11
Nuclear Instruments and Methods 208 (1983) 127-137 127 North-Holland Publishing Company UNDULATOR STUDIES AT SSRL H. WINICK l, R. BOYCE I, G. BROWN 1, N. HOWER l, Z. HUSSAIN 2,,, T. PATE i and E. UMBACH 3 i Stanford Synchrotron Radiation Laboratory (SSRL), P.O. Box 4349, SLA C Bin 69, Stanford, CA 94305, USA 2 Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA 3 Technische Universitiit, Miinchen, Germany In a collaboration between LBL and SSRL a permanent magnet (SmCos) undulator has been designed, constructed and installed into the SPEAR storage ring at SLAC during 1980. A report [1] on the design, construction and magnetic measurements has already been made. This is a report on subsequent work with the undulator. We describe (a) the performance of the undulator as a radiation source, (b) its effect on storage ring operation, (c) the instrumentation developed to facilitate its routine use as a radiation source for experiments, and (d) the experience gained in the first experimental study utilizing the undulator. 1. Introduction This is a report on the characterization and utiliza- tion of a permanent magnet undulator. This device was designed [1] as a high-brightness source of radiation in the soft X-ray part of the spectrum. It has been installed into the SPEAR ring, its radiation characterized and it has been used for an experiment. To our knowledge this is the first such device to be used in a storage ring as a radiation source for experiments. We, therefore, report in some detail the properties of its radiation, the instru- mentation developed to permit its use for experiments and the experience gained to date. Undulator magnets are attracting increasing scien- tific interest as generators of synchrotron radiation be- cause they offer higher brightness radiation than can otherwise be obtained. Several authors [2-7] have re- viewed the properties of undulator radiation. Briefly summarized, the radiation is emitted in quasi-mono- chromatic peaks at wavelengths given by X= 1 +~-+3'202 , (1) and harmonics of this wavelength. ~'u is the period of the undulator, 3' = EJ(mo c2) is the energy of the elec- tron in rest mass units, K= 0.934 B0(T ) X u(cm), B 0 is the peak value of the magnetic field, 0 is the angle of observation relative to the average electron beam direc- tion and the particle trajectory is assumed to be sinusoidal. * Presently at University of Minerals and Petrol, Dhahran, Saudia Arabia. For a sinusoidal trajectory K is also given by K = y3 where 28 is the full angular excursion of the electron beam traversing the undulator. Thus when K< 1 the angular excursion of the electron beam is comparable to the natural opening angle of synchrotron radiation and the high intrinsic brightness of synchrotron radiation is preserved and enhanced in the plane of deflection as well as in the perpendicular plane. When the electron beam undergoes larger deflections, as it does in bending magnets and wiggler magnets, the radiation is corre- spondingly spread over larger angles and there is less enhancement of the brightness in the plane of deflec- tion. For K << 1 the spectrum from each electron consists of a single peak at each observation angle at a wave- length given by eq. (1). For K= 1 the power radiated into this first peak is a maximum and some higher harmonics appear. For K >> 1 many strong harmonics are present, their spacing decreases and ultimately the spectrum approaches the smooth continuum character- istic of bending magnets and wigglers. The total power radiated by a current 1 from a source of length L is given by 2~ 2 4 P=~--hc(p- )3' LI/e, (2) where p is the instantaneous bending radius and the average is taken over the length L. In practical units P[W] = 0.127 E 2 [GeV 2] × (B 2 [kG2]) L[cm] I[A], (3) and the power radiated in the first peak is dP I 3P (1 + 1 (4) d,~ .... ,o, /¢2/2~2 ' J 0167-5087/83/0000-0000/$03.00 © 1983 North-Holland II. WIGGLERS/UNDULATORS/OTHER DEVICES

Undulator studies at SSRL

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Page 1: Undulator studies at SSRL

Nuclear Instruments and Methods 208 (1983) 127-137 127 North-Holland Publishing Company

U N D U L A T O R S T U D I E S A T S S R L

H. W I N I C K l, R. B O Y C E I, G. B R O W N 1, N. H O W E R l, Z. H U S S A I N 2,,, T. P A T E i

a n d E. U M B A C H 3

i Stanford Synchrotron Radiation Laboratory (SSRL), P.O. Box 4349, SLA C Bin 69, Stanford, CA 94305, USA

2 Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA

3 Technische Universitiit, Miinchen, Germany

In a collaboration between LBL and SSRL a permanent magnet (SmCos) undulator has been designed, constructed and installed into the SPEAR storage ring at SLAC during 1980. A report [1] on the design, construction and magnetic measurements has already been made. This is a report on subsequent work with the undulator. We describe (a) the performance of the undulator as a radiation source, (b) its effect on storage ring operation, (c) the instrumentation developed to facilitate its routine use as a radiation source for experiments, and (d) the experience gained in the first experimental study utilizing the undulator.

1. Introduction

This is a report on the characterization and utiliza- tion of a permanent magnet undulator. This device was designed [1] as a high-brightness source of radiation in the soft X-ray part of the spectrum. It has been installed into the SPEAR ring, its radiation characterized and it has been used for an experiment. To our knowledge this is the first such device to be used in a storage ring as a radiation source for experiments. We, therefore, report in some detail the properties of its radiation, the instru- mentat ion developed to permit its use for experiments and the experience gained to date.

