Tandem Mirror Research Articles

Study of the intermittent plasma structure around the divertor simulation experimental module in GAMMA 10/PDX

We have investigated the generation region of intermittent plasma structures, which could be due to the blob-like cross field transport around the divertor simulation experimental module (D-module) in the tandem mirror device GAMMA 10/PDX. A positive skewness of the ion saturation current was clearly seen when the electrode of the movable probe was located at the radial edge and just in front of the entrance limiter of the D-module. Fourier analysis and conditional averaging clarified that positive spikes intermittently appeared in the same region. This is the first indication that the phenomenon producing the high-density isolated plasma structures occurred in the upstream of the D-module. A negative skewness was also found, and large amplitude fluctuation was detected between regions showing positive and negative skewness. Furthermore, this and light emission fluctuations become stronger during the transient state from attached to detached state on the V-shaped target.

Scoping Studies for a Lead-Lithium-Cooled, Minor-Actinide-Burning, Fission-Fusion Hybrid Reactor Design

A feasibility study of a subcritical fission-fusion hybrid reactor using lead-lithium eutectic as a coolant and minor actinides (MAs) as fuel is presented. Such a reactor could support the fission community by transmuting MAs and the fusion community by breeding tritium. The feasibility of such a reactor for the burnup of MAs is assessed in terms of burnup performance, tritium breeding, and safety characteristics. Tandem mirrors are a promising neutron source technology, and a deuterium-tritium tandem mirror is considered here for the neutron source with power Psource = 1.13 MW assumed for scoping purposes. Subcritical reactivities from keff = 0.9800 to keff = 0.9950 were considered, representing the initial reference for subcritical reactivity and the assessed upper limit, respectively. Stability analyses indicated the reactivity would be stable under perturbations of fuel, coolant, and inlet temperatures, with a positive reactivity insertion expected during reactor shutdown. This range corresponded to nuclear heating values of 150 to 650 MW and mass burn rates of 53 to 216 kg/year. The upper mass burn rate limit would require 1110 reactor years with a capacity factor of 0.9 to fission the global supply of MAs and could offset the annual U.S. MA production with eight reactors. Tritium breeding was assessed for keff = 0.9800 and 3.795% 6Li enrichment in the coolant, and a tritium breeding ratio of 1.602 0.017 was tallied, suggesting the reactor could, without elevated 6Li enrichment, produce tritium to both sustain operation and supply tritium for other fusion devices. Time-series modeling of fuel burnup was conducted for a four-batch loading scheme and three different fuel residence times at keff = 0.9800, which showed system performance would drop with burnup, and that the rate of this drop was lower for longer fuel residence times, motivating a means of reactivity control. Last, changes in fuel composition with burnup were assessed for relative concentrations of MAs, transmutation products, and fission products. The breeding of plutonium in the blanket was calculated and found to be of minimal proliferation concern.

Experimental studies of cusp stabilization in Keda Mirror with AXisymmetricity (KMAX)

Stabilization of the axisymmetric magnetic mirror relies on the pressure-weighted magnetic field curvature. We report a new experiment by configuring a magnetic cusp structure to stabilize m = 1 interchange mode in the KMAX tandem mirror. The cusp configuration is formed by reversing currents in the two side cell coils, and a stronger cusp can lead to a more stable plasma once the null point of the cusp is less than 35–40 cm away from the device axis. The density fluctuations measured by four axial Langmuir probes are mitigated by 70%–80%. The stabilization effect is consistent with the prediction of a theoretical calculation.

Ku-band multichannel frequency comb Doppler reflectometer on the GAMMA 10/potential control and divertor simulating experiment (PDX) tandem mirror.

A Ku-band (12-18GHz) multichannel Doppler reflectometer (DR) has been developed in the GAMMA 10/potential control and divertor simulating experiment (PDX) tandem mirror device to improve the applicability of DR measurement for simultaneous monitoring of velocity of electron density turbulence at different locations. Our previous single-channel DR circuit has been replaced by the multichannel microwave system using a nonlinear transmission line based comb generator with heterodyne technique. The multichannel DR system has been installed in the central cell of GAMMA 10/PDX. Initial results of application to GAMMA 10/PDX plasma are presented, showing Doppler frequency shifts during an additional ion cyclotron resonance frequency heating and gas-puffing experiment.

