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Page last updated
March 28, 2003

Keeping the ALS at the Cutting Edge

Neville V. Smith and Michael C. Martin,
Lawrence Berkeley National Laboratory

1. Introduction

The Advanced Light Source (ALS) is a third-generation source that has been operating for almost a decade and is generating forefront science over a broad area. However, the ALS was one of the first third-generation machines to be designed, and its performance will be outstripped by the later, more advanced machines, such as the Swiss Light Source. Accelerator and insertion device technology have changed significantly since the conception of the ALS, and in order to remain competitive in the core areas of high-resolution spectroscopy, high-spatial-resolution soft x-ray microscopy, and experiments that exploit coherence, we must enhance the performance of the ALS to meet and then set the new standard. We offer here a very cost-effective scenario ($62M) for the ALS that will maintain us at the cutting edge for two to three decades.

First, let us dispel the notion that the "fourth generation" will supplant the third generation. The very exciting short-pulse machines presently under consideration represent a parallel and complementary development. There will be some overlap of the respective user communities, but not a lot. The term "fourth generation" is therefore misleading. There will be a continuing enormous demand for sources of the third-generation kind.

We envision, at Berkeley, a Light Source Campus comprising LUX (Linac-based Ultrafast X-ray source), an upgraded ALS ($42M), and a small new ring, CIRCE (Coherent InfraRed CEnter), dedicated to the production of coherent infrared and terahertz radiation ($20M). Integral to the complex will be our new User Building and the new User Housing Facility. Proximity to the Molecular Foundry and NCEM (National Center for Electron Microscopy) will facilitate advances in nanoscience and nanotechnology.

The case for LUX is presented elsewhere. We focus here on the case for the upgraded ALS and CIRCE. There are no significant technological obstacles, and so implementation could begin as soon as funds are made available. Moreover, there would be no major disruption since the upgrade could be accomplished in a phased sequence of short (six-week) shutdowns.

2. ALS Upgrade

Exploitation of the high brightness of a third-generation source translates into three areas: (1) high resolving power for spectroscopy; (2) high spatial resolution for microscopy and spectromicroscopy; and (3) high coherence for experiments such as speckle.  Here we explore the status of each and what it would take to make the next leap.  Briefly, we propose to go to full-energy injection and higher current for a cost of $8.2M, and to replace five obsolescent insertion devices (IDs) with nine state-of-the art IDs and four new beamlines at a cost of $33.5M.
 

2.1 Science pushing the limits

2.1.1 High resolving power applications.

A scientific area that has benefited enormously from the availability of high-brightness sources is the physics of complex materials with the use of high-energy and high-momentum-resolution photoemission. Improvement in the energy resolution from 50 meV to 10 meV has enabled investigation of low-energy excitations, such as the dispersion "kink" and bilayer splitting in the high-T_c superconductors. Five photoemission papers have made it to the "ten most cited physics papers". Further improvement in resolution down to the meV range will provide even sharper experimental incisiveness for the understanding of complex materials ranging from strongly correlated electron systems and magnetic materials to systems with reduced dimensionality.

Fig 2.1: "Kink" or self energy effects in the near-E_F dispersion of high-T_c superconductors. (a,b) Nodal data from Lanzara et al [1] indicating electron-phonon coupling . (c,d) (p,0) data from Gromko et al. [2] indicating coupling to the antiferromagnetic resonance mode observed in neutron scattering.

A complementary but more demanding spectroscopy is inelastic x-ray scattering (IXS). Since this is a photon-in/photon-out technique, it has the advantage over photoemission of greater penetration and the ability to look at bulk properties and buried interfaces. IXS is the only direct probe of charge-charge correlations having momentum resolution. Moreover, it is possible, unlike in photoemission, to apply a magnetic field to the sample. The disadvantage of IXS is its inherently low cross-section, so that the state-of-the-art resolution is 250 meV. We have dedicated our last remaining straight section at the ALS to a new beamline for IXS that will reach a resolution of 10 meV. This will occur only below a photon energy of 100 eV, so that only momentum transfers close to the Brillouin-zone center (q=0) can be investigated. To reach the zone edge it would be necessary to work closer to 1000 eV. This can in principle be done, but only after the upgrade described below.
 

