B-dot Probes Research Articles

Investigation of Magnetic Fields in Z-Pinches via Multi-MeV Proton Deflectometry

Proton deflectometry is a promising way for mapping electric and magnetic fields in high-density and high-temperature plasmas, where an application of the classical methods (B-dot probes, Faraday rotation, and Zeeman splitting) is limited. It is based on the detection of a multi-MeV proton beam deflected in examined B-fields. In the past years, it has been successfully utilized in laser-generated plasmas for E-field and B-field measurements. Using our numerical code, we investigate the capabilities of proton deflectometry as a diagnostic method of MA Z-pinches. We simulate proton trajectories propagating through typical Z-pinch B-fields in two fundamental experimental setups (radial and axial) in order to study synthetic images (deflectograms). We demonstrate where proton deflectometry might be beneficial for the Z-pinch research. We explain a formation of the key features of deflectograms, which give us information about a profile and strength of the Z-pinch B-fields. We introduce a $B_{L}$ parameter, denoting an effective B-field averaged along the deflected proton orbit and show its importance for the proton deflectometry.

Using of B-dot probe for z-pinch plasma diagnostics

We present experiments performed on the IMRI-5 generator (450 kA, 450 ns) with a metallic gas-puff Z-pinch with a power-law density profile. The experiments were carried out in a preembedded axial magnetic field Bz0 that was varied from 0 to 0.6 T. To determine the initial pinch radius r0, we used the function r(t) that was found from the time dependence of the pinch inductance L(t). The time-dependent inductance L(t), in turn, was determined as a function of load voltage Vload(t) and pinch current I(t). The function r(t) was verified by a B-dot probe diagnostics. Measurements showed that for the “first shot” the initial radius of the metallic gas-puff Z-pinch decreased from 4 cm at Bz0 = 0 to 2.1-1.7 cm at Bz0 = 0.15 T. We believe that the decrease it r0 is related to the field effect on the ion gyroradius.

The artificial neural network prediction algorithm research of rail-gun current and armature speed based on B-dot probes array

In this paper, based on the advantages of artificial neural network, such as good tolerance of data noise, strong ability of nonlinear mapping, multi-dimensional input variables, fast operation, low error, etc., a method of using artificial neural network for data prediction is proposed for the research of rail-gun. The results show that it is feasible to use the Back Propagation Neural Network, the Radial Basis Function Neural Network and the General Regression Neural Network to realize the method of prediction and simulation of the rail-gun current and the armature speed curve through relevant parameters. The General Regression Neural Network has superiority in error performance and time cost of neural network training and simulation.

A new B-dot probe circuit for magnetic diagnostics of radio frequency discharges.

Accurate magnetic measurements in radio frequency capacitively coupled plasmas (CCP) are challenging due to the presence of inherently strong electric fields and relatively weak magnetic fields. In this work, a new B-dot probe circuit is presented, comprising two variable capacitors in a tunable series resonance circuit, with a center-tapped, step-up transformer. The output characteristics of the probe are predicted using two distinct equivalent circuit models, one for the differential mode and the other for the common mode. A Helmholtz coil and a Faraday cup are used for experimental validation of the predicted probe output. By tuning the two variable capacitors in the circuit, the magnetic probe can achieve improved signal-to-noise ratio by amplifying the inductive signal, while suppressing capacitive coupling interference. Using the newly designed probe, magnetic measurements in typical CCP are presented.

Magnetic diagnostic suite of the C-2W field-reversed configuration experiment.

A fundamental component of any magnetically confined fusion experiment is a firm understanding of the magnetic field. The increased complexity of the C-2W machine warrants an equally enhanced diagnostic capability. C-2W is outfitted with over 700 magnetic field probes of various types. They are both internal and external to the vacuum vessel. Inside, a linear array of innovative in-vacuum annular flux loop/B-dot combination probes provide information about plasma shape, size, pressure, energy, temperature, and trapped flux when coupled with established theoretical interpretations. A linear array of B-dot probes complement the azimuthally averaged measurements. A Mirnov array of 64 3D probes, with both low and high frequency resolution, detail plasma motion and MHD modal content via singular value decomposition analysis. Internal Rogowski probes measure axial currents flowing in the plasma jet. Outside, every feed-through for an internal probe has an external axial field probe. There are many external loops that measure the plasma formation dynamics and the total external magnetic flux. The external measurements are primarily used to characterize eddy currents in the vessel during a plasma shot. Details of these probes and the data derived from their signals are described.

