Electron Phase Space Research Articles

A parametric study of the source rate for outer radiation belt electrons using a Kalman filter

Data assimilation methods have become increasingly popular to describe the outer radiation belt energetic electron environment. We use a Kalman filter with inputs of 1) electron phase space density (PSD) for constant first and second adiabatic invariants, μ = 2083[MeV/G] and K = 0.03[G1/2RE] respectively, from a five satellite data set (three LANL‐GEO, one GPS, and Polar), and 2) a one‐dimensional radial diffusion model with loss and source terms included. We augment the Kalman filter to include the intensity of local acceleration in the state vector. The output is an estimate of PSD for the radial range of the outer radiation belt and the time‐dependent amplitude parameter of a Gaussian shaped source rate term for given location and width. To further constrain the source rate parameters, a root mean square (RMS) analysis of the observation residual vector (a.k.a. innovation vector) is performed in a parameter space of source location and width. We analyze five storm periods spanning from July 30th to October 24th of 2002, and each period's unique solution in the location‐width parameter space is assimilated with the Kalman filter for a continuous reanalysis of the full 87 day period. The source amplitude parameter is analyzed for insight into time periods of enhanced local heating, suppressed loss, or, as the parameter can take negative values, additional loss. The source is found to peak in the recovery phases of the storms where the rate is sufficient to repopulate the radiation belt in approximately one day, suggesting that local heating is a major contributor to the electron radiation belts during the recovery phase.

Measurements of parallel electron velocity distributions using whistler wave absorption

We describe a diagnostic to measure the parallel electron velocity distribution in a magnetized plasma that is overdense (ω(pe) > ω(ce)). This technique utilizes resonant absorption of whistler waves by electrons with velocities parallel to a background magnetic field. The whistler waves were launched and received by a pair of dipole antennas immersed in a cylindrical discharge plasma at two positions along an axial background magnetic field. The whistler wave frequency was swept from somewhat below and up to the electron cyclotron frequency ω(ce). As the frequency was swept, the wave was resonantly absorbed by the part of the electron phase space density which was Doppler shifted into resonance according to the relation ω - k([parallel])v([parallel]) = ω(ce). The measured absorption is directly related to the reduced parallel electron distribution function integrated along the wave trajectory. The background theory and initial results from this diagnostic are presented here. Though this diagnostic is best suited to detect tail populations of the parallel electron distribution function, these first results show that this diagnostic is also rather successful in measuring the bulk plasma density and temperature both during the plasma discharge and into the afterglow.

Nonlocal, kinetic stimulated Raman scattering in nonuniform plasmas: Averaged variational approach

Excitation of continuously phase-locked (autoresonant) plasma waves in a nonuniform plasma via stimulated Raman backscattering is analyzed with a focus on the kinetic regime (kλD∼1). The dominant nonlinear effect in this regime is that of resonant particles, and the plasma wave excitation is a nonlocal process involving formation and transport of the electron phase space holes. Whitham’s averaged variational principle is applied in studying the coupled plasma, laser pump, and seed waves dynamics. A flat-top electron velocity distribution is used as the simplest model allowing a variational formulation within the water bag theory. The corresponding Lagrangian, averaged over the fast phase variable, yields evolution equations for the slow field variables. The adiabatic multiple water bag extension of the theory for application to autoresonant plasma waves in nonuniform plasmas with more realistic initial distributions is also discussed. Numerical solutions of the system of slow variational equations are compared with Vlasov-Ampere simulations.

Comparison of energetic electron flux and phase space density in the magnetosheath and in the magnetosphere

Whether energetic electrons (10s of keV) in the magnetosheath can be directly transported into the magnetosphere and further energized through radial diffusion is significant in understanding the physical mechanisms for producing the radiation belt electrons (>100s of keV) in the magnetosphere. In this study, we analyze more than two hundred magnetopause crossing events using the energetic electron and magnetic field measurements from Geotail and compare the flux and phase space density (PSD) of the energetic electrons on both sides of the magnetopause. It is found that for most of the events (>70%), the fluxes and PSDs of energetic electrons in the magnetosheath are less than those in the magnetosphere, suggesting that the energetic electrons in the magnetosheath cannot be a direct source sufficient for the energetic electrons inside the magnetosphere. In fact, our analysis suggests a possible leakage of the energetic electrons from inside to outside the magnetopause. By investigating the average energetic electron flux distribution in the magnetosheath, we find that the energetic electron fluxes are higher near the bow shock and the magnetopause than in between. The high energetic electron flux near the bow shock can be understood as due to energization of electrons when they go through the bow shock. The relatively low flux of the energetic electrons in between indicates that it is difficult for the energetic electrons to travel from the bow shock to the magnetopause and vice versa, possibly because the energetic electrons near the bow shock and the magnetopause are all on open magnetic field lines and these two relatively intense energetic electron populations in the magnetosheath rarely get mixed.