Undulator magnets are attracting increasing scien- tific interest as generators of synchrotron radiation be- cause they offer higher brightness radiation than can otherwise be obtained. Several authors [2-7] have re- viewed the properties of undulator radiation. Briefly summarized, the radiation is emitted in quasi-mono- chromatic peaks at wavelengths given by

X = 1 + ~ - + 3 ' 2 0 2 , (1)

and harmonics of this wavelength. ~'u is the period of the undulator, 3' = E J ( m o c2) is the energy of the elec- tron in rest mass units, K = 0.934 B0(T ) X u(cm), B 0 is the peak value of the magnetic field, 0 is the angle of observation relative to the average electron beam direc- tion and the particle trajectory is assumed to be sinusoidal.

* Presently at University of Minerals and Petrol, Dhahran, Saudia Arabia.

For a sinusoidal trajectory K is also given by K = y3 where 28 is the full angular excursion of the electron beam traversing the undulator. Thus when K < 1 the angular excursion of the electron beam is comparable to the natural opening angle of synchrotron radiation and the high intrinsic brightness of synchrotron radiation is preserved and enhanced in the plane of deflection as well as in the perpendicular plane. When the electron beam undergoes larger deflections, as it does in bending magnets and wiggler magnets, the radiation is corre- spondingly spread over larger angles and there is less enhancement of the brightness in the plane of deflec- tion.

For K << 1 the spectrum from each electron consists of a single peak at each observation angle at a wave- length given by eq. (1). For K = 1 the power radiated into this first peak is a maximum and some higher harmonics appear. For K >> 1 many strong harmonics are present, their spacing decreases and ultimately the spectrum approaches the smooth continuum character- istic of bending magnets and wigglers.

The total power radiated by a current 1 from a source of length L is given by

2 ~ 2 4 P = ~ - - h c ( p - )3' L I / e , (2)

where p is the instantaneous bending radius and the average is taken over the length L. In practical units

P [ W ] = 0.127 E 2 [GeV 2]

× ( B 2 [kG2]) L[cm] I [ A ] , (3)

and the power radiated in the first peak is

d P I 3P (1 + 1 (4) d ,~ . . . . ,o, / ¢ 2 / 2 ~ 2 ' J

0167-5087/83/0000-0000/$03.00 © 1983 North-Holland II. WIGGLERS/UNDULATORS/OTHER DEVICES

Page 2: Undulator studies at SSRL

128 H. Winick et al. / Undulator studies

F ' - . . . . . ; ' t r ' i . ] ! 1 ' :

ELECTRON / BEAM

VACUUM CHAMBER

ROTABLE ~ . =:~'3" END MAGNETIC / -~ : \~ ' \ )/SD>x ~ )

Fig. 1. Pictorial of the undulator.

END BLOCK ROTATOR

LBL - SSRL

/S

~ i t j, 4_ _- J , MAGNET GAP

ADJUSTING MECHANISM

,'~ BACKING BLOCK KEEPER (AI) i ~ - PLATE (AI)

S.~Co s MAGNET B L O C K S

BLOCK MAGNETIZATION ORIENTATION

U N D U L A T O R

2. D e s c r i p t i o n of th e u n d u l a t o r

The undula tor is shown schematically in fig. 1 and in a pho tograph in fig. 2. It has 30 periods, each of length

6.1 cm. The total length is 1.95 m. Each period has 4 rows of SmC% blocks magnetized in directions chang- ing by 90 ° f rom one row to the next. The blocks are 1.5 cm x 1.5 cm in cross-section and 2.5 cm long and have

Fig. 2. Photograph of undulator.

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H. Winick et a L / Undulator studies 129

~-- 80,841" ~ - 36,12"~

/ QUADRUPOL E --

ION PUMP

Fig. 3. Location of the undulator in SPEAR.

S,R, MASK 7

103,795" TYR STRAIGHT SECTION

+

~/~~-I 1.50" ~\ \ ~i, 50 "~~',

137,795" - - '\ \

UNDULATOR ~

S,R. MASK ~'" BENO MAGNET \

L I I k L ~ [ i l 0 I0" 20" 30" 40*

_ _

POSITION MONITOR

a remanent field, B r = 8.1 kG. Three blocks make up a row which is 7.5 cm long. More information about the design, construction and magnetic measurements on the undulator are given in ref. 1.

The magnetic field B 0 in the midplane of an array of magnetized rare-earth cobalt blocks (ignoring finite width effects) is given by [8]

sin ( ¢ z / M ) [ 1 - e - 2,~h / x . B° = 2Br e - ~ g / x " ,n'/M ], (5)

where g is the full magnet gap, M is the number of rows

per undulator period in each half (i.e.; upper and lower) of the magnet and h is the block height. The magnet gap is adjustable remotely from a minimum of about 3 cm (determined by the outside height of the fixed vacuum chamber) to 6 cm, corresponding to a field varying from 0.24 T to 0.05 T and a value of K varying from 1.37 to 0.28. Rotatable assemblies of magnet blocks are located at the ends of the magnet to null the field integral along the electron path as is required to produce no net deflection of the electron beam.

