Session TO4: Basic: Laboratory Techniques and Plasma Production

Behavior of a charged particle in contact with a chaotic thermostat

~A chaotic thermostat has been developed that extends the familiar Langevin model, in which transport results ~from an uncorrelated random force, to the domain of deterministic dynamics. ~The concept is based on supplementing the friction force of a deterministic thermostat (e.g., Nose-Hoover) with a self-consistent fluctuating force associated with coupling to the heat bath. The new system exhibits diffusive behavior, achieves symmetric Maxwellian distributions, and in the unmagnetized case, it satisfies the Einstein relation when a DC electric field is applied. The magnetized charge exhibits Hall and Pedersen mobilities with erratic variations around the predictions of the Langevin model. Significantly, the cross-field diffusion coefficient shows a non-monotonic dependence on the strength of the magnetic field, and absolute negative mobility (ANM) arises over a small range of magnetic field values. The application of a coherent AC field is found to reduce the zero-order chaotic transport.   

Density characterization of discharged capillaries through common-path spectral-domain interferometry

Pre-formed capillary-based plasma channels are well suited to extend the laser-plasma interaction, benefiting laser plasma accelerators. More recently, discharged plasma structures have been applied to focus relativistic electron beams exploiting the advantages of radial symmetry, tunability, and strong focusing gradients. Knowledge of the on-axis plasma density is of critical importance, since it dominates the plasma response time, self-injection threshold, accelerating field strength, electron beam dephasing length, and beam-driven wakefield effects. In this talk, a novel approach is presented and demonstrated [1-2], based on a common-path two-color interferometer. The approach relies on a single femtosecond laser pulse traveling through a frequency-doubling crystal, thus transforming into fundamental and 2nd-harmonic pulses (the pulses are intrinsically coupled and locked in timing, phase, pointing, and stability). The differences in phase and group velocity in the plasma can be recorded with a spectrometer. This technique allows for sensitive retrieval (from the phase velocity), without the need for phase tracking (from the group velocity). [1] van Tilborg et al. Opt. Lett. 12, 2776 (2018), [2] van Tilborg et al. Phys. Plasmas 26, 023106 (2019)   

Design and calibration of a solenoid used on magnetized plasma experiments and B-dot probes using commercial electronic components

Magnetic fields play an important role in many areas of plasma physics. Sometimes, such areas call for the generation of strong magnetic fields ($>$5T) in compact volumes. In addition to the engineering challenges of fabricating a powerful, reusable electromagnet design, measurement and calibration of such powerful magnetic fields and field geometries requires the use of precise and often disposable measuring devices that can be easily adapted to any experimental set-up. Here, we present our approach to both sides of this problem. First, we show the construction of a solenoid designed to produce an axial magnetic field with strength in the central gap in the order of 10T. Second, we show a method for fabricating B-dot probes using commercially available inductor elements commonly used in circuit board construction with a study of the performance in strong (10T) pulsed magnetic fields.   

Off-resonant RF Heating of Ultracold Plasmas to Measure Collision Rates

We have developed a new technique to measure electron-ion collision rates in ultracold plasmas. An off-resonant, high-frequency field is applied to an ultracold plasma. The electrons oscillate in response, and electron-ion collisions produce electron heating. The off-resonant nature of the oscillation reduces sensitivity to the plasma density as well as possible distortions. By using the known variation in photoionization energy with photoionization laser wavelength and applying controlled sequences of electric fields, the amount of heating imparted can be calibrated and precisely measured. This allows the comparison of electron-ion collision rates as a function of plasma parameters such as electron temperature/degree of strong coupling and magnetization. A description of this technique and the experimental results obtained with it will be presented.   

Measuring charge and weakly-ionized plasma densities over 12 orders of magnitude

Electron avalanche ionization driven by picosecond mid-IR lasers is a sensitive measure of the presence of individual charges in air and other gases, driving an exponential electron growth from a single electron seed analogous to the detection of single photons in a photomultiplier tube. A mid-IR drive wavelength reduces multiphoton ionization observed with shorter wavelength drivers, a process which generates excess charges and masks the signal from pre-existing, low charge densities. Imaging the location of plasma breakdowns---which for a picosecond pump are spatially limited around the original seed electron---permits the determination of extremely low charge densities in air. For higher electron and weakly-ionized plasma densities, the avalanche plasma generated from each seed overlap, but seed density can still be determined by measuring the temporal evolution of the breakdown. We present measurements of laser produced charge and plasma densities over 12 orders of magnitude (from 10$^{\mathrm{4}}$ to 10$^{\mathrm{16\thinspace }}$cm$^{\mathrm{-3}})$ measured using mid-IR driven avalanche breakdown.   

Dust Particle Pair Correlation Functions and the Coupling Parameter of a Vertical Dust Chain .

Dust kinetic energy is a measure of the stochastic motion of a dust particle and is a result of the combination of the Brownian motion and the fluctuations in the dust charge and confining electric field. The coupling parameter $\Gamma $, which is defined as the ratio of the interparticle Coulomb energy to the kinetic energy, can be used to predict phase transitions of the dust crystal structure. This talk will describe the relationship between the dust kinetic energy derived from the mean square displacement technique and a technique using the probability distribution of the displacements obtained from random fluctuations of the dust particle. A structural transition from 1D vertical dust chain to 2D or 3D in a glass box, the coupling parameter change can be investigated by measuring the dust kinetic energy and the Coulomb energy.   