Particle simulations on propagation and resonance of lower hybrid wave launched by phased array antenna in linear devices

In this work, we performed first-principles electromagnetic-kinetic simulations to study a phased antenna array and its interaction with deuterium plasmas within the lower hybrid range of frequency. We first gave wave accessibility and resonance results, which agree well with theoretical prediction. In addition, we further investigated the antenna power spectrum with different antenna phases in the presence of the plasma and compared it with that in a vacuum, which directly indicates wave coupling and plasma absorption. Furthermore, for the case with zero phasing difference, our simulation results show that, albeit the launch is away from the accessibility region, tunneling effect and mode conversion occurred, which enhanced coupling and absorption. Moreover, consistent interactions between the injected wave and the plasma concerning various antenna phase differences are shown. We presented the inchoate response of the plasma in terms of the launching directions. Our results could be favorable for the engineering design of wave heating experiments with a tunable phased antenna array in linear devices, such as simple magnetic mirrors or tandem mirrors.

Multilayer Polymer Photonic Aegises Against Near-Infrared Solar Irradiation Heating.

Preventing solar heating is nowadays of paramount interest in energy savings and health preservation. For instance, in building thermalization solar heating consumes an excess of energy leading to harmful CO2 emissions, while in food and beverage packaging it may lead to variation of organoleptic properties or even health issues. The phenomenon is attributed to the large presence of moieties with highly absorbing vibrational overtones and combination bands in the near-infrared spectral region that induces heating in water, moisture, and in polymers used in packaging. Thus, reducing and controlling the light absorbed by these materials with effective low-cost passive systems can play a major role in energy saving and health preservation. In this work, different polymer dielectric mirrors are reported, made of poly(N-vinylcarbazole) and either cellulose acetate or poly(acrylic acid), and able to selectively reflect near-infrared radiation while maintaining high transparency in the visible range. To this end, simple, tandem, and superperiodic mirrors are used to shield radiation impinging on samples of water and paraffin, demonstrating shielding efficiencies up to 52% with respect to unshielded references, promising a new paradigm to solve thermal management issues.

Publisher’s Note: “Study of detached plasma profile in the divertor simulation experimental module of tandem mirror GAMMA 10/PDX” [AIP Advances 11, 125231 (2021)

Publisher’s Note: “Study of detached plasma profile in the divertor simulation experimental module of tandem mirror GAMMA 10/PDX” [AIP Advances 11, 125231 (2021)

Fluid simulation of plasmas with anisotropic ion pressure in an inhomogeneous magnetic field of a tandem mirror device GAMMA 10/PDX

Fluid simulation of plasmas with anisotropic ion pressure in an inhomogeneous magnetic field of a tandem mirror device GAMMA 10/PDX

Study of detached plasma profile in the divertor simulation experimental module of tandem mirror GAMMA 10/PDX

A divertor simulation experimental module (D-module) in the tandem mirror GAMMA 10/PDX was used for the study of plasma detachment. In previous studies, it was difficult to measure far-upstream plasma parameters in the D-module, and only electrostatic probes on the target plate were used to perform electron temperature and density measurements. To study the detached plasma structure, a Thomson scattering (TS) system and a microwave interferometer system have been installed to measure the inside plasma parameters of the D-module, and a movable electrostatic probe has been placed at the inlet of the D-module to measure the inlet plasma density and temperature. The TS system in the central cell observed the electron temperature and density of the core plasma simultaneously. These measurements revealed the entire density and temperature structure from the core plasma to the divertor plate. The line average electron density measured by the microwave interferometer showed a rollover behavior during detachment. The results indicated that the ionization region was located around the center of the D-module, and it appears to move upstream along the axis.

Measurement of axial phase difference of density fluctuations owing to spontaneously excited waves by using microwave reflectometer on GAMMA 10/PDX.