The undulator magnet is located in a SPEAR 3 m straight section as shown schematically in fig. 3. To expedite the use of the undulator it was designed to be interchangeable with either of the two 8-pole, 1.8 T, 1.95 m long electromagnet wigglers [9] that have been in routine use in SPEAR since the summer of 1980, thus also permitting the wiggler beam line and end station to be used with the undulator.

To install the undulator, the wiggler magnet is split and the top and bottom halves are separated as shown in fig. 4. The change from wiggler to undulator or vice-versa can be made in a few hours without disturb- ing storage ring vacuum. In order to use the fundamen- tal peak ( - 1 keV) it is necessary to remove the beryl- l ium windows and helium system (including the two- crystal X-ray monochromator), and to install a high vacuum pipe connecting to a high vacuum experimental chamber.

Fig. 4. Undulator magnet in SPEAR tunnel, ready to be pushed into place over the vacuum chamber on the left. The top and bottom halves of the wiggler magnet have been separated to allow room for the undulator.

3. Effects of the undulator on the stored beam

The effects of a periodic magnetic insertion device such as an undulator or wiggler on a stored electron beam have been discussed by many authors [2,4,10,11]. For a rather weak field magnet, such as the device described here, there are no important adverse effects that cannot be rather easily compensated. For example,

11. WIGGLERS/UNDULATORS/OTHER DEVICES

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130 H. Winick et al. / Undulator studies

orbi t distort ions due to the insert ion are generally made negligible by proper choice of end fields [1] and other correcting magnets in the ring [12]. The non-zero verti- cal focussing effects can likewise be easily compensa ted [4,10].

O 2 4 6 @ IOmm

Fig. 5. Blowup of stored beam due to undulator induced reso- nance. Most of the beam is within the central elliptical pattern with whisps of beam extending in four outward directions as shown.

In using the undula tor an unexpected higher order effect was observed which, under certain restricted con- ditions, can produce an increase in beam emit tance and possible unstable growth in the transverse oscillations of the beam. This can occur when the be ta t ron tunes satisfy the equations:

3v X + v y - 2 1 = O or v + 3 5 . - 2 1 = 0 . (6)

The width of the first resonance was measured to be

c = 3v x + Vy- 21 = +0 .005 . (7)

The effect manifests itself by a blown up beam pa t te rn on a synchrot ron light moni to r as shown in fig. 5. Partial or total beam loss may result. The narrow-

ness of the resonance makes it easy to avoid by proper choice of the be ta t ron frequencies. The resonance has been studied theoretically by Mor ton [13], who shows that it could be produced by a rota ted octupole field with an integrated s t rength 1 cm off-axis of 6 G • m at 2.5 GeV.

In practice the undula tor is used freely, including varying the gap, with no noticable effect on ring opera- t ion or other users dur ing dedicated single-beam opera- t ion of SPEAR for synchro t ron radia t ion research, There has been no a t tempt yet to use the undula tor during col l iding-beam runs.

4. Spectral m e a s u r e m e n t s

4.1. Energy range between 3 and 7 ke V

We have measured the spectrum of radiat ion emit ted by the SSRL undula tor in the energy range 3 - 7 keV. The full cone of radia t ion passes first through a 15 ~ m graphi te filter assembly, two 250/.tin beryl l ium windows, and then some 2.4 m of a helium-fil led monochroma to r chamber . The monochromato r consisted of two parallel silicon crystals or iented to the ( l l l ) reflection. The Bragg angle and relative crystal parallelism were re- motely adjustable. The monochromat ized radiat ion passed through a 25 # m kap ton window, a 63 m m airpath, and then into a 150 m m nitrogen-fil led ion chamber . The pho ton intensity was computed from the known absorpt ion coefficient of ni t rogen and an as- sumed effective ionizat ion potent ia l of 30 eV per ion pair. Correct ion was made for all absorbers in the beam

0.025

0.020

--J 0.015 v

-6

0.010

0.005 -

0 . 0 0 0 '

3 0 0 0

f "

' I . . . . 1 . . . . I . . . .

K = I . O

• T H E O R E T I C A L S P E C T R U M

- - M E A S U R E D S P E C T R U M

° , . ° , , , °

4000 5000 6000

X-RAY ENERGY (eV)

7000

Fig, 6. The observed and theoretical angle-integrated spectrum for K = 1.0.

Page 5: Undulator studies at SSRL

H. Winick et aL / Undulator studies 131

line and for the energy-dependent crystal reflectivity. As an independent check, the spectral dependence of the absorption coefficients of air, beryllium, and kapton were independently measured.

The angle-integrated measurements were made for values of the parameter K ranging from 0.30 to 1.35. Sample spectra for K = 1.0 are shown in fig. 6. Also superimposed up on this figure is the theoretical angle-integrated spectrum, corrected for the polarization acceptance of the monochromator.

