Inertial Electrostatic Confinement Device Research Articles

Pulse operation mode of inertial electrostatic plasma confinement devices

The paper discusses the operation features of the inertial electrostatic plasma confinement (IEC) devices in the pulsed high voltage mode. The pulse operation mode offers significant advantages in IEC devices, which are associated with high discharge currents that are inaccessible for continuous operating modes. However, the implementation of such operating modes is associated with a number of physical limitations and technical difficulties.A stand with an IEC chamber powered by a high-voltage pulse transformer and a unit for gas preliminary ionization in the IEC chamber has been developed. In the chamber, ions are accelerated from a background glow discharge. A study of the burning background discharge in the IEC chamber, background discharge current Ipre influence on the duration of current and voltage pulses of the main discharge and the delay between them was carried out. The results of the neutron yield measuring in an IEC chamber operating in pulsed mode are presented at a charging voltage up to 105 kV, a discharge current amplitude of several tens of amperes, a preionization current of 8 mA, and a pulse repetition rate from single to 300 Hz. A stable neutron flux with an energy of 2.5 MeV at a level of 106 neutrons/s was experimentally obtained.

From theory to hands-on: teaching experimental plasma physics using small plasma devices at DTU

At the Technical University of Denmark (DTU), we have in recent years acquired or developed three small plasma devices. These consist of a small tokamak (NORTH), an inertial electrostatic confinement device, and a linear plasma device. This has enabled a restructuring of our teaching in plasma physics and nuclear fusion, allowing courses with a dedicated experimental focus. Here we describe the use of these devices in our teaching of fusion plasma physics, with particular emphasis on their integration in a new experimental Master’s level course. We also present examples of BSc and MSc projects completed at these experiments and offer some didactic reflections on student learning during our courses and projects. Our experience so far has validated the potential for student-driven activities at all levels to make useful scientific contributions to the experimental study of laboratory plasmas.

Variation of Plasma Properties in Cylindrical Inertial Electrostatic Confinement Device by Changing the Anode Transparency

Inertial electrostatic confinement (IEC) is investigated in terms of direct-current discharge in a cylindrical configuration using nitrogen gas in the pressure range between 0.028 and 0.09 Torr. Discharge characteristics are determined for different anode transparencies of 84%, 92%, and 96% corresponding to 24, 12, and 6 anode rods, respectively. I-V characteristic curves indicate that the electric discharge is in the abnormal glow discharge region. The discharge voltage has the highest values for the low anode transparency for the same value of the discharge current. A double electric probe has been used to measure electron temperature and ion density. The low anode transparency (24 anode rods) enhances field uniformity and aligns the motion of electrons into a chord so that better electrostatic confinement is achieved. This will raise the ion density and lead to thermalization of the plasma, which reduces the electron temperature. The behavior of the electron temperature and the ion density was studied as a function of the gas pressure at the center and near the edge. The variation of the density and temperature in both positions can confirm the plasma confinement. In the low-pressure regime, the confinement process is reinforced. Because of the longer mean free path, electrons cause ionization at the center, which raises the ion density to about 1.44 × 1015 m−3 and the electron temperature to about 2.9 eV.

Modeling of Inertial Electrostatic Confinement device processes for 3He–3He interactions

One of the most important characteristics of the IEC device is performing of advanced fusion reactions without radioactive products. In this work, the IEC device is simulated based on the primary conditions similar to the UW-IEC device with 3He as working gas using Particle in Cell (PIC) method for 7, 5, and 1.4 mA Ion Injection Currents (IICs). Once the different 3He plasma species have stabilized, the results show that on average, the number of ions inside the device and cathode increases by 67.5% and 57.3%, respectively, whereas the number of electrons increases just by 3%. The results show that nearly all the electrons in the device accumulate inside the cathode and the ions oscillate around the center of device. The obtained results show that the cathodic current for 1.4 mA IIC is comparable with that of measured by the UW-IEC device with 8% difference. The simulations also show 500 s−1 fusion reaction rates, which is very close to the measured value of 482 s−1. These results approve the validity of simulation. By calculating the electric field and potential, the number and exact location of nested virtual anodes and cathodes which are of important parameters for confinement quality are obtained. Also, this validated simulation could be used in future works in order to optimize the IEC device and its fusion reaction rate without any more experiments.

