0.3; the remaining parameters are given in section 4.2. b), (c) Shows probe readings at the center of the dipole with 1 μm difference in the y direction. The lower frequency, broadband part is probably from the dipole accumulation site. e-f) shows the spread and angular coherence of the radiated signal. The peaks match and the dipole emits radiation at the plasma frequency --- 12 4.2 Measuring the plasma density using radiation spectra involves three steps. a) Counterpropagation and declared pulses must be focused on the desired point.
Vertical axis indicates density normalized by cm; horizontal axis is the direction of the density gradient (the y direction of the simulation domain). The error bars derive from the spectral full bandwidth of the radiation at half maximum (FWHM) Obliquely propagated laser pulses and demonstrative density measurements (a) Schematic diagram of obliquely propagated lasers to induce the radiation. b) Reconstructed density profile for small (15°) and large (30° ) collision angles and (c) density reconstruction of three different gradients by sweeping shot angles of the pulses at fixed launch positions located far from the plasma. Laser wavelengths are = 800 nm, = 780 nm and normalized peak amplitude Reconstruction of the nonlinear density profiles, one is cosine (dotted, triangles) and the other is exponential (solid, rectangles).
The simulations were repeated, increasing the number of simulation particles per cell, , to reach the quasi-saturation of the numerical noise. The numerical-thermal noise decreases considerably for the larger ---19 5.2 Broadening of the spectra due to the extreme variation of the density along the transverse width of the dipole. The guaranteed range for the correct dipole oscillation is up to = 15%; If the density along the dipole width varies too much, the dipole cannot be formed correctly and therefore narrowband emission will not occur ---21.
Introduction
Plasma dipole oscillation (PDO) – novel idea
Background Theory of Embedded PDO
Dipole radiation
Time-dependent vector and scalar potentials can be calculated using current and dipole moments. In the remaining part of the thesis, a large number of coherently oscillating plasma dipoles will be discussed as an extension of the electric dipole radiation.
Mechanism - embedding plasma dipole oscillators (PDO) colliding detuned laser pulses
- Trapping the electrons
They should be distinguished for the sake of clarity, and the acquisition method is described below. If we manage to drag the electrons through an electric force, this will create charge separation between the ions. Once released, the electrons will oscillate around the background ions at plasma frequency as shown in figure 2.
The wave moves with a phase velocity of = ≪, capturing the beams of electrons in its path, forming a train of microbeams with longitudinal and transverse dimensions relevant to the spot radius and pulse width of the laser pulses. Ions, on the other hand, are very heavy and slow compared to electrons, so that in the high-frequency regime of electrons they can be considered stationary. From this point on, two regimes exist as candidates depending on the field strength of the incident lasers.
This technique uses low-intensity lasers (for a < 0.01, where a = normalized vector potential of the laser pulse). In the region where the pulses overlap, the pulsating potential wave excites the plasma and the response to this can be calculated from the fluid equations using linearization to higher orders. Only part of the direct current J remains in the steady state limit, where the fluctuating part is averaged out to fill the nonlinear component.
Jdcis is responsible for dipole accumulation as long as the pulse collision time is comparable to the dipole scales. The same pulse potential with a > 0.1 can exceed the wave breaking threshold where particle trapping is enabled for efficient PDO acquisition. A pulse wave moving through the region captures the electrons and pulls them together as microclusters.
Due to the charge separation, a restoring force for the electrons begins to act and after the potential train leaves the region, the phases of the micro-beams gradually mix and begin to oscillate in phase as a single beam as shown in Fig. This oscillating plasma dipole generates radiation with a plasma frequency that can be tuned by varying the background plasma density.
Radiation
Do plasma oscillations radiate?
0.3; other simulation parameters are given in section 4.2. b), (c) Shows the measurements of probes at the dipole center with a difference of 1 μm in the y direction.
New technique to measure local plasma density
Method
- Linearly density profiles
- Limitations
From figure 8, one can see the backscattering scheme to measure the density, where the lasers are set to be mobile, which is unlikely to be realized in real-life experiments, or at least greatly complicates the construction of the observing system. This resolves the laser wavelength by approximately 20 and 5. The resolution in the x-direction is higher because the laser propagates in that direction. Two probes were artificially placed to control the electrical and magnetic signals in the desired positions.
7c shows Ex and Bz values at probe 1 which construct non-zero curl which are the reasons for the radiation. This is the key idea to use to measure the density at that dipole location. The frequency spectra confirm this well, Fig. 8c shows the final step of reconstructing the density profile.
