Generation of whistlers by pulsed rf heating of electrons in a large laboratory magnetoplasma

The dynamics of narrow, field-aligned magnetoplasma irregularities is studied, which develop under the action of a short rf pulse. The laboratory experiment is aimed at demonstrating the rapid, so-called “unipolar” plasma transport mode, which is accompanied by excitation of eddy currents, in the case of localized rf heating of plasma electrons. The experimental parameters are chosen in a special way. The size of the heating spot, determined by the diameter of the loop antenna, exceeds the electron gyroradius significantly but is smaller than the ion gyroradius. The rf pulse duration encompasses several electron collision times but is shorter than the gyroperiod of ions. As a result, the electrons, which are strongly magnetized, acquire energy in rf antenna vicinity and can escape the heating region only along the magnetic field B0. In turn, collisionless ions can travel across B0 under the action of space-charge electric fields. For these conditions, redistribution of the plasma occurs with “unipolar” transport coefficients and is accompanied by excitation of electric currents. Weak plasma density disturbances, which are less than 5% of the background, are measured precisely with a microwave resonator probe. Parallel electron currents are obtained from magnetic probe measurements; the ion current across B0 is restored from the density profile modifications in their dynamics. It is shown that the ions traveling across B0 with a velocity about one third of the ion-acoustic velocity can easily close the current loop, which is driven by the parallel motion of heated electrons. This regime of plasma irregularities evolution is discussed in application to previous laboratory measurements, as well as to active ionospheric experiments.

The nonambipolar or “unipolar” particle transport accompanied by excitation of a system of eddy (short-circuit) currents can ensure the fast dynamics of small-scale magnetoplasma disturbances arising under pulsed localized rf heating, and evolving in electron magnetohydrodynamics regime of parameters. In this regime, redistribution of plasma density is possible, which is an order of magnitude faster than the classical mechanism of the ambipolar transport with the joint movement of electron-ion pairs. During the evolution of thermal plasma irregularity in the unipolar transport regime, magnetized electrons leave the heated plasma area along the magnetic field, while nonmagnetized ions drift predominantly across the field. The electric current arising in this case can be closed through the background plasma surrounding the irregularity. This regime can determine the times of development and decay of narrow field-aligned plasma density irregularities that arise, e.g., in the pulsed ionospheric heating experiments. The refined laboratory experiments with specially selected parameters, which were carried out in a large-scale Krot plasma device with localized (pointlike) short-pulse rf heating of electrons, demonstrated clearly the unipolar-cell dynamics. The “unipolar cell” is understood as a self-consistent, freely relaxing plasma-field structure, which is formed by the initial field-aligned plasma density depletion in a heated flux tube, the peripheral background plasma density enhancements and depletions, and a quadrupole system of electric eddy currents.

ObjectiveTo simulate the magnetic and electric fields produced by RF coil geometries commonly used at low field. Based on these simulations, the specific absorption rate (SAR) efficiency can be derived to ensure safe operation even when using short RF pulses and high duty cycles.MethodsElectromagnetic simulations were performed at four different field strengths between 0.05 and 0.1 T, corresponding to the lower and upper limits of current point-of-care (POC) neuroimaging systems. Transmit magnetic and electric fields, as well as transmit efficiency and SAR efficiency were simulated. The effects of a close-fitting shield on the EM fields were also assessed. SAR calculations were performed as a function of RF pulse length in turbo-spin echo (TSE) sequences.ResultsSimulations of RF coil characteristics and B1+ transmit efficiencies agreed well with corresponding experimentally determined parameters. Overall, the SAR efficiency was, as expected, higher at the lower frequencies studied, and many orders of magnitude greater than at conventional clinical field strengths. The tight-fitting transmit coil results in the highest SAR in the nose and skull, which are not thermally sensitive tissues. The calculated SAR efficiencies showed that only when 180° refocusing pulses of duration ~ 10 ms are used for TSE sequences does SAR need to be carefully considered.ConclusionThis work presents a comprehensive overview of the transmit and SAR efficiencies for RF coils used for POC MRI neuroimaging. While SAR is not a problem for conventional sequences, the values derived here should be useful for RF intensive sequences such as T1ρ, and also demonstrate that if very short RF pulses are required then SAR calculations should be performed.

