Particle-in-cell simulations of parametric decay instabilities at the upper hybrid layer of fusion plasmas to determine their primary threshold

Senstius, Mads Givskov; Nielsen, S. K.; Vann, R. G. L.; Hansen, S. K.

doi:10.1088/1361-6587/ab49ca

Cited by 18 publications

(18 citation statements)

References 26 publications

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“…The remaining spatial dimensions are assumed homogeneous. PDIs have previously been observed in EPOCH simulations of EBW conversion schemes 55 and of CTS experiments 54 .…”

Section: Decay Scatteringmentioning

confidence: 61%

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Numerical investigations of parametric decay into trapped waves in magnetized plasmas with a non-monotonic density background

2020

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Parametric decay instabilities (PDIs) exciting daughter waves trapped inside a magnetized plasma with a non-monotonic density profile are investigated numerically. The investigation is motivated in particular by observations of low threshold PDI signatures during 2nd harmonic electron cyclotron resonance heating (ECRH) experiments in magnetically confined fusion experiments. We use the particle-in-cell (PIC) code EPOCH to study conversion of a fast X-mode pump wave into a combination of half frequency X-mode and electron Bernstein waves (EBWs), and identify two regimes where PDIs can excite trapped electrostatic waves. Above the 2nd harmonic upper hybrid (UH) density, a PDI known also as a two plasmon decay (TPD) instability excites a pair of UH waves which we locate in frequency and wavenumber space. At lower densities, a PDI known as stimulated Raman scattering may produce one trapped and one returning X-mode daughter wave with a much slower growth rate than the TPD instability. In both cases, we show that the frequency separation of the daughter waves depends on the density in a predictable manner. With little loss from the decay region, the trapped daughter waves become unstable with respect to secondary parametric instabilities (PIs), leading to distinctly different phases of the UH spectrum. Unlike the primary instability, the secondary PIs are shown to depend on ion dynamics. Furthermore, we observe escaping waves near the 3/2 pump frequency resulting from tertiary PIs in agreement with recently proposed backscattering during magnetically confined fusion experiments.

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“…The remaining spatial dimensions are assumed homogeneous. PDIs have previously been observed in EPOCH simulations of EBW conversion schemes 55 and of CTS experiments 54 .…”

Section: Decay Scatteringmentioning

confidence: 61%

“…In this article, we use the particle-in-cell (PIC) code EPOCH 53,54 to study PDIs into one or two trapped UH waves due to a non-monotonic plasma density profile. Using Xmode and EBW dispersion relations, we investigate two processes, TPD and stimulated Raman scattering, which may produce the trapped waves from an X-mode pump wave.…”

Section: Introductionmentioning

confidence: 99%

Numerical investigations of parametric decay into trapped waves in magnetized plasmas with a non-monotonic density background

2020

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“…We use the particle-in-cell (PIC) code EPOCH [40,41,28,42] to simulate TPD into UH waves which are trapped in a 1D non-monotonic background density bump. Several simulations are performed with all plasma parameters kept constant except for the width of the density bump.…”

Section: Setupmentioning

confidence: 99%

Trapped upper hybrid waves as eigenmodes of non-monotonic background density profiles

Senstius

Nielsen

Vann

2021

Plasma Phys. Control. Fusion

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Non-monotonic plasma density structures such as blobs and magnetic islands give rise to trapped upper hybrid (UH) waves. Trapped UH waves which satisfy Bohr–Sommerfeld quantization can be thought of as eigenmodes of a cavity. Using fully kinetic particle-in-cell simulations, we verify the existence of these UH eigenmodes and demonstrate their significance as only eigenfrequencies become unstable to three-wave interactions. The eigenmodes can be excited through parametric decay instabilities (PDIs) of an X-mode pump wave at approximately twice the UH frequency, as could be the case for a gyrotron beam traversing a blob in a magnetically confined fusion plasma. We derive a closed expression for the wavenumber of UH waves, which is accurate both close to the UH layer and to the electron cyclotron resonance. This allows for fast analysis of eigenmodes in a non-monotonic structure. An expression for the amplification of PDI daughter waves in an inhomogeneous plasma is extended to a decay region where the first several derivatives vanish. From the amplification in a convective PDI, we estimate the growth rate of the absolute PDI involving the trapped waves. We show that the excitation of eigenmodes through PDIs in our simulations are indeed absolute rather than convective due to the trapping of the daughter waves. Additionally, we show that only eigenmodes get excited through the PDIs, and that we are able to predict the growth rates of the daughter waves and how they scale with the pump wave intensity. This is evidence supporting a fundamental assumption of analytical theory describing low threshold strong scattering observed in magnetically confined fusion experiments during second harmonic electron cyclotron resonance heating (ECRH). Such low threshold instabilities can degrade ECRH performance but also offer novel uses for ion heating or as diagnostics.

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“…Parametric Decay Instability (PDI) of dust lattice waves has been studied theoretically by Shukla 33 showing a phase transition of a dusty plasma medium from solid to gas-like state. PDI are also observed in many other aspects of plasma physics, e.g., ionosphere 34 , inertial confinement fusion 35 , magnetic confinement fusion 36 , laser-plasma interactions 37 , 38 , laser wake field acceleration 39 . The PDI is the nonlinear process of transferring the energy of a pump wave into other waves.…”

Section: Introductionmentioning

confidence: 99%

Parametric decay induced first-order phase transition in two-dimensional Yukawa crystals

Maity

Arora

2022

Sci Rep

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The melting process of two-dimensional (2D) Yukawa crystals for dusty plasma medium induced by external perturbations has been explored using molecular dynamics simulations. A 2D monolayer of particles interacting via Yukawa pair potential is formed in the presence of an external confinement potential. The confinement potential is a combined effect of the gravitational force and an externally applied electric force, which mimics the sheath electric field in dusty plasma experiments. The response of the 2D crystalline layer to an external perturbation is investigated. It is shown that transverse surface waves are generated below a particular threshold value of initial perturbation, but the crystalline order remains. However, above a threshold value of initial disturbance, the crystalline order structure of the 2D layer breaks, and it melts. The melting process is shown to be a first-order phase transition. We have demonstrated that the nonlinear amplitude modulation of initial disturbance through the parametric decay instability is responsible for the melting. Our proposed mechanism of first-order phase transition in the context of 2D dusty plasma crystal is distinctly different from the existing theoretical models. This research can provide a deeper understanding of the experimental observations in the context of plasma crystal.

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Particle-in-cell simulations of parametric decay instabilities at the upper hybrid layer of fusion plasmas to determine their primary threshold

Cited by 18 publications

References 26 publications

Numerical investigations of parametric decay into trapped waves in magnetized plasmas with a non-monotonic density background

Numerical investigations of parametric decay into trapped waves in magnetized plasmas with a non-monotonic density background

Trapped upper hybrid waves as eigenmodes of non-monotonic background density profiles

Parametric decay induced first-order phase transition in two-dimensional Yukawa crystals

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