Characterization of an ultra-compact neutron source based on an IEC fusion device and its prospective applications in radiography

Our group is developing an ultra-compact neutron source based on inertial electrostatic confinement (IEC) fusion device for various applications at Kyoto University. This IEC device is configured from a titanium anode and a molybdenum cathode with diameters of 17 and 6 cm, respectively. A high-intensity neutron source operated in a stable pulse shape is mandatory to increase the system’s reliability. Applying a higher voltage is a straightforward way to increase the neutron yield from the system. However, a contradiction between the increase of the applied voltage and the reduction of the system size limits such a proposal. A three-stage feedthrough system is employed in the developed compact IEC to address this contradiction. A feedback control system was developed and applied to the input and output parameters, such as the applied voltage and the neutron yield, to increase its stability in long-term operation. Characterization of the developed system was performed by scanning the neutron yield as a function of applied voltage and cathode current. To date, a maximum neutron yield of 9.2 × 107n·s–1 at 6.4 kW (80 kV and 80 mA) has been obtained. A study of the feasibility of using the IEC system for neutron radiography was performed. Preliminary analysis of the resulting images showed there was good contrast between the sample and the background. The results suggest that optimization of the experimental parameters is needed to perform higher accuracy neutron radiography.

Kinetic characteristics of ions in an inertial electrostatic confinement device.

The kinetic analyses are quite important when it comes to understanding the particle behavior in any device as they start to deviate from a continuum nature. In the present study, kinetic simulations are performed using the particle-in-cell method to analyze the behavior of ions inside a cylindrical inertial electrostatic confinement fusion (IECF) device which is being developed as a tabletop neutron source. Here, the lighter ions, like deuterium, are accelerated by applying an electrostatic field between the chamber wall (anode) and the cathode (cylindrical gridded wire), placed at the center of the device. The plasma potential profiles obtained from the simulated results indicate the formation of multiple potential well structures inside the cathode grid depending upon the applied cathode potential (from -1 to -5 kV). The ion density at the core region of the device is found to be of the order of 10^{16}m^{-3}, which closely resembles the experimental observations. Spatial variation of ion energy distribution function has been measured in order to observe the characteristics of ions at different cathode voltages. Finally, the simulated results are compared and found to be in good agreement with the experimental profiles. The present analysis can serve as a reference guide to optimize the technological parameters of the discharge process in IECF devices.

Measurements and modeling of ion divergence from a gridded inertial electrostatic confinement device using laser induced fluorescence

Inertial electrostatic confinement (IEC) is a method of confining and heating a plasma at benchtop scales to sufficient energies for nuclear fusion to occur. Ion velocity and flow direction were measured in an IEC discharge using laser induced fluorescence (LIF) on argon ions. A cathode of two parallel rings, with a common axis of symmetry, resulted in predominant discharge beams, otherwise known as microchannels, along this axis. The device was operated in the abnormal glow discharge regime where both current and voltage increase monotonically, replicating a conventional high voltage IEC device. It was found that argon ions accelerated and flowed outward from the midpoint between the rings along the axis; we have labeled this ion motion as being divergent. The predominant direction of ion flow in the discharge is opposite to the conventional ion focus model, where the discharge at the cathode center is assumed to be the result of ion flow toward it from outside of the cathode. An ion sheath model is shown to produce a virtual anode at the axial midpoint between the rings. The model also shows that ions within the virtual anode are accelerated outward with a spatial velocity profile that replicates those measured using LIF.

