Optimisation of a synchrotron radiation source

Nuclear Instruments and Methods 208 (1983) 1-10 North-Holland Publishing Company

1

Part. L Synchrotron radiation sources, facilities and beam fines OPTIMISATION

OF A SYNCHROTRON

RADIATION

SOURCE

D.J. T H O M P S O N

Science & Engineering Research Council, Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK

In 1978 the European Science Foundation set up an ad hoc Committee on Synchrotron Radiation and since then the Committee's two sub-groups (Machine and Instrumentation) have been developing the idea of an optimised X-ray synchrotron radiation source for Europe. The properties of different types of source element (bending magnet or special insertion) have been analysed and a storage ring lattice developed which accommodates the requisite number of each. The resulting "Pluralistic Synchrotron Radiation Facility" is described and mention is made of a number of other important features of such a machine.

1. Introduction In 1978 the European Science Foundation set up an ad hoc Committee on Synchrotron Radiation and since then the Committee's two Sub-Groups (Machine and Instrumentation) have been developing the idea of an optimised synchrotron radiation source for Europe. For many reasons it is preferable to provide separate sources for low-energy photons and X-rays respectively. This minimises problems due to the incidence of very high total power on the first optical element in the low-energy beam lines; and the smaller diameter and thinner shielding of a low-energy ring allow the beam lines to be shorter and the optical elements closer. State-of-the-art low-energy sources can be and are being built in Europe on a national basis (BESSY, S U P E R A C O ) and the ESF study is specifically concerned with an X-ray source. This does not mean there is a lower limit to the energy of the photons which may be used, but it means that the needs of users of low-energy photons are not important when optimising the design. The process of arriving at an optimum design consists first of considering the different types of source point, then of constructing a magnet lattice which accommodates the required source points and has the desired overall properties, and from there the design proceeds to more and more detailed levels.

2. Source elements for synchrotron radiation Two fundamentally different elements in an electron storage ring can be used as sources of synchrotron radiation: the bending magnets, which also control the particle orbit; and periodic magnets, each consisting of three or more dipoles, which can be inserted in a 0167-5087/83/0000-0000/$03.00 © 1983 North-Holland

straight section and which can, to a considerable extent, be designed and operated independently of each other and of the storage ring proper.

2.1. Bending magnets Neglecting special considerations which apply for short magnets and fringe fields, where the magnetic field is changing rapidly [1,2], the spectrum from a bending magnet is given by the well-known "universal curve" [3] (fig. 1). The flux of radiation, for any given value of ? ~ / ~ (where ?% = 18.6/BE 2, B = magnetic field in T, E = electron energy in GeV and ?'c is the characteristic wavelength in ,~) if defined per unit bandwidth and per unit angle of horizontal orbit, is proportional not only to the electron current but also to the electron energy. Moreover the vertical opening angle of the radiation for a given k / k c is inversely proportional to electron energy. This means that for a fixed choice of ~c and all other things being equal a high energy storage ring will provide a brighter source thana low-energy one. (For this reason a weak-field wiggler provides a very bright UV source and may be an important element even in a high energy storage ring intended mainly for X-rays). Altogether, 2~ horizontal radians of radiation are available from the bending magnets and the proportion of this which can be used (in total, and also for any one beam line) depends only on "engineering" criteria such as the dimensions of beam lines and experiments and the avoidance of obstructions due to downstream components, particularly quadrupoles and sextupoles. It is difficult to use more than 10% of the bending magnet radiation from a large diameter ring, but in principle this is mainly a matter of cost and ingenuity.

I. SR SOURCES/FACILITIES/BEAM LINES

D.J. Thompson / Optimisation of a SR source single-bump wiggler therefore lies in being able to shift the wavelength of the radiation c o m p a r e d with the b e n d i n g magnets (typically, to m u c h shorter values) whilst still providing b e a m simultaneously to several users [4].