2.1.2 High spatial resolution applications.

Understanding fundamental magnetic interactions is at the frontier of solid state physics, but is also driven by the technology of thin-film magnetic devices. At the ALS we have pioneered the development of new techniques [3], one example being the elucidation of exchange bias in the coupling of antiferromagnetic (AF) to ferromagnetic (FM) materials that lies at the heart of magnetic spin valves. This problem could only be solved with our new ability to image AF and FM domain structures at high spatial resolution, with interfacial sensitivity and elemental and chemical selectivity, using photoemission electron microscopy (PEEM). These techniques are revolutionizing our understanding of thin-film magnetism [4-8], and we are hard at work on PEEM3, the next-generation microscope.

Manipulation of spin directly by optical or electrical means leads to the concept of 'spintronics' [9]. The fundamental lateral length scale now shrinks to the magnetic exchange length of nanometers with a temporal scale set by the magnetization precession time of picoseconds. On the horizon are devices such as spin transistors, spin-transfer devices, or even spin-based quantum computers. Together with complementary laser probes, time-resolved soft x-ray probes will lead to major advances.

Fig. 2.2: X-ray magnetic circular dichroism and x-ray magnetic linear dichroism shows interfacial magnetic structure of ferromagnets and antiferromagnets on a nanometer scale using the ALS PEEM2 microscope

A new research topic along these lines would be spin transfer by a localized current from a ferromagnetic layer into another ferromagnetic layer. This promises to be an extremely fast way of manipulating magnetization. This fast manipulation is a result of the very strong interaction between spin-polarized electrons over very small distances, in comparison to the long-range Oersted field switching presently used. The mechanism of the reversal process, in particular the transfer of momentum from the polarized conduction electrons to the exchange-split states in the layer to be switched, is not well understood. The essentials for such work are (1) high spatial resolution due to the small-scale features needed to generate the enormous current densities required; (2) elemental specificity in order to distinguish different materials in a layered structure; (3) time resolution of a fraction of the precessional frequency; and (4) high magnetic interface sensitivity. Scanning Transmission X-ray Microscopy (STXM) is the most suitable probe as it has the capability for the highest flux density at the required resolution. Magnetic linear dichroism would directly determine the angular width of the precessional cone, and in a pump-probe style experiment the dynamics of the reversal would be visible. This, briefly, is an example of a class of exciting and ambitious new experiments in magnetism. The experiments are brightness limited and are at the edge of viability at the ALS today. They would become practical with the enhancements envisioned for this upgrade.
 

2.1.3 Coherence applications.

The high brightness of an optimized third-generation source translates directly into high average coherent x-ray flux, roughly the same per second as an x-ray FEL per pulse.  While the very high peak coherent flux of an FEL is useful for some experiments, the quasi-dc nature of synchrotron radiation is essential for many others. The diffraction of a coherent x-ray beam from a sample has information at the spatial scale of the wavelength, and over a lateral dimension of the coherence width. The information encoded in this speckle pattern can be used in numerous ways, two of which are illustrated below.

Figure 2-3. Upper, soft x-ray coherent diffraction pattern from 50 nm gold balls; lower, SEM image (left) and the reconstruction (right); Spence et al. PRL

Zone plate-based x-ray microscopes can now achieve spatial resolutions down to 18 nm at soft x-ray energies, and with improvements in fabrication, some further progress can be expected. However, for the imaging of 3D objects, the thickness of the object places severe restrictions on resolution, as the depth of field scales as the square of the resolution. For many applications such as cellular biology, materials engineering, in-situ study of materials in reactive conditions, and nanotechnology, there is a pressing need to find a technique that can look at thick objects at nanometer resolution. TEM can be used to examine the 3D structure of materials by single-particle or tomographic techniques, but there are severe restrictions on thickness and, of course, sample environment. It has recently been found that coherent diffraction patterns can be reconstructed back into real space without prior knowledge of phase information. The key is that the transform of a non-periodic object is continuous in Fourier space and can be 'oversampled'. Electron-density positivity and the sample boundary are then sufficient constraints for convergence on a unique phase set and hence an image [10]. We now have the prospect of being able to image thick objects at nanometer resolution in liquids or reaction conditions. This will give us a revolutionary new tool for application in cellular biology, materials and nanoscience, and many other areas. The key is high quasi-dc coherent flux, and the upgrade proposed here is key in providing much higher coherent flux at the optimum energies at the higher end of the soft x-ray range (2-4 keV).