Investigation of Ion Generation Rates in an Inertial Electrostatic Confinement Device by Spectroscopy-Based Inverse Analysis

Investigation of Ion Generation Rates in an Inertial Electrostatic Confinement Device by Spectroscopy-Based Inverse Analysis

Characterization of fusion plasmas in the cylindrical DTU inertial electrostatic confinement device

Inertial electrostatic confinement offers a relatively simple and cost-effective means of generating fusion plasmas for research and industrial applications. Here, we present the experimental setup and discharge characteristics of the inertial electrostatic confinement device at the Dept. of Physics, Technical Univ. of Denmark. Special features of this setup include a cylindrical anode and the novel use of 3D printed soccerball-like cathode grids of different sizes. Measurements with these grids show 25% higher fusion neutron rates than with manually manufactured grids with larger wire spacings. Additionally, we observe significantly higher neutron rates with smaller grids, with spherical rather than cylindrical cathodes, and when using the vacuum chamber, rather than a second spherical grid, as the anode. Ion–orbit simulations predict a core density in the ion distribution in good agreement with optical measurements, confirming that asymmetries in the cathode grid potential prevent a fully convergent ion flow. The simulations also demonstrate that the asymmetry of the electric field induced by the voltage stalk lowers the characteristic ion recirculation by a factor of four, and we discuss measures to circumvent this. Comparing measurements and simulations conducted with a spherical and cylindrical grid reveals tentative evidence that fusion reactivity is highly core-localized, pointing to ion–neutral fusion as the dominant reaction. We also quantify the thermionic and impact-induced secondary electron emission in the device, showing that only the latter can potentially suppress the ion current during normal operation.

Potential Profile Measurements Inside a Gridded Cathode at High Potential in a Spherical Inertial Electrostatic Confinement Device

This paper proposes a Langmuir probe–based diagnostics for plasma parameters inside gridded cathodes at high bias potentials in inertial electrostatic confinement devices. As the first step for the proof of concept, floating potential profiles were measured in deuterium and helium plasmas in a glow-discharge mode. The measurements with fusion-relevant cathode voltages up to 55 kV were carried out successfully. The results revealed that the positive potential buildup at the center ranges from 5% to 8% of the applied bias voltage to the gridded cathode, which is found to be much smaller than those in earlier works under cathode voltages lower than 5 kV. It was also shown that the floating potential profile is different significantly between deuterium and helium discharge plasmas.

Improvement of the Neutron Production Rate of IEC Fusion Device by the Fusion Reaction on the Inner Surface of the IEC Chamber

Neutrons are generated in the inertial electrostatic confinement (IEC) device through different types of fusion reactions of the fuel gas such as deuterium (D) and tritium (T). Fusion in the IEC device takes place via various kinds of collisions like beam-beam collision, beam–background gas collision, and beam-target collision on the electrode surfaces. Two identical anodes for the IEC chamber made from titanium (Ti) and SUS-316L stainless steel (SS) are used to study the effect of the anode material on the neutron production rate (NPR). The NPRs from the chambers are measured at different applied powers. The achieved NPRs, so far, for Ti and SS are 8.9 × 107 n/s at 5.25 kW (75 kV, 70 mA) and 2.8 × 107 n/s at 10.5 kW (70 kV, 150 mA), respectively. The normalized NPR (NPR rated to the cathode current) from the Ti chamber is three to four times higher than that from the SS chamber. We observed a better NPR for the Ti chamber compared with the SS chamber. This is explained by the fusion reaction occurring between the neutrals and D atoms adsorbed/embedded on the inner surface of the anode. Moreover, the Ti chamber shows an improvement of the NPR as a function of the operating time ranging from 1.5 to 1.75 after 25 h from the first discharge.

