Intense Reflection of a Relativistic Laser Pulse in Subcritical Plasmas

Interaction of relativistic electromagnetic (EM) (laser) pulse with plasmas has been investigated by means of electromagnetic particle-in-cell simulations. When a relativistic laser pulse, stronger than a critical intensity, is injected into a uniform plasma of sub-critical density (nc/4 < n0/γ < nc), strong reflection is observed. The frequency of the back-scattered wave is near the effective electron plasma frequency which is well below its unperturbed value. This novel stimulated scattering instability is recognized as a three-wave parametric resonance decay of the incident wave into an electron-acoustic wave (EAW) (ω << ωp) and a scattered EM Stokes sideband. The slow Stokes lightwave gradually builds up to eventually propagate through the plasma-vacuum interface in a form of short superintense reflectivity bursts of coherent low-frequency EM radiation

The Complexities on Ultra-Intense Laser Interaction with Plasmas

　　The interaction of intense laser electromagnetic (EM) wave with plasma has become a basic and important problem in plasma physics due to its potential applications, ranging from astrophysics to fusion science. Inertial confinement fusion (ICF) has been considered as an attractive energy source, which motivates many scientists to pay much attention to the research of intense laser-plasma interactions. On the other hand, the particle acceleration by intense laser interacting with plasma also became a very attractive research topic because of its widespread applications, such as laser-induced nuclear reaction, particle acceleration, medical treatment, radiography and so on.　　Intense laser pulse interacting with plasma is a source of various electronic instabilities. When an laser EM wave propagates in an underdense plasma, many electronic instabilities, such as stimulated Raman scattering (SRS) instability and stimulated Brillouin scattering (SBS) instability then can be excited and developed. These instabilities do not appear isolated but are often interconnected in the real intense laser-plasma interaction. In the past years, large efforts have been put into the studies of SRS and SBS, which produce energetic particles to preheat the core of a fusion pellet. Recently, a new type of stimulated scattering on the so-called Stimulated Electron-Acoustic Wave Scattering (SEAWS) instability was proposed by Montgomery et al., to reinterpret a underdense plasma data from the Trident laser facility. This novel SEAWS induced by relativistic laser interacting with a subcritical density plasma layer has been studied by Nikolic et al. by means of one-dimensional fully relativistic EM Particle In Cell (1D-PIC) simulations．　　When an ultra-intense laser pulse propagates in a plasma, a dispersion effect comes to play an important role due to the finite inertia with which plasma particles respond to the high laser EM field, while plasma density redistribution is caused by the ponderomotive force that pushes the plasma particles away from the region of maximum EM field. These effects can lead to well-known nonlinear phenomena such as self-focusing, transparency of an overdense plasma and the generation of EM soliton. Relativistic solitons are EM structures self-trapped by locally modified plasma refractive index through the relativistic plasma particle mass increase and the plasma density redistribution by the ponderomotive force of an intense laser pulse. These solitons are generated behind the front of the laser pulse and are made of nonlinear, spatially localized low-frequency EM fields with a close to zero group velocity. A fairly large part of the laser pulse energy can be transformed into EM solitons. The formation mechanism and spatial structure of EM soliton have been investigated by theoretical analysis and Particle-In-Cell (PIC) simulations． The solitons found in previous particle simulations consist of slowly or non-propagating electron density cavities inside which EM fields are trapped and oscillate coherently with frequencies below the unperturbed electron plasma frequencies. In homogeneous plasmas, solitons have been found to exist for a long time, close to the regions where they are generated and eventually decay due to their interaction with fast particles; as a result, the soliton energy is transformed into the fast particle energy. In inhomogeneous plasmas solitons are accelerated with the acceleration proportional to the plasma density gradient toward the low density side. When a soliton reaches some critical region, for example, the plasma-vacuum interface, it radiates away its energy in the form of a short burst of low-frequency EM radiation．　　　Particle acceleration by laser pulse propagating in plasmas has also become a very attractive research topic due to the advent of short-pulse, high-intensity lasers and their many potential applications. Various concepts of laser accelerators in a plasma, such as, beat-wave accelerator, laser wakefield accelerator, etc., are presently under discussion and investigation as possible approaches to accelerate to ultra-high energies. When an intense laser pulse propagates in underdense plasma, by backward and forward SRS, and other nonlinear processes e.g., the ponderamotive force of an intense laser pulse, a large amplitude electron plasma wave can be excited. This large amplitude plasma wave has a very high phase velocity close to the group velocity of a laser pulse, and can be used to accelerate electrons, protons or ions to high energies.　　　