Destruction of a Magnetic Mirror-Trapped Hot
Electron Ring by a shear Alfven Wave
Y. Wang1, W. Gekelman1, P. Pribyl1, D. Papadopoulos2
1 University of California, Los Angeles
2 University of Maryland, College Park
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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
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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.
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* Image from NASA website
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Rotating Magnetic Field (RMF) source -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
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Schematics of the experiment
Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave
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Hot electron ring generation
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Fast electrons are generated by Electron Cyclotron Resonant Heating (ECRH) at
fmicrowave = 2fce. (Peak Power = 15 kW, pulse duration = 30 ~ 50 ms)
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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.
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Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave
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Measurement of the ring size
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The size and position of the hot electron
ring is measured by inserting a
“luminator probe” along the positive xaxis. The thickness of the ring is ~10cm
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The axial extension of the ring is
measured to be Δz = 211 cm, which
corresponds to a minimum hot
electron pitch angle of 56o (loss cone
= 47o)
&
θ min = tan −1 $$ 1
%
B( z max ) #!
− 1 ! = 56° θ loss cone = 47°
Bmin
"
Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave
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Rotating Magnetic Field (RMF) antenna
Measured BAlfven vectors
2 m away from antenna
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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.
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Driven by 2 independent RF drivers,
capable of launching waves with arbitrary
polarity.
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The shear Alfvén wave dispersion relation
has been verified.
Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave
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Disruption of the hot electron ring
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The shear Alfvén wave significantly
enhances hot electron loss, as
evidenced by a burst of x-rays.
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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
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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
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Electrons lost axially travel ~ 11m along the
magnetic field line to the anode. X-rays are
generated on the mesh anode.
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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
Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave
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Alfvén ghost
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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
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Deformation of the hot electron ring
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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=fAlfvén.
Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave
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Deformation of the hot electron ring
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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=fAlfvén.
Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave
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Deformation of the hot electron ring
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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=fAlfvén.
Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave
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Role of Alfvén wave polarization
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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.
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LH and RH waves of same amplitude
and arbitrary phases are mixed.
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The x-ray oscillation is phase locked
to the RH component.
Destruction of a magnetic mirror-trapped hot electron ring by a shear Alfvén wave
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X-ray spectrum
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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
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Proposed de-trapping mechanism
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The hot electron ring is deformed in the non-uniform Alfvén wave field,
most likely by the Ewave×B0 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
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Summary
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The enhanced loss of fast electrons trapped in a magnetic mirror geometry irradiated
by shear Alfvén waves is studied by laboratory experiments.
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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.
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Magnetic mirror trapped fast electrons with energies up to 3 MeV are generated by 2nd 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 fAlfvén. The loss continues even after the termination the wave.
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.
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Experimental evidence indicates deformation of the hot electron ring, most likely due to the Ewave×B0
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
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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
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