Multipole wigglers

MCP experiments were performed at AR-NE1 station of KEK (National Laboratory of High Energy Physics), Japan, using circularly polarized X-rays with the incident X- ray energy of 60 keV emitted from the elliptical multipole wiggler. Figures 1 and 2 show MCPs ofUSe and UTe, which have been measured at 150 and 80 K, respectively. 

Finally we note a proposal for an European Synchrotron Facility (ESRF) with a 5 GeV storage ring which can accomodate about 14-15 multipole wigglers and 15 undulators. 

Multipole Wiggler Variable 0.7-2.5A Ca. 2.5 X 0.6 Dependent on wavelength RAPID ID SAXSAVAXS... 

Multipole wiggler/ Small molecule crystallography Wiggler... 

Wiggler Long beam line (75 m) Topography (possibly second Laue station) Multipole wiggler... 

Elliptical multipole wiggler, producing circularly polarised X-rays. 

The addition of more magnet poles to a wiggler results in a multiplication of the flux generated. One of the first multipole wigglers to be developed was on SPEAR (Winick and Spencer 1980, table 4.1) and had five full poles this was later replaced by a seven full pole device which is detailed in table 4.2. The flux multiplication factor is this number of poles. However, because of the extra magnet length, the source size as viewed off centre line is increased and the intensity enhancement of a focussed beam is somewhat less than the number of poles. 

Other multipole wigglers available, for example, include a 54-pole permanent magnet wiggler on SPEAR (Hoyer 1983) and a 6-pole wiggler on CHESS. There are others at SSRL, Photon Factory and the NSLS and wigglers are planned at ESRF and APS. 

An oscillation camera film data collection facility has been established at Stanford. This station (beam line VI1-1) uses 1 mrad of beam from an eight-pole multipole wiggler (Winick and Spencer 1980). The optics consists of a bent Ge(lll) triangular monochromator followed by a bent metal coated mirror. The small number of available mrad of beam is easily compensated by the number of poles in the wiggler. The station has been used extensively. Examples of structures reported using data collected on this station are given in chapter 10. 

This uses a multipole wiggler and will have operational modes for focussed Laue diffraction work and monochromatic experiments. The small source sizes should allow an equivalently small focal spot from a grazing incidence mirror system. Exposure times in the microsecond range for a macromolecular crystal should be feasible. Depending on the current achieved in single bunch mode it may be possible, at least for smaller unit cell sizes, to record a Laue pattern from one of the single bunches with an intrinsic time resolution therefore of the bunch width. (Feasibility experiments of this kind have been conducted at CHESS but on an undulator (Szebenyi et al 1989).)... 

This is for multiple energy anomalous dispersion (MEAD) data collection. It is proposed that this uses a multipole wiggler of 20 periods, critical energy 9.8 keV and a calculated flux at this energy of 4.5X1014. 

A promising application of multipole wigglers (at 0.5 A) or undulators (i.e. a harmonic at —0.3 A) is to solve the problem of data collection from very radiation sensitive crystals. For example, in virus data collection several hundred crystals are needed for structure determination. Of course, once the basic structure is known, a greatly reduced amount of data and far fewer crystals are needed in drug binding (difference Fourier) studies. 

Test experiments need to be made with these kinds of samples at these wavelengths, i.e. 0.5 A on a multipole wiggler and 0.3 A on an undulator (harmonic). The intensity of these beams will compensate for the increase in exposure time resulting from the X2 effect of the Darwin formula (section 6.1). At 0.5 A the absorption efficiency of an IP is still reasonable (50%) and at 0.33 A (37.5 keV) it is —44%. The Ba K edge at 0.331 A usefully enhances the stopping power of an IP from 19% just above the edge to 44% just below it. By comparison photographic film (Kodak DEF) would only absorb 8% of the photons at 0.33 A. 

Table 6.4. Promising possibilities with very short wavelength multipole wiggler and ultra-short wavelength undulator harmonic radiation for ultra-radiation sensitive samples.

To achieve the short wavelength may require use of a wiggler in the case of a relatively low energy machine. The use of a multipole device also leads to further reductions in exposure time. However, even a multipole wiggler has some undulator character, i.e. has a long fundamental wavelength of emission and so the smoothness of the emitted spectrum needs to be checked in a given case. 

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