Recent theoretical models and preliminary observations indicate that super small striations (SSS) in the plasma density with scale size of 10 cm can be excited by F region HF heating at frequencies close to multiples of the electron gyrofrequency. We present here new experimental results using the High Frequency Active Auroral Research Program ionospheric heater at a frequency close to the fourth electron gyroharmonic with simultaneous GPS, Stimulated Electromagnetic Emission, ionosonde, and occasional Incoherent Radar Scattering diagnostics. Differential phase measurements of GPS signals through the heated region indicated the presence of SSS with extremely high amplitude (delta n/n= 0.2-0.3) at scale size comparable to the electron gyroradius. The highest amplitude of GPS scintillations coincide with the highest level of the Broad Upshifted Maximum (BUM) and occurred when the HF frequency is slightly above the fourth harmonic of the electron cyclotron frequency. Frequency sweeps indicate that the scintillation amplitude exhibits hysteresis similar to that observed for the BUM amplitude when the HF frequency is cycled about the fourth harmonic of the cyclotron frequency. The results favor a four wave parametric process as the physical mechanism of the SSS. Additional experiments allowed the determination of the excitation and decay rates of the SSS.

We report results from the experiment aimed at the artificial excitation of extremely low-frequency (ELF) electromagnetic waves with frequencies corresponding to the frequency of Schumann resonance. Electromagnetic waves with these frequencies can form a standing pattern inside the spherical cavity formed by the surface of the Earth and the ionosphere. In the experiment the ELF waves were excited by heating the ionosphere with X-mode HF electromagnetic waves generated at the High Frequency Active Auroral Research Program (HAARP) facility in Alaska. The experiment demonstrates that heating of the ionosphere can excite relatively large-amplitude electromagnetic waves with frequencies in the range 7.8–8.0Hz when the ionosphere has a strong F layer, the frequency of the HF radiation is in the range 3.20–4.57MHz, and the electric field greater than 5mV/m is present in the ionosphere.

We present a theoretical and numerical study of the generation of extremely low frequency (ELF) and ultra‐low frequency (ULF) waves by the modulation of the electron pressure at the F2‐region with an intense high‐frequency electromagnetic wave. The study is based on a cold plasma Hall‐MHD model, including electron‐neutral and ion‐neutral collisions, which governs the dynamics of magnetostatic waves and their propagation through the ionospheric layers. Magnetosonic waves generated in the F2 region are propagating isotropically and are channeled in the ionospheric waveguide, while shear Alfvén waves are propagating along the magnetic field. To penetrate the ionosphere from the F2 peak at 300 km to the ground, the magnetostatic waves first propagate as magnetosonic or shear Alfvén waves that encounter a diffusive layer from about 150 km to 120 km where the Pedersen conductivity dominates, and then as helicon (whistler‐like) mode waves from about 120 km to 80 km where the ions are collisionally glued to the neutrals and the Hall conductivity dominates. By performing numerical simulations and studying the dispersive properties of the wave modes, we investigate the dynamics and penetration of ELF/ULF waves through the ionospheric layers to the ground and along the geomagnetic field lines to the magnetosphere. Realistic profiles of the ionospheric profiles of conductivity and density are used, together with different configurations of the geomagnetic field, relevant for both the high, mid and equatorial latitudes. Some of the results are compared with recent HAARP experiments.

We report results from numerical and experimental studies of the excitation of ULF shear Alfven waves inside the ionospheric Alfven resonator (IAR) by heating the ionosphere with powerful HF waves launched from the High Frequency Active Auroral Research Program (HAARP) facility in Alaska. Numerical simulations of the two-fluid MHD model describing IAR in a dipole magnetic field geometry with plasma parameters taken from the observations at HAARP during the October-November 2010 experimental campaign reveal that the IAR quality is higher during nighttime conditions, when the ionospheric conductivity is very low. Simulations also reveal that the resonance wave cannot be identified from the magnetic measurements on the ground or at an altitude above 600 km because the magnetic field in this wave has nodes on both ends of the resonator, and the best way to detect IAR modes is by measuring the electric field on low Earth orbit satellites. These theoretical predictions are in good, quantitative agreement with results from observations: In particular, (1) observations from the ground-based magnetometer at the HAARP site demonstrate no significant difference in the amplitudes of the magnetic field generated by HAARP in the frequency range from 0 to 5 Hz, and (2) the DEMETER satellite detected the electric field of the IAR first harmonic at an altitude of 670 km above HAARP during the heating experiment.

