. XUV MONOCHROMATOR SYSTEMS