In the GAMMA 10/PDX tandem mirror, plasma with strong ion-temperature anisotropy is produced by using the ion cyclotron range of frequency waves. This anisotropy of ion temperature causes several Alfvén-Ion-Cyclotron (AIC) waves to spontaneously excite in the frequency range just below the ion cyclotron frequency. In addition, difference-frequency (DF) waves are excited in the radial inner region of the plasma by wave-wave coupling among the AIC waves. The radial density profiles were measured at multi-axial positions using a frequency-modulation reflectometer with an axial array of microwave antennas, and an axial variation of the density was found to be significant. In addition, a relative phase difference of the DF wave between axially separated two points was first obtained by finely choosing the probing frequency of the reflectometers with a maximum coherence used as a measure, indicating that the DF wave is a propagating wave, while the pump AIC waves are standing waves in the axial region of measurement.

Suppression of flute-like fluctuation by potential formation in GAMMA 10/PDX

In the hot ion mode experiments of the tandem mirror GAMMA 10/PDX plasma, suppression of the flute-like fluctuation, for which the rotation direction is the same as the ion diamagnetic rotation direction, was observed during the axially confined potential formation while electron cyclotron heating (ECH) was applied at the barrier and plug cells for the first time. In the previous experiments, the electron diamagnetic rotation type fluctuations were clearly suppressed with the application of both barrier (B) and plug (P) ECH. We show that the flute-like fluctuation of ion diamagnetic rotation is also suppressed with the application of P/B-ECH.

Translation speed measurements of hydrogen, helium, and argon field-reversed configurations in the central cell of a KMAX mirror device

A series of experimental results of field-reversed configurations (FRCs) on a KMAX (Keda Mirror with AXisymmetricity) tandem mirror machine are reported. Single-side FRC translation processes with three different gas species were measured by avalanche photodiodes. Consistent with the theoretical prediction, the measured FRC speeds were inversely proportional to the square root of the ion mass. However, the speeds of the hydrogen FRC increased even in a uniform magnetic field region while the speeds of the helium and argon FRCs decreased. Possible mechanisms are discussed. The speed of the second pass due to the reflection of the mirror fields was found to be ∼1/3 of the first pass speed. The internal magnetic fields were measured for a colliding-merging argon FRC, and the results show that, even for very slow-moving FRCs, merging can occur.

Synergistic effect of nitrogen and hydrogen seeding gases on plasma detachment in the GAMMA 10/PDX tandem mirror

We have investigated the synergistic effect of a combination of various impurity gases and hydrogen gas on plasma detachment of high temperature plasma, equivalent to scrape-off layer plasma of tokamaks in the GAMMA 10/PDX end region, utilizing an open magnetic field configuration. A small puff of an impurity gas (N2, Ne, Ar, Kr, Xe) in combination with a puff of H2 gas is examined to evaluate their synergistic effect on the formation of detached plasma; the following results are obtained. (i) A combination of N2 and H2 puffs showed a clear decrease of electron density and ion flux; (ii) N2 and H2 puffs form a strong density gradient along the axial direction; and (iii) other noble impurity gases showed an insufficient synergistic effect. The new results indicate the possibility of achieving a reliable divertor operation scheme and the importance of a deeper understanding of the H2 and N2 assisted recombination process.

Analysis of Ar Impurity Transport in the Large Tandem Mirror Device GAMMA 10/PDX Plasmas

Analysis of Ar Impurity Transport in the Large Tandem Mirror Device GAMMA 10/PDX Plasmas

Impact of additional plasma heating on detached plasma formation in divertor simulation experiments using the GAMMA 10/PDX tandem mirror

The transition of a detached to attached plasma experiment has been performed in GAMMA 10/PDX by applying an additional plasma heating pulse of electron cyclotron heating (ECH). In a plasma detachment experiment, a short pulse (25 ms) of ECH was applied at the east plug-cell to examine the effects of electron heating on the plasma parameters in the D-module. It was found that the ion flux increased significantly during ECH injection. In the ECH injection period, the ion flux increased with the increasing impurity injection, which indicates the impurity ion components are enhanced by ionization in the D-module due to the application of ECH. During ECH injection period, the increase of the ion flux near the corner of the target plate shows a clear dependence on the gas species. The 2D visible emission has been captured by the high-speed camera and a bright emission near the corner of the target has been observed. The spectroscopic measurement in the D-module shows that the impurity ion emission increases remarkably during ECH injection. These results indicate the detached plasma change to the attached state.

