New beamline for electron bunch diagnostics

A new diagnostic beamline connected directly to the MAX IV linear accelerator is under construction.

It will enable time-resolved characterization of primarily the ultrashort electron bunches for the FemtoMAX beamline but will also be useful for other time-resolved experiments. The design of the highly specialized beamline components is to a large part done in-house.
Head up and tail down
The linear accelerator accelerates electrons up to high energies. Short bunches containing 109 electrons are delivered from the linear accelerator to make X-ray pulses for the FemtoMAX beamline. The duration of the bunches is in the femtosecond (10-15 s) regime to enable high temporal-resolution measurements at the beamline. The short duration makes the bunches very challenging to characterize with time resolution as conventional detection devices are too slow.
In the new setup, two so-called transverse deflecting cavities (TDC) will make the acquisition of time-resolved data possible. They will in principle add an electromagnetic field that deflects the head of the electron bunch upwards and the tail down so that the first electrons hitting the beam profile analyzer will end up at the top of the screen and the last ones will end up at the bottom. The resulting streak gives a time-resolved measurement of the shape of the bunch but the method will also be used to characterize for example how emittance and energy vary as a function of time.
– Today we rely on calculations and relative measurements for the bunch length delivered to FemtoMAX says project leader Erik Mansten, the TDC is a way for us to verify what we deliver. It also helps us preparing the linac for a possible free electron laser in the future.

>Read more on the MAX IV website

Image: These copper disks are going to become transverse deflecting cavities for the new diagnostic beamline.

Injecting relativity into Engineering

When you think about the theory of relativity, physics might be the first thing you think about.

But here at Diamond Light Source, our unique facility and state of the art instrument means that even our engineering teams must keep relativity in mind. In our last Year of Engineering spotlight piece, learn more about the unique engineering opportunities that present themselves when working at a synchrotron.
There are many areas where science and engineering work together, but relativity rarely makes an appearance. Most of our daily challenges can be solved by using simpler classical mechanics, where we (correctly) assume that objects travel at speeds which are a minute fraction of the speed of light, and weigh many times less than planets or stars. However, two engineering applications used every day at Diamond involve conditions which breach those assumptions, and so they must enter the strange world of relativity.
If you mention Einstein’s theory of relativity to a physicist, they will tell you how it provides a more accurate solution to any classical mechanics problem – but often with a lot more work involved! Inside Diamond’s linac and booster accelerators, the presence of relativistic effects instead allows for some clever engineering solutions which simplify the difficult task of controlling the movement of five billion electrons.

>Read more on the Diamond Light Source website

Image: The linac, with the gun at the far end and the accelerating structures coming towards us. The electrons are already more than 0.95 times the speed of light by the time they emerge from the copper rings at the back.

Linac team has reached major milestones

A big milestone was reached for the MAX IV linear accelerator end of May 2018.

The electron bunches accelerated in the linac was compressed to a time duration below 100 femtoseconds (fs). That means that they were shorter than 1*10^-13s. In fact, we could measure a pulse duration as low as 65 fs FWHM.

The RMS bunch length was then recorded at 32 fs. These results were achieved using only the first of the 2 electron bunch compressors in the MAX IV linac and shows not only that we can deliver short electron bunches, but also that the novel concept adopted in the compressors is working according to theory and simulations.

The ultra-short electron pulses are used to create X-ray pulses with the same short time duration in the linac based light source SPF (Short Pulse Facility). These bursts of X-rays can then be used to make time resolved measurements on materials, meaning you can make a movie of how reactions happen between parts of a molecule.

>Read more on the MAX IV Laboratory website

Picture: Linac team at MAX IV.

FemtoMAX makes first time-resolved X-ray diffraction measurement

The studied sample is an indium antimonide (InSb) coated with 60 nanometres of gold

This type of structure is called photo-acoustic transducer which is a device that can convert the energy in light to a sound wave. The sample is illuminated with light for a very short time (50 fs). The light is absorbed in the gold film and that energy is converted to heat within a few picoseconds. The rapid expansion due to heat creates sound waves both at the gold-vacuum interface and at the gold-InSb interface.

By using very short bursts of x-rays you can measure how the sound wave changes the local density of the InSb sample. The time-dependant intensity of the diffracted X-rays gives information about the shape of the acoustic waves which in turn sheds light on how the wave was generated. The particular design allows for modulating the intensity of X-rays with light. We have demonstrated an on-switch which allows reflection of X-rays for only 20 ps.

>Read More