The measured undulator harmonic intensities were found to be in qualitative agreement with the theoretical values, although there is a significant discrepancy (about a factor of 5) between the two for increasingly high K values for the fifth harmonic. Part of this discrepancy is certainly due to the failure of the infinite-pole ap- proximation, and the subsequent rounding of the peaks. In the infinite pole approximation, the odd harmonic peaks are characterized by an infinitely sharp drop on the high energy side of the peak. This step will be rounded, and hence attenuated, by the effect of the finite number of poles. We estimate that this rounding, for the fifth harmonic at K = 1.0, can be as large as about three.

4.2. Energy range below 3 k e V

In the soft X-ray range below about 3 keV an all-vacuum beam line and experimental set-up was re- quired because of the strong absorption of beryllium windows and He atmosphere. A soft X-ray monochro- mator was not available; thus the simplest way of analyzing the photon distribution appeared to be its conversion into a photoelectron distribution which can easily be measured with conventional electron spec- trometers. Rare gases as Ne or He would have been ideal samples because their spectra contain very few and narrow photoelectron or Auger structures and nearly no background from inelastically scattered electrons. How- ever, a gas phase spectrometer was not available and thus graphite has been chosen as the most appropriate solid state sample. Its electron spectrum contains three different types of spectral features: 1) The Cis derived photoelectron structure at a kinetic

energy which depends on the photon energy distribu- tion (Eki n = hp - 282 eV),

2) the C valence level derived structure which appears at a kinetic energy similar to the photon energy, and

3) the broad C KLL Auger structure at fixed kinetic energy (Eki n --- 275 eV). For weak magnetic fields (K < 1) most of the (quasi-

monochromatic) photon intensity is radiated in the fundamental. Therefore, at 3 GeV only three spectral structures appear in the spectrum and are well separated from each other. For high magnetic fields ( K > 1) the higher harmonics gain intensity, and thus additional

photoelectron structures appear in the spectrum. The possible overlap between Cls and valence band

structures excited by different harmonics is not a severe problem because the cross section of C~s excitation is more than one order of magnitude higher than that of valence band excitation. The intense Cls line can be used to characterized the photon distribution because, in principle, the latter is much broader (AE > 50 eV) compared with all other contributions to the line width (AE < 1 eV) besides the background.

Two typical examples of such Cls spectra are shown in the upper part of fig. 7. They have been taken under different conditions: undulator fully open ("h~, = 1350 eV") and fully closed ("h~, = 750 eV"; note different energy scales). Unfortunately, the shape of these curves does not directly reflect the shape of the photon distri- bution. This is due to the fact that in the case of solid samples a rather high background of inelastically

800 900 1000

cr 400 500 600

E ( . . _ ~ . ~ _ ~ / / " - ~ "hv : 830 eV" f / / /

= y ' (mm)

400 500 600 Kinetic Energy {eV}

Fig. 7. Cls photoelectron spectra from a graphite sample ex- cited with the undulator fundamental (Ee = 3 GeV). Upper part: Gap fully open (K = 0.3, h~, -- 1350 eV) and fully closed (K = 1.3, h~' 1 = 7 5 0 eV) , Note different kinetic energy scales. The inset shows the same C~s spectrum taken with a Mg K,~ lab source. Lower part: C~s spectra taken at different vertical detector positions y ' with respect to the average beam axis ( y ' = 0). The intensities are normalized to about equal peak heights.

II. WIGGLERS/UNDULATORS/OTHER DEVICES

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132 H. Winiek et aL / Undulator studies

scattered electrons contributes to the curves. For com- parison a Cls spectrum taken with a narrow Mg K~ line source is shown as an inset in fig. 7. The convolution of the narrow C is peak with a broad photon distribution in the presence of the background increase below the C1., peak (see inset) yields spectra like the ones displayed in fig. 7 [14]. The peak asymmetry, the peak width and the height of the background on the low kinetic energy side depend on the width of the photon distribution. This has been described theoretically [14] and could be ob- served in our experiments by changing the size of our effective sample area or by changing the gap and hence the energy and width of the photon distribution (see upper part of fig. 7). A computer deconvolution has not been completed because some parameters were not ac- curately known. However, due to the steep high energy tail of the photon distribution [2,3] the onset (or cutoff) E ' of the photoelectron curve can be utilized to de- termine the actual photon energy (see fig. 7). We found excellent agreement between measured and calculated photon energies [according to eq. (1)] for the first three harmonics if we allowed for a 10-30 eV correction of h v~. This rather small offset was not constant and might be due to an energy of the electron beam slightly smaller (0.5-1.5%) than assumed, or to a slightly higher peak magnetic field, or to a combination of both.

Furthermore, the dependence of photon intensity and spectral distribution upon the position of the detec- tor relative to the average beam axis has been tested. According to eq. (1) larger wavelengths and thus lower photon energies are expected for finite observation an- gles (0 > 0) relative to the beam axis. This angle depen- dence has been checked experimentally by moving the whole spectrometer chamber together with sample and pinhole collimator up or down, or by changing the steering current in well defined steps thus effectively moving the photon beam vertically or horizontally with respect to the detector. Both procedures gave equivalent results. As an example five Ci. ~ spectra are displayed in the lower part of fig. 7. They were taken at different vertical positions y ' (in mm) of the detector relative to the average beam axis, as indicated in the figure. A marked shift of the Cls curves of more than 150 eV to lower kinetic energies could be observed for a vertical displacement of the detector of 4.5 mm.

