Scattering of Magnetic Mirror Trapped Fast Electrons by a Shear Alfvén Wave

Laboratory observations of enhanced loss of fast electrons trapped in a magnetic mirror geometry irradiated by shear Alfvén waves (SAW) are reported. A population of runaway electrons generated by second harmonic electron-cyclotron-resonance heating, as evidenced by the production of hard x rays with energy up to 3 MeV, is subjected to SAW launched with a rotating magnetic field antenna. It is observed that the SAW dramatically affect the trapped fast electrons and scatter them out of the magnetic mirror despite any obvious resonance. The results could have implications on the techniques of artificial reduction of energetic electrons in the inner radiation belt.

Figure 1

Schematic of the experiment (not to scale). The cathode-anode separation is 0.55 m. The plasma column is 17 m long and 0.6 m in diameter. The magnetic coils are not shown. The center of the magnetic mirror defines the $z=0$ position. The length of mirror section is about 3.5 m. The Alfvén wave launcher is at $z=-2.00\mathrm{m}$, and the cathode at $z=10.75\mathrm{m}$.

Figure 2

A plot of the plane at $z=0$, showing the relative size and location of the microwave waveguide and the hot electron ring (the purple annulus). Also shown is a measurement of the Alfvén wave ${\overrightarrow{B}}_{\perp }$ vectors on this plane 0.15 ms after the start of the Alfvén wave burst. (b) X-ray flux as a function of luminator probe tungsten tip position at different times during the ECRH. The probe is inserted radially along the positive $x$ axis.

Figure 3

Time series of the x-ray flux. (a) Comparison of x-ray measurement with or without the presence of a 100-cycle SAW. The ECRH is on from $t=0$ to 30 ms, but only after about 20 ms are there sufficient high energy electrons to produce a measurable x-ray flux. (b) Overlay of 19 traces (designated $A\mathrm{\text{−}}S$), each measured with a 100-cycle SAW launched at different time delays (marked by an arrow on the top). A population of fast electrons persists after the shutoff of the ECRH, and can be detrapped by application of the SAW to produce x-ray bursts. (c) 1200 shot averaged signal of the x-ray burst during SAW propagation. $B_x$ of the SAW measured at the center of the magnetic mirror is shown on the bottom trace.

Figure 4

(a) Schematic of the “soda-straw probe.” Electrons collected by this probe have gyroradii larger than a threshold value set by $L$ (the distance the copper wire recessed into the ceramic tube) and $d$ (ceramic tube inner diameter). (b) Current measured using soda-straw probe with 3 different recessed distances. The corresponding threshold energies are 140, 240, and 480 eV. The small positive signal at the outset of the $L=0.64\mathrm{mm}$ trace is due to ions collected by the probe. (c) Comparison of cases with or without a SAW launched from $t=1.5\mathrm{ms}$ to 1.7 ms.