Merger and reconnection of Weibel separated relativistic electron beam

Abstract

The relativistic electron beam (REB) propagation in a plasma is fraught with beam plasma instabilities. The prominent amongst them being the collisionless Weibel destabilization which spatially separates the forward propagating REB and the return shielding currents. This results in the formation of REB current filaments which are typically of the size of electron skin depth during the linear stage of the instability. It has been observed that in the nonlinear stage the filaments size increases as they merge with each other. With the help of 2-D PIC simulations in the plane perpendicular to the REB propagation, it is shown that these mergers occur in two distinct nonlinear phases. In the first phase, the total magnetic energy increases. Subsequently, however, during the second phase, one observes a reduction in magnetic energy. It is shown that the transition from one nonlinear regime to another occurs when the typical current associated with individual filaments hits the Alfvén threshold. In the second nonlinear regime, therefore, the filaments can no longer permit any increase in current. Magnetic reconnection events then dissipate the excess current (and its associated magnetic energy) that would result from a merger process leading to the generation of energetic electrons jets in the perpendicular plane. At later times when there are only few filaments left the individual reconnection events can be clearly identified. It is observed that in between such events the magnetic energy remains constant and shows a sudden drop as and when two filaments merge. The electron jets released in these reconnection events are thus responsible for the transverse heating which has been mentioned in some previous studies [Honda et al. Phys. Plasmas 7, 1302(2000].

Ion effects on the scaling of magnetic field amplification in plasmas with the system size

“…Time evolutions of the transverse magnetic field energy E B ⊥ and beam electron energy E be are shown in figure 2 for the two plasma cases. The growth rate of the instability is [12,28,36,37], where β = v be0 /c, α is the beam-to-plasma density ratio, as indicated in figure 2(a). Good agreement is found between the theory and simulation results.…”

“…One of the typical characteristics of CFI is the strong magnetic field generation. Both the linear and nonlinear stages of the instability are detailed investigated using PIC simulations [11][12][13][14][15][16][17][18][19][20][21]. During the linear stage of CFI, the magnetic field energy grows exponentially and then becomes saturated when beam electrons inside the filament are trapped in the magnetic field.…”