In the lower part of fig. 8 the results of the experi- mental energy analysis and in the upper part the corre- sponding normalized intensities are plotted as a func- tion of vertical detector position. The intensity data have been corrected for cross section differences, decay- ing primary fluxes and varying detector sensitivity. The photon energies closely follows a symmetrical curve but the intensity ratio data scatter considerably. This is due to small misalignments of the effective detector area relative to beam and collimator caused by an insuffi- cient mechanical set-up. Such misalignments could

-5 -4 -3 -2 -I 0 +I *2 +3 ~ +5 O I l I ~ 0 I I I I I I I !

i0-2

10 .3

8OO

~ 700 o l

uJ ~

~ BOO [3_

-5 -4 -3 -2 -I 0 +I *2 *3 +4 *5

Vertical Detector Displacement (mm)

Fig. 8. Normalized intensities (upper curve) and cutoff energies E' (lower curve) as a function of the vertical detector position y ' relative to the beam axis. The circles represent experimental data.

change the intensity by one order of magnitude while the energy cutoff E ' remained unaffected.

In conclusion, the spectral measurements in both energy ranges showed excellent agreement between ex- pected and experimentally tested properties of the un- dulator. The observed small differences are easily ex- plainable. Much of the experience gained in these characterization experiments could be utilized for the first experiment with the undulator (see section 7).

5. Beam position monitoring and control

An undulator beam, because of its extremely small size and divergence in both the vertical and horizontal planes, presents special problems in beam position monitoring and control and new instrumentation has been developed for this purpose. In the case of the present undulator the beam at the experimental station (25 m from the undulator) is only about 7 mm wide by 1 mm high (fwhm). It is, therefore, not feasible to devote about 1 cm horizontally exclusively to position monitoring as is done on SSRL bending magnet or wiggler beams which have much larger horizontal diver- gence. Furthermore, the extremely small size of an undulator beam makes it necessary to monitor and control horizontal as well as vertical beam coordinates, whereas with beams from bending magnets and wigglers

Page 7: Undulator studies at SSRL

H. Winick et aL / Undulator studies 133

FILTER ASSY- -~

BEAM FINING PADDLES

"\x \ \

"4

) MASK - - ~o~ ~o~

/ / /

"--Iem x 2cm

FLUORESCENT SCREEN

VERTICAL POSITION MONITOR WIRES

"~--'T V CAMERA

Fig. 9. Pictorial of undulator beam diagnostic system.

normally only the vertical position is monitored and controlled. An additional complication in the case of the present undulator is that all beam diagnostics must be done in the high vacuum beam line because the spectral range of the undulator (hP ~< 3 keV) prohibits the use of vacuum windows.

A pictorial of the undulator beam diagnostic system assembly, which is located about 2 m upstream of the experimental end station, is shown in fig. 9. Its major components are (a) a set of 4 beam defining paddles, (b) a pair of wires used to sense the vertical beam position, (c) a variable thickness filter assembly, and (d) a re- movable fluorescent screen. All components are desig- ned for use in ultra high vacuum (~< 10 - 9 Torr) and are capable of operation with the highest power that can be radiated by the undulator (60 W with 100 mA at 3 GeV and undulator fully closed so that the peak field is about 2.4 kG). The beam diagnostic system assembly is preceded by a water cooled mask which limits the beam to 1.0 cm vertically and 2.0 cm horizontally.

The beam defining paddles are used primarily to mask out synchrotron radiation from the fringe fields of the bending magnets up and downstream of the undu- lator so that this radiation does not contribute signifi- cantly to the signals from the position monitor wires.

With these paddles fully withdrawn the pattern of the beam on the fluorescent screen is as shown in fig. 10. The central elliptical lobe is due to the undulator and is much more intense than the upper and lower lobes which are due to radiation produced in the fringe fields of the adjacent bending magnets. These bending magnet lobes are usually separated vertically from the undulator lobe, as shown in the figure, because of quadrupole magnets which are located between the undulator and the adjacent bending magnets. The stored electron beam traversing these quadrupole magnets off axis undergoes a deflection in both vertical and horizontal directions.

I i J I I 0 2 4 6 8 I 0 mm

Fig. 10. Radiation patterns due to undulator and bending magnet fringe fields.

II. WIGGLERS/UNDULATORS/OTHER DEVICES

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134 H. Winick et a L / Undulator studies

By inserting the four paddles while viewing the pattern in the fluorescent screen, the bending magnet radiation can be rejected almost completely. The paddles can be further inserted if only a small spot is desired for the experiment. The paddles are inserted by manually con- trolled micrometer drives.

Free access to the micrometer drives, filter mover and fluorescent screen mover (all of which are manually controlled) is available with the beam on. This is possi- ble because the undulator radiation is very soft (hJ, ~< 3 keV) and its radiation does not penetrate the vacuum enclosure. High energy radiation due to gas brems- strahlung by the electron beam traversing the 5 m long straight section results in dose rates which are typically less than 5 m R / h in accessible regions. The much higher levels of gas bremsstrahlung present along the beam axis are absorbed in a 20 cm thick lead backstop situated at the end of the beam line. Higher radiation levels that are unlikely but potentially possible during injection are protected against by a pair of 45 cm lead beam stoppers that close automatically during injection.

