6th Asia-Pacific Conference on Plasma Physics, 9-14 Oct, 2022, Remote e-conference

Study of Plasma Oscillator as a Radiation Source for Compact ElectronAccelerators
Min Sup Hur1
1Dept. Physics, UNIST
e-mail (speaker): mshur@unist.ac.kr
Introduction
Plasma is a medium equipped with rich opticalproperties due to its electromagnetic susceptibility.
Among them, plasma oscillation is a simple andfundamental characteristic of a plasma. One good
example of its successful application might be thewakefield electron acceleration in a plasma, whereas its
usefulness as a radiation source has been overlooked.
Recently several ideas for using the plasma oscillation asa THz radiation source have emerged, such as inverse
mode conversion [1] or superluminous plasma wave [2].
In our research, we devised a couple of exotic ideas for
isolating a single plasma oscillator, which can directly
emit a radiation with almost a hundred percent (actually,theoretically) coupling efficiency between radiation andoscillator. In this presentation, I will brief a couple ofideas to accomplish the isolation of a plasma oscillator
and THz emission from that. Also, though it is not quite
on the context of radiation source, I will brief our recent
study for compression of laser pulses in a density
gradient plasma.
Idea
We make two laser pulses collide in the plasma. The
lasers are slightly detuned in their frequencies by anamount of plasma frequency, but they need not exactly
the same to each other. Because of the detuning, in theoverlapped region of the two laser pulses, a slowly
moving ponderomotive beat potential is formed. Whenthe beat potential is deep enough (say, by using intenselaser pulses such that a0 > 0.1), the background electrons
are trapped into the beat potential train. As the trappedelectrons, riding on the potential train, move together
slowly in the direction of the higher frequency laser
pulse (but much slower), i.e. a bunch of electrons(trapped electrons) is dragged slowly by the laser pulses.
As the laser pulses pass by each other, the beat potential
is weakened, and the displaced bunch of electrons getsno more displacing force and is released to commence
the plasma oscillation. The overall procedure is like a
drag-and-release of a mass attached to a spring.
The plasma oscillation generated in this way is unique.
The plasma oscillation exists usually in the form of aplasma wave; electrons oscillate around their equilibrium
positions but with different phases (mostly sinusoidallydistributed phase). In contrast, our plasma oscillation isan oscillator rather than a wave; all the electrons in theoscillating bunch have the same phase. In that sense, wecall it a plasma dipole oscillator (PDO). PDO electrons
move in-phase, so it can emit electromagnetic wave as in
the dipole antenna, with a very high coupling efficiency
between the oscillator and emission.

As the radiation is emitted from the plasma oscillation,
the emission frequency is well collimated to the plasma
frequency. Furthermore, the plasma frequency of typicalunder-dense laser-produced plasma is in the terahertz(THz) range. Hence the emission from PDO can be ideal
for THz-driven compact electron accelerators.
Results
We conducted simulation studies on PDO-THz using
particle-in-cell (PIC) codes. From 2D PIC simulations,
we obtained up to 1.5 % conversion efficiency from theenergy of the driving laser pulses to the THz emission.
The peak field strength of the emitted THz reaches
several GV/m with total energy of order mJ. Such ahighly efficient, strong THz emission could be obtainedby optimal shaping of the driving laser pulses, which I
brief in this presentation.
Other than this result, I also present another novel idea
for generation of a similarly isolated bunch of electron
plasma oscillation, using the resonance driving of theplasma motion. In this new configuration, the emission isconsiderably more directional, radially polarized, andhighly narrowbanded, especially useful for the THz
accelerators.
At the same time, I brief a bit different concept for
using the plasma as a pulse compression grating [3]. The
idea can be a potentially useful candidate for thecompact exawatt laser systems.
Conclusion
In this presentation, I introduce a couple of novel ideas
for generating narrowband, high-efficiency THz
emission from plasma oscillation. The idea is expected tobe useful in development of THz-driven, next-generationelectron accelerating scheme. Along with that, I brief our
newly conceived idea for getting compression ofultraintense laser pulses in a plasma. peak power isexpected.
References[1] Z.-M. Sheng et al., Phys. Rev. Lett. 94, 095003(2005).
[2] A. Pukhov et al., Phys. Rev. Lett. 127, 175001(2021).
[3] M.S. Hur et al., Nat. Phot. (recommended forpublication in 2023).