The interplay between spin, charge, lattice, and orbital degrees of freedom produces very complex phase diagrams and nanoscale phase behaviors in transition metal oxides. These microphases are thought to play a key role in the properties of high-temperature superconductivity (HTSC) in the cuprates and colossal magnetoresistance (CMR) in the manganites. For example, the equilibrium configuration of doped manganites is often a heterogeneous mix of phases on a length scale of tens of nanometers. Various models suggest that these nanoscale domains play a key role in CMR. Another example of microphases observed in transition metal oxides is offered by the stripe phases. Charge separation occurs when the holes doped into these oxide materials segregate into one-dimensional stripes separated by insulating antiferromagnetic regions a few lattice constants wide. Length scales of the stripes themselves are 1-2 nm, and those of the domains are 20-30 nm. Again, these length scales map directly to soft x-ray probe wavelengths. The role of the stripe phases is a forefront topic in solid state physics. While they have been observed by neutron and x-ray scattering, coherent soft x-ray scattering offers important advantages such as high coherent flux, excellent chemical contrast, an adequate scattering wave vector to probe nanoscale features, and access to core levels of interest, such as oxygen and the transition metals. These advantages should enable a much more complete characterization of these materials, including their response to external control parameters such as field, temperature, and current as well as their related dynamical properties.
 

2.2 The top-off panacea, increased brightness and advanced undulators

Machine performance of the ALS is lifetime limited.  Substantial improvements in brightness and current have always been quite feasible, but they incur the penalty of a much reduced lifetime totally unacceptable to our users.  In top-off mode, injection would be quasi-continuous, and so the lifetime impediment disappears.

At present the ALS is operated in a mode where beam is injected three times daily to 400 mA. In the eight hours between fills, the beam decays to 200 mA with a time-averaged current of about 250 mA. With an upgrade of the RF system it is possible to increase the current in the machine to 750 mA. In terms of raw flux, top-off would therefore increase our capacity to the equivalent of three ALSs, which must surely rank as a dramatic example of cost effectiveness. More important than capacity, however, is enhanced capability. The proposed upgrades will enable the newer and more revolutionary experiments briefly outlined above.

The ALS has five long insertion devices (IDs), which were designed and installed about a decade ago. In the meantime, undulator technology has undergone significant advances. The currently favored design is the two-meter elliptically polarizing undulator (EPU). Of these, we have two installed, one under construction, and one proposed. However, ID technology continues to advance, with small-gap short-period in-vacuum undulators and superconducting undulators emerging as the IDs of choice. For the ALS, the "workhorse" is a 4.45-m-long, 50-mm-period linearly polarizing undulator. Such a device requires one storage-ring straight section. Significant brightness improvements can be realized in the core soft x-ray region by going to top-off operation with higher average current, reducing the vertical emittance and beta function, and installing small-gap permanent-magnet or superconducting insertion devices. In Table 2.3 we compare the brightness of the workhorse using the present storage-ring parameters with three other devices using the upgraded ring parameters. The new devices all have a magnetic gap of 5 mm. The first two are 28-mm-period devices of lengths 4.45 m (full length) and 2.0 m (half length), respectively. These two devices would be possible with hybrid permanent magnets [11,12]. The third is a 14-mm-period 1.5-m-long (half length) superconducting device. The parameters of this device are similar to that which is being constructed by ACCEL[13].

Table 2.3 Comparison of present and future storage-ring parameters and present and future brightness

The figure below compares the brightness of the "workhorse" using the present machine parameters with candidate new undulators using the future machine parameters.

