Nuclear Instruments and Methods in Physics Research A266 (1988) 13-15 13 North-Holland, Amsterdam

S S R L D E V E L O P M E N T - R E C E N T A N D P L A N N E D

A r t h u r B I E N E N S T O C K

Stanford Synchrotron Radiation Laboratory, P.O. Box 4349, Stanford, California 94305, USA

Recent developments at the Stanford Synchrotron Radiation Laboratory include the addition of six VUV/soft X-ray branch lines, including the multiundulator line and a lithography line as well as three X-ray insertion device lines, of which two are undulator/wiggler lines on PEP. A new 3-GeV booster synchrotron injector for SPEAR is anticipated, which will also allow routine operation in SPEAR's low emittance (127 nm rad) mode through the addition of a third kicker. PEP has been operated in a low emittance ( < 12 nm rad at 8 GeV) mode. Methods for reducing that further to 5-6 nm rad at 8 GeV with no hardware changes and to 0.7 nm tad at 6 GeV with hardware additions have been proposed. Implementation of the former is anticipated during the fiscal year 1988.

I. Introduction

During the past two years, there have been striking developments on both the SPEAR and PEP storage rings. While these constitute major advances in them- selves, making possible significant new research, they are also important as harbingers of even more dramatic improvements anticipated for both rings.

This paper begins with a review of the V U V / s o f t X-ray and X-ray beam and branch fines which have been introduced at SSRL during the past two years. A description of anticipated improvements of SPEAR fol- lows. The paper continues with a description of tests performed thus far on PEP, which indicate that it can perform now at the level expected for third generation rings presently being planned. Finally, ideas for modify- ing PEP so that it can perform as a diffraction limited source (i.e., a fourth generation synchrotron radiation source) at X-ray wavelengths are presented.

2. VUV and soft X-ray branch lines

Relative to 1985, SSRL's capabilities in the VUV and soft X-ray spectral regions will have more than doubled by the end of 1987. The new facilities are shown in table 1 and are described in more detail by Waldhauer [1] in these Proceedings.

The most novel of these is the mult iundulator on Beam Line V, which has been implemented and used effectively for experimentation. This device allows an experimenter to use any one of four undulators, so that the spectral region from 10 to 1200 eV is covered by extremely bright radiation. The change from one undu- lator to another can be made without interrupting SPEAR operation, so that the entire spectral region is

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Table 1 VUV/soft X-ray beam and branch fines under commissioning, installation or construction and expected to be operational by the end of 1987

Beam line Monochromator Spectral region (eV)

1-2 Toroidal grating 8- 170 III Seya-Namioka (18 °) 4- 50 III Multilayer (lithography) 2-3000 V Locust 10-1200 VIII Toroidal grating 8- 170 VIII Spherical grating 50-1000

available to the experimenter. The multiundulator and the L O C U S T monochromator , which is presently being assembled, are described in the paper by Bachrach [2].

3. X-ray insertion device branch lines

In the summer of 1985, SSRL had seven X-ray experimental stations on three wiggler beam lines. The 8-pole electromagnetic wigglers on Beam Lines IV and VII i l luminated three branch lines each, while the 54- pole permanent magnet wiggler on Beam Line VI il- himinated one. The new insertion device X-ray fines are shown in table 2.

Beam Line X, a University of California, D O E Na- tional Laboratories and SSRL collaboration, will pro- vide the second multipole, permanent magnet wiggler of the Beam Line VI type, but with 31, rather than 54, poles. Under LLNL's leadership, the beam transport and X-ray end station are expected to be ready for commissioning by the end of 1987. This will be a most welcome addition, since demand for Beam Line VI has been far in excess of time available, and would be, even

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14 A. Bienenstock / SSRL development

Table 2 X-ray insertion device beam lines developed since 1985

Beam lines Insertion device Status

X 31-pole wiggler Under construction (SPEAR) Permanent magnet completion in 1987

PBL5B 26-period undulator Completed (PEP) 11.5-22.5 keV

PBL1B 26-period undulator Under construction (PEP) 11.5-22.5 keV completion in 1987

if SPEAR were fully dedicated to synchrotron radiation production.

