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
Motivations and Background } 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 -- * Image from NASA website The RMF source is an innovative method to efficiently launch waves in space plasmas. Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave � 2
Schematics of the experiment Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave � 3
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. Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave � 4
Measurement of the ring size } The size and position of the hot electron } The axial extension of the ring is ring is measured by inserting a measured to be Δ z = 211 cm, which “luminator probe” along the positive x- corresponds to a minimum hot axis. The thickness of the ring is ~10cm electron pitch angle of 56 o (loss cone = 47 o ) B ( z ) & # 1 = 47 tan 1 max 1 56 − $ ! θ ° θ = − = ° min loss cone $ ! B % " min Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave � 5
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. Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave � 6
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 Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave � 7
} 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. Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave � 8
Electrons lost axially travel ~ 11m along the Electrons lost radially are most likely to strike } } magnetic field line to the anode. X-rays are the waveguide, which is the closest metallic generated on the mesh anode. object to the magnetic mirror. * Graph normalization: Parallel loss is about 2% of radial loss Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave � 9
Alfvén ghost } The fast electrons loss is observed to continue even after the termination of the Alfvén wave. Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave � 10
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 . Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave � 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 . Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave � 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 . Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave � 13
Role of Alfvén wave polarization } LH and RH waves of same amplitude } LH and RH waves of arbitrary and arbitrary phases are mixed. amplitudes are mixed together to scatter the hot electrons. } The x-ray oscillation is phase locked to the RH component. } The x-ray intensity is only related to the amplitude of the RH component. Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave � 14
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 Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave � 15
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. Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave � 16
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. Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave � 17
Planning next experiment… • Frequency 8.5-9.6 GHz • peak power 225 kW maximum • pulse width 0.5 us (or 2.4 us) • 0.1% duty cycle maximum � 18
� 19
Recommend
More recommend
