The Linac Coherent Light Source (LCLS) is a revolutionary new machine for the production of hard x-rays. The x-rays are emitted in the form of a laser beam, with a brightness that is 10 billion times greater than that of any existing x-ray source on earth. X-rays are already our most widely used and essential tool for studying and understanding the arrangement of atoms in materials such as metals, semiconductors, ceramics, polymers, catalysts, and plastics, and in biological molecules. The structural knowledge obtained with x-rays holds the key to understanding the properties of matter such as mechanical strength, magnetism, transport of electrical currents and light, energy storage, and catalysis. Likewise, in biology much of what we know about structure and function on a molecular level comes from x-ray studies. Such knowledge forms the basis for the development of new materials and molecules and the enhancement of their properties, which in turn will advance technology, fuel our economy, and improve our quality of life. LCLS will bring a completely new dimension to the use of x-rays to study matter through its unique properties never before available.

In general, the shorter the wavelength of the light used, the smaller the sample one can see with it. Visible light has a wavelength that is far too large to resolve single molecules. (When a large ocean wave rolls over a small stone the waves are not perturbed enough to detect that the stone is there. However, the same small stone can have a large effect on small ripples.) To see an atom, a very short wavelength is needed. Today, scientists often use the x-rays produced by a synchrotron light source to study how their atomic structures affect the properties of materials (for example, ferromagnetic nanostructures in electronic devices, the binding of contaminants to soil particles, and the active sites of biomolecules that bind drugs). But the synchrotron light sources cannot produce ultra-short pulses, so they cannot resolve the ultra-fast motions of atoms during chemical reactions. LCLS is a revolutionary advance within the synchrotron radiation world, since it produces the x-rays associated with synchrotron light sources, and these x-rays are produced in ultra-short, ultra-intense pulses.

Almost all of the experiments at today's x-ray sources primarily give a "static" picture of materials averaged over relatively long time scales. Basic physical principles tell us that small building blocks of matter like nano-sized man-made structures, molecules, and atoms are able to rapidly change in time, the smaller the faster. In computers, for example, nano-sized bits are written and read today in several billionths of a second. Atomic motions are even faster -- by a factor of about 10,000 (a timescale of less than a trillionth of a second). LCLS will emit pulses that are this fast, a factor of 1000 or more shorter than pulses from existing x-ray sources. In addition, a single pulse will contain enough x-rays to record a single shot picture. It will be possible to record time-resolved images of chemical reactions, even to the point of following the change in the chemical bond as the reaction proceeds. This will give scientists a glimpse on a time scale never before possible, and open untold opportunities for understanding catalysis, chemical processes, and molecular assembly.

Stated differently, LCLS will be able to photograph atomic motion, much as a "strobe" flash is used to photograph the motion of a bullet in flight. This latest advance in stop-action imaging at Stanford has roots going back more than 100 years. Around 1872, Eadweard Muybridge started making stop-motion photographs of people, animals, and trains in motion on Leyland Stanford's farm. He is famous for showing that all four horse's feet leave the ground during a gallop. To be able to click a shutter fast enough to show each stride a horse makes when galloping required tremendous engineering ingenuity. LCLS will provide x-rays of such shortness and precision that stroboscopic experiments can be done with materials on the nanoscale, and even with individual molecules and atoms. In a more technical sense, this will provide the means to directly observe how the fundamental properties of materials change as their constituent atoms move, and how the clouds of electrons that glue atoms together shift and flow in response.

The tremendous brightness of the LCLS x-ray pulse will also be invaluable for imaging the atomic structures of small static objects. Individual single molecules or small clusters of molecules may be able to be imaged. When perfected, this new approach would enable biologists to study the structures of macromolecules that cannot be coaxed into forming the periodic arrays known as crystals (the basis for almost all work today in this area around the world). In fact, many biological macromolecules, including a very important class that occur imbedded in membranes, are very resistant to crystallization and hence their structures are not very accessible with today's methods. Such membrane-bound proteins are very important targets for many of the drugs that are important in treating disease today, and wider access to their structures could bring great benefits to human health.