1~ 1 .

lO~Oo~

109 -

o 108 ,~ <

10 7-

2.3. Multipole wigglers [5]

106 -

.~

The natural extension of the simple wiggler is to increase the n u m b e r of poles and hence the intensity of the radiation. The angular extent of the radiation is given by

lo 5-

o

o'1

11o

i

i ¸'

i

lo

10 2

10 3

104

h/Xc

2c~ = 2K/~,,

Fig. 1. Universal curve for photon flux as a function of wavelength

2.2. Single-bump wigglers The simplest form of insertion is a 3-pole wiggler producing a single bump. The differences from a source p o i n t in a b e n d i n g magnet are: a) the magnetic field ( a n d hence )to) is i n d e p e n d e n t of the storage ring proper a n d can be set at any value limited only by the capability of the wiggler itself (in a suitably designed storage ring); b) the electron b e a m at the source point may be smaller in cross section, a n d hence the source brighter, (if the larger electron divergence is still small c o m p a r e d with the p h o t o n opening angle); c) the extent of the horizontal fan of radiation available depends on the length of the wiggler poles a n d o n the magnetic field, b u t in most storage rings it should be easy to find space for a wiggler providing several tens of milliradians of radiation within the high-field region - as m u c h as can b e a c c o m m o d a t e d by a b e a m line of reasonable dimensions feeding several experiments. The usefulness of a

where K = eB )tc/2~rm0c = 0.0934 B)t 0 if )t 0, the wiggler period, is in ram. E = grno c2, a n d 2 a = 0.0955 B ) t o / E = 1.78 ) t o / ( ) t c E 3) m r a d using previously defined units. The angular width of the b e a m of radiation of specified )to is limited absolutely by the space, N)t 0 ( N = n u m b e r of periods) available in the straight section, and is p r o p o r t i o n a l to B. Examples of wiggler parameters are given in table 1. Because radiation is received at the experiment from peaks on b o t h sides of the quasi-sinusoidal trajectory the source size in the plane of the wiggler (wigglers may be horizontal or vertical) is increased by an a m o u n t 2 x = K )t0/~ry = c~ )t0/~r. In a high field large-period wiggler, x m a y be greater t h a n the electron b e a m cross section and the radiation source can be resolved into two separate peaks. In general, the distribution in phase space (in the plane of the wiggle) from a m u h i p o l e wiggler is quite different from that from a b e n d i n g magnet arc. Figs. 2 a n d 3 show typical distributions, obtained by referring all the radiation to an effective source point at the centre of the wiggler. Two different cases are given: one where the

Table 1 Examples of wiggler parameters

Maximum field (B0) Characteristic wavelength Xc Periods Total length Period (X0) K-value Total energy loss Useful energy loss (0.8 Bo < B < B0) Full angle of useful radiation 2 Xm,~ (broadening in wiggler plane) 2.35 ox (fwhm of beam)

W0 Wp

Multipole wiggler

Single-bump wiggler

ADONE (for comparison see fig. 3)

0.75 T 1.0 6 1.1 m 176 mm 12.3 11.0 keV 8.9 keV 2 mrad 0.07 m 0.19 mm 1 . 2 7 mm

3T 0.25T 0.5 0.5 m -

1.85 T (2.3 A at 1.5 GeV) 3 1.95 m 650 mm 112.3

-

- 31 keV 18 keV 12 mrad -

0.19 mm

7.9 mm 4.3 mm

D.J. Thompson / Optimisation of a SR source

3

X (ram) -2.5

-2

-1.5

-1

0

-0.5

0.5

1

1.5

2

2.5

I

0.20.40.6E

-><

o.8-

1.01.2DO
{~.2
~.4
[~.6
~.8
Fig. 2. Distribution of brilliance in horizontal phase space for a 6 period wiggler in the PSRF. K = 12.3. n is the ratio between local and maximum brilliance (integrated in the vertical plane).

amplitude of the wiggle is small compared with the source size, and one where it is large. It is apparent that a multipole wiggler is a device ideally suited to produce a narrow beam of intense radiation and if )% is chosen to make ct large, then x too will be large and the distribution in phase space may be rather irregular. The term "wiggler" is usually reserved for devices with relatively few poles and a high field intended to produce radiation with a different )'c to that of the bending magnets. The spectral curve still generally follows that given in fig. 1. However, interference effects do occur and these will be noticeable at longer wavelengths. Ref. 5 gives an estimate of the frequency v* below which structure may be seen in the spectrum:

X (ram) -30 0

i

-20

-10

0

10

20

30

]

5-

1520-

40

EZ2Z

\

O
u62 ?