We see that there is one to two orders of magnitude improvement in brightness that can be had in the soft x-ray range. It should also be noted that the proposed upgrades extend the high-energy range of the undulator radiation, which is currently limited at 2000 eV. The devices that were chosen are devices that can be built now and have the promise to provide substantially higher performance.

One important aspect of the upgrade is to take some full-length linearly polarized undulators and replace them with chicaned straights containing two shorter, more advanced devices. With the increased flux and brightness available, these devices will perform far better than the current devices, but at the same time allow simultaneous operation of two fully optimized independent beamlines. The ALS, unlike the larger x-ray machines, has a severely limited number of straights. This has meant that several world-class programs that would normally command one or more IDs have multiple endstations sharing a single beamline. One example is the combination of the condensed matter physics and atomic and molecular physics programs on BL10.0, each of which is world leading and would merit a beamline of its own. It would be preferable to design optical systems fully optimized to the type of science on each beamline.  Specifically, we intend to replace five workhorse IDs with nine newer, more advanced IDs plus four new beamlines. We therefore propose an orderly upgrade in which obsolescent IDs are progressively replaced by state-of-the-art IDs that are responsive to the scientific drivers, such as those discussed above. All upgrades envisioned can be executed in a phased sequence of short  (six-week) shutdowns.

3. CIRCE: a dedicated coherent infrared ring

For the last two years we have been exploring the virtues of a small ring dedicated to the production of coherent far-infrared and THz radiation and have determined conclusively that such a machine is entirely feasible and will create a true leap forward. Specifically, we have experimentally demonstrated that coherent THz emission produces very high powers [14], we experimentally verified the regime of stability for coherent emission in a storage ring [15], and we have performed the first scientific experiment using coherent synchrotron radiation in a ring measuring for the first time the Josephson plasma frequency in the high-temperature superconductor Bi₂Sr₂CaCu₂O₈ [16]. CIRCE (Coherent InfraRed CEnter) will be a revolutionary source for a traditionally difficult spectral region at the border between optics and electronics, namely the "THz gap."

Figure 3.1. Calculated flux for CIRCE, a dedicated coherent THz ring, compared to other sources.

Synchrotron radiation becomes coherent when the electron bunch length is smaller than the wavelength being emitted [17,18]. In this regime, the radiating fields of individual electrons add in phase, producing an intensity that scales with the square of the number of electrons instead of linearly, as is the case for the more familiar synchrotron emission. The calculated flux for the CIRCE source is compared to other sources in Figure 3.1. The many-orders-of-magnitude increase in far-IR intensity is the basis of our project and enables new kinds of science.
 

3.1 New science with CIRCE

A high peak and average power THz source is critical for driving and measuring novel nonlinear phenomena with excellent signal-to-noise, and for studying ultrafast dynamical properties of materials, both of which are central to future high-speed electronic devices [19,21]. It will also be useful for studies of molecular vibrations and rotations, low-frequency protein motions, phonons, superconductor bandgaps, electronic and magnetic scattering, and collective electronic excitations (e.g., charge-density waves). The absorptive and dispersive properties of materials in the THz and sub-THz spectral range provide contrast for a unique type of imaging [22,23]. The striking improvement in power CIRCE will provide could revolutionize this application by allowing full-field, real-time image capture.
 

3.1.1 Pumping novel states of matter

Tabletop far-IR setups are typically too weak for exciting ("pumping") the sample, while the CIRCE coherent THz synchrotron source can provide pulses with energies of ~100 nJ and corresponding peak electric fields of E ~ 10⁵ V/cm. This, combined with a tunable pulse spacing as fast as 600 ps, enables synchronization with available femtosecond lasers. At the same time, the spectrum exhibits a broad bandwidth in contrast to tunable but spectrally narrow free-electron lasers. The combination of defined-wavelength powerful pump pulses (tunable via spectral filtering) and a broadband probe will make possible novel far-IR non-linear studies previously impossible.