Ionospheric heaters located in Polar Regions (EISCAT, HAARP, HIPAS) have been used extensively to generate electromagnetic waves in the ULF/VLF/ELF frequency range and this concept has been referred as the Polar Electrojet (PEJ) antenna. A major problem with the PEJ is its reliance in the presence and strength of the electrojet current. This restricts not only the location of the heater but makes their generation highly unpredictable. A new concept of using HF heating to generate a virtual antenna in the ionospheric Hall region at ULF/ELF frequencies without relying in the existence of electrojets, was advanced recently by Papdopoulos et al. (2010). The current drive and the creation of a virtual ELF antenna are accomplished in two steps. First, F region heating generates a diamagnetic current with a field aligned magnetic moment. Second, the electric field of the magnetosonic wave radiated by the F region magnetic moment drives a Hall current when it reaches the bottom of the ionosphere. In a fashion similar to the PEJ the HAARP driven virtual antenna excites the Earth-Ionosphere Waveguide (EIW) as well as injects helicon and Shear Alfven (SA) waves upwards into the radiation belts. The paper presents the first experimental demonstration of the ICD concept using the HAARP facility. Strong signals at frequencies between .5-50 Hz have been detected at sites near HAARP as well as far sites located at Homer and Poker Flat in the absence of electrojet currents. While during electrojet modulation the maximum signal occurs in the vicinity of 2-4 kHz, during ICD the peak fields occur at a frequency of 12 Hz and the maximum frequency is limited to 50 Hz by the slow response time of the F-region ionosphere. During strong ICD generation the 2kHz signal, used as a proxy for the presence of electrojet currents remains below .1 pT, more than 30-40 dB smaller than the 1-10 Hz signals. A comparison of the waveforms between the PEJ and ICD waves generated at 20-40 Hz frequency shows that while the former are characterized by the usual sequence of overshoots due to temperature saturation at .2 msec times and the associated reflection of the signals every .5 msec, these features are completely absent from the ICD waveforms as theoretically expected. A diagrammatic representation that shows the measured signals at 1-50 Hz frequencies normalized to simultaneously measured kHz signals has been used to clearly distinguish ICD from PEJ generated waves. Furthermore, the paper presents the first simultaneous ground and DEMETER satellite measurement of ICD generated shear Alfven waves at frequencies 2-5 Hz. The paper concludes with suggestions of similar ICD experiments using the Arecibo heater and the advantages of a heater located at deep equatorial latitudes.

Ionospheric heaters located in the polar-regions (EISCAT, HAARP, HIPAS) have generated waves in the ULF/ELF/VLF range by modulating the auroral electrojet at D/E region altitudes in a concept termed by the EISCAT group as the Polar Electrojet (PEJ) antenna. The PEJ antenna requires an ionospheric heater located under regions of naturally driven D/E region currents, such as in the auroral or equatorial electrojet. In a simplified fashion the PEJ antenna operates as following: In the presence of an electrojet current periodic electron heating in the Hall dominated region of the lower ionosphere reduces the conductivity of the heated spot building oscillatory polarization charges at the boundaries. This acts as an essentially Horizontal Electric Dipole (HED) at the modulation frequency. The HED excites the Earth-Ionosphere Waveguide (EIW) from above while the current closes by launching upwards helicon (whistler) waves carrying field-aligned current. A major problem with the PEJ is its reliance in the presence and strength of the electrojet current. This restricts not only the location of the heater but makes their generation highly unpredictable. We will present theoretical/computational results indicating that modulated HF heating can generate ionospheric currents in the ULF/ELF range even in the absence of electrojet currents. The new concept relies in using HF heating of the F-region to modulate the electron temperature and has been given the name Ionospheric Current Drive (ICD). The ICD is a two-step process: First, the pressure gradient associated with anomalous or collisional F-region electron heating drives a local diamagnetic current that acts as an antenna to inject mainly Magneto-Sonic (MS) waves in the ionospheric plasma. Second, the electric field associated with the MS wave drives Hall currents when it reaches the E region of the ionosphere. The Hall currents act as a secondary antenna that inject waves in the Earth-Ionosphere Waveguide (EIW) below and Sh- - ear Alfven (SA) waves upwards towards the conjugate regions. We will examine the scaling and limitations of the concept with simulation and suggest proof-of-principle experiments using the HAARP ionospheric heater. We will discuss the implications of freeing the ionospheric ULF/ELF generation from the requirement of electrojet availability and strength.