Summary form only given, as follows. Magnetrons, with efficiencies greater than 70% when operated at low voltages, are of great interest to the high power microwave (HPM) community. However, when operated at higher voltages (>100 kV), relativistic magnetrons suffer from short output RF pulses, as well as large decreases in efficiencies (typically less than 30%). Recent 3-D computer simulations have addressed the issue of efficiency limitations. Short rf pulses could be caused by the large electric fields that exist in the vane structures. Using a large slot to vane ratio may lead to the reduction of the electric field stresses, however doing so leads to severe mode competition. The Air Force Research Lab has laid out a method using 2- and 3-D electromagnetic codes to investigate different magnetron structures, which may lend themselves to 'large' mode separation between the Pi-mode and its nearest adjacent modes, while minimizing the electric field stress.

High-gradient acceleration is a key research area that could enable compact linear accelerators for future colliders, light sources, and other applications. In the pursuit of high-gradient operation, rf breakdown limits the attainable accelerating gradient in normal-conducting rf structures. Recent experiments at the Argonne Wakefield Accelerator suggest a promising approach: using short rf pulses with durations of a few nanoseconds. Experimental studies show that these O ( 1 ns ) rf pulses can mitigate breakdown limitations, resulting in higher gradients. For example, an electric field of nearly 400 MV / m was achieved in an X -band photoemission gun driven by 6-ns-long rf pulses, with rapid rf conditioning and low dark current observed. Despite these promising results, the short-pulse regime remains an underexplored parameter space, and rf breakdown physics under nanosecond-long pulses requires further investigation. In this paper, we present analytical and numerical simulations of dark current dynamics in accelerating cavities operating in the short-pulse regime. We study breakdown-associated processes spanning different time scales, including field emission, multipacting, and plasma formation, using simulations of the X -band photogun cavities. The results reveal the advantages of using short rf pulses to reduce dark current and mitigate rf breakdown, offering a path toward a new class of compact accelerators with enhanced performance and reduced susceptibility to breakdown.

Selective and non-selective NMR excitation of quadrupolar nuclei in the solid state

NMR strong off-resonance irradiation without sample overheating

In order to generate an MR signal from the net magnetisation, the RF oscillating magnetic field is used to deliver energy into the population of protons at the Larmor Frequency. The Larmor frequency (resonant frequency) is proportional to the strength of the magnetic field and is typically in the Megahertz range. The RF field is normally applied as a short pulse, known as an rf pulse. When it is applied, the proton magnetic moments start to rotate together about the main magnetic field, causing the net magnetisation to move away from its alignment with the main magnetic field and to rotate around it. The greater the amount of energy applied by the RF pulse, the greater the angle that the net magnetisation makes with the Bo field. This angle that the net magnetisation is rotated through by the RF pulse is known as the flip angle. Commonly used RF pulses for imaging are a low flip angle RF pulse, a 90° RF pulse, 180° RF refocusing pulse and a 180° RF inversion pulse. The low flip angle and 90 RF pulses are used as excitation pulses and generate a transverse component of magnetisation which rotates about the main magnetic field direction, generating an oscillating magnetic field that is detected by the RF receiver coil as an MR signal. The MR signal generated by a single RF pulse gradually decays and is known as a free induction decay or (FID).

Context. The Parker Solar Probe (PSP) measures solar wind protons and electrons near the Sun. To study the thermodynamic properties of electrons and protons, we include electron effects, such as distributed turbulent heating between protons and electrons, Coulomb collisions between protons and electrons, and heat conduction of electrons. Aims. We develop a general theoretical model of nearly incompressible magnetohydrodynamic (NI MHD) turbulence coupled with a solar wind model that includes electron pressure and heat flux. Methods. It is important to note that 60% of the turbulence energy is assigned to proton heating and 40% to electron heating. We use an empirical expression for the electron heat flux. We derived a nonlinear dissipation term for the residual energy that includes both the Alfvén effect and the turbulent small-scale dynamo effect. Similarly, we obtained the NI/slab time-scale in an NI MHD phenomenology to use in the derivation of the nonlinear term that incorporates the Alfvén effect. Results. A detailed comparison between the theoretical model solutions and the fast solar wind measured by PSP and Helios 2 shows that they are consistent. The results show that the nearly incompressible NI/slab turbulence component describes observations of the fast solar wind periods when the solar wind flow is aligned or antialigned with the magnetic field.

