Research
Our research group focuses on the development of advanced laser diagnostic and manipulation techniques for the characterization of neutral gas and plasma flows, and the manipulation of the velocity distribution function (VDF) of species, respectively. We achieve these goals by designing and developing in-house the required laser systems, or by utilizing cutting edge commercial equipment.
A major part of our research concentrates on using single shot coherent Rayleigh-Brillouin scattering (CRBS) for the thermodynamic characterization (temperature, density, flow velocity) of neutral gas as well as plasma flows.
Coherent Rayleigh-Brillouin scattering (CRBS)
Coherent Rayleigh-Brillouin scattering is a non-resonant, four-wave mixing technique in which a moving optical lattice is created in a medium by the interference of two, equipolarized, high intensity laser beams of precisely tailored relative frequency difference between them, called the pumps. A third beam, termed the probe, normally polarized to the pumps, is incident upon this lattice at the Bragg angle. This results in a fourth beam, termed the signal (Fig. 1). In comparison to spontaneous Rayleigh-Brillouin scattering (SRBS), where the scattering occurs in a 4π solid angle, the resulting CRBS signal beam is another laser beam, maintaining the characteristics of the probe. This gives a high signal-to-noise ratio (SNR) and high spatial resolution, which allows for remote measurements in areas where the placement of a detector right next to the scattering region would be impractical or even impossible (such as in an arc discharge, a flame, a wind-tunnel etc), without sacrificing signal collection.
By scanning the velocity of the lattice, CRBS effectively scans across the thermal velocity distribution function of a neutral gas (Figure 2).
The end result is the CRBS spectrum which is the summation of three peaks (Figure 3):
- The Rayleigh peak due to the particles’ individual motion
- The two Brillouin peaks, equidistant from the Rayleigh, due to the acoustic waves that are driven by the lattice.
From this lineshape the following quantities can be estimated in a gas:
- Temperature; no limit on the detectable range
- Density; from 0.1 Torr to several atmospheres for practically all gases
- Flow velocity; from 0.5 ms-1 up to several Mach, with a resolution of 0.5 ms-1
- Polarizability Importantly, CRBS works with all gases, as long as these exhibit some polarizability. Practically this translates to all gases, apart from Helium, which has a very small polarizability (only 0.2 Å3).
Measuring neutral gas airflows (project FRAGOLA)
We have recently demonstrated the capability of CRBS to characterize airflows in the lab. For this purpose we have built an in-house sub-sonic open circuit wind tunnel that allows us to generate and control the airflow over an airfoil, as schematically depicted in Figure 5. By focusing two same-polarization pumps on the test section, we create an interference pattern of alternating low and high electric field regions, resulting in an optical lattice. The latter is created as the particles in the air are attracted to the high field regions due to the optical dipole force. A probe that is scattered off this region at the Bragg angle yields a coherent signal beam carrying information about the density and velocity of the particles in the focused area. When the two pumps have the same frequency, a standing wave is created and thus an optical lattice at 0 m/s is formed. However, we can also create a moving optical lattice by introducing a frequency difference between the two pumps during the pulse duration. In this case, only particles in the gas medium with velocity equal to that of the lattice interact with it, yielding the CRBS signal. Therefore, different flow conditions around the test section yield different CRBS spectra. By chirping the frequency difference of the two pumps, we effectively perform a fast scan of the lattice velocity, scanning from -750 m/s to +750 m/s in 100-200 ns. This enables a single-shot measurement of the velocity distribution function in the test section and allows us to extract the flow velocity field around the airfoil in real time, in a single shot.
We are currently working on extending this work towards hypersonic airflows and multi-point measurements as well as measuring the translation temperature and relative number density in the gas.
Probing the dynamics of plasma discharges (projects FRAGOLA & ULTRAION)
We are additionally working on employing the CRBS technique as a tool for plasma diagnostics. In particular, CRBS offers the opportunity to measure important thermodynamics properties of plasma such as translational temperature and particle densities, while having the advantages of a remote and non-intrusive measurement that does not perturb the plasma dynamics. This line of research can lead to new insights on the dynamical behaviour of charged particles in plasma, as well as a better understanding of the heating and thermalization mechanisms that are responsible for the occurrence of instabilities in atmospheric pressure plasmas. Moreover, in contrast to current state-of-the-art methods to non-perturbatively probe plasma dynamics, such as optical emission spectroscopy (OES), the CRBS measurements do not rely on any assumptions regarding the thermal distribution of plasma. This feature in combination with the very high temporal resolution of the CRBS technique, can enable us to probe plasma physics in extreme conditions and shed light to currently unexplored plasma dynamics.