We describe a new model for calculating the ac space charge in a linear-beam traveling wave tube (TWT) large-signal simulation code when the true three-dimensional (3-D) geometry of the interaction circuit is taken into account. We use the 3-D electromagnetic simulation code CTLSS to characterize the ac space charge fields generated by a set of test currents placed inside the periodic interaction structure. This information, expressed in matrix form, is used by the large-signal simulation codes CHRISTINE-1D and CHRISTINE-3D to compute self-consistently the additional space charge field terms due to the structure in response to the beam evolution during the simulation. We present the formulation, and describe the implementation in both CTLSS and the large-signal codes. We validate the model by comparison with the analytical sheath helix model, and evaluate the importance of including this correction for modeling a number of helix and coupled-cavity TWT devices.

Two lumped element models of coupled-cavity slow-wave structures, one due to Curnow (1965) and another due to Malykhin, Konnov, and Komarov (MKK) (2003), are compared. The basis of comparison is the level of accuracy with which the models reproduce the cold circuit phase velocity and characteristic impedance of the structures as functions of frequency, as obtained from numerical solutions of Maxwell's field equations. Two distinct types of coupled-cavity structure are analyzed using these two models. These bare a C-band double staggered ladder circuit and an L-band slot coupled "Hughes-type" structure. We find for the C-band case, both the Curnow and MKK models give excellent representations of both the cavity and slot bands, but for the L-band case the Curnow model has no solution when an attempt is made to fit both the cavity and slot bands, while the MKK model accurately represents both bands in this case.

A fully two-dimensional (2-D) dc space charge model has been implemented in a large-signal traveling-wave amplifier code. The simulation algorithm takes an iterative approach by alternately solving the Poisson equation and the beam trajectory equations to converge toward a self-consistent steady-state solution. This approach is similar to that employed in steady-state gun codes. However, it is well known from gun simulations that the iterative algorithm can be slow to converge. We have found the slow convergence is due to a convective numerical instability. To speed up convergence, we implemented and tested stabilization schemes based on mixing one-dimensional and 2-D Poisson potentials during the iteration cycles. These schemes are shown to accelerate convergence considerably. The fully 2-D dc space-charge model permits accurate treatment of the axial dc space-charge field in the computation of the large signal gain and efficiency, taking into account the fast variation of beam parameters along the device axis. Therefore, it can be applied to a mismatched beam with large scalloping motion. The methodology of incorporating dc space charge is general and could be incorporated in other large signal codes.

Simulation codes are commonly used in the design and enhancement of TWT performance. A suite of TWT design codes developed under ONR-NRL sponsorship has been validated against 750 watt Ku-band communication TWTs manufactured by CPI (VTU-6397). The design code suite includes: MICHELLE, a 3D gun and collector code; CTLSS, a 3D cold test code; CHRISTIN, 1D & 3D large signal codes; and mPPM/lesPPM, magnet design code. The corresponding role of each code in TWT simulation is described in this paper.

