Session YO08: MFE: Alternate Configurations and Magnetized Target Fusion

Proposal of a linked mirror stellarator configuration for magnetic confinement experiments

A new linked mirror stellarator device for magnetic confinement experiments is proposed. The new linked mirror device consists of two straight magnetic mirrors connected by two half-torus. The device is constructed by just circular coils with a fixed radius. The structure of the configuration as a whole is three dimensional because the two linear mirror sections are not parallel. The angle between the two mirror sections generates rotational transform makes it a stellarator, which results in good magnetic confinement of toroidally passing particles. In this way the usual loss cone of the traditional linear mirror machines is eliminated. Most of the trapped particles are also well confined. The calculated neoclassical confinement is very good and is comparable with that of an equivalent tokamak. The scaling law of particle neoclassical confinement time shows that it meets Lawson's Criterion under typical fusion parameters.   

Device for stable plasma confinement by cross-helicity generation at the Alfvén velocity

A new device configuration is described to stably confine plasma. Helical fluid flow driven at the Alfvén velocity by spatially-periodic cusped magnetic fields and edge currents is combined with magnetic helicity injection to generate the conserved quantity of cross-helicity for the formation of a stable Taylor-Couette-like magnetohydrodynamic singular structure. The basic outline of the device is shown, its nominal use case being for generating fusion power, and to further illustrate the generality of cross-helicity generation, a laminar kinematic dynamo generator is also shown.   

The Novatron mirror concept – theory and simulation

The Novatron mirror fusion concept offers, for the first time, mirror plasma confinement at high mirror ratio in axially symmetric magnetic field geometry with uniformly good curvature. It is designed to significantly improve upon previous mirror devices by enhancing stability against anisotropic MHD interchange modes and microinstabilities, specifically DCLC modes. Currently, a proof-of-principle experimental facility is being constructed, and initial experiments are scheduled for late 2023. If funding and construction of a series of experiments N1-N4 are successful, this could lead to the development of a compact commercial reactor by the end of the next decade. Moreover, the Novatron design reduces the capital expenditure due to both the efficient use of the axisymmetric magnetic field, and the absence of need for superconducting coils. The plasma confinement physics of the innovative Novatron magnetic field geometry, which incorporates ring-shaped magnetic mirrors at both ends of the device, is explored through theoretical models, as well as from particle-in-cell (PIC) and MHD simulations. We will present our current understanding of plasma equilibrium formation and MHD stability at high beta. A number of planned measures for Q enhancement will be presented, being of particular importance since mirrors are open field line devices where energy confinement to a large extent depends on well confined particles.   

Long Pulse Operation of the Centrifugal Mirror Fusion Experiment (CMFX)

The goal of CMFX is to investigate the stability and scalability of centrifugally confined plasmas for fusion energy production. An applied voltage across the magnetic field yields an azimuthal E x B drift with supersonic speeds that creates velocity shear that stabilizes and heats the plasma.  A pair of superconducting magnets are used to produce 3 T mirror fields and 0.375 T at midplane. The cylindrical chamber with a length of 6.7 m and diameter of almost 0.8 m contains a high-voltage center electrode, tungsten-coated circular grounding limiters, and bucket-shaped insulators to allow for applied voltages of up to 100 kV. A hydrogen gas-puff system allows for discharges exceeding 200 ms with peak voltages of 20 kV and densities of order 10¹⁸-10¹⁹ m^-3 with momentum confinement times measured at 20 – 30 ms and higher. Ion Doppler spectroscopy, interferometric density diagnostics, and neutron detectors are now operational. Plans for x-ray diagnostics, Thomson scattering, and preliminary data from deuterium discharges at peak voltages of 50 kV are presented.   

A Deuterium-Fueled FRC Fusion Reactor

As previously noted [1, 2], between the mainline fusion concepts represented by ITER and NIF, a cost minimum exists at about a megagauss. Operation at such a high field can be accomplished by imploding liner or plasma flux compression. Three main challenges [3] are: stability, cost for repetition, and protection from the copious number of high-energy neutrons. To these we add a fourth: the large size of the tritium facility for reactors based on D-T [4]. The first three challenges may be met by the Stabilized Liner Compressor (SLC), which provides repetitive implosion and recapture of a liquid metal liner stabilized by rotation and free-piston drive, compressing an elongated FRC. Recent calculations indicate that the SLC technique can usefully apply to so-called advanced fuels (D-3He, catalyzed D-D) to avoid the fourth challenge. In particular, use of only deuterium as fuel in the temperature range 15-75 keV offers complete burning of the 3He and T produced. The SLC accesses this temperature regime by strong adiabatic compression, while the liquid liner protects the reactor from high-energy D-D and D-T neutrons.