Radial distributions of equatorial phase space density for outer radiation belt electrons

We present the first direct observations of equatorial electron phase space density (PSD) as a function of the three adiabatic invariants throughout the outer radiation belt using data from the Solid State Telescopes on THEMIS‐D. We estimate errors in PSD that result from data fitting and uncertainty in the calculation of the second and third invariants based on performance‐weighted results from seven different magnetic field models. The PSD gradients beyond geosynchronous orbit (GEO) are energy dependent, revealing different source regions for the relativistic and non‐relativistic populations. Specifically, the PSD distribution of outer belt relativistic electrons is peaked near L* ≈ 5.5. These features are typical for the outer belt, based on a survey of a two‐month period from 01 Feb.–31 Mar. 2010. The results are consistent with previous studies, which were based on off‐equatorial observations, but remove the high uncertainties introduced from mapping by using truly equatorial measurements (i.e., within only a few degrees of the magnetic equator) and quantifying the error in PSD. The newly calibrated THEMIS‐SST dataset forms a powerful tool for exploration of the near‐Earth magnetosphere, especially when combined with the upcoming RBSP mission.

Collisionless magnetic reconnection in a plasmoid chain

Abstract. The kinetic features of plasmoid chain formation and evolution are investigated by two dimensional Particle-in-Cell simulations. Magnetic reconnection is initiated in multiple X points by the tearing instability. Plasmoids form and grow in size by continuously coalescing. Each chain plasmoid exhibits a strong out-of plane core magnetic field and an out-of-plane electron current that drives the coalescing process. The disappearance of the X points in the coalescence process are due to anti-reconnection, a magnetic reconnection where the plasma inflow and outflow are reversed with respect to the original reconnection flow pattern. Anti-reconnection is characterized by the Hall magnetic field quadrupole signature. Two new kinetic features, not reported by previous studies of plasmoid chain evolution, are here revealed. First, intense electric fields develop in-plane normally to the separatrices and drive the ion dynamics in the plasmoids. Second, several bipolar electric field structures are localized in proximity of the plasmoid chain. The analysis of the electron distribution function and phase space reveals the presence of counter-streaming electron beams, unstable to the two stream instability, and phase space electron holes along the reconnection separatrices.

The role of drift orbit bifurcations in energization and loss of electrons in the outer radiation belt

[1] Radiation levels in Earth's outer electron belt (L ≳ 2.5) vary by orders of magnitude on the time scales ranging from minutes to days. Multiple acceleration and loss processes operate across the belt and compete in defining its global variability. One such process is the drift orbit bifurcation effect. Caused by coupling of the drift and bounce motions, it breaks the second adiabatic invariant of radiation belt electrons producing their transport in radius and pitch angle. In this paper we investigate implications of drift orbit bifurcations to the global state and variability of the outer electron belt. For this purpose we use three-dimensional test particle simulations of electron guiding center motion in a realistic magnetic field model. We show that even at most quiet solar wind conditions bifurcations affect a broad range of the belt penetrating inside the geosynchronous orbit. This has an important practical implication for the analysis of experimental particle data: since the third adiabatic invariant is undefined for bifurcating orbit, the electron phase space density cannot be expressed in terms of three adiabatic invariants. We show that long-term transport of electrons due to drift orbit bifurcations is a complex combination of large ballistic jumps and small-amplitude diffusion in the second invariant and radial location. To model long-term transport, we derive an empirical map of the second invariant and radial jumps at bifurcations. The map can also be implemented by other radiation belt models, which cannot directly account for the physics of drift orbit bifurcations. Drift orbit bifurcations can produce electron losses through the magnetopause escape and through pitch angle scattering into the atmospheric loss cone. Most electrons, however, can stay quasi-trapped in the bifurcation regions for very long time periods. The pitch angle and radial transport due to drift orbit bifurcations lead to their meandering back and forth across the region producing mixing and recirculation of particle populations with different initial conditions. We show that this recirculation can greatly amplify electron energization by radial diffusion. Compared to the diffusion alone, the combined action of radial diffusion and drift orbit bifurcations can double electron energization at each recirculation cycle. Our results suggest that drift orbit bifurcations can play an important role in the buildup of increased electron fluxes in the storm recovery phase.