The extremely high power of the LCLS x-ray pulse can also be used to both create in a controlled fashion and study new states of matter called warm dense plasmas. Such plasmas are thought to be associated with the interiors of planets, cool dense stars, and in plasma reactions initiated from solids. Today's sources are inadequate to study the very important properties of such materials.

LCLS construction and operation will build upon and utilize DOE's extensive knowledge, strengths and experience as the steward of the world's greatest collection of shared, multidisciplinary scientific user facilities. It will also leverage upon core competencies in accelerator science and technology at the collaborating institutions. No other facility on earth will be able to match the scientific power of LCLS as a new x-ray tool for discovery.

	Atomic Physics	^ ^ ^

Atomic physics, the study of the behavior of small groups of atoms and of electrons within atoms, is essentially the study of Quantum Mechanics. Great advances in our understanding of atomic physics have come about through the use of precision tools such as optical lasers, and Quantum Mechanics has proved to be one of the most successful theories of all time. LCLS, an x-ray laser, allows us to study atomic physics in a currently inaccessible regime, thereby extending our understanding and knowledge of Quantum Mechanics. [more technical details]

	Femtochemistry	^ ^ ^

Chemical science is highly evolved; with great precision we can predict the outcome of simple chemical reactions. To expand this capability to much more complicated reactions requires new tools; LCLS is one such tool. The chemical reactions between small molecules are by nature ultra-fast, but the time sequence of these reactions can be captured with the ultra-fast pulses of LCLS. Photosynthesis involves such ultra-fast reactions. A better understanding of photosynthesis (a highly efficient use of the Sun's energy) has implications for future energy sources and for agriculture. [more technical details]

	Nanoscale Dynamics in Condensed Matter Physics	^ ^ ^

Modern technology (electronic devices, computer chips, and liquid crystal displays on watches) uses nanoscale materials. Those materials consist of simple constituents arranged in complex, man-made ways, all on a very tiny scale. Often, what is interesting about these molecules is that they change with time in a useful way. (For example, the molecules in an LCD display change their alignment, dictating which numerals appear on the face.) How those states change and how the change is induced can be better studied with the ultra-fast x-ray pulses from LCLS. As technological devices continue to get smaller and faster, better understanding such nanoscale processes will help to build better technology. [more technical details]

	Structural Studies on Single Particles and Biomolecules	^ ^ ^

Using x-rays to study the atomic structures of biological molecules such as proteins has turned out to be invaluable for understanding their roles in life processes. Often, the atomic structure of a molecule is crucial to its biological activity. Molecules often fit together something like a light bulb in its socket. Only something with the shape of the end of a light bulb can fit in the socket. Nowadays, drug molecules can be created to fit the shape of certain human biological molecules and thereby deliver their effect in a very specific way. This is only possible with the knowledge of the structures of the molecules, knowledge which is most often gleaned from synchrotron radiation science. But the x-ray diffraction process used to study molecular structure has its limitations. The radiation quickly destroys the molecule being studied. Researchers have found one way to work around this destruction: the molecules are formed into crystals (containing many neatly ordered, identical copies of the molecule) so that many molecules are simultaneously examined, thus spreading the radiation damage around. Though this technique has been extremely useful, the crystals are often very difficult to create. With many molecules, it may be impossible. But LCLS offers another way to work around the radiation damage problem: the ultra-short x-ray pulse can give a picture of the molecule nearly instantly, before it is destroyed. Thus LCLS offers the possibility to study the structures of a wide range of molecules and other atomic structures that cannot be examined using conventional techniques. [more technical details]

	Plasma and Warm Dense Matter	^ ^ ^

Conditions inside a proto-star (brown dwarf) or a super planet such as Jupiter involve extremely high pressure and extremely high temperatures. These conditions are beyond anything we can create on Earth now. Much of the matter in the universe is locked in these cosmological bodies. LCLS should offer a way to create similar conditions on a minute scale, allowing us to study these conditions and thus learning more about these important astronomical bodies.[more technical details]

	X-ray Laser Physics	^ ^ ^

The study of the x-rays themselves will form the basis of the future capabilities of the LCLS. [more technical details]