Poor mans X-ray Free Electron laser for only half a million dollars

Thijs van Oudheusden has developed a machine that in many respects can compete with billion-euro X-Ray free electron laser facilities, based on ideas from his co supervisor Jom Luiten. The essence of their ‘poor man’s X-FEL’ is that it uses electrons instead of X-rays. “Why convert electrons into X-rays if you can use the electrons themselves?”, asks Van Oudheusden. “As well as that you only need to give the electrons a low energy, so you can accelerate them in just a centimeter. That’s why the whole system fits on a tabletop.”

Stanford University in the USA has an X-FEL (X-ray Free Electron Laser) with a pricetag of hundreds of millions. It provides images of ‘molecules in action’, using a kilometer-long electron accelerator. A European X-FEL, which will cost a billion euro, is currently under construction in Hamburg (Germany).

The problem with electrons in electron bunches is that they repel each other. This causes the electron bunches to expand, making them longer than the desired 100 femtoseconds (1 femtosecond is 10-15 second).

The key was to create bunches of exactly the right shape, so they can be controlled and focused by means of electrical fields into bunches of the desired type and length. All with a number of electrons (1 million) that is sufficient to create a diffraction pattern in just a single shot. Supervisor prof.dr. Marnix van der Wiel believes that half to three-quarters of the kind of research that can be done on an X-FEL can also be done with the ‘poor man’s X_FEL’.

The X-FEL at Stanford works non-stop, all year round, and is used by thousands of research groups over several decades. So if you’re allocated time on the system you have to take all your equipment to the USA, where you have to stick to a very strict schedule. Our finding is a good alternative for people who want to have the freedom to do research in their own labs. As far as the costs are concerned, it depends on the user if our system will turn out to be cheaper on a per publication basis

134 page (7 megabyte) thesis – Electron source for sub-relativistic single-shot femtosecond diffraction

In this thesis we show full control and subsequent inversion of the Coulomb explosion of space-charge dominated electron bunches at sub-relativistic speeds. In particular, we demonstrate longitudinal compression in a single step by more than two orders of magnitude -a record by itself- to sub-100-fs bunch durations. To control the fourth dimension (time), i.e. to compress the electron bunches, we have introduced the use of a radio-frequency (RF) cavity. This constitutes a radical change in approach compared to existing ultrafast electron diffraction (UED) experiments. To show that high-quality diffraction patterns can be obtained with » 10^6 electrons in a single sub-100 fs shot, we have carried out a diffraction experiment on a gold nanolayer.

Compared to state-of-the-art UED experiments we have improved the temporal resolution by a factor five, and combined this with an increased bunch charge by at least two orders of magnitude. Thereby we have provided a `poor man’s X-FEL’ (X-ray Free Electron laser) in the sense that single-shot, sub-relativistic, femtosecond electron diffraction can be performed. The bunch compression results and single-shot diffraction patterns presented in this thesis pave the way for the study of structural dynamics with atomic spatio-temporal resolution.

Ideally the bunch should be a uniformly charged ellipsoid (or `waterbag’ bunch), because this is the only charge distribution with linear space-charge fields. The preliminary transverse phase-space measurements presented in this thesis indicate that we have created waterbag-like bunches. Strictly speaking, to claim the realization of waterbag bunches also the longitudinal phase-space has to be measured. This can be done with, e.g., the streak cavity (that we have used for bunch length measurements) in combination with a constant transverse magnetic field perpendicular to the streaking magnetic field. In this way the energy-time-correlation of a bunch can be obtained.

Further, by measuring all three 2D phase-spaces of bunches of different initial charge densities (at the cathode) the `waterbag existence regime regime’ can be mapped in which a thin sheet of electrons will develop into a waterbag bunch. Outside the waterbag existence regime nonlinearities of the image-charge field are expected to degrade the beam quality.

This can be examined in more detail both analytically and by particle tracking simulations with, e.g., the gpt code.

Besides these two more academic issues there is quite some practical potential to create even brighter bunches, i.e. shorter bunches with improved transverse coherence. To explore the limits we recommend further research on potential sources of aberrations, on RF compression, and on the electron source itself. These issues are discussed below.

When having created a high-quality waterbag bunch the (transverse) coherence should
not be spoiled by nonlinear electro-magnetic fields, i.e. the charged particle optics have to be aberration-free. The main component to be explored in this sense is the RF compression cavity. Our experiments convincingly show that the bunches we have realized are of sufficient quality for single-shot electron diffraction, in agreement with our expectations based on gpt simulations that include the detailed field map of the cavity. For further beam quality optimization however, the effect of the nonlinear fringe fields (at the cavity apertures) and of (spherical) aberrations of the field inside the cavity should be studied.

Also the compression limits have to be further looked into. We measured bunch durations as small as 67 fs, but compression of 0:1 pC bunches down to 10 fs should be possible according to gpt simulations. To achieve this, the amplitude of the RF ¯eld has to be increased, resulting in a shorter bunch at a focal position closer to the cavity. The energy spread in the focus will then be larger, leading to a smaller longitudinal coherence length. As the longitudinal coherence length of our bunches is an order of magnitude larger than the transverse coherence length, a small decrease in favor of a shorter bunch can be allowed. Furthermore, when looking into the limits of longitudinal compression combined with transverse focusing (which is usually the case in UED experiments) path length differences have to be considered.

Finally, if a true waterbag bunch has been created and if all optics are aberration-free, the only source for deviation from a linear phase-space would be the thermal, i.e., uncorrelated velocities related to the creation of free electrons. It is therefore the electron source that limits the bunch quality: the initial (thermal) energy spread has to be lowered. In our group promising research is being carried out on an ultracold plasma, from which electron bunches are extracted that have a 1000 times lower effective temperature compared to photoemitted bunches. The transverse coherence length of such `ultracold’ bunches is on the order of 10 nm. Combining this ultracold electron source with the RF compression technique would extend the applicability of the poor man’s X-FEL presented in this thesis to the study of femtosecond dynamics of relatively large molecules, like proteins and viruses. This would undoubtedly generate new insight into the building-blocks of life.

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