-x 2530354D-

Fig. 3. Distribution of brilliance in horizontal phase space for the ADONE wiggler (3 periods, K = 112). n is the ratio between local and maximum brilliance (integrated in the vertical plane).

. . = 2v~¢/Xo(1 + K ~ / 2 ) ×

+ (2 a ~ , / y ) 2

1 + K2/2 where Ni is the angular spread of the electrons in the beam with respect to a point-like detector at a given distance; Nd is the spread of collected angles due to the finite dimensions of the experimental acceptance; and A~,/~, is the electron beam energy spread. This is the frequency above which the inhomogeneous line-broadening exceeds the line separation. 2.4. Undulators [6] If the number of poles in a wiggler is increased to the point where interference effects dominate and the spectrum consists of a few lines, the device is called an undulator. This is when the homogeneous line broadening ( 1 / N when N is the number of periods) is much less than unity and K, which is proportional to the magnetic field, is sufficiently small ( < 10) that relatively few harmonics appear with high intensity. An undulator is an important source of synchrotron radiation because in the limit of an electron beam with zero emittance and a magnet with a large number of poles the brightness is proportional to N 2 and even in practical machines the brightness can be very large. Examples are given in table 2. There is a gradual transition from multipole wigglers to undulators and an undulator may be very useful even if the lines cannot be resolved completely because of a combination of small length, high field and large electron beam divergence [7]. However ideally an undulator is conceived as a device to produce separated very bright spectral lines. I. SR SOURCES~FACILITIES~BEAM LINES

4

D.J. Thompson / Optimisation of a SR source

Table 2 Examples of photon flux and spectral brilliance for one particular undulator operating in the PSRF at two different field levels. X0 = 56 mm, N = 180 poles. Harmonic

B = 0.23 T K = 1.2 Total emitted power = 1.2 kW

B = 0.38 T K= 2 Total emitted power = 3.6 kW

X (A)

Flux photons/s/0.1% bandwidth)

Brilliance ( flux/mm2/ mrad 2)

X (,h,)

Flux (photons/s/0.1% bandwidth)

Brilliance (flux/mm 2 mrad z )

1

5

24 x 10 ~4

46 × 1017

3 5 7 9

1.7 1.0 0.7

8.7 2.9 1.8 1.3 1.0 0.8 0.67 0.58 0.51 0.46

32 X 1014 15 9 5.6 3.5 2.2 1.4 0.9 0.6 0.4

35 × 10 iv 17 10 6,2 3,9 2,4 1.5 1,0 0,66 0,44

6 1.6 0.4

11 3 0.9

11

13 15 17 19

The wavelength from an u n d u l a t o r is

)~i

1 Xo ( 1 + K 2 / 2 ) i 2y 2

1+

- 1 + K2/2

'

where i is the h a r m o n i c n u m b e r (odd harmonics only, at 0 = 0), 0 is the angle of observation from the axis, K = ay = 0.0934 BX 0 for B in T a n d X 0 in mm. The h o m o g e n e o u s line-broadening is always 1 / N of the line spacing a n d so to get narrow lines N must be large. However, the observed wavelength depends also on O and hence the observed b a n d w i d t h depends on the acceptance of the experiment and on the divergence of the electron beam. It also depends on the energy spread in the electron b e a m and the transverse field uniformity in the magnet. Because this inhomogeneous b r o a d e n i n g is proportional to frequency the higher harmonics tend to merge. To obtain very bright narrow lines from undulators a s y n c h r o t r o n radiation source therefore needs the following: a) b e a m lines with a very small acceptance, 0 < 1/y. So if E = 5 GeV, y = 1 0 4 , 0 < 0 . 1 mrad, i.e. if the experimental aperture is 10 m m it must be 100 m from the source. b) an electron b e a m with a very small divergence at the u n d u l a t o r location; o ' < 1 / y ( < 0.1 m r a d for E = 5 GeV) c) energy high enough to give the required m i n i m u m o u t p u t wavelength: (see section 3.2 below). d) space for a large n u m b e r of magnet periods. If the u n d u l a t o r period is a few cm, then for 100 periods a space of a few m is required. Useful undulators can of course be m u c h shorter but in a new source in which a variety of undulators may be placed (in different straight

sections or from time to time) an available space of several metres per u n d u l a t o r is clearly a p r u d e n t provision. 3. G e n e r a l specification for an optimised X-ray source for E u r o p e ]8]