Fig. 3.2. Stimulation of Intrinsic Localized Modes—driven nonlinear localized motions of atoms in a lattice.

Intrinsic localized modes (ILMs) have been predicted and modeled by Sievers [24]. ILMs are driven, nonlinear, high-amplitude motions of atoms within a lattice (Figure 3.2). Unlike solitons, these modes are predicted to remain localized. CIRCE will produce high intensities in a wavelength region spanning important collective excitations in solids and could be used to pump and study these novel ILMs.

Using CIRCE as a pump will also enable the study of surprising photon-induced electronic state transitions such as the very recent observation of zero-resistance states induced by electromagnetic radiation in GaAs/AlGaAs two-dimensional gas quantum well systems [20]. In this work, a high enough intensity of ~100 GHz radiation induced zero-resistance states at 4/5 and 4/9 Bf, where B is the magnetic field and f is the frequency. CIRCE will provide appropriate pumping intensity at yet higher frequencies, meaning that zero resistance can be achieved at only ~100 Gauss, field levels of technological interest.
 

3.1.2 Magnetospectroscopy—Full-spectrum electron spin resonance (ESR)

Traditional ESR is limited to single or a few frequencies, typically in the GHz. Spin resonances are found by scanning the magnetic field and searching for the field at which a specific microwave frequency is absorbed by the sample. By moving to higher magnetic field (many Tesla via superconducting magnets), ESR resonances can be moved up to the THz frequency regime where CIRCE will allow broadband spectroscopy.

Combining the expertise and instrumentation of FTIR spectroscopy, high-field Nb₃Sn superconductor magnet technology, and most importantly a high-intensity broadband THz source, a new kind of ESR is possible: a full-frequency spectrum obtained at once for any magnetic field. This will increase the scientific power and ease of use of ESR as a tool for exploring spins in numerous materials.

Fig. 3.3. Electron spin resonance measurements in LaMnO₃ showing an antiferromagnetic resonance signal below the ordering temperature.
(Mihaly et al., unpublished)
 

Laszlo Mihaly and collaborators recently demonstrated this concept using a conventional synchrotron far-IR source, a commercial FTIR spectrometer, and a 16 T magnet. The test sample was LaMnO₃, a parent compound to colossal magnetoresistance (CMR) materials, which has free electron spins on the Mn sites and large ESR signals. The first demonstration full-spectrum ESR spectroscopy results are presented in Figure 3.3, indicating how resonances can be mapped in the frequency-field phase—something not possible with traditional ESR equipment.

An enticing twist on this idea is to use the high B-field component in the intense CSR electromagnetic pulses from CIRCE directly to induce spin alignment. Combined with coherent time-resolved detection of the emitted spin resonance photons, one can obtain subpicosecond information about spin relaxation processes in a wide variety of intriguing materials from magnetic technology to metalloproteins.
 

3.1.3 Coherent Spectroscopy

The coherent and pulsed nature of the CIRCE light source provides new opportunities for studying complex materials: (i) On ultrashort timescales, fundamental microscopic interactions in condensed matter occur between quasiparticles, phonons, vibrons, spin-excitations, and photons. Understanding the dispersions and quantum-mechanical interactions in a broader class of materials, many of whose excitations are in the infrared, necessitates a new source. Such experiments are of relevance from a viewpoint of basic science but also provide important input for potential applications. (ii) The generation and detection of coherent THz radiation via nonlinear optical mixing was revolutionized in recent years [23]. Since the shape of the electric field is retained coherently pulse-to-pulse in CIRCE, synchronization with optical femtosecond pulses allows one to measure the electric field E(t) via Electro-optic (EO) sampling. Both phase and amplitude—and thus the complex optical constants e(w) = e₁(w) + ie₂(w) of the material—are then obtained without any need for error-prone Kramers-Kronig transformations. The possibility to determine the complex conductivity directly strongly improves the quality of the data and allows one to derive detailed conclusions.