Observations of core ion cyclotron emission on ASDEX Upgrade tokamak.

The B-dot probe diagnostic suite on the ASDEX Upgrade tokamak has recently been upgraded with a new 125 MHz, 14 bit resolution digitizer to study ion cyclotron emission (ICE). While classic edge emission from the low field side plasma is often observed, we also measure waves originating from the core with fast fusion protons or beam injected deuterons being a possible emission driver. Comparing the measured frequency values with ion cyclotron harmonics present in the plasma places the origin of this emission on the magnetic axis, with the fundamental hydrogen/second deuterium cyclotron harmonic matching the observed values. The actual values range from ∼27 MHz at the on-axis toroidal field BT = -1.79 T to ∼40 MHz at BT = -2.62 T. When the magnetic axis position evolves during this emission, the measured frequency values track the changes in the estimated on-axis cyclotron frequency values. Core ICE is usually a transient event lasting ∼100 ms during the neutral beam startup phase. However, in some cases, core emission occurs in steady-state plasmas and lasts for longer than 1 s. These observations suggest an attractive possibility of using a non-perturbing ICE-based diagnostic to passively monitor fusion alpha particles at the location of their birth in the plasma core, in deuterium-tritium burning devices such as ITER and DEMO.

Role of return currents in the dynamics of a magnetically rastered plasma torch

The role of return currents in the plasma dynamics of a dc non-transferred arc plasma torch has been investigated. The anode arc root connects the main plasma column and return current in the anode and its motion governs the return current formation in the anode. Thus by investigating the return currents, we can explore complex spatio-temporal evolution of the plasma in detail. In this work, we have used electrical and magnetic diagnostics. The electrical diagnostic comprises of a high voltage probe, and the magnetic diagnostic consists of a garland of several B-dot probes incorporated inside the anode water cooling channels. The magnetic (B-dot) probes are designed to pick up the dominant component of the time-varying magnetic fields. Results show that, beyond a threshold, volume return currents transform to constricted line currents in the anode return path. This is attributed to eddy currents arising due to space varying rather than time varying return current components in the anode. The probes capture fast rotation of the arc root that is not captured by fast imaging. Also, arc root shunting (restriking) phenomena and its location on the anode periphery are clearly captured by the garland of magnetic probes. A physical model explaining the processes is also presented.

Determining the Effect of an Arc in a Railgun Bore on Emission Efficiency Using B-Dot Probes

The unexpected arcs generated in a railgun bore are one of the major issues affecting emission efficiency. To study the impact of such arcs, a series of experiments with the same launch parameters has been conducted. B-dot probes positioned along the length of the launcher provide data for detecting arc formation phenomena and for measuring the armature velocity. In this paper, two sets of typical signals are discussed. The first set of signals is normal, whereas the other is abnormal in terms of the existence of an arc in the bore during launch. The time interval during which the arc generated and the state of the arc are determined from the magnetic induction curves by integrating the B-dot probe signals. Using a fitting analysis, the displacements and velocities of the armatures are obtained, and the energy distribution of each shot is computed. The results indicate that the generation of an arc in the bore causes a hash launch environment and greatly reduces the emission efficiency.