Vertical axis indicates density normalized by horizontal axis is the direction of the density gradient (y-direction of the simulation domain). The error bars derive from the spectral full bandwidth of the radiation at half maximum (FWHM). It was also discussed in Chapter 3 that obliquely collided lasers have complex overlapping regions, which improves the efficiency of radiation generation accordingly.
All the simulation parameters are the same; the only difference is in the plasma density distribution over the simulation domain. 8a, the threshold angle can be set to 15° for which the dipole shows narrowband and reliable data, otherwise the radiated emission has unreliable characteristics. Vertical axes in (b) and (c) represent the density normalized by horizontal axes being the y-position along the gradient.
When the density shows extreme changes, the dipole fails to oscillate homogeneously, so reliable local density data with a broad peak spectrum is not available, making it difficult to distinguish the main peak. The emitted signal is radiated at a plasma frequency, which is simply cut off from the surrounding plasma when there is a density bump.
Conclusion
- Higher harmonic radiation
- Magnetic field effects
- Thermal effects
- Propagation of the emitted wave
- Dipole formation – density steepness limitations and broadening of the spectrum
We have studied 2D PIC simulations in settings relevant to laser-plasma interaction system, where the plasma density is in scales of 1018 cm-3, and the driving wavelength is ~1μm. Preparations are underway to apply the dipole concept to low intensity, low density plasma cases. It is therefore argued that the electron neutral collisions are not enough to critically affect the PDO emission in most of the plasma sources.
Moreover, the electron beam of the plasma dipole is thermal, since the dipole is generated from the electron capture, and it is prone to stochastic heating. 1], it is observed that the dipole bundle has the thermal velocity on the order of ~50 eV, and is high enough that the electron-ion collision effect is negligible. The intensity decrease of the radiated wave over distance is relevant to the question of the sensitivity of the detector.
1], it was demonstrated that the emission of the PDO obeys the pattern of the two-dimensional dipole radiation. In addition, we performed similar analysis (unpublished) that showed the decay of the field strength over the propagation distance. In the three dimensions, the field strength of the dipole radiation can be estimated by.
Note that the transverse size is approximately the same as the spot size of the laser pulse. Note that if ∝ , ~ 10 V/m then the field measured 1 m away from the dipole will be of the order of a few hundred V/m, implying that it will be very strong and measurable. If the plasma density varies very widely, the plasma dipole cannot be formed coherently.
This can be seen by the broadening as the slope of the density gradient becomes too high. The guaranteed region of the real dipole oscillation is up to = 15%; If the density varies too much along the dipole width, the dipole cannot be formed properly and therefore narrowband emission will not occur.
Conclusion
I want to thank him as he has always supported us and treated his students with special fatherly care and love. Finally, I want to thank the faculty members and the Korean government for providing a scholarship and funds to do basic science. 1] Kwon K B, Kang T Y, Song H S, Kim Y K, Ersfeld B, Jaroszynski D A and Hur M S 2018 High-Energy, Short-Duration Bursts of Terahertz Coherent Radiation from an Embedded Plasma Dipole Sci.
2] Cherrington BE 1982 The use of Langmuir probes for plasma diagnostics: a review of plasma chemistry and plasma processing2, 113–140. 10] Kang T, Kwon K B, Cho MH, Kim Y K, Hur MS, Park H K, Lee W 2018 Envelope-PIC hybrid method for the simulation of microwave reflectometry IEEE Trans. Kylychbekov S et al2020 Reconstruction of plasma density profiles by measuring spectra of radiation emitted by oscillating plasma dipolesPlasma sources Sci.
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22] Chen F F 2003 Langmuir Probe Diagnostics EE, UCLA; Minicourse on plasma diagnostics IEEE-ICOPS meeting, Korea. 27] Song H S, Cho M H, Kim Y K, Kang T Y, Suk H and Hur M S 2016 Measurement of local density and magnetic field of a magnetized plasma using Raman scattering from a focused laser pulse Plasma Phys. 28] Cho M H, Kim Y K and Hur M S 2014 Measuring the magnetic field in a magnetized plasma using Raman scattering Appl.
29] Jang H, Hur M S, Lee J M, Cho M H, Namkung W and Suk H 2008 A method for measuring the electron temperature and density of laser-produced plasma by Raman scattering Appl. 32] Cho M H, Kim Y-K, Suk H, Ersfeld B, Jaroszynski D A and Hur M S 2015 Strong terahertz emission from near-boundary electromagnetic diffusion in plasma Novo J.