Summary form only given, as follows. The Cold-Test, Large-Signal Simulation Code (CTLSS) is being developed to provide a 3D electromagnetic simulation tool that is designed to interoperate with large-signal codes employed in microwave and millimeter-wave vacuum electron device design. In this presentation, we describe capabilities recently introduced in CTLSS that directly support features of the large-signal simulation codes CHRISTINE ID and CHRISTINE 3D. For large signal models that operate in the frequency domain, it is necessary to specify device characteristics at selected operating frequencies. A new eigensolver capability has been developed in CTLSS to determine the eigenmode fields and related parameters (phase velocity, interaction impedances & admittances, etc.) of a dispersive periodic structure at a predetermined frequency. This contrasts with the more common approach, in which the frequency of a traveling wave is computed for a specified phase advance per period (Floquet boundary condition), and results later interpolated to the frequencies of interest. The new method reduces the total computation time required to obtain parameters for the large-signal models. The CHRISTINE 3D code simulates the large-signal characteristics of slow-wave devices using a fast, parametric model that includes a fully three-dimensional representation of both particle motion and electromagnetic fields. The traveling-wave circuit field and the RF space-charge field are treated separately, but self-consistently, and in common with many existing parametric large-signal models, the space-charge fields are computed assuming that they exist only within a cylindrical pipe at the inner radius of the circuit structures. We describe a method for correcting the space-charge field to take account of the true 3D geometry, using correction terms that are precomputed from the full circuit structure using CTLSS.

The results of the TSS‐1R mission generated several scientific puzzles. First the current collection was much more efficient than predicted on the basis of theoretical models, and previous laboratory and rocket experiments. Second, a sharp transition in the interaction physics occurred at threshold potentials between 5–10 V. Third, a significant population of suprathermal electrons, heated ionospheric ions, and enhanced plasma waves were observed in the ram direction, following the transition. The letter contains a preliminary examination of the extent to which these phenomena are related to the interaction of the impinging ambient ram O+ ions with the sheath surrounding the TSS satellite.

The TSS-1R science mission was conducted on the space shuttle STS-75 at the end of February 1996. During the flight, the Tethered Satellite was deployed to a distance of 19.7km and current was collected by the Satellite. Over the course of science operations, a rich dataset of current-voltage measurements was collected from instruments onboard both the satellite and the shuttle. We have constructed I-V curves including the uncertainties from Mission data. Comparing the TSS IV characteristics with known theoretical models, we find that the level of electron current collected exceeds those predicted by the steady state models. At high voltages (> 300 V), the I-V characteristics scale with potential according to the Parker-Mutphy model, implying a magnetically insulated collection process. The absolute value of the current collected greatly exceeds predictions of present models. The fact that TSS collected large currents at relatively low potentials bring many of the tether applications, such as power and thrust generation, closer to realization.

A comprehensive theoretical analysis of direct Cerenkov excitation of the Earth ionosphere waveguide using ionospheric heating is presented. The model relies on transient ionospheric heating with a heater spot moving horizontally at the bottom of the waveguide with speed close to the speed of light. The cases of isotropic ionospheric conductivity, corresponding to heating altitudes below 70 km, and of anisotropic conductivity, corresponding to higher heating altitudes, are examined separately. It is found that enhanced radiation coupling requires that the speed of the heater approaches the speed of light. For the anisotropic case, such enhancement occurs independently of the direction of motion, while for the isotropic case, motion parallel to the ambient electric field is required.

Over the past several years, the ARGUS code has been used to simulate a variety of ion beam phenomena. ARGUS is a general purpose three-dimensional simulation code. It can handle fields-only electromagnetic and electrostatic simulations, as well as electrostatic (steady-state and time-dependent) and electromagnetic time-dependent particle-in-cell (PIC) simulations. In the fields-only electromagnetic regime, ARGUS can be run in the time domain as well as the frequency domain (eigenmodes). The steady-state electrostatic PIC model is essentially a three-dimensional gun code. This paper will present results of using the steady-state PIC module in ARGUS to model two related ion beam devices that use electrostatic quadrupole fields to focus and accelerate an H− ion beam. The results of simulations of Lawrence Berkeley Laboratory's (LBL) Constant-Current Variable-Voltage (CCW) H− accelerator will be presented first. For this device, ARGUS has been used to simulate electron trapping and H− stripping between the ion source and the CCW accelerator, acceleration and transport through the CCW modular electrostatic, discrete quadrupole sections. The second device is a helical electrostatic quadrupole used for low-energy beam transport in the SSC.