The general requirements are a specified spectral range, an adequate n u m b e r of b e a m lines, and the highest possible brightness. M a n y applications of sync h r o t r o n radiation need p h o t o n energies of the order of 10 keV and most are satisfied by p h o t o n energies below 50 keV, so using the rule of t h u m b that good fluxes are o b t a i n e d to Xc/4 this indicates that the majority of b e a m lines should have X~ - 1 A,, including undulators. Some applications require higher energies and so some ports should be specified with X c - 0.25 A. It is desirable that this should not be the limit of the machine but that X c = 0 . 1 i~ should be feasible (e.g. for medical applications). A E u r o p e a n source must allow many simultaneous experiments a n d a n u m b e r of the order of 100 seems reasonable in relation to the existing d e m a n d on present-day machines. The d e m a n d for the highest possible brilliance cannot be divorced from a high flux by having an extremely small source size. Experiments vary enormously as to the nature of their limiting factors a n d it is by no means always source size or even source brilliance. 3.1. Types of source

If - 100 b e a m lines are required it is unreasonable a n d unnecessary to provide these all from undulators. It

D.J. Thompson / Optimisation of a SR source

is even difficult to provide them all from multipole wigglers. A source of this nature, the " A l l - W i g g l e r - M a chine" has been studied [9] but found not to be completely satisfactory. Locations for wigglers must have a small electron b e a m cross section a n d preferably zero dispersion, otherwise the operation of the wiggler affects the storage ring behaviour. The b e a m width from a multipole wiggler is (section 2.3): 2c~ = 1.78 X o / E 3 X c mrad and with Xc = 1 A, E = 5 G e V (see below) X 0 = 150 m m (say) one finds 2 a = 2 mrad which c a n n o t be usefully divided between two or more b e a m lines. In this respect therefore a multipole wiggler is similar to an undulator. If a is increased by increasing X 0 then not only must more space be provided or fewer poles used b u t the source b r o a d e n i n g increases and reduces the brightness. F r o m these considerations the concept of the "Pluralistic Synchrotron R a d i a t i o n Facility" ( P S R F ) emerged. This machine has m a n y straight sections, some suitable for undulators (long length, small electron b e a m divergence) some suitable for wigglers (small b e a m cross section a n d zero dispersion) and others adequate though n o t ideal for more wigglers. Some wigglers will be single-bump with large a a n d several b e a m lines, a n d in addition, the b e n d i n g magnets will be used as radiation sources. As some experiments require lower energy radiation, a convenient a n d power-saving a r r a n g e m e n t is for the b e n d i n g magnets to have lower X~, say 2 ,~. Wigglers, which provide higher energies, will only be energised when in use. The total flux spectra from the various source points are shown in fig. 4.

5

The critical consideration is of undulators. One can achieve a short wavelength from a n u n d u l a t o r of a given energy by using a high h a r m o n i c a n d / o r a short period which implies a small magnet gap. Both of these are disadvantageous if they can be avoided. Use of a high h a r m o n i c requires K large [6] to achieve a high power in that harmonic. The inhomogeneous line-broadening is greater at high harmonics, a n d the total power from the u n d u l a t o r is proportional to K 4 / i for a given wavelength and when K is large. The ratio of total power to power in the i t h h a r m o n i c is approximately p r o p o r t i o n a l to K2. Use of a very small magnet gap presents practical difficulties of alignment and injection a n d also an increase in v a c u u m c h a m b e r impedance which will limit the peak b u n c h current. D e t e r m i n a t i o n of the m a x i m u m permissible gap in a given storage ring straight section is complex a n d perhaps not unambiguously feasible at present but in a source intended to serve a wide comm u n i t y for a long time and to have good development potential it is wise to be cautious in this respect. The conclusion [6] is that 5 G e V is an o p t i m u m choice of energy, allowing ~c = 0.12 A (100 keV) from a 6 T wiggler, a n d 1 ,~ in the 5th h a r m o n i c from an u n d u l a t o r with K = 1.2 a n d magnet gap of 45 m m or in the f u n d a m e n t a l with K = 1 if the gap can be reduced to 13 mm. 3.3. Source brilliance