The first X-ray undulator beam line, PBL5B, was commissioned on PEP in the autumn of 1985 [3]. It provides over an order of magnitude greater brightness than any other X-ray beam line in the world. The effectiveness of this brightness for new experimentation was demonstrated in a 3-week period early in 1986, when two experiments were performed. One involved inelastic X-ray scattering, including 40 meV monochro- matization of the incident beam and 40 meV resolution analysis of the scattered beam. The second involved grazing-incidence-scattering structural analysis of amorphous films as thin as 100 ~,. As indicated in table 2, a second such beam line, PBLIB, is close to comple- tion.

Each of these beam lines contains a 2-m undulator with 26 periods and a wavelength of 7.72 cm. These yield spectra with fundamentals ranging between 11.5 and 22.5 keV with PEP operating at 14.5 GeV. Under normal parasitic operating conditions, including 15 mA beam current, the brightness ranges between 8 × 1014 and 3 × 1014 photons/(s-0.1% bandwidth-mm2-mrad 2) over this spectral region.

In series with each undulator is a 2-m, 3-pole wiggler magnet which can provide synchrotron radiation with a critical energy as high as 42 keV when the ring is run at 14.5 GeV in the normal parasitic mode. This critical energy is limited by heating of beam line components. We anticipate starting Compton scattering measure- ments with these insertions to assess their effectiveness for such experiments. Initial calculations indicate that major advances in this field should become possible, with the increased flux yielding increases in momentum space resolution which are very much needed to test theoretical predictions.

4. Anticipated SPEAR improvements

By the end of 1987, SPEAR will have five insertion device beam lines containing eight wigglers and undu- lators. It has space for at least another ten insertion device beam lines. In this sense, it is the most modem

and advanced of the existing storage rings presently utilized in a dedicated mode for X-ray synchrotron radiation. Two shortcomings, however, limit its use- fulness: its relatively large emittance (440 nm rad) and the dependence on the SLAC linear accelerator. Over the next few years, both of these shortcomings should be eliminated.

The President's fiscal year 1988 budget contains funding for a new injector which will allow SPEAR to function independently of the SLAC linear accelerator. The injector will allow more running of SPEAR in the synchrotron radiation dedicated mode, and will eliminate interference between such dedicated operation and operation of the SLAC linear colhder.

The injector will consist of a 3-GeV booster synchro- tron which, in turn, is injected into by a 150-MeV linear accelerator. Hence, "at-energy" injection will be achieved, eliminating the need for time-consuming ramping and permitting the storage of higher currents.

SSRL also expects to install a third kicker in SPEAR as part of this project. This kicker will make it practical to run with low emittance (127 nm rad) on a regular basis. The resulting SPEAR emittance will be close to that (110 nm rad) of the NSLS X-ray ring.

With these improvements, further development of SPEAR as an insertion device storage ring becomes most appropriate. SSRL has proposed a soft X-ray undulator beam line and an X-ray wiggler beam hne for construction in future years. Also proposed are additions to the two beam line buildings which surround SPEAR. Construction of these buildings would make space available for these two beam lines plus some PRT beam lines.

5. PEP low emittance operation

In March 1986, seven shifts of machine physics were made available to SSRL. During the last three of these, a low emittance mode of operation was implemented at 8 GeV. In that mode, an upper bound to the beam emittance of 12 nm rad was measured. This corresponds to 9.2 nm rad at 7 GeV, which is slightly greater than that expected for the Advanced Photon Source at 7 GeV. It should be pointed out, however, that only an upper bound to the emittance was measured. Since the calculated emittance was 8 nm rad at 8 GeV, the actual emittance could be closer to that value.

