High μ core ELF/VLF Miro-Array Transmitters (MAT) for Space Platforms

ELF/VLF waves with frequencies from few Hz to several kHz are a ubiquitous component of the ionosphere, the near-Earth magnetosphere, and the Radiation Belts (RB). They are generated by instabilities, lightning discharges, and ground VLF transmitters. Observations by the Van-Allen probes confirmed the role of nonlinear interactions driven by anthropogenic VLF sources in controlling space weather phenomena and the population of MeV electrons in the Radiation Belts (RB). The MeV electrons are known as “killer electrons” due to their adverse effects on the operation and lifetime of LEO communications and reconnaissance satellites. However, despite their importance their properties and especially their non-linear interaction has never been tested in space. This is due to the inefficiency of conventional electric and magnetic dipole transmitters. In experiments at the UCLA LAPD chamber, we tested a Rotating Magnetic Field (RMF) as an antenna. The RMF consisted of two orthogonal coils with magnetic moment (M) perpendicular to the B field, driven by AC currents with phase difference ± π/2, and frequency to drive RH Whistlers above the ion cyclotron frequency (Ω
i
) and LH Electro-Magnetic Ion Cyclotron (EMIC) below Ω
i
. It was found
¹
that contrary to the ELF/VLF dipoles the RMF waves had helical phase surfaces described by Exp(±mϕ+kz-ωt). The magnetic topology is a combination of a loop with M ⊥ B, that propagates along B while rotating about it. The combination of phase rotation and translation results in spiraling phase surfaces like Helicons
²
(H). RMF antennae: (i) Introduce Orbital Angular Momentum (OAM) much larger that due to polarization; (ii) Control the radial wavenumber ±m; (iii) The antenna determines the transverse phase and amplitude profile while the highly aligned group velocity (V
g
) prevents radial energy spread; (iv) Generates large OAM and large k
⊥
that result in large transverse magnetic field gradients and transverse Doppler shifts of energetic electrons with rotating waves that facilitate breaking the adiabatic invariance of the “killer electrons”, as verified in LAPD experiments
³
. The drawback of the RMF antenna was its low radiation resistance. The LAPD experiments utilized loops with 8 cm diameter, driven by 600 A each, resulting in M
⊥
= 11.5 A-m
²
and generated power P=250 W. The resulting radiation resistance was R
r
=2x10
^-4
Ohm. A value very low to generate B field amplitude exceeding the threshold for non-linear wave-particle interactions that result in triggering
⁷
of Whistlers and EMIC waves. In LAPD experiments
⁴
and current experiments in the NRL chamber we introduced a novel type of antenna the consisting of a long solenoid with small diameter d=O(cm) and aspect ratio larger than 30, into which a core of a non-conducting high μ material was inserted. The first set of experiments used a Ferrite Loop Antenna (FLA) in which a high μ ferrite was inserted in the solenoid. The experiments demonstrated that phasing individual FLA rods with their M perpendicular to B reproduced all the helicon properties of the RFM properties mentioned previously. It was further demonstrated that that radiation resistance contributed by each rod R
r
(μ)=μ
²
R(μ=l). Both RH W/H and LH EMIC/H waves were driven. Using a two element anti-phased array FLA rods perpendicular to B resulted in m=±1 H-modes like those driven by the RMF. Using ferrite with μ=30 the two-element array driven by I=1.6 A had M=10
³
A-m
²
and generated power P=.34 W. The measured R
r
=.14 Ohm was more than 30 dB higher than the RMF under similar plasma parameters. Control of k
⊥
by using ten-element phase-array and controlling the angle of the array with respect to B. Examples of radially and azimuthally phased arrays will be presented. The presentation will conclude with the first proof of principle experiments of the Magnetic Nano-Transmitter
⁵
(MNT). The MNT is a phased array whose elements are in which the core of the solenoid is filled by superparamagnetic nanoparticles embedded in a non-conducting solid, such as epoxy or paraffin. Theoretical estimates indicate that using a mixture 50%-50% Fe-Co, MNT can achieve performance by more than 20 dB higher than the FLA arrays, and importantly for space-based transmitter achieve it by an order of magnitude lower weight.