The vertical beam position monitor consists of a pair of horizontally mounted 1 mm diameter tungsten wires. The free space for the beam between these wires may be adjusted from 0 to 5 mm before the position monitor is installed into the vacuum system. Since the vertical size of the undulator beam is only about 1 mm (fwhm) a 3 mm free aperture for the beam passes virtually all the radiation and this is the aperture that has been used to date.

The photo-electron currents from the fringes of the beam striking the upper and lower wires are differen- tially amplified to provide a signal which is used in a feedback system which controls the amplitude of a local vertical orbit distortion (beam bump) which keeps the electron beam pointed at the center of the position monitor as it traverses the undulator. In this way any drifts of the vertical electron position during a run, or changes from fill to fill are automatically compensated and the undulator beam is stabilized and reproduced to an accuracy of about + 10/~m at the position monitor location. The photo-current from the wires is collected on plates with an adjustable positive bias voltage. In practice the current levels off at bias voltages above about 50 V and the system functions well with zero bias voltage. Horizontal centering of the beam is now done manually using a local horizontal beam bump while observing the beam spot on the fluorescent screen.

When the beam is centered vertically, the position monitor wires are located in the fringes of the beam and hence absorb very little power. A ntis-steered beam can strike a position monitor wire directly, causing a tem- perature rise calculated to be about 2500°C under the highest radiated power conditions (60 W). The heat is radiated from the wire and also conducted out through copper.

The filter assembly is a 10 cm linear motion feedthrough which can be used to insert one of five different filters into the beam. The following filters have been used: pyrolytic graphite, 5 #m and 15 /~m; stain- less steel 65 /~m, 110 /~m and 750 /~m. In the fully withdrawn position no filters are in the beam. Filters are inserted manually and may be observed through a viewport during beam-on conditions.

Just downstream of the filter assembly is a fluo- rescent screen which may be manually inserted using a linear motion feedthrough. The beam pattern on the screen may be observed through a glass viewport. Since close access to the screen is possible with the beam on, direct viewing of the screen is possible. The screen pattern may also be viewed remotely (e.g. in the SPEAR control room) using closed circuit TV. With no filters inserted the screen is swamped with radiation and saturated so that no beam pattern is discernable. By inserting one of the filters (usually the 25 /~m stainless steel) a clear elliptical lobe pattern of undulator radia- tion may be observed. This is used to monitor the beam as it is being vertically and horizontally centered.

6. Radiation from adjacent bending magnets

Radiation from the fringe fields of the bending mag- nets adjacent to the undulator is emitted very close to the direction of the undulator beam. Using measured

,01 9~

7-

m

6~

~ 4 -

0 / i

B / B m a x - ;

// / J i /

,' / /1' I I" /I

/ / //

// / i I /

22

21

20

19

IB

I?

15

14

x: E

o x /

/ /

: / k'

IO <~.r/ / /

2- / /

, - m - , 60 50 40 30 -2.0 tO 0 I0 20

D is tance f r o m Physicol E d g e of B e n d i n g M a g n e t ( inches}

Fig. l l. Fringing magnetic field, electron beam deflection, 0, and displacement, X as a function of distance from the physical edge of the SPEAR bending magnet.

Page 9: Undulator studies at SSRL

11. Winick et al. / Undulator studies 135

values of the magnetic field of the SPEAR bending magnets as a function of distance from the physical edge (kindly supplied by G. Fischer), the displacement and deflection of the electron beam traversing the fringe field have been calculated. The results are shown in fig. 11 which can be used to estimate the usually negligible amount of radiation originating in these fringe fields that appears close to the main lobe of undulator radia- tion.

Furthermore, as mentioned earlier, the fringe field radiation is usually vertically displaced from the undu- lator lobe (see fig. 10) because of the quadrupole mag- nets situated between the undulator and the bending magnets (see fig. 3). These quadrupoles are about 50 cm long and operate at a gradient of about 300 G / c m at 3 GeV. Thus, if the electron beam traverses the quadru- pole displaced vertically by only imm, it passes through a horizontal field of 30 G and is deflected by about 0.15 mrad leading to a vertical displacement of 3.75 mm at 25 m. Since the r.m.s, vertical orbit distortion is 1-2 mm we would normally expect the bending magnet lobes to be displaced by 3-6 mm from the undulator lobe as shown in fig. 11. This vertical separation facili- tates the rejection of the radiation from the bending magnets.

7. First use of the undulator for an experiment

Among the possible applications of undulator radia- tion without using a monochromator, Auger spectros- copy appears to benefit much from the properties of the undulator. First of all, soft X-rays are superior to elec-

trons as a primary excitation source for many high resolution Auger experiments, in particular for samples such as molecular overlayers or biological substances, which are very sensitive to radiation damage. Second, experiments with laboratory sources or conventional beam lines often suffer from low photon fluxes and hence from long measuring times or /and poor quality. In particular, high energy and angular resolution in angle resolved Auger experiments of two-dimensional adsoprtion layers require photon fluxes which are at least one to two orders of magnitude higher than those from conventional sources. Third, undulator radiation is quasi-monochromatic and partly tunable, which can be used to improve the experimental conditions consider- ably (see below). These properties make the undulator highly suited for angle resolved studies of the Auger fine structure of adsorbed molecules [15]. In the following, some experience gained before and during the realiza- tion of such an experiment will be discussed.