A new regime in radiofrequency (rf) breakdown, named the breakdown insensitive acceleration regime (BIAR), was observed in an 11.7 GHz metamaterial structure for wakefield acceleration driven by rf pulses with a duration of a few nanoseconds. In the BIAR, rf breakdown occurs without interrupting potential beam acceleration, resulting in greater resilience to breakdown. We have investigated the possibility that BIAR can support higher gradients by characterizing the breakdown in a high-power test. The peak gradient reached 190 MV/m when the structure was powered by 6 ns long rf pulses with 115 MW peak power. The short rf pulses were extracted from 65 MeV electron bunch trains with a total charge of up to 210 nC. This work has revealed the benefits of short-pulse acceleration by characterizing rf breakdown in the previously unexplored parameter space. Published by the American Physical Society 2024

Comment on “The magnetic response of the ionosphere to pulsed HF heating” by K. Papadopoulos, T. Wallace, G. M. Milikh, W. Peter, and M. McCarrick

The effect of small magnetic perturbation propagating through the magnetoplasma has been considered. The collisional Boltzmann transport equation has been linearized and solved for small magnetic field perturbations. The energy acquired by electrons due to magnetic perturbation is obtained from kinetic consideration. The energy equation has been solved for thermal equilibrium. The variation of electron temperature with time has been obtained. It has been shown that heating of electrons mainly depends on plasma parameters, steady magnetic field and its perturbation. The electron heating with different collisional terms has been considered. It is seen that electrons acquire higher temperature in weakly ionized plasma as compared to fully ionized plasma.

Traditional MRI scanners acquire samples of the encoded image in a spatial frequency (Fourier) domain, known as k-space. The recently developed theory of Compressed Sensing (CS) has shown that a natural image could be acquired and reconstructed from a reduced number of linear projection measurements at sub-Nyquist sample rates. CS is well suited to fast acquisition of MRI by sparse encoding in k-space. However, the commonly used Fourier-encoding MRI scheme only weakly satisfies the incoherent measurements constraint required for CS. MR images can be reconstructed from a more sparse wavelet-encoded k-space than from Fourier-encoded k-space with the same resolution. Wavelet-encoded MRI is flexible in spatially-selective excitations to yield an encoding scheme similar to the universal encoding. In this work, a CS-MRI scheme has been investigated to accurately reconstruct MR images from sparsely wavelet-encoded k-space. This sparse encoding is achieved with tailored spatially-selective RF pulses, which are designed with Battle-Lemarie wavelet functions. The Fourier transforms of the Battle-Lemarie wavelet functions used as RF pulse profiles are smooth and decay rapidly to zero. This technique provides short RF pulses with relatively precise spatial excitation profiles. Simulator results show that the proposed encoding scheme has different characteristics than conventional Fourier and wavelet encoding methods, and this scheme could be applied to fast MR image acquisition at relatively high resolution.

RF pulses are used in radar and sonar to detect and locate targets in extended media. Short RF pulses (and impulses) can be used to find buried objects or voids by echo sounding, or can be used to probe snow fields or the depths of the earth. Although similar to radar in principle, there are important differences in these applications that can lead to significant variations in the design approach. A novel system is described, with a computer-programmed tunable RF source, that tunes rapidly through all the frequencies of a nanosecond pulse spectrum, makes individual "CW" measurements, stores them, and finally computes a synthetic echo. Target signatures can be "recognized" by calculating correlation functions. Experimental results are presented.

In the Heliotron E device, a non-axisymmetric helical system, ICRF heating experiments were carried out for the first time, using fast-mode and slow-mode waves. In the fast-wave heating experiment, ICRF power of up to 550 kW was emitted during 15 ms by four antenna loops. Effective heating of a current-less ECRH-produced target plasma was observed over a wide density range. The plasma loading resistance of an antenna loop reached about 5 Ω. This is a value comparable with that of tokamak experiments. The increments of ion and electron temperatures by fast-wave heating were about 200–230 eV at an electron density of about 3 × 1019m−3. Minority heating and pure second-harmonic heating have almost the same efficiency ((1–2) × 1019eV·m−3·kW−1) during the short RF pulse used (t ≤ 15 ms). The energy transfer rate from the waves to ions and electrons could be explained by mode conversion. The signals of toroidal eigen-modes were experimentally observed and radial mode numbers could be determined using a simple model. In the slow-wave heating experiment, the upper density limit of effective heating appeared to be in qualitative agreement with wave theory.