Formation of field-reversed configuration using an in-vessel odd-parity rotating magnetic field antenna in a linear device.

A Field-Reversed Configuration (FRC) is formed by an in-vessel odd-parity rotating magnetic field (RMF) antenna in a tandem mirror device, Keda mirror with axisymmetricity. The 40-cm diameter antenna is fed independently by four IGBT-based power units with an output current of 1500 A each at 84 kHz, and their phases are adjustable to launch odd or even parity RMFs. A medium-sized washer gun is utilized to generate a highly ionized seed hydrogen plasma. Driven by RMF, the resultant FRC is formed with a separatrix radius of ∼17 cm, external field of ∼50 G, and trapped poloidal flux of ∼0.15 mWb. The formation process of FRCs is evidenced by the measurement of an array of internal two-dimensional probes; a comparison with the rigid rotor model is presented in this paper. In addition, substantial toroidal electron current is found to be driven, despite the partial RMF penetration. Moreover, the dependence of the driven current on the antenna current is reported and is found to be consistent with the RMF current driving model.

Ion cyclotron resonant heating in the central cell of the Keda Mirror with AXisymmetricity (KMAX)

In this paper, we report the results of ion cyclotron resonance frequency (ICRF) heating in the central cell of a fully axisymmetric tandem mirror. With a total power of 100 kW radiated by double half-turn and half-turn antennas, the plasma diamagnetism increases by 15-fold, with a corresponding peak β⊥ ∼2%, density ∼1.5×1018m−3, and total temperature ∼60 eV. The effects of the magnetic configuration on resonance heating and wave emission are studied by varying the magnetic fields at the midplane and at the location of the antennas, respectively; the results confirm that the magnetic beach configuration is key to successful ICRF heating. The axial phase speed measurements suggest that the excited wave is a slow wave in the plasma core and a fast wave at the edge.

Blob- and hole-like structures outstanding during the transition from attached to detached divertor states in GAMMA 10/PDX

We have measured multipoint fluctuations inside a divertor simulation experimental module (D-module) during the transient discharge from attached to detached states in the tandem mirror device GAMMA 10/PDX. It was first observed that blob-/hole- like positive/negative spikes of the ion saturation current appeared non-periodically around the radially outer side edge on the V-shaped target plate. This tendency became outstanding in the transient state, and the typical speed and size of the isolated blob-like structures in the azimuthal direction were estimated. Additionally, lower-frequency periodic mode fluctuations and their changes due to plasma detachment were investigated not only in the D-module but also in the central cell with the microwave interferometer, etc. Odd and even modes continuously appeared during the discharge, in the range of several kilohertz.

Demonstration of fast-electron populations in a low-pressure, low-power, magnetized RF plasma source

Using x-ray spectroscopy, we demonstrate the existence of 0.3–6 keV electrons in a low-pressure, low-power, magnetized plasma source, heated by an external radio-frequency antenna located at one end of an axisymmetric tandem mirror. X-ray measurements on low-bulk-temperature, Te,b ∼ 4 eV, hydrogen, neon, argon, and gas-mixture plasmas show spectra with high-energy tails having a near-Maxwellian form with Te,f up to 650 eV. The fast electrons producing these x-rays have densities in the range of 107–109 cm−3, 0.01%–1% of the bulk plasma density (up to 3 × 1011 cm−3), and so carry a significant fraction of the plasma energy and represent an important mechanism relevant to power coupling and heat flow in similar plasma sources.

Using Poisson-regularized inversion of Bremsstrahlung emission to extract full electron energy distribution functions from x-ray pulse-height detector data

We measured Electron Energy Distribution Functions (EEDFs) from below 200 eV to over 8 keV and spanning five orders-of-magnitude in intensity, produced in a low-power, RF-heated, tandem mirror discharge in the PFRC-II apparatus. The EEDF was obtained from the x-ray energy distribution function (XEDF) using a novel Poisson-regularized spectrum inversion algorithm applied to pulse-height spectra that included both Bremsstrahlung and line emissions. The XEDF was measured using a specially calibrated Amptek Silicon Drift Detector (SDD) pulse-height system with 125 eV FWHM at 5.9 keV. The algorithm is found to out-perform current leading x-ray inversion algorithms when the error due to counting statistics is high.