Reanalysis of radiation belt electron phase space density using various boundary conditions and loss models

Data assimilation is becoming an increasingly important tool for understanding the near Earth hazardous radiation environments. Reanalysis of the radiation belts can be used to identify the electron acceleration mechanism and distinguish local acceleration from radial diffusion. However, for any practical applications we need to determine how reliable is reanalysis, and how significant is the dependence of the results on the assumptions of the code and choice of boundary conditions. We present the sensitivity of reanalysis of the radiation belt electron phase space density (PSD) to the assumed location of the outer boundary, using the VERB code and a Kalman filter. We analyze the sensitivity of reanalysis to changes in the electron-loss throughout the domain, and the sensitivity to the assumed boundary condition and its effect on the innovation vector. All the simulations presented in this study for all assumed loss models and boundary conditions, show that peaks in the phase space density of relativistic electrons build up between 4.5 and 6 R E during relativistic electron flux enhancements in the outer radiation belt. This clearly shows that peaks build up in the heart of the electron radiation belt independent of the assumptions in the model, and that local acceleration is operating there. The work here is also an important step toward performing reanalysis using observations from current and future missions.

Reflective optical system for time-resolved electron bunch measurements at PITZ

The Photo-Injector Test facility at DESY, Zeuthen site (PITZ), produces pulsed electron beams with low transverse emittance and is equipped with diagnostic devices for measuring various electron bunch properties, including the longitudinal and transverse electron phase space distributions. The longitudinal bunch structure is recorded using a streak camera located outside the accelerator tunnel, connected to the diagnostics in the beam-line stations by an optical system of about 30 m length. This system mainly consists of telescopes of achromatic lenses, which transport the light pulses and image them onto the entrance slit of the streak camera. Due to dispersion in the lenses, the temporal resolution degrades during transport. This article presents general considerations for time-resolving optical systems as well as simulations and measurements of specific candidate systems. It then describes the development of an imaging system based on mirror telescopes which will improve the temporal resolution, with an emphasis on off-axis parabolic mirror systems working at unit magnification. A hybrid system of lenses and mirrors will serve as a proof of principle.

Modeling the TrueBeam linac using a CAD to Geant4 geometry implementation: Dose and IAEA‐compliant phase space calculations

To create an accurate 6 MV Monte Carlo simulation phase space for the Varian TrueBeam treatment head geometry imported from CAD (computer aided design) without adjusting the input electron phase space parameters. GEANT4 v4.9.2.p01 was employed to simulate the 6 MV beam treatment head geometry of the Varian TrueBeam linac. The electron tracks in the linear accelerator were simulated with Parmela, and the obtained electron phase space was used as an input to the Monte Carlo beam transport and dose calculations. The geometry components are tessellated solids included in GEANT4 as GDML (generalized dynamic markup language) files obtained via STEP (standard for the exchange of product) export from Pro/Engineering, followed by STEP import in Fastrad, a STEP-GDML converter. The linac has a compact treatment head and the small space between the shielding collimator and the divergent are of the upper jaws forbids the implementation of a plane for storing the phase space. Instead, an IAEA (International Atomic Energy Agency) compliant phase space writer was implemented on a cylindrical surface. The simulation was run in parallel on a 1200 node Linux cluster. The 6 MV dose calculations were performed for field sizes varying from 4 x 4 to 40 x 40 cm2. The voxel size for the 60 x 60 x 40 cm3 water phantom was 4 x 4 x 4 mm3. For the 10 x 10 cm2 field, surface buildup calculations were performed using 4 x 4 x 2 mm3 voxels within 20 mm of the surface. For the depth dose curves, 98% of the calculated data points agree within 2% with the experimental measurements for depths between 2 and 40 cm. For depths between 5 and 30 cm, agreement within 1% is obtained for 99% (4 x 4), 95% (10 x 10), 94% (20 x 20 and 30 x 30), and 89% (40 x 40) of the data points, respectively. In the buildup region, the agreement is within 2%, except at 1 mm depth where the deviation is 5% for the 10 x 10 cm2 open field. For the lateral dose profiles, within the field size for fields up to 30 x 30 cm2, the agreement is within 2% for depths up to 10 cm. At 20 cm depth, the in-field maximum dose difference for the 30 x 30 cm2 open field is within 4%, while the smaller field sizes agree within 2%. Outside the field size, agreement within 1% of the maximum dose difference is obtained for all fields. The calculated output factors varied from 0.938 +/- 0.015 for the 4 x 4 cm2 field to 1.088 +/- 0.024 for the 40 x 40 cm2 field. Their agreement with the experimental output factors is within 1%. The authors have validated a GEANT4 simulated IAEA-compliant phase space of the TrueBeam linac for the 6 MV beam obtained using a high accuracy geometry implementation from CAD. These files are publicly available and can be used for further research.