The spectral brilliance of a s y n c h r o t r o n radiation source is conventionally expressed as [10]: SB = [ n u m b e r of p h o t o n s / s ] [ u n i t area of source

3.2. Electron energy

× unit solid angle × 0.1% b a n d w i d t h ] The smallest X c specified from wigglers is 0.25 A, a n d a field of 6 T would achieve this with E = 3.5 GeV. However the possibility of shorter Xc a n d hence a higher energy is very desirable. •: ld a

a n d a sort of average can be calculated for a particular port on a given source as a function of wavelength ()~) by ( S B ) = [ n u m b e r of p h o t o n s / s (vert. i n t e g r a t e d ) / 0.1% b a n d w i d t h ] [2.362 oxoz × 2.36 oj (total) × m r a d horiz.] -

N la g,0

{

)o"

n

2

I Z l l l q1~)6

10

Photon energy (eV}

Fig. 4. Photon flux from different ports on the same storage ring. Curves calculated for the PSRF. Curve l : normal bending magnet; Curve 2: multipole wiggler; Curve 3: single bump wavelength shifter wiggler; Curve 4: undulator ( K - 1.2, Xs l A).

i

with of(total) = [o~'(photon) 2 + a'(electron)2] 1/2 a n d o ' ( p h o t o n ) = ( O . 2 8 9 / E ) ( X c / X ) o425(0' in mrad, E in GeV, • in A). ox. z are electron b e a m sizes, horizontal a n d vertical (one s t a n d a r d deviation) o" is the vertical divergence. To achieve a high spectral brilliance one requires a large total flux a n d m i n i m u m values of ox, oz a n d o'. The former is achieved by having a high energy (this reinforces the choice of 5 GeV) a n d a high electron current (see section 5). Small b e a m dimensions d e p e n d o n the horizontal emittance, the coupling to the vertical plane, and the values of the fl-functions at the source points. Van Steenbergen [ 11 ] has discussed this in detail. I. SR SOURCES/FACILITIES/BEAM LINES

6

D.J. Thompson / Optimisation of a SR source

Table 3 Beam dimensions in the PSRF. E = 5 GeV, e~ = 8.3 × 10 9, ez =0.1Ex a).

1018. Source point

Bending magnet 1 Bending magnet 2 Wo wiggler Wp wiggler Undulator

ox [mm]

oj [mrad]

o~ [mm]

o" [mrad]

0.19

0.07

0.11

0.008

0.15 0.070 0.54 0.43

0.07 0.13 0.033 0.019

0.11 0.035 0.065 0.08

0.008 0.023 0.013 0.011

-5

1017.

o~

4. P S R F m a g n e t lattice

The original E S R F lattice [12,13] was modified first to the A W M [9] and more recently to the P S R F format.

/

~

I \ \

1016.

E

-~10

-

o

1014 ill IJ o

i

3

Wavelength {,~) Fig. 5. Spectral brilliance for different ports on the PSRF. Curve 1: bending magnet; Curve 2: wavelength shifter; Curve 3: Wo multipole wiggler; Curve 4: Wp multipole wiggler; Curve 5: undulator.

T h e circumference has been increased to 900 m and the lattice has 8-fold symmetry allowing, as before, for 6 long undulators but also for up to 48 wigglers. It is not possible to provide, in addition to the 8 long straight sections (6 for undulators, one for r.f. a n d one for injection), 48 more with zero dispersion. Instead the wiggler positions are of two kinds, W 0 where the dispersion is zero and the highest field wigglers would be placed, and Wp where the dispersion is not zero but nevertheless short wigglers have a minimal effect. The lattice is shown in fig. 6 a n d its optical functions given in fig. 7. The b e a m cross section is very small at the W o positions a n d somewhat larger for Wp. The criteria in the design have been:

I-I"1-1 Dw B

bl~Us 04

Q5

psQ nr-I ~-

/

1

0

a) This is a very cautious assumption. In smaller existing rings, e~ = 0.01 ex. This would reduce all oz and o~' values by a factor of 3.16. The horizontal emittance is a property of the overall m a g n e t lattice and should be reduced to the m i n i m u m . This is easier the larger the circumference. The ]~-functions vary around the circumference and there are severe practical limitations on the degree to which they can be adjusted. Generally the greater the n u m b e r of quadrupoles (and of different families of quadrupoles) the greater the degree to which the B-function can be optimised. The vertical emitance is in practice defined by the degree of accidental h o r i z o n t a l / v e r t i c a l coupling a n d is in the region 1-10% of the horizontal emittance. Skew quadrupoles and other devices can be included to control it. The b e a m emittance and the/3-functions for the PSRF are given in section 4 a n d the electron b e a m sizes for 10% coupling in table 3. Curves of spectral brilliance as a function of wavelength for the P S R F are given in fig. 5, neglecting the source b r o a d e n i n g due to the multiple wigglers, which depends on the wavelength chosen for the wiggler.

iI

B

6'

•

\ \

~/IAppr°xlmately

1/32 °f the t°tal c i r c u m f e r e n c e ~ ~ z

/~..~.

.

//

/

Fig. 6. One-quarter of a super-period of the lattice. A super-period consists of this, its mirror image reflected in the Wo straight, and in between, two similar sections with Und replaced by Wo.

D.J. Thompson / Optimisation of a SR source m

0 40

10

20

30

40

50

I

I

I

I

I

60

30-

1.o E

ca. 2 0 -

10-

0.5

0

Fig. 7. Optical functions fix, ft, and ~7for a lattice half-period. energy circumference

= 5 GeV (explained above); = 900 m (slightly arbitrary but necessary to accommodate this particular lattice); ?~c in bending magnets = 2 ,~ (useful, and uses less magnet power and less r.f. than for 1 .A); small beam dimensions in both planes at all wiggler positions; minimum beam divergence in both planes at the undulator positions;

minimum beam emittance (using techniques described by Van Steenbergen [11]). The beam sizes achieved are given in table 3 and a parameter list is provided in section 7 of the paper.

5. B e a m current

In the E S R F [8] the nominal beam current of 565 m A was the value which gave 10 k W / m radiation density on the vacuum chamber walls. With the reduced

. - - J k..--f~oj _ / J

,,o

j

~

o % ~ o

Bgeam f ~V - ~ull~'po~ wiggler sta~;ons | WS -Single-bump wavelength shifter " LBM-Bending magnet Magnets not to scale

Figures indicate distance along beam line (m)

Fig. 8. A possible layout of experimental stations. The shield wall is not shown, for clarity, but extends to about 25 m measured from the tangent point along the beam lines. I. SR SOURCES/FACILITIES/BEAM LINES

8

D.J. Thompson / Optimisation of a SR source

field in the P S R F bending magnets the density in the bending magnets is lower and by proper design of the b e a m extraction ports the very high flux density from wigglers and undulators can all be channelled down b e a m lines. The m a x i m u m current is now likely to be limited by beam instabilities, or by the heat loads on experimental equipment a n d beam line optical components. It is not possible to estimate how high the limits due to the latter will rise in the future. The matter of instabilities is the subject of an ongoing study but so far, assuming that care is taken to keep the vacuum c h a m b e r impedance low, the maxim u m current in the multi-bunch mode is believed to be similar to that in the original ESRF. There will be a severe limitation on the current which can be achieved in a single b u n c h without longitudinal turbulence and hence an increase in b u n c h dimensions. This is still being examined, as is the threshold for transverse turbulence, but it will not easily be counteracted. A n i m p o r t a n t aspect of the design, therefore, will be to minimise increases in c h a m b e r impedance due to u n d u l a t o r insertions or b e a m extraction slots. The latter will be very long ( 1 - 2 m) and must be accurately on the m e d i a n plane of the b e a m a n d as narrow as possible. In the figures and tables, a current of 343 m A has been assumed. This is the value which gives the same total n u m b e r of p h o t o n s from all the ports which have X c = 1 ~, as in the case of the original ESRF. In the latter machine there were 36 such ports, each with 16 m r a d aperture, all on bending magnets (576 mrads). In the present machine there are 42 such ports, all on 6-period wigglers with 2 mrad aperture each (84 mrad in all). The flux per milliradian and the spectral brightn e s s / b r i l l i a n c e are thus much higher for the PSRF than in the original ESRF, whilst the intensities from singleb u m p wigglers, wavelength shifters, bending magnets a n d undulators are not much less. This current should be achievable and is not necessarily an upper limit.