Evidence for surface fusion in inertial electrostatic confinement devices

Inertial electrostatic confinement is a method of producing nuclear fusion in which concentric spherical electrodes are used to accelerate ions to fusion relevant energies. Fusion events are generally attributed to collisions between accelerated ions and neutral gas molecules in the centre of the device, with ion–grid collisions considered detrimental. In this paper, we present data that indicate that collisions between ions and neutral gas molecules adsorbed on the grid surface may, in fact, contribute significantly to the observed fusion rate in deuterium fuelled systems. When operating in the 1 × 10−4–1 × 10−3 Torr, V ≤ 40 kV regime, fusion on the grid surface is found to contribute up to 80% of the measured fusion rate, as determined from hysteresis effects between the fusion rate and system pressure. Surface fusion measurements were also carried out for a selection of cathode materials, with graphite found to produce a fusion rate that is an order of magnitude greater than the highest performing metal targets.

A study on neutron emission from a cylindrical inertial electrostatic confinement device

The adaption of new generation portable neutron sources has been increasingly marked in a wide range of research fields compared to the large-scale neutron generators. In this context, we have successfully demonstrated some essential parameters required for the emission of 2.45 MeV DD fusion neutrons from a steady state portable linear neutron source based on inertial electrostatic confinement scheme. The parameters that control the production of neutrons are the working pressure of the fuel gas, applied voltage, measured current and cathode geometries. The neutrons emitted from the source are confirmed using neutron monitor, bubble dosimeters, nuclear track detectors, and He-3 proportional counter. Presently, the device produces neutrons up to the order of ∼ 106 n/sec at discharge voltage ranging from -60 kV to -80 kV, and discharge current of 20 mA to 30 mA.

Jet extraction and characterization in an inertial electrostatic confinement device

Characterization of IEC tight jet mode is achieved through Faraday probe measurements. Preliminary results indicate that the tight jet is a highly energetic electron beam with scattering of secondary ions and electrons, which result from electron beam impact ionization. A novel analytical model is proposed to evaluate this non-Maxwellian plasma beam, including a compensation of the secondary electron emission effect on the probe's surface. The results show prominent features on the extracted jet's kinetic energy and the respective (electron) current of IEC. In addition, they demonstrate a practical method to characterise non-Maxwellian plasma jets through Faraday probes.

Investigation of gas breakdown in cylindrical inertial electrostatic confinement device with inner cage anode

The gas breakdown in the Inertial Electrostatic Confinement (IEC) device has been studied by DC discharge of Nitrogen gas in the pressure range between 0.03 Torr and 0.7 Torr. Paschen curves and Townsend coefficients are obtained in a cylindrical system with inner anode in shape of rods and the outer grid cathode. The breakdown occurs and the plasma is formed at the center of the electrodes. Townsend coefficients are calculated for anode transparencies of 84%, 92%, and 96% (24, 12, and 6 rods respectively). On the left hand side of Paschen curves, the high transparency has a higher breakdown voltage. More electrons are allowed to pass the anode. So, the collisions increase, leading to a slight increment of the first Townsend coefficient (α) decreasing of the second Townsend coefficient (γ) while the ionization efficiency (η) is not affected. For high reduced electric field E/P, i.e. a high electric field with low pressure, the probability of ionizing collision events decreases. This causes a decrement of α while γ is raised because the ions' energy is not exhausted in collisions at the center. The coefficients α and γ are working in harmony to balance the rates of electrons generated in gas ionization so that the relation between the two coefficients is satisfied.

Energy distributions and radiation emissions in an inertial electrostatic confinement (IEC) device under low and moderate magnetic fields

The energy distributions and electromagnetic emissions of deuterium ions and electrons in an inertial electrostatic confinement (IEC) unit are reported for low and moderate magnetic field cases. The IEC device has a central wire system inside the grid system of the chamber for the production of the azimuthal magnetic field. The real-time simulations are performed by time integration method by using a finite difference method (FDM) for the physical geometry of device. It is observed that the field induces helical trajectories on the particles especially at the central region where the magnetic force is maximized in a fully ionized Deuterium media. The results prove that the particles have rich dynamics in terms of their trajectories. The effect of negative potential and particle–particle interaction play important roles to determine the trajectories even for low number of ions and electrons. Ion temperatures are calculated as Ti = 6.9 keV after 6 μs for low field case and Ti = 1.517 MeV after 22 μs for medium field case.