Laser-driven strong magnetostatic fields with applications to charged beam transport and magnetized high energy-density physics

Powerful nanosecond laser-plasma processes are explored to generate discharge currents of a few 100 kA in coil targets, yielding magnetostatic fields (B-fields) in excess of 0.5 kT. The quasi-static currents are provided from hot electron ejection from the laser-irradiated surface. According to our model, which describes the evolution of the discharge current, the major control parameter is the laser irradiance Ilasλlas2. The space-time evolution of the B-fields is experimentally characterized by high-frequency bandwidth B-dot probes and proton-deflectometry measurements. The magnetic pulses, of ns-scale, are long enough to magnetize secondary targets through resistive diffusion. We applied it in experiments of laser-generated relativistic electron transport through solid dielectric targets, yielding an unprecedented 5-fold enhancement of the energy-density flux at 60 μm depth, compared to unmagnetized transport conditions. These studies pave the ground for magnetized high-energy density physics investigations, related to laser-generated secondary sources of radiation and/or high-energy particles and their transport, to high-gain fusion energy schemes, and to laboratory astrophysics.

EMP control and characterization on high-power laser systems

Giant electromagnetic pulses (EMP) generated during the interaction of high-power lasers with solid targets can seriously degrade electrical measurements and equipment. EMP emission is caused by the acceleration of hot electrons inside the target, which produce radiation across a wide band from DC to terahertz frequencies. Improved understanding and control of EMP is vital as we enter a new era of high repetition rate, high intensity lasers (e.g. the Extreme Light Infrastructure). We present recent data from the VULCAN laser facility that demonstrates how EMP can be readily and effectively reduced. Characterization of the EMP was achieved using B-dot and D-dot probes that took measurements for a range of different target and laser parameters. We demonstrate that target stalk geometry, material composition, geodesic path length and foil surface area can all play a significant role in the reduction of EMP. A combination of electromagnetic wave and 3D particle-in-cell simulations is used to inform our conclusions about the effects of stalk geometry on EMP, providing an opportunity for comparison with existing charge separation models.

The discharge characteristics in nitrogen helicon plasma

Discharge characteristics of helicon plasma in nitrogen and argon-nitrogen mixtures were investigated experimentally by using a Langmuir probe, a B-dot probe, and an optical emission spectrum. Helicon wave discharge is confirmed by the changes of electron density and electromagnetic signal amplitude with the increasing RF power, which shows three discharge stages in nitrogen, corresponding to E-mode, H-mode, and W-mode discharges in helicon plasma, respectively. Discharge images in the radial cross section at different discharge modes through an intensified charge coupled device (ICCD) show a rapid increase in luminous intensity along with the RF power. When the nitrogen discharge is in the W-mode, the images show that the strongest luminance locates near the plasma boundary and no blue core appears in the axial center of tube, which is always observed in argon W-mode discharge. The “big blue” or blue core is a special character in helicon plasma, but it has not been observed in nitrogen helicon plasma. In nitrogen-argon mixtures, a weak blue core is observed in ICCD images since the nitrogen content is increased. The electric field turns to the periphery in the distribution of the radial field and the electron temperature decreases with the increasing nitrogen content, especially when the blue core disappears. The different behaviors of the electron impact and the energy consumption in nitrogen helicon plasma are suggested to be responsible for the decrease in electron energy and the change in the electric field distribution.

Applied axial magnetic field effects on laboratory plasma jets: Density hollowing, field compression, and azimuthal rotation

We experimentally measure the effects of an applied axial magnetic field (Bz) on laboratory plasma jets and compare the experimental results with numerical simulations using an extended magnetohydrodynamics code. A 1 MA peak current, 100 ns rise time pulse power machine is used to generate the plasma jet. On application of the axial field, we observe on-axis density hollowing and a conical formation of the jet using interferometry, compression of the applied Bz using magnetic B-dot probes, and azimuthal rotation of the jet using Thomson scattering. Experimentally, we find densities ≲5 × 1017 cm−3 on-axis relative to jet densities of ≳3 × 1018 cm−3. For aluminum jets, 6.5 ± 0.5 mm above the foil, we find on-axis compression of the applied 1.0 ± 0.1 T Bz to a total 2.4 ± 0.3 T, while simulations predict a peak compression to a total 3.4 T at the same location. On the aluminum jet boundary, we find ion azimuthal rotation velocities of 15–20 km/s, while simulations predict 14 km/s at the density peak. We discuss possible sources of discrepancy between the experiments and simulations, including surface plasma on B-dot probes, optical fiber spatial resolution, simulation density floors, and 2D vs. 3D simulation effects. This quantitative comparison between experiments and numerical simulations helps elucidate the underlying physics that determines the plasma dynamics of magnetized plasma jets.