6. Other important attributes of a dedicated SR source

G o o d b e a m stability and long beam lifetime are essential to efficient use of a s y n c h r o t r o n radiation source. Like all large storage rings with high b e t a t r o n wave n u m b e r s the P S R F is rather sensitive to alignment tolerances, and an rms error of 1 bt in the quadrupole positions will cause vertical closed orbit errors in the b e n d i n g magnets which have a 5% probability of exceeding 360 /Lm. The corresponding angle change is of the order of 0.04 m r a d which means the b e a m at an experimental station 50 m away would move 2 mm. At the wiggler and u n d u l a t o r positions the sensitivity is somewhat less. This indicates that the machine be provided with very good b e a m steering controls and, because the b e a m position detectors in the ring are un-

likely to be accurate and sensitive enough, good positional information from the beam lines. A system providing b e a m b u m p s for each b e a m line which are as i n d e p e n d e n t as possible [14] will be required. This is particularly i m p o r t a n t because of the m a n y wigglers, which are likely to have finite, though small and repeatable effects on b e a m position which will require correction. The sensitivity to errors in the dipole current is not unduly high. A stability of 1 in 10 4 will achieve a horizontal beam stability of better than 0.25 mm. Long b e a m lifetimes (over 20 hours with b e a m currents over t00 mA) are being achieved in existing rings. A n i m p o r t a n t contributing factor is glow discharge cleaning of all vacuum c o m p o n e n t s prior to installation a n d in situ bakeout to recover quickly after opening the ring. The philosophy of the PSRF is to regard each u n d u l a t o r and wiggler as the first element in its beam line, not specified until the purpose of the line has been decided. In this case, to avoid frequent openings of the main v a c u u m system, every w i g g l e r / u n d u l a t o r position must have isolation valves, it will be i m p o r t a n t that these do not add significantly to the c h a m b e r impedance. Very dense bunches cause short lifetimes due to the Touschek effect, or there is a relatively low threshold current at which the b u n c h dimensions grow, due to the multiple Touschek effect. The calculated lifetime for the P S R F at 5 G e V is 14 hours and if the storage ring is operated at lower energy (for instance to tune the u n d u l a t o r spectra) this effect could be significant. It is i m p o r t a n t to have access to the experimental area at all times, even during injection and machine studies and preferably for non-radiation workers, (to avoid complex administrative procedures in a laboratory with m a n y visitors). The shielding estimates m a d e for the original E S R F [15] will need careful re-examination on this basis. Unfortunately, for a synchrotron radiation source with a very large radius such as the PSRF, the shielding adds considerably to the m i n i m u m distance from the tangent point to the nearest experimental station. One possible layout of b e a m lines and experiments is shown in fig. 8. The shield wall has been omitted for clarity but the beam lines are approximately 25 m from the tangent points at the exits from the wall.

7. Parameter list for the P S R F 7.1 Machine parameters

Energy Circumference Superperiods Dipole magnets Field Bending radius Number of quadrupoles

5 GeV 905.6 m 8 128 0.37 T 44.8 m 304

D.J. Thompson

Number of q = 0 straights 6.4 m long Number of q = 0 straights 3 m long Number of TJ* 0 straights 2.5 m long Number of sextupoles Horizontal betatron wavenumber Vertical betatron wavenumber Momentum compaction Horizontal beam emittance R.m.s. energy spread Logitudinal damping time constant R.f. frequency Harmonic number Energy loss per turn Peak r.f. voltage (100 h quantum lifetime) Energy acceptance Horizontal chromaticity Vertical chromaticity Horizontal p at undulators Horizontal p at r) = 0 wiggler Horizontal fl maximum Vertical p maximum Nominal beam current

8 for undulators, and injection 24 for wigglers

/ Op~imisa~ion of a SR source

r.f.