Energetic particle phase space densities at Saturn: Cassini observations and interpretations

[1] Saturn's magnetosphere has been studied extensively by the Cassini spacecraft during the last 6 years. We present mission-averaged energetic proton and electron measurements obtained by the MIMI/LEMMS instrument onboard Cassini in an energy range from several 10 keV to several 10 MeV separated by equatorial pitch angle. We discuss the resulting radial profiles and energy spectra. The measured intensities are converted to phase space densities. The distribution of energetic particles is governed by a large variety of processes. For instance, moons absorb energetic particles, creating macrosignatures or microsignatures. We have found that the moon Rhea is partly responsible for a change in gradient of electron phase space densities. We show that, in contrast to larger distances, the particle distribution for L < 8 is not driven by radial diffusion alone. There, the particle profiles are significantly modified due to Saturn's Neutral Torus, plasma environment, E ring, injection events, and cosmic ray albedo neutron decay. Large parts of our analysis are focused near L = 7. There, protons are lost within the Neutral Torus and not the E ring. For electrons, we find that these two losses are of comparable rate but have discovered that neither process is the dominant driver of loss. We point out that intensity measured by a energy channel, such as in a particle instrument, can actually increase in the region of ring and torus instead of decrease. The importance of injection events is shown to be at least of similar importance as radial diffusion.

Electron streams formation and secondary two stream instability onset in the post-saturation regime of the classical Weibel instability

The electrostatic activity in the post-saturation regime of the velocity anisotropy driven Weibel instability is investigated by means of 1D 3V particle in cell simulations. Two different initial simulation configurations have been chosen to characterize the electrostatic activity in the post-saturation stage. A secondary two stream instability arises in both cases. However, significant differences occur in the thickness of the electron streams, in their initial locations, and in their effects on the bulk electron phase space distribution. An Hamiltonian description of particle motion in a 1D setting explains these differences in terms of the effective potential experienced by particles as a function of their initial perpendicular velocity. The different roles of the longitudinal electric field and the Lorentz force in the formation of electron streams are discussed.

Transverse instability and magnetic structures associated with electron phase space holes

Electron phase space holes (electron holes) are found to be unstable to the transverse instability. Two-dimensional (2D) electromagnetic particle-in-cell simulations are performed to investigate the structures of the fluctuating magnetic field associated with electron holes. The combined actions between the transverse instability and the stabilization by the background magnetic field (B0=B0e⃗x) lead a one-dimensional electron hole into several 2D electron holes which are isolated in both the x and y directions. The electrons trapped in these 2D electron holes suffer the electric field drift vE=E×B0/B02 due to the existence of the perpendicular electric field Ey, which generates the current along the z direction. Then, the unipolar and bipolar structures are formed for the parallel cut of the fluctuating magnetic field along the x and y directions, respectively. At the same time, these 2D electron holes move along the x direction, and the unipolar structures are formed for the parallel cut of the fluctuating magnetic field along the z direction.