Although storage of only microamperes was antic- ipated in this short period in the new lattice, 4 mA were, in fact, achieved during 16 h of trials. At this current, the lifetime was too long to be measurable, but certainly greater than 20 h. This is consistent with calculations which indicate that the dynamic aperture in this lattice is equal to or greater than the physical aperture and, therefore, quite large. All of this is in keeping with the

A. Bienenstock / SSRL development 15

expectation that this particular lattice will be quite tolerant.

The achievement of this emittance means that PEP may be utilized as a forerunner for the Advanced Pho- ton Source and the European Synchrotron Radiation Facility, providing experience with beam lines and experimentation that will allow rapid acceleration of those programs. As discussed below, it is our expecta- tion that the emittance may be reduced by approxi- mately a factor of 2 with no hardware changes and by approximately an order of magnitude with significant hardware changes. Thus, running PEP in a dedicated mode at low emittance should provide those planning both facilities with information about both machine and beam line problems which may be avoidable with care- ful design.

In the low emittance mode achieved thus far, the calculated brightness of the two PEP beam lines is 8.2 × 1016 photons/(s-0.1% bandwidth-mm2-mrad 2) at 3.7 keV, with a stored current of 4 mA. An expected increase in current by an order of magnitude will, of course, increase the brightness by the same amount. Appreciable brightness increases will also be obtained with the use of insertion devices optimized for 7-8 GeV operation.

Based on experimentation performed thus far and theoretical estimates, we anticipate that approximately 50 mA could be stored in this mode with the present vacuum system. It is possible, however, to double the number of pumps around the ring at a modest cost, which should increase the possible stored current.

6. PEP's future potential - Diffraction limited X-ray source

Finally, Wiedemann [4] has proposed that PEP be transformed into a fourth generation storage ring with an emittance as low as 0.6 nm rad at 6 GeV, providing extreme brightness (102°-10 21 photons/(s-0.1% band- width-mm2-mrad 2) at 100 mA) and significant coherent power at wavelengths extending to a few angstroms. PEP would probably then remain the most advanced X-ray synchrotron radiation storage ring in the world throughout this century, and would provide guidance for the construction of more advanced rings in the 21st.

The emittance reduction would be achieved through the installation of damping wigglers. Fig. 1 shows the emittance reduction which could be achieved through use of various lengths of readily achieved permanent magnet wigglers.

A beauty of this approach is that the low emittance, high brightnesses and short-wavelength coherent powers

q

w

u m

58 188 150

Total L e n g t h of D a m p i n g Wigg le r ( m )

Fig. 1. Calculated PEP electron beam emittance as a function of the total length of added damping wigglers.

could be achieved in an evolutionary fashion without major individual expenditures. As a result, it would be possible to determine whether significant limitations to this approach exits. At the same time, corresponding advances in beam control and beam line design could proceed. These wigglers would also be used as synchro- tron radiation sources for experimentation. Hence, SSRL has proposed construction and installation of the first damping wiggler, starting in the fiscal year 1989.

It is apparent that this goal of producing useable coherent X-rays is fraught with potential difficulties. Nevertheless, one may anticipate major scientific ad- vances if it can be achieved. Thus, the coming decade is expected to be one of considerable excitement for SSRL.

Acknowledgement

SSRL is supported by the Department of Energy, Office of Basic Energy Sciences; and the National In- stitutes of Heatlh, Biotechnology Resource Program, Division of Research Resources.

References

[1] A. Waldhauer, these Proceedings (5th Nat. Conf. on Syn- chrotron Radiation Instrumentation, Univ. of Wisconsin- Madison, 1987) Nucl. Instr. and Meth. A266 (1988) 16.

[2] R. Bachrach et al., ibid., p. 83. [3] G. Brown, Nucl. Instr. and Meth. A246 (1986) 147. [4] H. Wiedemann, ibid., p. 24.

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