Angular beam width Experiments per port

2 mrad

Total flux per port at X,

6.4 X 1014 photons/s/O.

32 for sextupoles wigglers 128 34.54

Total flux per port at 1.5 A Spectral brilliance at 1.5 .A

and

Number

xc

500 MHz 1520 1240 keV (no wigglers on) 1550 kV (no wrigglers on) 4.1 x10-3 - 93.4 -31.5 23.8 m 0.6 m 21.3 m 19.3 m 343 mA

Angular beam wdth ‘Typical experiments per port Total flux per port at X,

24

W,)

Number of ports Total number of mrads

I

Number

6.4 x 10” photons/s/O. AA/i 8.5 x lOI photons/s/O. AA/i 1.8 x IO” flux/mm2 mrad’

1.5A Spectral brilliance 1.5 A

at

Number

Angular beam width Typical experiments per port Total flux per port at X,

I%

Total flux per port at 1.5 A Spectral brilliance at 1.5 A c) 9 f 0 mullipole wiggler ports Number A,

(

W,)

say 18 (actually rA

34 - W,) I

2.7 x 1014 photons/s/O. AX/A 2. I x 10 I4 photons/s/O. JX/X 1.6~ lOi flux/mm2/ mrad’

IR I%

34 - BM)

Total flux at 1.5 A from all ports except undulators

6 4 x 10 I4 photons/s 2.4 X IO’s flux/mm2/ mrad’

64 256(or more, depending on the model) 86 (or more, depending on the model) 4.1 X IO” photons/s/O.l% Ah/h

The material presented herein, though entirely the responsibility of the author, represents the work of the Machine Sub-Group and grateful acknowledgement is made. In particular, figs. 3 and 4 are taken from ref. 5, which is a publication of part of the work of the Sub-Group. I must thank Dr. P.J. Duke for the data for fig. 5 and Professor B. Buras for the original version of fig. 8.

say6 (actually 24 - W,,) 0.25 A (but could be as short as 0.12 k) 12 mrad 3 3.3 x IO I4 photons/s/O. Ah/X 4.6 x 10I4 photons/s/O. AX/X 9.4 X lOI flux/mm2/ mrad’

of experiments

1%

h) IJ = 0 wavelength shifrer wigglers (IV’)

A,

I%

e) Undulator ports (U)

f) 7btoi

Total flux per port at

say 10 (actually 2A 10 mrad 2

Total flux per port at 1.5 A Spectral brilliance at 1.5 A

a) 7 = 0 muttipofe wrggler ports (W(j) say I8 (actually IA 2 mrad

8.5 X lOI photons/s/O. AX/h 1.3X 10’h flux/mm2/ mrad2

d) Bendmg magnet ports (BM)

21.90 5.42 x IO- 4 8X 10e9 m.rad 6.4X 1o--4 12.2 ms

7.2 Beam port parameters

Number

I%

AX/h

Number Approx. flux per port at 1 A in 0.1% bandwidth Approx. spectral brilliance at 1 A

A, Angular beam width Experiments per port Total fiux per port at X,

I

I%

1%

References

[I] R. Caisson. Optics Commun.

22 (1977) 135. [2] V.G. Bagrov, M.B. Moiseev, M.M. Nikitin and N.I. Fedosov, Nucl. Instr. and Meth. 195 (1982) 569. (31 V.P. Suller, Daresbury Laboratory Internal Report, DL,‘TM 118, ( 1973). [4] N. Marks et al., these Proceedings, p. 97; G.N. Greaves et al., these Proceedings, p. 139. I. SR SOURCES/FACILITIES/BEAM

LINES

10

D.J. Thompson / Optimisation of a SR source

[5] R. Coisson, S. Guiducci and M.A. Preger, Nucl. Instr. and Meth. 201 (1982) 3. [6] European Synchrotron Radiation Facility - Supplement II - The Machine, eds., D.J. Thompson and M.W. Poole (ESF, Strasbourg, 1979) Ch. IV. [7] M.W. Poole et al., these Proceedings, p. 143. [8] European Synchrotron Radiation Facility - The Feasibility Study, ed. Y. Farge (ESF, Strasbourg, 1979). [9] The ESRF Machine Sub-Group, D.J. Thompson et al. IEEE Trans. Nucl. Sci. NS-28 (1981) 3153.

[10] European Synchrotron Radiation Facility - Supplement Ill - Instrumentation, eds., B. Buras and G.V. Mart (ESF, Strasbourg, 1979)p. 15. [11] A. van Steenbergen, Nucl. Instr. and Meth. 177 (1980) 53. [12] Ref. 6, Ch. II. [13] D.J. Thompson, Nucl. Instr. and Meth. 177 (1980) 43. [14] Ref. 6, Ch. X. [15] Ibid, Ch. XIV.