Analysis of radiation belt energetic electron phase space density using THEMIS SST measurements: Cross-satellite calibration and a case study

[1] In this study we perform an energy channel dependent cross-satellite calibration of Time History of Events and Macroscale Interactions during Substorms (THEMIS) solid state telescope (SST) flux measurements based on electron phase space density (PSD) conjunctions at fixed phase space coordinates. By comparing PSD around L* = 6 between THEMIS SST and Los Alamos National Laboratory (LANL) satellite LANL-01A synchronous orbit particle analyzer (SOPA) for a half year period starting from 1 July 2007, we evaluate systematic errors in the THEMIS SST measurements and quantify the cross-instrument calibration factors for the 11 SST energy channels from 40 to 2159 keV. Good consistency in electron PSD conjunctions between the five THEMIS probes indicates that the SST instrument aboard each spacecraft responds quite similarly to the ambient electron radiation environment. Compared to the LANL-01A SOPA instrument, the THEMIS SST underestimates the electron fluxes within a factor of 2 for the 40–140 keV energy channels and overestimates the electron fluxes within a factor of 3 for the 204–2159 keV energy channels. Using the cross-satellite calibrated SST flux data for the five THEMIS spacecraft and the SOPA measurements from the LANL-01A and 1989–048 satellites, we analyze the response of radiation belt electrons to a sudden solar wind pressure enhancement event. The electron PSD conjunctions between the THEMIS probes and the two geostationary satellites show good agreement, suggesting a reasonable cross-satellite calibration of the SST measurements. Our results also indicate a clear correlation between electron PSD dropouts and the solar wind pressure pulse, which is likely due to the combination of magnetopause shadowing and outward radial diffusion. The gradual buildup of electron PSD after the abrupt pressure enhancement most likely results from a combined effect of local acceleration and inward and outward radial diffusion that refills the main phase PSD dropout when the magnetopause moves outward. A longer-term quantitative analysis of the temporal evolution and radial profile of electron PSD based on multiple satellite measurements, including the cross-satellite calibrated THEMIS SST data, will be required to improve our understanding of the dynamic responses of radiation belt electrons to solar activity.

Simulation of low frequency Buneman instability of a current-driven plasma by particle in cell method

The nonlinear dynamics of low frequency Buneman instability in a current-driven cold unmagnetized plasma are studied using particle in cell simulation. Simulations of the Buneman instability show that the electron density profile has sharp peaks and the amplitude of the peaks changes with time. Also, the nonlinear evolution of this instability and saturation time is investigated by the time variation of the electric potential energy in the plasma. Moreover, the electron trapping phenomena and the formation of phase space electron holes are presented by considering the phase space diagram and electron distribution function. Finally, the ion mass effect on saturation time is investigated and it is found that the saturation time increases by increasing the ion mass and the saturation level is not very sensitive to the ion mass.

Statistical analysis of pitch angle distribution of radiation belt energetic electrons near the geostationary orbit: CRRES observations

[1] A statistical analysis of energetic radiation belt electron pitch angle distributions (PADs) at the radial distances of 6 RE and 6.6 RE is performed on the basis of the pitch angle resolved flux observations from the Medium Electrons A (MEA) instrument onboard the Combined Release and Radiation Effects Satellite (CRRES). While previous studies of Vampola (1998) and Gannon et al. (2007) have used CRRES MEA data to investigate the general variations in electron PAD at particular energies, in this study we present a detailed statistical analysis of electron PADs including the dependence on electron kinetic energy, magnetic local time (MLT), and the level of geomagnetic activity. By fitting the measured PADs with a power law function of sine of local pitch angle, the power law index n that relates to the category of radiation belt electron PAD is quantified in detail as a function of electron kinetic energy, MLT interval, and geomagnetic index Kp. Statistical averaged n values vary considerably with respect to MLT, ranging from n ∼ 0 within 0000–0400 MLT to n ∼ 1.5 within 1200–1600 MLT, because of the MLT dependence of wave scattering and the effects associated with drift shell splitting and magnetopause shadowing. Drift shell splitting and magnetopause shadowing result in often observed negative values of n. At lower energies of a few hundred keV the pitch angle distributions are more flat than at MeV energies, which is consistent with faster pitch angle scattering at low energies by chorus waves. These quantitative results of radiation belt electron PAD, consistent with the previous studies by Vampola (1998) and Gannon et al. (2007), provide further insight into the global dynamics of energetic radiation belt electrons near the geostationary orbit and also are useful for inferring electron phase space densities and assimilating their radial profiles using omnidirectional electron flux measurements.