 

1. P.J. Turchi, IEEE Trans. on Plasma Sci., 36, 1, 52 (2008).

2. P.J. Turchi, S.D. Frese, M.H. Frese, IEEE Trans. on Plasma Sci., 45, 10, 2800 (2017).

3. P.J. Turchi, in Proc. of 16^th Intern. Conf. on Megagauss Magnetic Field Generation and Related Topics, 25 – 29 Sept 2018, Tokyo, Japan. ieeexplore.ieee.org/.xpl/tocresult.jsp?isnumber=8722602.

4. S. Wilms, ITER UID THR7JS (Karlsruhe, 2016).   

Detection and Analysis of Low energy X Ray Emission from the Princeton-Field-Reversed Configuration (PFRC-2).

The PFRC-2 is an RF-formed and heated field-reversed-configuration magnetic confinement experiment. X rays produced in its plasma have been studied with AMPTEK, X-123 FAST SDDs that view the plasma column in four regions, two near the axial midplane. These detectors provide X-ray energy and arrival-time information. Their detectable X-ray energy range is 0.05-30 keV and resolution 30 eV at 150 eV and 125 eV at 5 keV. From the detected x-ray spectrum (XED), the electron energy distribution (EED), average electron energy and electron density are extracted. We present recent measurements of the XED from 59 to 300 eV and the inferred electron temperature in the PFRC device as functions of magnetic field, heating power, gas pressure and aperture scan based on the Pulse Pile up.   

Fundamental Scaling of Adiabatic Compression of Field Reversed Configuration Thermonuclear Fusion Generators with Magnetic Energy Recovery

Field Reversed Configuration (FRC) plasmas are plasma devices that have demonstrated that through magnetic compression they can be heated to thermonuclear fusion conditions in the parameter space of an energy-producing generator[i]. Of particular interest, FRCs are high-beta, in that the plasma particle kinetic energy is in balance with an externally applied magnetic field at all stages of operation. By using the simplified cylindrical model, detailed fusion reaction, radiation, and energy transport equations are now numerically-tractable and can be modelled over a wide parameter space. In the second section of this work, a detailed numerical model will be presented with the key theoretical performance of the compression of high-beta fusion plasmas in both deuterium-tritium (D-T) and deuterium-helium-3 (D-He-3) fuels. As will be shown, a high-beta D-He-3 plasma outperforms a low-beta D-T fuel and can theoretically yield a net-positive fusion generator. Further, this work will include the effects of magnetic energy recovery on overall energy balance, which enables net positive electricity production with significantly lower Lawson criteria than previously reported for fusion generators.

The TexatronTM Concept for creation and confinement of hot toroidal plasmas

The Texatron^TM concept for creation and confinement of hot toroidal plasmas is presented along with results of promising early experiments. The ultimate goal of the Texatron^TM Project is to create a compact source of pulsed fusion power.  The Texatron^TM is based on the concept of a “fast pulsed” Torastron  toroidal magnetic plasma confinement device [1] where toroidal plasmas will be created and imploded using fast pulsed magnetic coils. The helical configuration of the pulsed coils creates a rotational transform and high temperatures in the plasma when it implodes to form a toroidal , high beta,  plasma. Formation of shock  heated toroidal plasma configurations have been demonstrated in the Los Alamos Syllac experiments [2] The plasma is created and imploded  with two global flux invariants:  the Wells-Taylor invariant A·B  and the Leaf Turner invariant Ψ² .  MHD relaxation of the resulting hot toroidal plasma has been shown theoretically  to result in Grad-Shafranov toroidal equilibria  with helical magnetic fields and finite beta [3]  Initial experiments and MHD simulations have shown these equilibria occur and will be discussed.

Novel Hybrid Reactor Concepts Based on Ignitor Technology and Physics*

A development of the Ignitor program, aimed at making fusion energy of near term relevance, is that of starting from nuclear engineering, technology and physics advances, on which the Ignitor effort is based, to conceive novel hybrid reactors. High field compact machines have produced record high density plasmas with excellent confinement properties that can be utilized as neutron sources for power producing reactors with Thorium as its fissile component (E. P. Velikhov and B. Coppi, 2019). The Columbus concept [1], that had been studied as a follow-up to Ignitor in order to investigate the burn conditions of Tritium deprived plasmas, is being reconsidered as a neutron source to start with. Advances in relevant Molten Salt Reactor technology are followed in this context. Moreover, following a suggestion by C. Bolton (2020) the adoption of pure, or nearly pure, D plasmas for which the high field approach is appropriate is being analyzed together with relevant advances in materials science. *Sponsored in part by C.N.R. of Italy.

