| People involved: | Wilfred Fullagar |
| Former members: | Sophie Canton, Jens Uhlig, Monika Walczak, Niklas Gador |
This project is related to the following Fields, Subjects and Techniques:
A strong contemporary interest is the measurement of increasingly small objects on ultrafast timescales. Watching chemistry in action at a molecular level, and on the timescale of fundamental reactions is achievable. This can be done using sub-picosecond pulsed X-ray sources. Large scale electron acceleration facilities play a vital role in the assembly of a wealth of expertise and infrastructure associated with generating and manipulating X-rays. It is possible to produce sub-picosecond X-ray pulses at such facilities using a variety of schemes and for several years there has been increasing pressure for them to do so. However, molecular X-ray structure solution involves procedures that were established in relatively humble and ill-equipped laboratories many decades before the advent of such facilities. In recent years the generation and manipulation of extremely intense, ultrabrief laser light pulses using chirped pulse amplification schemes has become a fully mature technology available to any optical laboratory. These light pulses can be used to generate correspondingly short (and indeed far shorter) X-ray pulses, by a handful of different approaches, to meet a range of experimental requirements. Such environments are indeed typically the source of the materials that are ultimately studied. The construction of laser-based short pulse X-ray sources in laboratory settings is therefore a very natural and inevitable outcome. Particularly in an ultrafast context, it can relieve users of the need to produce competitive beamtime proposals with associated frustrations of delays, costs and considerable unknowns. While large scale electron acceleration facilities will always play a key role in many X-ray developments, there always has been and will be a place for the development of in-house X-ray sources.
Our group has local access to nearby large scale facilities and enjoys many of the associated benefits. In particular we have on-site general access to the MAXlab facility and the user beamlines there, occasional collaborative access to the D611 ultrafast X-ray beamline, ultrafast and high power lasers through the Lund Laser Centre (LLC), involvement in the development of the MAX-IV linac for short X-ray pulse and free electron laser developments, an interest in the potential nearby construction of the European Spallation Source (ESS), and also external synchrotron facilities. We are both laser physicists and chemists! This puts us in a strong position to develop and use existing and novel sources of radiation.
A number of approaches for generating ultrabrief X-ray pulses and beams are being developed or may be viewed as feasible within the Lund Laser Centre and we pursue such local developments vigorously. Through the latter collaboration, we have enjoyed a very high level of access to the low power branch of the LLC's Terawatt Laser. Following several years of earlier development of hard X-ray plasma sources at this facility, and based on perceived needs for sub-picosecond pulses and broadband X-rays in a chemical structure context, we have used this collaboration to develop a novel and very compact laser plasma X-ray source.
Focusing ultrashort laser pulses onto condensed matter can generate essentially isotropic X-rays, these being the emission lines of the target material along with bremsstrahlung. The associated processes have been studied by many groups for some decades. Emission lines can in principle have longer temporal characteristics than bremsstrahlung (illustrated below), since nuclei must reacquire electrons before X-ray fluorescence is possible. Two of the most alluring and informative ultrafast X-ray techniques (Laue crystallography and EXAFS) are fundamentally polychromatic techniques, while other ultrafast X-ray approaches are generally capable of polychromatic adaptation. Emission lines are exploited in certain experiments and particular ways, but in the present context only complicate temporal measurements and add to radiation safety concerns. To eliminate observable emission lines in our apparatus and difficulties associated with handling and debris, we chose to use a simple water jet as target. This would usually be considered a poor choice for bremsstrahlung production, however its general advantages cannot be overlooked. Apart from non-toxicity and universal familiarity, it is an exceedingly versatile solvent, the target surface is exceptionally smooth and X-ray transmission is also good.
The properties of the source were largely explored using a helium gas atmosphere surrounding the jet, since this gave excellent and reliable results with a stable jet and a minimum of experimental difficulty. Cooled degassed water can also be used, drawn in from atmospheric pressure using a water aspirator, to give stable jet operation with considerably higher X-ray flux and temperatures. Grazing p-polarised incidence gives best yields, using a high temporal contrast ~800 nm laser pulse. Optimal yields and highest X-ray temperatures are observed for non-optimally chirped pulses of ~200 fs duration, which along with the interaction geometry indicates a resonant vacuum heating mechanism.
As is immediately apparent from the photos below, considerable emphasis was placed on simplicity, this being recognised as the key to characterisation, duplication, modification, and use.
 One variant of the setup we have often used, here with the lead shielding chamber in place. |
 The lead (shielding) chamber is simply lifted away to give direct access to the target jet and any associated components. |
| Animated version (1.7Mb) |
The following publication has resulted from work on this source: Review of Scientific Instruments. It is a bremsstrahlung source with measured (upper bound) dimensions of ~30x30x20 microns and X-ray pulse duration within the resolution function of an available X-ray streak camera. There are some other reasons to be optimistic that the X-ray pulse duration is comparable to the laser pulse duration. The development is exciting because of its adaptability to lower power, higher repetition rate lasers, general ease of use and the fact that (unlike synchrotrons) samples can be positioned in arbitrary proximity (indeed contact) with the isotropic X-ray source.
From the outset, development of the source was directed towards the observation of ultrafast chemical processes. The properties of the source - in particular its isotropic nature - motivates positioning samples in physical contact with, or otherwise extremely close to the source, with subsequent dispersion of energies in an EXAFS experiment. The manner in which we anticipate using this source and variants adapted to kilohertz repetition rate lasers are outlined in our detector development pages. As we indicate there, we expect to obtain an exceptionally powerful tool for in-house chemical structure studies using the EXAFS technique. Sub picosecond pump-probe EXAFS requires only trivial extensions.
A significant aspect of the characterisation was to establish the temporal characteristics of the laser pulse leading to optimum and stable X-ray production. Deliberate prepulses or accidental temporal impairment of the laser pulse leads to generation of other radiation (in particular high energy electron beams) in addition to the X-rays, along with a change of the optimum interaction geometry for X-ray production. Prepulses or other temporal impairments leads to changes in the density gradient from sharp (short scale length) to diffuse (long scale length) in the exploding plasma. This variable gradient, as well as frustration of the interaction geometry experienced by later temporal features of the laser pulse, allows different laser-plasma interactions to occur. The electron beams are of interest in their own right, while the water jet leads to a very versatile arrangement with which to study them.
To date, associated results have been presented at CAPS meetings, MAXlab User meetings, Nordic Femtochemistry meetings, MAXlas seminars and department seminars. This poster was presented at the Emerging Sources workshop, arranged by MAXlab and the Lund Laser Centre.
An X-ray shadow image of a leaf is shown below, purely for fun! It was taken using a direct-detection CCD array detector placed ~7cm from the source and required several hundred laser shots in a synchronised readout. It should be pointed out that other lab-based sources are better adapted to imaging of this nature, while other laser-based X-ray sources are more appropriate for crystallographic studies, in particular Laue.
General overview and motivation for X-ray based measurements
X-ray absorption spectroscopy
X-ray Diffraction and the argumentation for broad bandwidth
Overview and motivation for ultrafast X-ray measurements
Developments done on ultrafast X-ray sources in Lund (This page)
Developments on X-ray Detectors
Measurements done on ultrafast synchrotron user facilities
Steady state measurements done on synchrotrons
