24
P. R. Willmott
facility where they are installed gain little or nothing from the emittance of the storage ring being further improved (i.e. reduced by more sophisticated electron optics).
This is the true meaning of the diffraction limit: the fundamental lower value set by
the photon beam rather than the electron beam.
DLSRs are thus defined by their use of MBAs. They gain most at largecircumference storage rings, at medium storage ring energies and indeed the two
greenfield DLSR facilities to date (MAX-IV and Sirius) plus the first major upgrade
from third to fourth generation (ESRF-EBS) reflect these characteristics. This seemingly obvious and long-understood approach to improve the horizontal emittance via
the use of MBAs and thereby the brilliance has only recently been pursued because,
until now, the costs (both in hardware and real estate) and the introduction of potential mechanical misalignments associated with increasing the number of elements in
the magnet lattice, were considered unacceptable. Miniaturization of these magnet
lattice components and the development of multifunctional magnets machined from
a single yoke block have decreased both costs and the necessary circumference of
the ring to accommodate the MBAs and thus achieve these goals. The PETRA-IV
project is especially interesting, as the circumference is 2.3 km, allowing exceptionally shallow angles for the arcs, promising diffraction-limited photon energies well
into the hard X-ray regime, at approximately 6 keV.
Another serious obstacle to reducing the size and separation of the magnets is
that the lateral dimensions (that is, the cross-section) of the storage ring vacuum
vessels containing the electron beam need to become so small that pumping them with
traditional pumping equipment, in particular, ion-getter pumps, becomes increasingly
difficult. In recent years, however, a novel approach to achieve ultrahigh vacuum
(UHV) conditions has been developed, namely the use of non-evaporable getter
(NEG) coating of the inner walls of the storage ring vacuum vessels [14, 15]. NEGs
are porous alloys of Al, Ti, Fe, V and Zr sintered onto the inner walls of the vacuum
vessel to a thickness of the order of a micrometre, or even as little as 100–200 nm.
After installation, the vacuum vessel is pumped to a moderately high vacuum using
traditional pumps and then heated out to temperatures below 200
◦ C. This activates the
NEG material, allowing pressures to drop to approximately 10
−10 mbar in the UHV
regime. Importantly, even very narrow spaces can be readily coated. Moreover, recent
developments in computer numerical control of machining storage ring components
allow micrometre accuracy such that geometrically near-perfect miniature vacuum
vessels with cross-sections of the order of a square centimetre can be constructed.
With the advent of DLSRs and improvements in magnetic materials, undulator
spectra have undergone important transformations. Figure 1.16 compares the brilliance of the same undulator at a third-generation source and at a DLSR having a
40 times smaller total horizontal emittance. In addition to the expected 40 times
increase in peak brilliance, the DLSR-spectrum is substantially cleaner: the lobes
seen on the low-energy flanks of the spectral peaks for the third-generation source
are completely absent in the DLSR spectrum. This is because the horizontal width
of the electron beam is much smaller (typically by an order of magnitude). In thirdgeneration facilities, the horizontal electron beam width is, at approximately 100 µm,
two orders of magnitude larger than the oscillation amplitude A, which is of the order
P. R. Willmott
facility where they are installed gain little or nothing from the emittance of the storage ring being further improved (i.e. reduced by more sophisticated electron optics).
This is the true meaning of the diffraction limit: the fundamental lower value set by
the photon beam rather than the electron beam.
DLSRs are thus defined by their use of MBAs. They gain most at largecircumference storage rings, at medium storage ring energies and indeed the two
greenfield DLSR facilities to date (MAX-IV and Sirius) plus the first major upgrade
from third to fourth generation (ESRF-EBS) reflect these characteristics. This seemingly obvious and long-understood approach to improve the horizontal emittance via
the use of MBAs and thereby the brilliance has only recently been pursued because,
until now, the costs (both in hardware and real estate) and the introduction of potential mechanical misalignments associated with increasing the number of elements in
the magnet lattice, were considered unacceptable. Miniaturization of these magnet
lattice components and the development of multifunctional magnets machined from
a single yoke block have decreased both costs and the necessary circumference of
the ring to accommodate the MBAs and thus achieve these goals. The PETRA-IV
project is especially interesting, as the circumference is 2.3 km, allowing exceptionally shallow angles for the arcs, promising diffraction-limited photon energies well
into the hard X-ray regime, at approximately 6 keV.
Another serious obstacle to reducing the size and separation of the magnets is
that the lateral dimensions (that is, the cross-section) of the storage ring vacuum
vessels containing the electron beam need to become so small that pumping them with
traditional pumping equipment, in particular, ion-getter pumps, becomes increasingly
difficult. In recent years, however, a novel approach to achieve ultrahigh vacuum
(UHV) conditions has been developed, namely the use of non-evaporable getter
(NEG) coating of the inner walls of the storage ring vacuum vessels [14, 15]. NEGs
are porous alloys of Al, Ti, Fe, V and Zr sintered onto the inner walls of the vacuum
vessel to a thickness of the order of a micrometre, or even as little as 100–200 nm.
After installation, the vacuum vessel is pumped to a moderately high vacuum using
traditional pumps and then heated out to temperatures below 200
◦ C. This activates the
NEG material, allowing pressures to drop to approximately 10
−10 mbar in the UHV
regime. Importantly, even very narrow spaces can be readily coated. Moreover, recent
developments in computer numerical control of machining storage ring components
allow micrometre accuracy such that geometrically near-perfect miniature vacuum
vessels with cross-sections of the order of a square centimetre can be constructed.
With the advent of DLSRs and improvements in magnetic materials, undulator
spectra have undergone important transformations. Figure 1.16 compares the brilliance of the same undulator at a third-generation source and at a DLSR having a
40 times smaller total horizontal emittance. In addition to the expected 40 times
increase in peak brilliance, the DLSR-spectrum is substantially cleaner: the lobes
seen on the low-energy flanks of the spectral peaks for the third-generation source
are completely absent in the DLSR spectrum. This is because the horizontal width
of the electron beam is much smaller (typically by an order of magnitude). In thirdgeneration facilities, the horizontal electron beam width is, at approximately 100 µm,
two orders of magnitude larger than the oscillation amplitude A, which is of the order