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Abstract The matter in an accretion disk must lose angular momentum when moving radially inwards but how this works has long been a mystery. By calculating the trajectories of individual colliding neutrals, ions, and electrons in a weakly ionized 2D plasma containing gravitational and magnetic fields, we numerically simulate accretion disk dynamics at the particle level. As predicted by Lagrangian mechanics, the fundamental conserved global quantity is the total canonical angular momentum, not the ordinary angular momentum. When the Kepler angular velocity and the magnetic field have opposite polarity, collisions between neutrals and charged particles cause: (i) ions to move radially inwards, (ii) electrons to move radially outwards, (iii) neutrals to lose ordinary angular momentum, and (iv) charged particles to gain canonical angular momentum. Neutrals thus spiral inward due to their decrease of ordinary angular momentum while the accumulation of ions at small radius and accumulation of electrons at large radius produces a radially outward electric field. In 3D, this radial electric field would drive an out-of-plane poloidal current that produces the magnetic forces that drive bidirectional astrophysical jets. Because this neutral angular momentum loss depends only on neutrals colliding with charged particles, it should be ubiquitous. Quantitative scaling of the model using plausible disk density, temperature, and magnetic field strength gives an accretion rate of 3 × 10−8solar mass per year, which is in good agreement with observed accretion rates.
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Model for binary collisions in strongly magnetized plasmas: Repulsive interactions
A reduced model is developed to describe the outcome of collisions between two like-charged particles in the presence of a strong magnetic field. Two cases are considered: large mass ratio (e.g., positron–proton or electron–antiproton) and unity mass ratio (e.g., electron–electron or ion–ion). The model applies to the asymptotic regime of strong magnetization, where the gyroradius of the low-mass particle is small compared to the interaction spatial scale (of the order of the Debye length in a weakly coupled plasma). The ion is assumed to be weakly magnetized in the two-component case. The positron (or electron) magnetic moment is assumed to be conserved during the collision, satisfying the first adiabatic invariant. The model then solves for other aspects of the charged particle motion perturbatively in orders of the inverse magnetic field strength. For the positron–ion case, this includes the velocity vector of the ion, the change in velocity of the positron parallel to the magnetic field, and the spatial shift of the positron gyrocenter. For the identical particle case, this includes the relative speed of the two particles in the parallel direction and the shift of the relative gyrocenters of the particles. An important aspect of the model is the identification of a generalized conserved momentum. The results enable the determination of the outcome of collisions with far lower computational resources than required for full orbit calculations, and can be utilized to rapidly evaluate transport rates for kinetic theories. The regimes considered are expected to be particularly relevant to experiments that trap antimatter.
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- Award ID(s):
- 2205506
- PAR ID:
- 10643085
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- Physics of Plasmas
- Volume:
- 32
- Issue:
- 10
- ISSN:
- 1070-664X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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