Destruction of a Magnetic Mirror-Trapped Hot Electron Ring by a shear Alfven Wave

Transcription

1 Destruction of a Magnetic Mirror-Trapped Hot Electron Ring by a shear Alfven Wave Y. Wang 1, W. Gekelman 1, P. Pribyl 1, D. Papadopoulos 2 1 University of California, Los Angeles 2 University of Maryland, College Park Work supported by ONR MURI award (Fundamental Physics Issues on Radiation Belt Dynamics and Remediation), performed at the Basic Plasma Science Facility which is supported by DOE and NSF

2 Motivations and Background * Image from NASA website Radiation Belt Remediation Artificial method to control and drain the energetic particles trapped by the Earth s magnetic field, which can be fed by natural source or human activities (such as High-altitude nuclear explosions) and pose hazards to space satellites.! Rotating Magnetic Field (RMF) source -- The RMF source is an innovative method to efficiently launch waves in space plasmas.!2

3 Schematics of the experiment!3

4 Hot electron ring generation Fast electrons are generated by Electron Cyclotron Resonant Heating (ECRH) at f microwave = 2f ce. (Peak Power = 15 kw, pulse duration = 30 ~ 50 ms) X-rays are measured outside the 3/8 stainless steel vacuum chamber, which cut off x-ray transmission below 100keV. The perpendicular E field generates a hot electron population with large perpendicular energies, which grad-b and curvature drift in the θ-direction and form a hot electron ring in the magnetic mirror.!4

5 Measurement of the ring size The size and position of the hot electron ring is measured by inserting a luminator probe along the positive x- axis. The thickness of the ring is ~10cm The axial extension of the ring is measured to be Δz = 211 cm, which corresponds to a minimum hot electron pitch angle of 56 o (loss cone = 47 o ) θ min = tan 1 & $ 1 % B( z B max min ) # 1! = 56 " θ loss cone = 47!5

6 Rotating Magnetic Field (RMF) antenna Measured B Alfven vectors 2 m away from antenna The RMF antenna is composed of 2 orthogonal coils, placed in x-z and y-z planes, with diameters of 8 cm and 9 cm. Driven by 2 independent RF drivers, capable of launching waves with arbitrary polarity. The shear Alfvén wave dispersion relation has been verified.!6

7 Disruption of the hot electron ring The shear Alfvén wave significantly enhances hot electron loss, as evidenced by a burst of x-rays. The x-ray signal is modulated at the Alfvén wave frequency. Signal averaged over 1200 shots!7

8 A population of fast electrons persists after the shut-off of the ECRH, and can be de-trapped by application of the shear Alfvén wave to produce X-ray bursts.!8

9 Electrons lost axially travel ~ 11m along the magnetic field line to the anode. X-rays are generated on the mesh anode. Electrons lost radially are most likely to strike the waveguide, which is the closest metallic object to the magnetic mirror. * Graph normalization: Parallel loss is about 2% of radial loss!9

10 Alfvén ghost The fast electrons loss is observed to continue even after the termination of the Alfvén wave.!10

11 Deformation of the hot electron ring The ring becomes asymmetric in the Alfvén wave field. The deformation of the ring gives rise to the oscillations in the x-ray signal at f=f Alfvén.!11

12 Deformation of the hot electron ring The ring becomes asymmetric in the Alfvén wave field. The deformation of the ring gives rise to the oscillations in the x-ray signal at f=f Alfvén.!12

13 Deformation of the hot electron ring The ring becomes asymmetric in the Alfvén wave field. The deformation of the ring gives rise to the oscillations in the x-ray signal at f=f Alfvén.!13

14 Role of Alfvén wave polarization LH and RH waves of arbitrary amplitudes are mixed together to scatter the hot electrons. The x-ray intensity is only related to the amplitude of the RH component. LH and RH waves of same amplitude and arbitrary phases are mixed. The x-ray oscillation is phase locked to the RH component.!14

15 X-ray spectrum X-ray spectrum is measured by analyzing pulse heights from the NaI(Tl) scintillator x-ray detector. The Alfvén wave de-trapping effect is observed for electrons with a broad range of energy. Radial loss Axial loss!15

16 Proposed de-trapping mechanism The hot electron ring is deformed in the non-uniform Alfvén wave field, most likely by the E wave B 0 drift. It is proposed that the deformation accumulates if the ring azimuthal drift speed matches that of the rotation of the Alfvén wave pattern. Collective modes of the ring, with three dimensional spatial distortion, can affect its confinement and lead to losses.!16

17 Summary The enhanced loss of fast electrons trapped in a magnetic mirror geometry irradiated by shear Alfvén waves is studied by laboratory experiments. Magnetic mirror trapped fast electrons with energies up to 3 MeV are generated by 2 nd harmonic Electron Cyclotron Resonance Heating Shear Alfvén waves are launched by a Rotating Magnetic Field antenna with arbitrary polarity Irradiated by a right-handed circularly polarized shear Alfvén wave, the electrons are lost in both the radial and axial direction with a modulated at f Alfvén. The loss continues even after the termination the wave. Test particle simulation confirms that the single particle motion of the trapped fast electrons in presence of a shear Alfvén wave is not adequate to explain the experimental observation. No axial loss is observed in the test particle simulation with a wave amplitude measured in the experiment It is proposed that the deformation of the hot electron ring drives a collective mode of the ring that leads to electron losses from the magnetic mirror. Experimental evidence indicates deformation of the hot electron ring, most likely due to the E wave B 0 drift in the Alfvén wave field. The deformation grows when the electron azimuthal (grad-b and curvature) drift matches the rotation of the RH shear Alfvén wave. The non-uniform 3D charge distribution in the deformation builds up a large scale global electric field and leads to electron loss.!17

18 Planning next experiment Frequency GHz peak power 225 kw maximum pulse width 0.5 us (or 2.4 us) 0.1% duty cycle maximum!18

19 !19