Optimizing Neutron Production Rates from D-D Fusion in an Inertial Electrostatic Confinement Device

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

An inertial electrostatic confinement (IEC) fusion device accelerates ions, such as deuterium (D) or tritium (T), to produce nuclear fusion and generate neutrons. The IEC's straightforward configuration consists of a concentric spherical transparent cathode at a negative bias surrounded by a grounded spherical anode. The effects of cathode properties on the neutron production rate (NPR) remain, to date, inadequately studied. This study aims to determine the impact of the cathode material on the NPR by investigating fusion reactions on the cathode surface. Two buckyball-shaped cathodes made of stainless steel (SS) and titanium (Ti), both of 5 cm diameter, fabricated by selective laser melting and 3D printing, are used for this investigation. A SS spherical chamber of 25 cm inner diameter is used as an anode in this experiment. A performance evaluation of surface fusion reaction in the IEC using SS and Ti grids is conducted by examining the NPR as a function of the applied voltage and grid currents at different gas pressures. So far, IEC with Ti and SS cathodes achieves NPRs of 2.32 and 1.41 × 107 n/s, respectively, at 5.6 kW (70 kV, 80 mA). The normalized NPRs (NPR/I-cathode) from IEC using SS and Ti cathodes are compared. The results demonstrate that fusion reaction occurs on the cathode surface, and fusion increases with the applied voltage. The measured NPR/I-cathode using the Ti cathode is higher than that of the SS cathode by factors of 1.36–1.64 across the 20–70 kV range. Moreover, fusion on the Ti cathode surface enhances the total NPR significantly compared to the SS cathode under the same conditions. The Ti's considerable ability to accumulate D ions and molecules compared with that of SS explains the difference of measured NPR results.

A two-dimensional numerical code has been developed for simulating dc discharges in inertial electrostatic confinement (IEC) fusion devices. Unlike the existing IEC codes, the developed code is not based on Monte Carlo scheme by the use of random number nor time-domain particle-in-cell (PIC) method, aiming at a drastic reduction of computational efforts. It is based on a time-independent scheme, i.e. iterative calculations of particle-tracking and induced self-field, leading to a much faster convergence than the time-domain PIC scheme on steady-state self-consistent solutions. Also, a new scheme for atomic process treatment is proposed, which is completely free from the inherent difficulty of Monte Carlo method, i.e. requiring many particles for simulating rare events. Preliminary numerical result from the this code showed agreement with experimental helium discharge characteristics in an IEC device.

We are developing, the first of its type, a portable active interrogation system for special nuclear materials (SNMs) detection such as U-235 and Pu-239. The system is based on the recently developed technique, threshold energy neutron analysis (TENA) method. Mandatory aspects for the neutron generator to be used for on-site inspection as well as for container screening at ports of entry is intense neutron as 5×107 n/s neutron yield in a very compact configuration such as ∼25 cm diameter, 60 cm height, and lightweight ∼30 kg. The criteria mentioned above are matching the neutron generator based on the D-D fusion reaction from Inertial Electrostatic Confinement (IEC) fusion device. Two prototypes of the DD-IEC fusion device, with 17 cm anode diameter, have been designed, fabricated and tested. The anode of the first device was made from stainless steel, while titanium was chosen to build the second version. The cathode for both versions was made from molybdenum with 6 cm diameter. High voltage and current are required to achieve the target neutron yield from the compact configuration. To cope with that, we used a developed technique called a multistage feedthrough method to enable applying higher voltage and current, up to 120 kV and 300 mA, in a compact configuration. The neutron production rate (NPR) achieved from the 1st prototype was ∼2.8×107 n/s by applying 70 kV and 150 mA, and from the 2nd prototype was 8.9×107 n/s, by applying 75 kV and 70 mA current, which exceeds the target NPR designed for the SNMs interrogation system. The technical developments and challenges to construct and operate the prototypes of the DD-IEC fusion device are discussed together with the experimental results for the NPR measurements.

