Indranuj Dey

Experimental investigation of microwave interaction with magnetoplasma in miniature multipolar configuration using impedance measurements

A miniature microwave plasma source employing both radial and axial magnetic fields for plasma confinement has been developed for micro-propulsion applications. Plasma is initiated by launching microwaves via a short monopole antenna to circumvent geometrical cutoff limitations. The amplitude and phase of the forward and reflected microwave power is measured to obtain the complex reflection coefficient from which the equivalent impedance of the plasma source is determined. Effect of critical plasma density condition is reflected in the measurements and provides insight into the working of the miniature plasma source. A basic impedance calculation model is developed to help in understanding the experimental observations. From experiment and theory, it is seen that the equivalent impedance magnitude is controlled by the coaxial discharge boundary conditions, and the phase is influenced primarily by the plasma immersed antenna impedance.

Highly efficient broadband terahertz generation from ultrashort laser filamentation in liquids

Generation and application of energetic, broadband terahertz pulses (bandwidth ~0.1–50 THz) is an active and contemporary area of research. The main thrust is toward the development of efficient sources with minimum complexities—a true table-top setup. In this work, we demonstrate the generation of terahertz radiation via ultrashort pulse induced filamentation in liquids—a counterintuitive observation due to their large absorption coefficient in the terahertz regime. The generated terahertz energy is more than an order of magnitude higher than that obtained from the two-color filamentation of air (the most standard table-top technique). Such high terahertz energies would generate electric fields of the order of MV cm-1, which opens the doors for various nonlinear terahertz spectroscopic applications. The counterintuitive phenomenon has been explained via the solution of nonlinear pulse propagation equation in the liquid medium.Developing simple and efficient table-top sources of intense terahertz radiation is an ongoing pursuit. Here, Dey et al. demonstrate broadband terahertz generation from laser filamentation in liquids with an order of magnitude higher energy than from conventional two-color filamentation in air.

Megagauss magnetic fields in ultra-intense laser generated dense plasmas

Table-top terawatt lasers can create relativistic light intensities and launch megaampere electron pulses in a solid. These pulses induce megagauss (MG) magnetic pulses, which in turn strongly affect the hot electron transport via electromagnetic instabilities. It is therefore crucial to characterize the MG magnetic fields in great detail. Here, we present measurements of the spatio-temporal evolution of MG magnetic fields produced by a high contrast (picosecond intensity contrast 10−9) laser in a dense plasma on a solid target. The MG magnetic field is measured using the magneto-optic Cotton–Mouton effect, with a time delayed second harmonic (400 nm) probe. The magnetic pulse created by the high contrast laser in a glass target peaks much faster and has a more rapid fall than that induced by a low contrast (10−6) laser.

Intense femtosecond laser driven collimated fast electron transport in a dielectric medium–role of intensity contrast

Ultra-high intensity (> 1018 W/cm2), femtosecond (~30 fs) laser induced fast electron transport in a transparent dielectric has been studied for two laser systems having three orders of magnitude different peak to pedestal intensity contrast, using ultrafast time-resolved shadowgraphy. Use of a 400 nm femtosecond pulse as a probe enables the exclusive visualization of the dynamics of highest density electrons (> 7 × 1021 cm-3) observed so far. High picosecond contrast (~109) results in greater coupling of peak laser energy to the plasma electrons, enabling long (~1 mm), collimated (divergence angle ~2°) transport of fast electrons inside the dielectric medium at relativistic speeds (~0.66c). In comparison, the laser system with a contrast of ~106 has a large pre-plasma, limiting the coupling of laser energy to the solid and yielding limited fast electron injection into the dielectric. In the lower contrast case, bulk of the electrons expand as a cloud inside the medium with an order of magnitude lower speed than that of the fast electrons obtained with the high contrast laser. The expansion speed of the plasma towards vacuum is similar for the two contrasts.

Development of a miniature microwave electron cyclotron resonance plasma ion thruster for exospheric micro-propulsion

A miniature microwave electron cyclotron resonance plasma source [(discharge diameter)/(microwave cutoff diameter) < 0.3] has been developed at Kyushu University to be used as an ion thruster in micro-propulsion applications in the exosphere. The discharge source uses both radial and axial magnetostatic field confinement to facilitate electron cyclotron resonance and increase the electron dwell time in the volume, thereby enhancing plasma production efficiency. Performance of the ion thruster is studied at 3 microwave frequencies (1.2 GHz, 1.6 GHz, and 2.45 GHz), for low input powers (<15 W) and small xenon mass flow rates (<40 μg/s), by experimentally measuring the extracted ion beam current through a potential difference of ≅1200 V. The discharge geometry is found to operate most efficiently at an input microwave frequency of 1.6 GHz. At this frequency, for an input power of 8 W, and propellant (xenon) mass flow rate of 21 μg/s, 13.7 mA of ion beam current is obtained, equivalent to an calculated thrust of 0.74 mN.

Probing ultrafast dynamics of solid-density plasma generated by high-contrast intense laser pulses

We present ultrafast dynamics of solid-density plasma created by high-contrast (picosecond contrast ∼10−9), high-intensity (∼4 × 1018 W/cm2) laser pulses using time-resolved pump-probe Doppler spectrometry. Experiments show a rapid rise in blue-shift at early time delay (2–4.3 ps) followed by a rapid fall (4.3–8.3 ps) and then a slow rise in blue-shift at later time delays (>8.3 ps). Simulations show that the early-time observations, specifically the absence of any red-shifting of the reflected probe, can only be reproduced if the front surface is unperturbed by the laser pre-pulse at the moment that the high intensity pulse arrives. A flexible diagnostic which is capable of diagnosing the presence of low-levels of pre-plasma formation would be useful for potential applications in laser-produced proton and ion production, such as cancer therapy and security imaging.

Strong power upscaling of THz sources based on laser filamentation in transparent media

Intense, ultra-broadband terahertz (THz) pulses can drive major advances in ultrafast dynamics, nonlinear THz optics, and bio-material imaging [1,2]. Single and two-color filamentation of ultrashort laser pulses in gases have been extensively used for the generation of such pulses [3,4]. However, filament formation in gases is hindered by the need of high laser pulse energies, motivating the exploration of alternate generation mechanisms [5]. On the other hand, filamentation can be achieved more easily in alternative target materials such as transparent solids and liquids [6,7]. Though, THz emission through either single or two-color filamentation in such materials, has not been investigated yet. Here we demonstrate, for the first time, strong THz emission through single and two-color filamentation in liquids and in fused silica (SiO2) respectively and a significant THz field enhancement when using high repetition rate laser sources.

Development of a miniature microwave-multicusp plasma source as an electron neutralizer for space propulsion

Summary form only given. The advent of micro space-probes and nano-satellites has prompted the development of miniature plasma based electric thrusters for space propulsion and orbital manouvering1. Plasma thrusters are capable of providing high specific impulse and greater exhaust velocities with much less fuel load compared to chemical propulsion2. Plasma ion thrusters provide the highest specific impulse among the different types of electric thrusters in use today2. An electron neutralizer is required in tandem to neutralize the exhaust ion beam and prevent the charging of the space craft. DC and filament based thruster and neutralizer have lifetime limitation, and wave based systems have high input power requirement. Therefore, the development of a wave based plasma source operating at low power is important for modern space applications and is being pursued by various research groups3,4.