Plasma Physics

Head of the Group: Dzsotján Gagik


Time-resolved diagnostics of rubidium plasma generated by ultra-short laser radiation.
This research is closely related and correlated with the AWAKE (advanced wake field
acceleration) experiment at CERN and is continuation of our research conducted in 2016.
The main goal of the AWAKE experiment is construction of a novel plasma-based particle
accelerator that will utilize the proton bunch available at Large Hadron Collider (LHC) to
accelerate electrons (positrons) to TeV energies in a single acceleration stage.
An extended volume of extremely homogeneous plasma is an indispensable
part of the acceleration scheme. This plasma will be used for splitting
the LHC proton bunch into microbunches using self-modulation instability
in the plasma to provide coherent wake-field acceleration of electrons
by the proton bunch. Our experimental setup may be considered as a
tabletop analogy of the laser plasma source for the AWAKE experiment. In
2017 the experiments were conducted in our newly renovated “clean room”
lab with a new Ti:Sa laser system Hydra of Coherent Co. with pulse energy
about 25-30 mJ and pulse duration of 30-40 fs (see Fig.1) .

Cold plasma

Figure1. The experimental setup (a)
with the generated plasma (b) and the
scheme of the real-time interferometric
diagnostics (c).

We applied our interferometric diagnostics scheme to determine parameters of the created
Rb plasma at different values of temperature (density) of the Rb vapor and different values
of intensity of the ionizing laser pulses. The diagnostics of the generated plasma is
performed using a Mach-Zehnder-type interferometry, Fig.1. The measurements were
provided by a CW diode laser with frequency close to D2 line of Rb. Accordingly, the probe
diode laser beam propagates in direction opposite to the ionizing laser pulse through a glass
cell filled with Rb vapor located in a one arm of the interferometer. This probe beam creates
a fringe pattern with a reference beam from the same laser propagating in the air in the
other arm of the interferometer. Time variation of the interferometric signals is measured
by fast detectors in a real-time regime. Our fitting technique allows us to measure the
plasma density-length product and its variation in time, along with the recombination time
constants. An example of the interferometry signal is shown in Fig.2 with the cosine fitting
function applied. Results of our studies allow us to understand the physical mechanisms of
generation of extended laser plasma, as well as to characterize the induced plasma
instabilities. These results will be used to create optimal conditions for generation of highly
homogeneous plasma for application in the AWAKE project at CERN.

Modeling of propagation of an ionizing femtosecond laser pulse in Rb vapors. — In 2017
we investigated the propagation of an ultrashort, ionizing laser pulse through rubidium
vapor, a project associated with the previously mentioned AWAKE experiment. An initial
calculation that employed a relatively simple atomic model to calculate the effect of the
ultrashort pulse on the rubidium atoms and to compute the optical response of the atoms
was perfected in two respects. First, the atomic model was made more elaborate by
including a more precise description of the internal atomic structure, using more atomic
levels for the calculation. Instead of the very simple four-level atomic model, an 18-level and
a 10-level model was developed and their accuracy evaluated. It was confirmed that the
approximation that uses the 10 lowest lying atomic levels of rubidium is accurate enough for
computing the optical response, yet it is still simple enough to be used in propagation
calculations. Next, the propagation calculations (using the original, four level model) were
generalized to include two spatial dimensions, one along the propagation direction and one
perpendicular to it. Assuming cylindrical symmetry, we started studying the propagation of
the ionizing laser beam in order to evaluate the intensity and focusing requirements to
produce a 10 m long plasma channel with very close to 100% ionization in its central region.
Self-channeling of the ultrashort laser pulse, as well as a nontrivial pulsing of the plasma
channel radius was observed in the calculations, (see Fig.3), where the ionization probability
Fig.2. An example of an interferometry signal measured by the fast detector with a cosine
fitting curve:

Cold plasma

dependence on the propagation length and dependence of the radiant fluence of the
ionizing field versus propagation length are presented.