The authors examine the electrodynamics of charged platforms in the ionosphere with a variety of analytical and numerical models. These models have been specifically designed to study the tethered satellite system (TSS-1) due to be launched in early 1992. One of the objectives of TSS-1 is to determine the potential of tethers for electrical power generation from orbital motion across the earth's magnetic field. The author identifies and explores important aspects of the interaction between the ambient ionospheric plasma and moving charged orbital platforms: (1) the formation of energetic particles in the wake of a nominally neutral satellite, (2) transient current collection by a highly charged platform, (3) the current closure paths in the ionosphere between multiple polarized platforms, and (4) the conditions for rapid neutralization by enhanced plasma formation in the presence of effluent gases.

Instabilities due to a cross-field ion flow are reexamined by including the electromagnetic response of the ions, which has been ignored in existing discussions. It is found that this effect can lead to significant enhancement of the growth rates. Among the new results, we find a purely growing, electromagnetic unstable mode with a wave vector k parallel to the ambient magnetic field. The plasma configuration under consideration is similar to that used in the discussion of the well-known modified-two-stream instability. This instability has a growth rate faster than the ion cyclotron frequency, and is not susceptible to high-plasma-\ensuremath{\beta} stabilization.

Optical guiding in the free-electron laser is studied for a configuration in which a relativistic electron beam propagates through a rectangular waveguide in the presence of a planar wiggler. The wiggler model includes the effect of parabolically tapered pole faces for enhanced focusing of the electron beam. A set of coupled nonlinear differential equations is solved in three dimensions, which governs the evolution of the fields in a loss-free rectangular waveguide as well as the trajectories of an ensemble of electrons that are integrated using the complete Lorentz force equations. The interaction in a free-electron laser arises from the beating of the wiggler and radiation fields, which produces both upper and lower beat waves. The upper beat wave results in a slowly varying ponderomotive wave, while the lower beat wave introduces an oscillation at half the wiggler period. The lower beat wave cancels out entirely for a helical wiggler, but is present in planar configurations and causes the power and phase of the wave to oscillate at half the wiggler period. This effect also results in an oscillation in the guiding of the signal. The three-dimensional, multimode waveguide code WIGGLIN has been adapted to treat the short-wavelength regime and is capable of examining the effect of the lower beat wave because the orbit equations are not averaged over a wiggler period. The analysis assumes the injection of a Gaussian mode from a master oscillator into the wiggler in synchronism with the electron beam. The Gaussian mode is assumed to be focused down to the minimum spot size upon entry to the wiggler and is then decomposed into the modes of the rectangular waveguide, which satisfy the boundary conditions. Results indicate that the optical guiding of the signal (i) is oscillatory at one half of the wiggler period, and (ii) is more marked in the direction of the wiggle motion rather than transverse to it. In addition, side lobes about the principal radiation signal are observed to grow, which may adversely affect the bulk guiding of the wave and could result in increased wall loading. Implications of these results upon future free-electron laser designs are discussed.

Argus is a system of three-dimensional codes which share the same utilities for structure input and grid generation, memory management, data handling, and diagnostics. The field module of Argus can solve for electrostatic fields or for the complete electromagnetic fields in either the frequency or time domain. This module can be used as a stand-alone code for impedance and other cold-test calculations. In addition, Argus includes a multispecies particle-in-cell (PIC) simulation module. This module can push particles in specified fields and can function with the field module to generate self-consistent calculations of collective effects in particle beams. The module runs in a transient mode as well as in a steady-state mode (as a gun code). Applications to the four-finger cross-bar RFQ (radio-frequency quadrupole) linac and the Pierre CCVV ESQ (constant-current variable-voltage electrostatic quadrupole) accelerator and Pierce gun are considered.< >