An external helicon ion source is used on HELIOS, an inertial electrostatic confinement (IEC) fusion device in which ions are accelerated radially to fusion-relevant energies by the electrostatic field between the spherical chamber wall and a concentric cathode grid. The external ion source has allowed for the demonstration of 3He-3He fusion in an IEC device for the first time, as part of the advanced fuels program at the University of Wisconsin.1

Inertial Electrostatic Confinement (IEC) fusion reactors are fusion-powered devices that produce energy and radioactivity using electric fields to confine ions and induce particle collisions.The primary objective of this research is to simulate a multi-grid IEC device to evaluate the neutron production rate (NPR) using XOOPIC software and to study the variables affecting neutron production within the IEC, including the voltage supplied to grid, the ion currents supplied to the system, and the radius of the innermost cathode grid.The scope of this study includes simulation of an IEC device with 1 mm grid diameter, 25 cm exterior grid radius as anode, a continuous injection of deuterium ions into the system, and a high vacuum of the background gas.From the simulation results, it was found that supplying highly negative voltage to the cathode grid affected the depth of potential well and ion confinement.The neutron production rate tended to increase linearly as more negative potential was applied to the grid.The IEC device was able to increase the neutron production rate from 1.73 x 104 n/s to 1.41 x 105 n/s when the supply voltage to the cathode grid was reduced from -200 kV to -500 kV.Increasing the depth of the potential well results in greater ion confinement in the core region of the IEC device, where neutrons are the most generated because the kinetic energy of the ions is at their highest, thus maximizing the probability of fusion reactions.By modifying the radius of the cathode grid, the neutron production rate was increased from 1.21 x 10 5 n/s to 2.19 x 10 5 n/s when the cathode radius was increased from 50 mm to 80 mm.The effect of increasing fusion reactions in the core region is the same as adding a more negative potential to the cathode grid.However, the difference is that it does not increase the ion density of the core.Rather, it increases the extent to which the reaction can take place.Increasing ion current affected the ion density within the system.The IEC device was able to increase the neutron production rate from 1.38 x 10 5 n/s to 1.96 x 10 5 n/s when the ion current was increased from 0.1 A to 2 A. This is due to increased ion density in the system, which increases the possibility of ions colliding and causing fusion reactions.

A particle-in-cell calculation code was made to simulate the operation of an inertial electrostatic confinement (IEC) fusion device. The computation includes the effects of ionization by electron impact. Several techniques to save computational time are introduced in this program code. One of them is time-dependent fine space meshes used in the regions where the particles concentrate. Several superparticles that have similar radial position as well as similar energy are merged, while one superparticle is divided into several particles with a somewhat different velocity when the total number of superparticles decreases. The methods enable more precise determination of the characteristics of an IEC device in a shorter time than by previous methods.

Confinement of ions in an inertial electrostatic confinement fusion (IECF) device and its influence on neutron production rate

Among many facilities in the field of nuclear fusion devices, inertial electrostatic confinement (IECF) device has the specific character of tendency to generate fusion products continuously. Besides the distinctive characteristics, it has become an outstanding focus of interest for many scientists because of several applications such as the ability of performing hydrogen boron fusion. This paper summarizes primary results of the design and construction of the first Iranian IECF device (IR-IECF). It consists of 13.5 cm diameter stainless steel cathode, 41 cm diameter anode with a 60 cm diameter and 60 cm height vacuum chamber. The outcomes of neutron detection rep- resent more than 10 7 neutron/s at the maximum biased voltage of -140 kV and 70 mA current with deuterium operational filling gas in the steady state regime.

For the past 15 years, the Inertial Electrostatic Confinement (IEC) fusion group at the University of Wisconsin-Madison has been conducting experiments to demonstrate that there can be many near term applications of fusion research long before the production of electricity in commercial fusion power plants. This research has concentrated on three fuel cycles: DD, D3He, and 3He3He. Some of the major accomplishments are listed below:a. The production of > 108 DD neutrons per second on a steady state basisb. The production of pulsed DD neutrons to over 1010 per second in 10Hz, 100 μs bursts.c. The production of 14.7 MeV protons at > 108 per second (steady state) from the D3He reaction.d. Demonstrated the detection of the explosive C-4 with steady state DD neutrons.e. Demonstrated the detection of Highly Enriched U (HEU) with pulsed DD neutron fluxes.f. Production of the positron emission tomography (PET) isotopes, 94mTc and 13Nusing D3He protons.g. Production of the first measured 3He3He fusion reactions in an IEC device.h. Development of unique diagnostic techniques to measure the rate, spectrum, and location of fusion reactions in IEC devices.i. Use of an IEC device to study the behavior of materials at high temperature during charged particle bombardment.The accomplishments above were carried out in 3 devices HOMER, 3HeCTRE, and HELIOS that have operated up to 180 kV and meter currents of 65 mA. New applications are currently being explored and expanded roles for the IEC device will be described.