Chorus-Driven Outer Radiation Belt Electron Dynamics at Different L-Shells

Energetic outer radiation belt electron phase space density (PSD) evolution due to interaction with whistler-mode chorus at different L-shells is investigated by solving the diffusion equation including cross diffusion terms. It is found that the difference of diffusion rates for different L-shells occurs primarily at pitch angles 0°-50° and around 90°. In particular, diffusion rates for L = 6.5 are found to be 5–10 times larger than that for L = 3.5 at these pitch angles. In the presence of cross terms, PSD for ∼ MeV electrons after 24 h decreases by about 25, 12, 10 and 8 times at L = 3.5, 4.5, 5.5 and 6.5 near the loss cone, and increases by about 55, 45, 30 and 20 times at larger pitch angles, respectively. After 24 h, the ratios between ∼ MeV electron PSDs from simulations without and with cross diffusion at L = 3.5, 4.5, 5.5 and 6.5 are about 350, 600, 800 and 800 near the loss cone, and become 5, 5.5, 6.5 and 8 at pitch angle 90°, respectively. These results demonstrate that neglect of cross diffusion generally results in the overestimate of PSD, and the cross diffusion plays a more significant role in the resonant interaction between chorus waves and outer radiation belt electrons at larger L.

Estimation of radial gradients of phase space density from POLAR observations during a quiet period prior to a sudden solar wind dynamic pressure enhancement

We have analyzed POLAR electron flux data to estimate the radial gradients of electron phase space density (PSD) immediately prior to a sudden solar wind dynamic pressure enhancement at ∼1135 UT on 12 August 2001. In this event, the instantaneous flux changes from the magnetospheric compression at L ∼ 7.6 in the postmidnight magnetic local time sector are observed to be both energy and pitch angle (PA) dependent: a substantial flux increase for lower energies (&lt;∼200 keV) particularly at smaller local PA (&lt;∼50°) and a large flux decrease for relativistic energies that is more pronounced for a larger PA. For this event, because of the extremely steady and quiet solar wind conditions prior to the pressure impact, we can reasonably assume that the temporal variation is negligible and so deduce the spatial and pitch angle distribution of initial electron fluxes to construct the event‐specific electron flux model for a localized region of the nightside outer radiation belt. PSD radial profiles are then estimated for 6.5 &lt; L &lt; 8.5 using the constructed electron flux model and Tsyganenko magnetospheric model for quiet times. The estimates show a positive PSD gradient for low energies (&lt;∼a few 100 s keV at the POLAR location right after the pressure impact) and a negative gradient for relativistic energies. In addition, a rather gradual transition of the radial gradient from highly positive to more negative is found with increasing energy. The expected immediate flux responses using these estimated PSD radial gradients are qualitatively consistent with those observed, partially validating the veracity of the estimated profiles.

Excitation of electron cyclotron harmonic waves in the inner Saturn magnetosphere within local plasma injections

Strong electron cyclotron harmonic (ECH) waves have been observed by Cassini associated with local plasma injection regions in the Saturnian magnetosphere. Using measured electron distributions from the Cassini plasma spectrometer, we calculated local growth rates and path‐integrated wave gain of ECH waves inside an injection event using the HOTRAY code. We showed that electrons with energy near a few eV have a collision frequency comparable to their bounce frequency; thus, they cannot have an empty loss cone. We then demonstrated that the growth of ECH waves inside the injection event can be driven by electron phase space density gradients associated with the loss cone distribution of injected electrons at energies between a few hundred eV and a few keV. This conclusion is contrary to previous results that the source of free energy for growth of ECH waves is provided by electrons near a few eV. Results in this work are helpful for understanding the generation of ECH waves and their roles in electron dynamics in the Saturnian magnetosphere.

Electron kinetics in a laser plasma with increased collisionality

Laser ion sources are based on a free expansion of the plasma generated in the ns laser focus into the vacuum. This expansion is guided by various electron groups present in the plasma and, correspondingly, groups of fast and thermal ions are registered at the end of the expansion path. The fast electron group may be generated by various accelerating mechanisms. One of the key processes leading to the electron acceleration is the Raman scattering in its non-linear phase. The development of the Raman instability requires an unimpeded kinetic evolution in the electron phase space, which is a condition well fulfilled in the outer corona of the plasma generated by the 1st harmonics of a near-infrared laser. In this case, the generated plasma is only weakly collisional and the Raman scattering is accompanied by a complex kinetic evolution in the electron phase-space, such as a saturation of Landau damping, which is then translated in an unfavourable time dependence of the reflected light intensity and several fast ion groups. However, even when working with the 3rd harmonics illumination in the visible or near-UV range, by which the generated plasma is rendered more collisional and a part of the mentioned kinetic behaviour is suppressed, some residual fast groups may still be observed.