My research motivations come from inertial confinement fusion and particle acceleration． In the thesis, the researches are mainly concentrated on instabilities, relativistic EM soliton and electron acceleration, induced by linearly-polarized intense laser interacting with underdense plasmas, by means of fully relativistic EM 1D-PIC simulations.　　　The first part is the generation of accelerated large amplitude relativistic EM solitons in a long underdense homogeneous plasma. In simulations, ions are initially placed as a neutralizing background and are kept immobile. When laser enters the plasma layer, the first stage is dominated by SRS interactions. In our low density and long plasma condition, Stimulated Backward Raman Scattering (B-SRS) has shorter growth time than Stimulated Forward Raman Scattering (F-SRS). A nonlinear interplay between B-SRS and F-SRS produces a strong spatial modulation of the laser pulse. After that, there is typically the stimulated Raman cascade in the EM frequency spectra and wavenumber spectra both for backscattered and transmitted EM waves; which effectively scatter incident laser energy to higher order (Stokes and anti-Stokes) EM modes. In the later time, the continuing instability growth through stimulated Raman cascade downshifts the power maximum from the fundamental to the bottom of EM wave spectra. They clearly reveal a tendency of a transition from the stimulated Raman cascade regime to the regime of energy accumulation at the about electron plasma frequency, the so-called photon condensate. The cascade-to-condensate transition becomes more pronounced with increasing laser intensity. After the photon-condensate process, the standing, backward- and forward-accelerated large amplitude relativistic EM solitons are observed. As a new research results, we found that the acceleration of EM soliton depends upon the incident laser intensity in a homogeneous plasma. The accelerated solitons are accelerated toward the plasma-vacuum interfaces and it radiate their energy in the form of low-frequency intense EM bursts. The frequency of the EM wave trapped inside soliton region is about the half of the unperturbed electron plasma frequency, while the corresponding ES frequency is about four and half times the unperturbed electron plasma frequency. The transverse electric and magnetic field have half- and one-cycle structure in space, while the corresponding ES field has one-cycle structure in space, respectively.　　　The second part is the generation of ion-vortices in phase-space in subcritical density plasmas (ncr/4＜n/γ＜nc, γ-relativistic factor). When intense laser light enter a subcritical plasma, a stimulated trapped electron-acoustic wave scattering (T-SEAWS) instability takes place. It can be well-explained by a resonant three- wave parametric decay of the relativistic laser pump into the slowed Stokes EM wave with ωs～ωpe and the trapped electron-acoustic wave (EAW) with ωeaw＜ωpe in the early stage, where ωpe is the electron plasma frequency. There appear a rapid growth and strong localization of the Stokes wave by forming narrow intense EM soliton-like structures with downshifted laser light. The train of EM soliton-like structures get irradiated through the front vacuum-plasma boundary in a form of intense coherent reflection of the downshifted laser light. Large trapped EAW quickly heats up electrons to relativistic energies, which eventually suppresses the T-SEAWS instability. The ion dynamics does not play a significant role on the early physics behaviors of T-SEAWS. However, the ion wave created in the upstream region breaks in time and generates a large amplitude relativistic EM soliton in its breaking place. Thus this forms a large ES field inside. As a new phenomenon, we found that an ion-vortex (ion-hole) structure in phase-space is created because the large part of ions are accelerated and trapped by the regular EM and ES fields inside soliton. As this large amplitude EM soliton is accelerated in the backward direction, several ion-vortices in phase-space are generated due to the continuing ion acceleration and trapping.　　　In the third part of this thesis, the formation of high-quality and well-collimated return relativistic electron beam in long underdense homogeneous plasma is studied. A short ultra-relativistic electron beam acceleration by an intense laser pulse in a finite plasma is examined by 1D-PIC simulations. The mechanism is the combined effect of the electron acceleration by longitudinal field: synchrotron radiation source (SRS) and driven oscillatory relativistic electron plasma wave and the electrostatic (ES) Debye sheath field at the plasma-vacuum interface. The standard dephasing limit and the electron acceleration process are briefly discussed. The novel point is that, at relativistic laser intensities, a phenomenon of pushed short high-quality and well-collimated return relativistic electron beam with thermal energy spread in the direction opposite to laser propagation, is observed. It operates like a two-stage accelerator. In the initial phase: rapid electron heating by the SRS driven relativistic plasma wave allows a massive initial electron blow-off into a vacuum. Large potential Debye sheath fields are created which further accelerate electrons (second stage) to ultra-relativistic beam energies. The mechanism of the beam formation, its characteristics and the time history in x and px space for selected test electrons in a beam, are analyzed and clearly exposed