Non-contact and contact measurement system for detecting projectile position in electromagnetic launch bore

This paper introduces a new measurement system for measuring the position of a projectile within a rapid fire electromagnetic launching system. The measurement system contains both non-contact laser shading and metal fiber contact measurement devices. Two projectiles are placed in the rapid fire electromagnetic launch bore, one in the main accelerating segment and the other in the pre-loading segment. The projectile placed in the main accelerating segment should be shot first, and then the other is loaded into the main segment from the pre-loading segment. The main driving current (I-main) can only be discharged again when the second projectile has arrived at the key position (the projectile position corresponds to the discharging time) in the main accelerating segment. So, it is important to be able to detect when the second projectile arrives at the key position in the main accelerating segment. The B-dot probe is the most widely used system for detecting the position of the projectile in the electromagnetic launch bore. However, the B-dot signal is affected by the driving current amplitude and the projectile velocity. There is no current in the main accelerating segment when the second projectile moves into this segment in rapid fire mode, so the B-dot signal for detecting the key position is invalid. Due to the presence of a high-intensity magnetic field, a high current, a high-temperature aluminum attachment, smoke and strong vibrations, it is very difficult to detect the projectile position in the bore accurately. So, other measurements need to be researched and developed in order to achieve high reliability. A measurement system based on a laser (non-contact) and metal fibers (contact) has been designed, and the integrated output signal based on this detector is described in the following paper.

A Small-Sized and Battery-Supplied Data Acquisition System for the Electromagnetic Launcher to Measure Pulsed Magnetic Fields

In rail-gun launchers, the main current is pulsed and can reach up to several MA. The width of the pulse is about 2 - 4ms. When pulsed currents are supplied from the pulse power, it will generate a large transient magnetic field around the armature. Meanwhile, the armature will be accelerated up to 2km/s by a large electromagnetic force. In order to research the features of the fields around contact surfaces between rails and the armature, it needs to use a small-sized and battery-supplied data acquisition system to measure pulsed magnetic fields. The B-dot probe acts as a sensor to acquire the transient magnetic fields. All data will be stored in a SD card. The armature and SD card will be caught and fetched back after the launching. The system would be suitable and useful for the electromagnetic launcher design.

Control of unsteady laser-produced plasma-flow with a multiple-coil magnetic nozzle

We report an experimental demonstration of controlling plasma flow direction with a magnetic nozzle consisting of multiple coils. Four coils are controlled separately to form an asymmetric magnetic field to change the direction of laser-produced plasma flow. The ablation plasma deforms the topology of the external magnetic field, forming a magnetic cavity inside and compressing the field outside. The compressed magnetic field pushes the plasma via the Lorentz force on a diamagnetic current: j × B in a certain direction, depending on the magnetic field configuration. Plasma and magnetic field structure formations depending on the initial magnetic field were simultaneously measured with a self-emission gated optical imager and B-dot probe, respectively, and the probe measurement clearly shows the difference of plasma expansion direction between symmetric and asymmetric initial magnetic fields. The combination of two-dimensional radiation hydrodynamic and three-dimensional hybrid simulations shows the control of the deflection angle with different number of coils, forming a plasma structure similar to that observed in the experiment.