Jet extraction modes of inertial electrostatic confinement devices for electric propulsion applications

The Inertial Electrostatic Confinement (IEC) method is a plasma confinement principle with different operation modes. The work presented investigates the jet mode, which occurs with a jet extraction relevant for space propulsion applications. Two different jet modes were observed, the tight jet mode and the spray jet mode. Depending on propellant, discharge pressure and power these modes occur at different operation conditions and characteristics. Emission spectroscopic investigations show a different plasma species composition of confinement and extraction plasma and the transition between those modes has been examined with a high-speed camera. Finally, the applicability of IEC plasma sources for space propulsion systems is discussed with respect to the experimental results.

Many-body solution to the D2 gas filled inertial electrostatic confinement device

The simulations of the electrostatic confinement fusion unit have been presented in low magnetic field case, which is produced by a central wire system inside the grid system of the chamber. The time-dependent simulations have been realized by the time integration together with the finite difference elements (FDE). Especially, FDEs have been used to compute the chamber potential and magnetic field interaction of the particles, namely electrons and ions with the chamber structures. The central wires exert a magnetic field in the azimuthal direction inside the central grid. It is found that this field induces helical trajectories on the particles. The model unit has six cathodes around the center. The system is simulated in a Deuterium media which is fully ionized. Considering the boundaries of the unit, the electrical and magnetic forces are determined by using the many-body technique with the particle-chamber and particle–particle interactions. According to the results, many of the electrons can be repelled by the negative potential; however the ions have changeable trajectories with slower attitudes. The ion temperature has been found as Ti = 6.9 keV at the end of 6 μs. The ion distribution proves that 45% of ions exist inside the grid however the increasing trend of ion temperature proves that this value is to be increased further. The velocity distribution shows a maximum around 5 × 105 m/s, however there are also highly energetic particles.

Fusion energy in an inertial electrostatic confinement device using a magnetically shielded grid

Theory for a gridded inertial electrostatic confinement (IEC) fusion system is presented, which shows a net energy gain is possible if the grid is magnetically shielded from ion impact. A simplified grid geometry is studied, consisting of two negatively biased coaxial current-carrying rings, oriented such that their opposing magnetic fields produce a spindle cusp. Our analysis indicates that better than break-even performance is possible even in a deuterium-deuterium system at bench-top scales. The proposed device has the unusual property that it can avoid both the cusp losses of traditional magnetic fusion systems and the grid losses of traditional IEC configurations.

Progress in the Understanding of Gridded Inertial Electrostatic Confinement Devices at the University of Wisconsin

For nearly two decades, as many as 4 Inertial Electrostatic Confinement (IEC) devices have been operated simultaneously at the University of Wisconsin-Madison. Over that time period we have learned that the early perceptions of how IEC devices operate are quite different from the actual performance in the Laboratory. Over the past 2 years we have gained even more understanding of IEC physics and technology. Experimental measurements and theoretical improvements have better characterized both the negative ions that contribute up to ~10% of the fusion rate in some cases and the neutral energy distributions in IEC devices at moderate pressure (0.07-0.7 Pa ≈ 0.5-5 mTorr). We also now understand more of why operation with helium plasmas has such a detrimental effect on high voltage performance of the traditional tungsten alloy grid wires. Most of the previous IEC work had been confined to < 100 kV with short operation times up to 150 kV. We have recently expanded our operating regime to ≈ 200 kV anode-cathode potential difference, which is, to our knowledge, the highest-voltage IEC operation reported in the worldwide IEC literature. Several design modifications were required to achieve steady state operation at these high voltages and some are described in this article.