Response of photosynthesis, the xanthophyll cycle, and wax in Japanese yew (Taxus cuspidata L.) seedlings and saplings under high light conditions

In order to understand the adaptative changes of the Japanese yew (Taxus cuspidate L.) to high light conditions, this study investigated gas-exchange, chlorophyll fluorescence, chlorophyll, and the impact of epicuticular wax on the gas-exchange and photoinhibition of Japanese yew seedlings and saplings. The chlorophyll content per unit area and photosynthetic rate in seedling leaves were significantly lower than in sapling leaves. When leaves from seedlings and saplings were exposed to 1,200 μmol·m−2·s−1 photon flux density (PFD) for 2 h, seedling leaves exhibited a greater down-regulation of maximum quantum yield (Fv/Fm) and actual photosystem II efficiency ( $\Phi$Φ PSII). Non-photochemical quenching (NPQ) and high energy quenching (qE) in sapling leaves were much higher than in seedling leaves when both were exposed to 1,200 μmol·m−2·s−1 PFD for 2 h. At a low level of O2, the photorespiration rate (Pr) and the ratio of photorespiration/gross photosynthetic rate (Pr/Pg) in seedling leaves were lower than in sapling leaves when both were exposed to 1,200 μmol·m−2·s−1 PFD, but this difference did not reach statistical significance (P < 0.05). Compared with sapling leaves, seedling leaves exhibited lower levels of xanthophyll pool. Epicuticular wax content on seedling leaves was significantly lower than on sapling leaves. The results of this study showed that wax coverage on the leaf surface decreased the photosynthetic rate in sapling leaves as a consequence of decreased stomatal conductance. Epicuticular wax is related to tree age and photoinhibition prevention in the Japanese yew. It is possible that lower photosynthetic rate, lower NPQ depending on the xanthophyll cycle, and lower deposition of epicuticular wax results in seedling plants that are not adapted to high light conditions

Different Growth Responses of an Invasive Weed and a Native Crop to Nitrogen Pulse and Competition.

Resource pulses are a common event in agro-ecosystems. A pot experiment was conducted to assess the effects of nitrogen (N) pulses and competition on the growth of an invasive weed, Amaranthus retroflexus, and a native crop, Glycine max. A. retroflexus and G. max were planted in pure culture with two individuals of one species in each pot and in mixed culture with one A. retroflexus and one G. max individual and subjected to three N pulse treatments. The N treatments included a no-peak treatment (NP) with N applied stably across the growing period, a single-peak treatment (SP) with only one N addition on the planting date, and a double-peak treatment (DP) with two N additions, one on the planting date and the other on the flowering date. N pulse significantly impacted biomass and height of the two species across the whole growing season. However, only the relative growth rate (RGR) of A. retroflexus was significantly affected by N pulse. A. retroflexus had the greatest biomass and height in the SP treatment at the first harvest, and in the DP treatment at the last three harvests. Pure culture G. max produced the greatest biomass in the DP treatment. In mixed culture, G. max produced the greatest biomass in the NP treatment. Biomass production of both species was significantly influenced by species combination, with higher biomass in mixed culture than in pure culture at most growth stages. Relative yield total (RYT) values were all greater than 1.0 at the last three harvests across the three N treatments, suggesting partial resource complementarity occurred when A. retroflexus is grown with G. max. These results indicate that A. retroflexus has a strong adaptive capacity to reduce interspecific competition, likely leading to its invasion of G. max cropland in China