The three-dimensional positioning system at the VINETA.II experiment—a multipurpose tool for in situ plasma diagnostics

The manipulator systems installed at the VINETA.II magnetic reconnection experiment are essential elements for experimental investigation of local plasma parameters. A novel three-dimensional (3D) probe manipulator has been designed, implemented and successfully operated at VINETA.II. This work presents its design and performance for three-dimensional measurements of VINETA.II plasmas. Its design consists of three vertically stacked independent and mutually perpendicular linear motion stages which allow flexible positioning of diagnostic tools such as electrical and magnetic probes or optical diagnostics within the vacuum vessel. Its design features include a wide spatial coverage, sub-millimeter positioning accuracy and the capability to operate in a harsh environment under the influence of microwaves, radio-frequency waves and direct contact with plasma. Manipulator performance is assessed by measuring a volumetric distribution of plasma parameters by a B-dot probe. A typical discharge of the magnetic reconnection setup in VINETA.II with a pulse time of τ=600 μs is chosen for this purpose. The azimuthal magnetic field distribution measured with the 3D manipulator agrees favorably with measurements obtained by the two-dimensional (2D) manipulator, used at VINETA.II as a standard reference diagnostic tool, thereby demonstrating its reliability and performance. A programmable stepper motor controller (TMCM-1110) that is operated remotely by a PC drives all possible features of the manipulator system.

Judging the Abnormal Rail-Armature Contact States With Waveforms of B-Dot Probes

In this paper, two kinds of abnormal rail-armature contact states are assumed and simulated, and the simulated B-dot probe waveforms are compared with the results from the experiments. Through theoretical analysis, simulations, and experiments, relations between the abnormal contact states and the output waveforms of the B-dot probe are investigated. Results show that with the help of the B-dot probe output waveforms, the transition position can be obtained and the intervals between the parallel circuit branches, which are created by the arcs arising from the abnormal contact states, can be primarily measured.

Improvement of a Low-Cost Combined Fitting Method for the Armature Speed and the Rail Current Measurement

Measuring the armature speed in bore and the rail current is very important in doing a further research for the launcher design. The B-dot probe method we often used has limited number of probes and its mathematical fitting curve has no actual physical meanings. Therefore, the current waveform’s shape information was used to improve the B-dot probe method’s result. The combined method could give an armature’s velocity at an arbitrary position, and the current waveform could also be calculated out at the same time. Although the combined method was suitable for the measurement, the transient situation limited the accuracy of the results. According to the transient current, the inductance gradient and the distance between rails and a probe are changing during the launching process. In this paper, this two varied parameters that influence the combined method are discussed. The experimental data shows that the improvement of the combined method is more reliable and suitable.

Analysis of the Velocity and Current Measurement Method Based on B-Dot Probes for the Rail Gun

Measuring the velocity and displacement of the armature and current for a rail gun is important. The common method uses B-dot probes to acquire the discrete time points when the armature passes by them. These points can be interpolated into a curve of the armature’s displacement. Actually, the common interpolation method has some disadvantages. Some probes may break down, and the interpolation method is regardless of the process between the discrete points. So it is difficult to get the armature’s position at any time and the exact velocity at any position. Aiming at these disadvantages, this paper presents an improved method, which based on both B-dot points and the main current waveform. A B-dot probe beside the rails easily gives this waveform. The shape of the current waveform helps us to fit the curve with enough physical information. The new method can give a more precise velocity of the armature at an arbitrary position between two B-dot probes’ positions. It is also practical outside of two B-dot probes’ positions too. And it still works when some B-dot probes fail. Moreover, the current waveform with the adjusted amplitude is also calculated out. The launching experimental results and currents measured by Rogowski coils show that the improved method is highly suited for the velocity and current measurement for a rail-gun system.

Measuring the compression velocity of a Z pinch in an axial magnetic field

This paper presents the results of measuring the velocity of the plasma boundary during the compression of a metallic gas-puff Z pinch in an axial magnetic field. The experiment was conducted on the IMRI-5 facility (current pulse of 450-kA amplitude with a 450-ns rise time); the initial magnetic field Bz0 was varied in the range of 0.15–0.6 T. To measure the compression velocity, B-dot probes were used successfully. The data obtained with the B-dot probes agree with the results obtained by other methods [visible imaging and determination of the pinch radius as a function of the time-varying pinch inductance L(t)]. It is shown that the plasma compression velocity is (1–1.5) × 107 cm/s at Bz0 = 0 and 0.6 × 107 cm/s at Bz0 = 0.6 T.