Time-resolved studies allow one to excite a specific initial nonequilibrium condition and to subsequently probe the transient microscopic state of the system. In the past, coherent and ultrafast spectroscopy in the mid-infrared (3-30 µm) has provided fundamental insights into electronic transitions within semiconductor nanostructures, superconductors, vibrational excitations, and structural dynamics of liquids or biomolecules [24]. Ultrafast dynamics in the far-infrared are much less studied primarily due to the sparse availability of suitable pulsed sources. The subpicosecond durations and powerful intensities of coherent THz pulses from CIRCE will be used for new dynamical studies in fundamental research but are also quite useful in applications. For instance, after the successful development of mid-infrared quantum cascade lasers, much research is currently being focused on demonstrating semiconductor THz lasers [25]. For a successful operation, however, the dynamical aspects of these structures need to be better understood experimentally. Ultrafast pump-probe experiments using CIRCE will probe the evolution of carrier populations in these and other novel low-dimensional nanostructures.

Fig. 3.4. Model showing the CIRCE coherent THz ring (purple & red) located on top of the existing ALS booster ring. Space exists for experimental stations (shown in green) just outside the CIRCE shielding.
 

3.2 CIRCE: Ready to build

Stable CSR in a ring was observed for the first time at BESSY II [26] with a special machine setup incompatible with standard user operation. In such a configuration, a gain in flux of about 4 orders of magnitude was obtained. The ALS CIRCE team recently led collaborations with BESSY II and SLAC that have provided a very good understanding of the CSR mechanism. This model allows us to optimize and predict the performance of a reliable coherent THz source, CIRCE, which will have enormous flux gains over present sources (see Figure 3.1). The key effect is the interaction of the electron beam with its own radiation. The benefit of this interaction is a self-focusing of the beam resulting in a stable distortion of the bunch distribution. This distortion gives a sharp edge to the bunch that emits coherently at significantly shorter wavelengths and shorter pulse lengths than an undistorted Gaussian bunch. At higher currents, this self-interaction can result in amplification of small modulations in the bunch, causing quasi-chaotic bursts of CSR in a process very similar to that of self-amplified spontaneous emission (SASE). Both of these regimes have been experimentally demonstrated [15,26], and CIRCE is optimized to produce CSR in the stable region of emission.

Most synchrotron light sources require a large floor space outside the main ring to accommodate long x-ray beamlines. However, IR beamlines require relatively little space and prefer to be located as close to the source as possible. Given the layout of the ALS facility, the ideal location for an IR ring is on top of the booster shielding, as shown in Fig. 3.4. A detailed preliminary design and evaluation has been done resulting in a 66-m-circumference ring that fits on the existing booster shielding. Installation and commission of the CIRCE ring will have minimal impact on the operation of the ALS storage ring. Full-energy injection to the ring can be done from the ALS booster without interfering with injection to the ALS main ring, even under continuous top-off operation. The use of the ALS injector as well as existing ALS utilities and general infrastructure allows one to design an extremely cost-effective project. The optimized electron beam energy is around 600 MeV using a 1.5 GHz radio-frequency (rf) system. Nominal bunch lengths of 1-2 psec can be achieved using a combination of high-frequency rf and a modest reduction of the lattice momentum compaction. The optimization of CIRCE for the THz region includes enhanced photon beam stability and very-large-aperture vacuum chambers (140 mrad vertical by 300 mrad horizontal) for the best possible acceptance of the large-divergence coherent THz synchrotron radiation. Our study has shown that there are no technical reasons why such a source cannot be built, and we are ready to commence with a full conceptual design upon receiving funding approval.

 4. Costs, schedule, scope and technical management

Costs have been estimated for these projects based upon recent feasibility studies and budgetary estimates. All costs are escalated at the latest DOE Field Management rates, and include all overheads and contingency. The projects will be managed by the ALS Division, with independent reviews, project milestones, risk control, and change control procedures implemented. All applicable rules will be followed in the design and construction of the facility.

Schedule of Project Funding (Dollars in thousands)

^aIncludes commissioning and startup.

5. References and links

We have created a special web site at http://www-als.lbl.gov/als/20. We trust that this site and the links therein will provide the committee with the references and additional information that it needs.

References