The experimental set-up was as normally used for surface studies. The undulator beam coming from the diagnostic and beam defining system described in sec- tion 5 passed through a phosphor coated collimator with selectable pinholes of different size and an I 0 monitor grid into the experimental (SSRL-VG) cham- ber. Only the two smallest pinholes (1.3 and 2 mm diameter) were needed; even smaller ones (0.3, 0.6 mm) would have been useful.

The electrons emitted from the Ni(100) sample were analysed in a conventional electron spectrometer with 4 ° acceptance angle which could be rotated around the sample in the horizontal plane.

A typical total scan extending over a large range of

\ Onset \ Ni 2p Ni-Auger O/Ni(100} Ee = 3.2 GeV lxl0s ~ ] ~1 I Bo = 1.3 kG

l K =0.74 I hvt = 1250 eV

x5 / Onset (1st Harmonic} of Ni 3p Ni VB

0 • I , i , • / . . . . I . . . . I . , . i I

500 I O 00 1500 2000 2500 Kinetic Energy (eV} =

Fig. 12. Total kinetic energy scan of an oxygen covered Ni(100) surface. Auger structures and photoemission onsets (or cutoffs) of Ni 2p, 4p, and valence band excited with first and second order undulator radiation are marked.

a., 5 xlO~'

U

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136 H. Winiek et al. / Undulator studies

kinetic energies is shown in fig. 12. Sharp structures from Ni LMM-Auger transitions between 600 and 850 eV as well as much weaker peaks from O KLL-Auger transitions of adsorbed oxygen at about 500 eV can be observed. All photoelectronic structures are smeared out because of the relatively large effective detector area ( - 1 mm2). Their remnants can be identified as kinks in the smooth background indicating the onset (or cutoff) of Ni 2p, 3p and valence band emission excited with radiation from the fundamental and even from the second harmonic. It is clear from eq. (1) that changing the undulator parameters Ee, K(gap) and 0(detector position) shifts these photoemission onsets with respect to the Auger structures, Hence, the shape of the back- ground underneath the Auger structures may change markedly. Furthermore, the total photon intensity and the partial intensity radiated in the fundamental strongly depend on Ee, K and 0 as well (see sections 1 and 4).

It is obvious that the right choice of all parameters ( E e, gap, position, collimators, pass energy of analysers, etc.) can be extremely important for the quality of the experimental results. The criteria which determine this choice are as follows:

(1) Photon energy. A simple recipe is not possible in most cases. The spectral range of the fundamental is mainly given by SPEAR energy E~ [hl, - E 2, see eq. (1)] which usually cannot be changed during an experimen- tal period because of the requirements of other experi- ments on other beam lines. Within a given range, how- ever, the photon energy can be tuned by changing the gap a n d / o r the sample-detector position relative to the beam axis. Photon energies near the threshold of the Auger transition under consideration result in a high ionization cross section. Simultaneously emission from deeper levels can be suppressed which otherwise would contribute to an inconvenient background. On the other hand, interference of Auger structures with steep back- ground slopes may be undesirable and can be avoided by tuning the photon energy. Small gaps (i.e. high K-values) produce low photon energies of the funda- mental but enhance the intensity of higher orders which may add steps and enhance the background consider- ably.

(2) Photon densities. The oustanding characteristic of undulator radiation is its enormous brightness. Photon fluxes which were about 5 orders of magnitude higher than those obtainable with conventional lab sources or monochromatized beam lines have been measured (see below). Therefore, for some applications it may be advantageous to reduce the photon density in order to avoid noticeable radiation damage of the sample during the measuring time or to prevent saturation of the detector. This can be achieved by opening the gap or by moving sample and detector out of the center of the beam (see section 4) which can be done by (a) moving the experimental chamber relative to the beam, (b)

changing the steering current manually, or (c) shadow- ing one of the position monitor wires with one of the paddles (see fig 9) thus causing an automatic resteering of the beam.

In any case it is worth trying to find the optimum set of parameters and thus the best experimental condi- tions. It should be mentioned, that we were able to enhance the signal to background ratio (and thus the signal-to-noise ratio) of the oxygen Auger structure by more than a factor of 10 in this way. Considering the limited counting rates which can be handled by normal channeltrons or channelplates (about 2 × 105 c / s ) such an optimization may be extremely important for the reduction of measuring time and the enhancement of the S / N ratio.