1. B. Coppi and M. Salvetti, MIT (RLE) Report PTP 02/06, December 2006.   

Electrostatic Orbitron Fusion Reactor: Overview of Scientific Program

The Orbitron is a small compact plasma device that confines high-energy ions electrostatically in orbits around a cathode, as is the case in an Orbitrap, while confining electrons in an E×B weak magnetic field, like a Magnetron. The confined electrons mitigate electrostatic ion space-charge density constraints, enabling a high density of ions orbiting with trajectories and energies sufficient for ion-ion fusion. Orbitrons employ "crossed-fields", similar to relativistic microwave devices, and co-rotating ions and electrons to achieve high plasma energy, density and long energy confinement times. They combine aspects of particle accelerators, electrostatic ion traps, relativistic magnetrons, rotating centrifugal mirror plasmas and have the potential to be small compact bright neutron sources (>1E13 n/s) and possibly even net-energy fusion devices that could be operated in a small lab. We are currently investigating the science of Orbitron plasma devices and how they scale in terms of neutron flux and power balance with different fusion fuels. This talk will give a high-level overview of the various theoretical, simulation and experimental work that is ongoing along with a discussion of the current status and next steps.   

One-dimensional model of power balance in an Orbitron fusion reactor

The Orbitron is a small scale fusion reactor that relies on crossed electric and magnetic fields for co-confinement of ions and electrons at fusion-relevant energies. To confine ions, a >100 kV bias is applied across an azimuthally symmetric anode-cathode gap modelled around the geometry of an orbitrap. An ion source injects particles azimuthally into elliptical orbits that precess around the cathode to collide with each other at high energies and fuse. An axial magnetic field confines electrons radially, building density to prevent a space charge limited ion density.

This work presents a simplified analytical ion model that predicts power balance in such a device for a variety of operating parameters and input species. In an assumed cylindrically symmetric configuration, we apply fundamental conservation laws to predict species velocities and densities as functions of radius. We then apply known reaction rate laws to predict fusion and energy generation rates for deuterium-tritium and proton-boron 11 species composition. We further analyze several loss mechanisms including Coulomb collisions, energy loss to background neutral species, and bremsstrahlung radiation and discuss their relative importance. Finally, we discuss operating regimes that would be required to achieve net energy gain and the potential impact of phenomena not captured by the model, such as plasma instabilities.   

Suppression of electron temperature due to an axial magnetic field in multi shell multi species gas puff z-pinches

We report results from collective optical Thomson scattering in multi shell and multi species gas puff z-pinch plasmas investigated on the 1-MA, 200 ns COBRA pulsed power generator at Cornell University. The gas puff configuration consists of concentric outer (Ne gas) and inner (Ne gas) annular gas puffs with an Ar center jet. An axial magnetic field of either 0.1 or 0.2 Tesla was applied. We used a combination of multiple shells and external magnetic field to suppress Magneto Rayleigh Taylor Instability (MRTI).  We used the ion acoustic wave (IAW) feature to obtain both the electron and ion temperature of the plasma. Our results indicate that the use of multiple shell gas puffs and an external magnetic field reduces the amplitude of instabilities. Optical Thomson scattering results show that the electron temperature (in the range 0.02-0.08 keV) is suppressed (in the range 0.01-0.04 keV) when an axial magnetic field is applied. We will discuss a more detailed analysis of electron and ion temperature measurements and underlying physics behind electron temperature suppression.

    

Modular Theta-pinch Experiment (MTX) overview and initial results

The Modular Theta-pinch Experiment (MTX) at Los Alamos National Laboratory consists of multiple independently-triggered theta-pinches around a quartz vacuum vessel, each of which is driven by several capacitor/switch modules, enabling straightforward expansion and reconfiguration to drive a wide range of experiments. Initial work on MTX will explore the physics of high-density Field-Reversed Configuration (FRC) plasmoid formation using a variety of techniques. Areas of investigation include the efficacy of several pre-ionization methods and implications for subsequent FRC formation, various formation approaches (e.g., "static"/simultaneous vs. "dynamic"/sequential field-reversal), and a widely range of tunable formation parameters (reversal voltage, bias field, length of formation section, etc.). A program overview, scaling theory, and preliminary experimental results will be presented.