Posted: Sep 18, 2018
(Nanowerk News) When light pulses from an extremely powerful laser system are fired onto material samples, the electric field of the light rips the electrons off the atomic nuclei. For fractions of a second, a plasma is created. The electrons couple with the laser light in the process, thereby almost reaching the speed of light. When flying out of the material sample, they pull the atomic cores (ions) behind them. In order to experimentally investigate this complex acceleration process, researchers from the German research center Helmholtz-Zentrum Dresden-Rossendorf (HZDR) have developed a novel type of diagnostics for innovative laser-based particle accelerators.
Their results are now published in the journal Physical Review X ("Observation of Ultrafast Solid-Density Plasma Dynamics Using Femtosecond X-Ray Pulses from a Free-Electron Laser").
With the aid of the powerful X-ray free-electron laser at SLAC in California, HZDR researchers were able to investigate the plasma processes on the small scales of a few nanometers and femtoseconds on which the turbulent laser interaction with the particles to be accelerated takes place. (Image: Juniks/HZDR)
"Our goal is an ultra-compact accelerator for ion therapy, i.e. cancer irradiation with charged particles," says physicist Dr. Thomas Kluge from the HZDR. Besides clinics, the new accelerator technology could also benefit universities and research institutions. However, much research and development work is needed before the technology is ready for use. The DRACO laser at the Helmholtz Center in Dresden currently achieves energies of around 50 megaelectronvolts. However, 200 to 250 megaelectronvolts are required to irradiate a tumor with protons.
Thanks to its ultrashort pulses in the range of a few femtoseconds - a time during which a light beam crosses just a fraction of a human hair - the DRACO laser achieves a power of almost one petawatt. This corresponds to one hundred times the average electrical power generated worldwide.
"We need to understand the individual processes involved in accelerating electrons and ions much better," stresses Kluge. Together with colleagues from Dresden, Hamburg, Jena, Siegen and the USA, the HZDR researchers have now succeeded for the first time in observing these extremely fast processes virtually in real time at the SLAC National Accelerator Laboratory of Stanford University in the USA.
To achieve this feat, the scientists need two special lasers at the same time: The high-intensity laser at SLAC has a power of around 40 terawatts - that is, about 25 times weaker than DRACO. When striking the material sample (target), it ignites the plasma. The second laser is an X-ray laser, which is used to precisely record the individual processes: from the ionization of the particles in the target and the expansion of the plasma, to the plasma oscillations and instabilities that occur when the electrons are heated to several million degrees Celsius, up to the efficient acceleration of the electrons and ions.
"Using the small-angle scattering method, we have realized measurements in the femtosecond range and on scales ranging from a few nanometers to several hundred nanometers," says HZDR doctoral student Melanie Rödel, who played a leading role in the experiment. Several years of work were necessary to access these areas and obtain clean signals on the scattering images of the X-ray laser.
"The new diagnostics for laser-based accelerators has excellently confirmed our expectations regarding its spatial and temporal resolution. We have thus paved the way for the direct observation of plasma-physical processes in real time," says Dr. Josefine Metzkes-Ng, head of one of the participating junior research groups at the HZDR's Institute of Radiation Physics.
The high-intensity laser pulse (red) is focused on a silicon grating target under 45° parallel to the grating ridges. The X-ray pulses (blue) probe the laser-plasma dynamics under 90° over time. The scattering patterns below show the complex particle-acceleration processes. (Image: Juniks/HZDR) (click on image to enlarge)
Starting in 2019, the Helmholtz International Beamline for Extreme Fields (HIBEF), which the HZDR is currently setting up as part of an international collaboration at the world's strongest X-ray laser, the European XFEL near Hamburg in Germany, will provide a next-generation experimental setup with a significantly more powerful short-pulse laser.
With the aid of the powerful X-ray free-electron laser at SLAC in California, HZDR researchers were able to investigate the plasma processes on the small scales of a few nanometers and femtoseconds on which the turbulent laser interaction with the particles to be accelerated takes place. (Image: Juniks/HZDR)
"Our goal is an ultra-compact accelerator for ion therapy, i.e. cancer irradiation with charged particles," says physicist Dr. Thomas Kluge from the HZDR. Besides clinics, the new accelerator technology could also benefit universities and research institutions. However, much research and development work is needed before the technology is ready for use. The DRACO laser at the Helmholtz Center in Dresden currently achieves energies of around 50 megaelectronvolts. However, 200 to 250 megaelectronvolts are required to irradiate a tumor with protons.
Thanks to its ultrashort pulses in the range of a few femtoseconds - a time during which a light beam crosses just a fraction of a human hair - the DRACO laser achieves a power of almost one petawatt. This corresponds to one hundred times the average electrical power generated worldwide.
"We need to understand the individual processes involved in accelerating electrons and ions much better," stresses Kluge. Together with colleagues from Dresden, Hamburg, Jena, Siegen and the USA, the HZDR researchers have now succeeded for the first time in observing these extremely fast processes virtually in real time at the SLAC National Accelerator Laboratory of Stanford University in the USA.
To achieve this feat, the scientists need two special lasers at the same time: The high-intensity laser at SLAC has a power of around 40 terawatts - that is, about 25 times weaker than DRACO. When striking the material sample (target), it ignites the plasma. The second laser is an X-ray laser, which is used to precisely record the individual processes: from the ionization of the particles in the target and the expansion of the plasma, to the plasma oscillations and instabilities that occur when the electrons are heated to several million degrees Celsius, up to the efficient acceleration of the electrons and ions.
"Using the small-angle scattering method, we have realized measurements in the femtosecond range and on scales ranging from a few nanometers to several hundred nanometers," says HZDR doctoral student Melanie Rödel, who played a leading role in the experiment. Several years of work were necessary to access these areas and obtain clean signals on the scattering images of the X-ray laser.
"The new diagnostics for laser-based accelerators has excellently confirmed our expectations regarding its spatial and temporal resolution. We have thus paved the way for the direct observation of plasma-physical processes in real time," says Dr. Josefine Metzkes-Ng, head of one of the participating junior research groups at the HZDR's Institute of Radiation Physics.
The high-intensity laser pulse (red) is focused on a silicon grating target under 45° parallel to the grating ridges. The X-ray pulses (blue) probe the laser-plasma dynamics under 90° over time. The scattering patterns below show the complex particle-acceleration processes. (Image: Juniks/HZDR) (click on image to enlarge)
Starting in 2019, the Helmholtz International Beamline for Extreme Fields (HIBEF), which the HZDR is currently setting up as part of an international collaboration at the world's strongest X-ray laser, the European XFEL near Hamburg in Germany, will provide a next-generation experimental setup with a significantly more powerful short-pulse laser.