Finally, fig. 13 may demonstrate the unique capabil- ity of the undulator. It shows Ni LMM-Auguer spectra measured at two different beamlines under similar con- ditions. The upper spectrum was taken at the crystal monochromator J U M B O (BL III-3) equipped with Ge crystals (3.0 GeV, 45 mA, hu - 2000 eV), and recorded

6x10 3

l 4×103

6×10 ~ o =

C.3

.4x104

2×10 ~

2.98

xtO 5

2.96

I I I

Ni - LMM Auger

Source :Crys ta l Monochromotor

Pass Energy 80 eV

Time 80 mln ' ""

v"

". .. I s .:

! /

Source UnduLator

Pass Energy I0 eV

Time 20 mm " ~ J ~

J I n I I I

750 800 850

I I i

× 100

i I i | ~ I

750 800 850

Kinetic Energy (eV) ----4,,- Fig. 13. NI LMM-Auger spectra excited with radiation from the crystal monochromator JUMBO (upper curve) and the undulator (lower curve) are compared. Note the different inten- sity scales. The spectrum at the bottom shows the 10 signal recorded simultaneously with the undulator spectrum above it (intensity scale stretched by factor 100).

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H. Winick et al. / Undulator studies 137

with a high efficiency, posi t ion sensitive detector. The lower spectrum was measured with the undu la to r (3.5 GeV, 18 mA, K = 0.8, hu 1 - 1420 eV) using a normal 150°-sector analyser. The compar ison between the two spectra shows that the undula tor spectrum has much higher S / N quality, be t ter energy resolution (10 vs 80 eV pass energy) and higher countra tes (factor 10) while the overall condi t ions under which it was taken were worse (measur ing time, current, detector). A detai led compar i son of all parameters yielded an est imate for the p h o t o n flux radiated from the undula tor of about (2 -4 ) × 1015 p h o t o n s / m m 2- s at 3.5 GeV and K - 0.8. This mus t be compared with normal lab sources or mono- chromat ized beam lines (e.g. J U M B O ) which provide abou t 101° p h o t o n s / m m 2 • s.

In the lower par t of fig. 13 the I 0 moni tor signal is displayed which has been recorded s imultaneously with the undula tor spectrum above it. No te that its intensi ty scale has been stretched by a factor of 100, In this case, the I 0 noise-to-signal rat io was extremely small (0.03%). Al though it was not always so good, it seemed to be considerably bet ter compared to other beam lines at SSRL. This is mainly due to the very effective diagnos- tic and beam defining system described in section 5.

Unde r these condi t ions many angle resolved spectra of Auger fine s tructures from adsorbates could success- fully be taken. While stable adsorbates such as CO and a tomic oxygen could be measured wi thout problems, rapid degradat ion of the adsorpt ion layer by dissocia- t ion and desorpt ion took place in the case of molecu- larly adsorbed N 2 and N O on Ni within a few minutes. Therefore, we had to reduce the pho ton intensi ty as described above, and we had to move the sample after each scan to expose and measure a fresh par t of the overlayer. Al though this effect was t roublesome for our measurements , it highly recommends the undu la to r for pho ton s t imulated desorpt ion experiments.

We are grateful for the excellent support provided by the SSRL staff, part icular ly R. Hettel, R. Mayer, H. Morales and J. Yang. The efforts of J. Harris, P. Mor- ton and J. Spencer of SLAC in s tudying the interact ion

of the undula tor with the stored beam are much appre- ciated. J. Chin, K. Ha lbach and E. Hoyer of LBL are pr imari ly responsible for the concept, design and con- s t ruct ion of the undulator . We would like to thank B. Pate for making a graphite sample and a Ni crystal available to us. One of us (E.U.) gratefully acknowl- edges a research grant f rom the Deutsche Forschungs- Gemeinschaf t .

Work done at SSRL was suppor ted by the NSF through the Division of Materials Research and the N I H through the Biotechnology Resource Program in the Division of Research Resources in cooperat ion with the Depa r tmen t of Energy.

Work done at LBL was suppor ted by the U.S. Depa r tmen t of Energy under contract S-7405-ENG-48.

References

[1] K. Halbach, J. Chin, E. Hoyer, H. Winick, R. Cronin, J. Yang and Y. Zambre, IEEE Trans. Nucl. Sci. NS-28 (1981) 3136.

[2] G. Brown, K. Halbach, J. Harris and H. Winick, these Proceedings, p. 65.

[3] H. Winick, G. Brown, K. Halbach and J. Harris, Phys. Today 34 (1981) 50.

[4] J. Spencer and H. Winick, in Synchrotron radiation re- search, eds., H. Winick and S. Doniach (Plenum, New York, 1980) ch. 21.

[5] Y. Farge, App. Optics 19 (1980) 4021. [6] D.F. Alferov, Yu. A. Bashmakov, K.A. Belovintsev, E,G.

Bessonov and P.A. Cerenkov, Part. Accel. 9 (1979) 223. [7] A. Hofmann, Nucl. Instr. and Meth. 152 (1978) 17. [8] K. Halbach, Nucl. Instr. and Meth. 187 (1981) 109. [9] SSRL Activity Report 81/01 (May 1981).

[10] R.H. Helm, PEP Note 272 (1978). [11] C.Y. Yao, SPEAR Note 220 (1980) and references therein. [12] A.S. King, M.J. Lee and P. Morton IEEE Trans. Nucl. Sci.

NS-20 (1973) 878. [13] P. Morton, SPEAR Report 223 (July 1981). [14] S. Tougaard and P. Sigmund, Phys. Rev. B25 (1982) 4452. [15] E. Umbach and Z. Hussain, SSRL Activity Report

82/01-Proposal 655; and to be published.

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