Plasma parameters of conductors electric explosion by the external electric field near the critical point

With a view to modify and control the plasma parameters of conductors electrical explosion near the critical point the application of strong electric fields of thermostat to the reaction volume is used. In this case, the polarizing external field influences on the system critical parameters and electric field strength is a control parameter providing modification of the ionization characteristics of plasma near the critical point. Theoretical model of thermal ionization of plasma products from the metals explosion in the critical region in the presence of an external electric field of thermostat has been developed. The specific features of the liquid metal - vapor phase transition near the critical region and the influence of external field on the local electrochemical potential of plasma electron subsystem and its radiation characteristics have been discussed. Degree of ionization and plasma and power of its braking radiation under external polarizing field have been determined in a wide range of temperatures, concentrations and sizes of submicron metal particles.   

Investigation of light ion fusion reactions in metal hydrides with plasma discharges

Fusion at relatively low energies is important for our understanding of stellar fuel chains and the development of future energy technologies. Experiments are challenging due to the exponential drop of fusion cross sections below the Coulomb barrier. We report on experiments on D-D fusion with ion pulses from glow discharge plasmas [1]. With this approach we can deliver relatively high peak ion currents (0.1 to several A/cm\textasciicircum 2) to metal wire cathodes for several days. With Pd targets, we find neutron yields that are over 100 times higher than expected for bare nuclei fusion at ion energies below 2 keV (cm-frame). A possible explanation is a correction to the ion energy due to an apparent electron screening potential of 1000$+$/-250 eV, which increases the probability for tunneling through the repulsive Coulomb barrier. But such a high value is not consistent with theoretical descriptions of electron screening potentials. We discuss possible explanations, ideas of treating metals as analogs of cold, dense plasmas and follow-up experiments aimed at understanding this effect. [1] T. Schenkel, et al., \underline {https://arxiv.org/abs/1905.03400}; C. P. Berlinguette, et al., Nature 570, 45 (2019)   

Measurements of DD Neutron Yield and Down-scattered DT Neutrons Using Cherenkov Detectors

Nuclear diagnostics are essential to infer inertial confinement fusion (ICF) plasma conditions, such as ion temperature, areal density and implosion shape, during burn and stagnation. Traditional neutron time-of-flight (nToF) detectors use plastic scintillators to detect neutrons with high efficiency, but with the complexity of a multi-component \textgreater 1 ns decay tail. To study details in the down-scattered neutron spectrum, such as the n-T and n-D edges, and the DD neutron signal, requires the disambiguation of the 14.1 MeV DT neutron scintillator decay tail which can be \textgreater 1000X brighter and persists for several hundred nanoseconds. In this work we present the physics basis of using quartz, sapphire and/or undoped Yttrium Aluminium Garnet (YAG) to detect low energy neutrons (\textless 10 MeV) through the Cherenkov effect, where the light emission is below the response time (\textasciitilde 100 ps) conventional 10 mm micro-channel plate (MCP) photomultiplier tubes (PMT), thereby removing the complexity of the tail and allowing clean measurements of the DD yield, and n-T/D edges to be made. Preliminary measurements at Omega using the Diagnostic for Areal Density (DAD) as a surrogate neutron detector are also presented. British Crown Owned Copyright 2019/AWE.   

Ion friction at small values of the Coulomb logarithm

We report velocity relaxation measurements in a Yb$^+$/Ca$^+$ dual species ultracold neutral plasma. The nearly 4:1 mass ratio of the ion species in our plasma is similar to the alpha:proton mass ratio important for fusion-class systems. Our system provides a platform for using Ca$^+$ and Yb$^+$ ions to test expressions for the Coulomb logarithm used in momentum transfer collisions in a strongly coupled plasma environment. The spatial profiles and velocity distributions are determined using laser-induced fluorescence. Measurements are compared to a two-fluid code calculation that include convection, adiabatic expansion, pressure acceleration, ion friction, ambipolar field acceleration, and Joule heating. We compare our measurements with simulations using expressions for the Coulomb logarithm from the literature.   

Negative Hydrogen Ion Production in a Helicon Powered Magnetized Plasma Column

A new generation of neutral beam systems will be required in future fusion reactors, such as DEMO, able to deliver high power (up to 50 MW) with high neutral energy (\textgreater 1 MeV). Negative ions have a higher neutralization fraction (compared to positive ions) in a gas cell at energies greater than 50 keV. They are generated mostly on cesiated metal surfaces inside a magnetized high brightness plasma source but cesium consumption must be limited to a minimum in a fusion power plant to reduce the maintenance of the source. There is hence a strong research focus to optimize the production of negative ions via dissociative attachment of the gas molecule inside the source volume. To achieve this, one must generate a plasma with a hot (\textasciitilde 10 eV) and cold (\textasciitilde 1 eV) electron temperature regions and confine the electrons magnetically. In this work, we will analyse the properties of a hydrogen plasma produced in a thin (20 cm radius and 1.8 m length) magnetized (\textasciitilde 150G) plasma column powered by a helicon discharge [I. Furno et al., EPJ Web of Conferences \textbf{157}, 03014 (2017)]. The numerical simulations are performed with a 2.5D Particle-in-Cell algorithm with Monte-Carlo Collisions (PIC-MCC) [G. Fubiani et al., New J. Phys. \textbf{19}, 015002 (2017)]. The model will be compared to experiments.