New diagnostics are required to understand the physics operation of an inertial electrostatic confinement (IEC) device. In an attempt to understand the fusion source regimes within the IEC device, a new diagnostic called the eclipse disk has been introduced. This diagnostic was used to exploit the byproduct protons’ energy difference between the deuterium-deuterium (D–D) and deuterium—an isotope of helium with two protons and one neutron (D–He3) reactions to study the contributions of the protons generated from various source regimes. These source regimes are divided into five categories namely: converged core, embedded, beam background, volume, and wall-surface sources. The eclipse disk diagnostic has provided the first confirmed evidence that D–He3 reactions are predominantly embedded reactions. It has been observed that at the present operating power levels (6–10kW) most of the D–D reactions occur in the volume of the chamber caused by the charge exchanged neutrals, and the converged core contribution is significant only for D–D reactions. Since the branching ratio for the proton and neutron generation in a D–D fusion reaction is ∼50%, it is inferred that the proton to neutron count ratio is a better parameter to monitor than either proton or neutron counts measured alone while studying the source regimes. This parameter may also be used for studying the potential wells within the cathode grid.

An inertial electrostatic confinement (IEC) fusion device is studied for a compact fusion neutron/proton source using a magnetron discharge as an external ion source. The addition of ion sources to the IEC device is to enhance the efficiency of fusion reactions by lowering the operating gas pressure and by providing beam-like ion energy distribution. At lower gas pressures, charge exchange collisions are reduced, resulting in longer ion lifetime and enhanced ion re-circulation. An improvement in normalized neutron yield about a factor of ten has been observed with respect to grid current and the operating gas pressure at greatly reduced operation gas pressure of 0.1 Pa

Summary form only given. The PDS1 particle-in-cell Monte-Carlo-Collision (PIC-MCC) code has been modified for simulating the Inertial Electrostatic Confinement (IEC) device which has multiple collision processes and electrical grids in a spherical geometry. PDS1 can give a detailed account of the transient and steady-state plasma physics occurring in the IEC device. Two experimental single-grid devices and one triple-grid conceptual device were simulated.

The design of the cathode grid for a spherical inertial electrostatic confinement (IEC) fusion device is described based upon the results of SIMION, an electric field and ion trajectory program. Cathode grids of varying radii and geometric transparency were simulated with this code. Two-dimensional models of energetic ions in the IEC that were produced with SIMION show the formation of ion microchannels and the Star-mode. The SIMION results also show that smaller, less transparent grids produce smaller cores and more focused ion channels.

Inertial electrostatic confinement fusion (IECF) devices often use two concentric spherical electrodes to converge ions in a plasma electrostatically. Using a highly transparent inner cathode, the ions can move through the cathode and collide at the center to undergo fusion reactions. This is a simple method to build a neutron source. Past research has focused chiefly on cathode “grids” manufactured by joining metal wire loops or disc-shaped elements via spot welding. There are two common geometries: “Globe” grids with a distinct latitude-longitude structure and “symmetric” grids with even-sized triangular-shaped apertures. Recent advances in additive manufacturing have opened the way to manufacturing a third class of grids in which the apertures are evenly distributed over the grid surface and have either circular or polygonal shaped apertures - here called “regular” grids. These three types are analyzed and compared based on a set of metrics, including transparency, homogeneity of aperture size, and the regularity of aperture distribution. It is shown that every type of grid comes with different advantages and disadvantages. The analysis focuses on grid geometries with 6 to 120 apertures.