Mykonos Load Physics Research

The Mykonos pulsed power system is at the forefront of advancing load physics research, with a focus on advancing the understanding and efficiency of pulsed power technologies through innovative experimental techniques and diagnostic capabilities. One of the key areas of exploration is the Auto-Magnetized (AutoMag) liner studies, which utilize specially designed liners with helical conducting paths to generate strong axial magnetic fields internally, eliminating the need for external magnetic field coils. This advancement not only enhances magnetic field generation but also improves current coupling and simplifies experimental setups. Complementing this, the Applied B-Field on Mykonos (ABM) subsystem employs pulsed resistive electromagnets to create axial magnetic fields, facilitating enhanced diagnostic access and operational flexibility. Additionally, research into ElectroThermal Instability (ETI) and the development of x-pinch radiography techniques are critical for understanding plasma dynamics and improving imaging capabilities. Together, these research thrusts at Mykonos contribute to the broader goals of advancing pulsed power technology and enhancing scientific understanding in various applications, ultimately supporting national security and energy sustainability initiatives.

Auto-Magnetized (AutoMag) liner studies

AutoMag is a novel approach to magnetizing the fuel column in Magnetized Liner Inertial Fusion (MagLIF) without relying on external magnetic field coils. It utilizes specially designed liners that contain helical conducting paths, which allow for the generation of a strong axial magnetic field directly within the liner itself. This method is significant because it addresses several limitations associated with traditional external coil systems.

Importance of AutoMag to MagLIF:

  1. Enhanced Magnetic Field Generation:
    • AutoMag can achieve higher magnetic field strengths (potentially exceeding 100 T) directly within the liner during the fuel magnetization stage. This is crucial because stronger magnetic fields can improve the magneto-thermal insulation of the preheated fuel, reducing thermal losses during the implosion process.
  2. Improved Current Coupling:
    • By eliminating the need for external coils, AutoMag reduces the inductance in the circuit. This allows for more efficient current delivery to the liner, enhancing the overall performance of the MagLIF process.
  3. Better Diagnostic Access:
    • The absence of external coils improves access for x-ray diagnostics, which are essential for monitoring the conditions and dynamics of the fusion fuel during experiments. This access is vital for understanding the behavior of the fuel and optimizing the fusion process.
  4. Streamlined Design:
    • AutoMag’s design simplifies the experimental setup by integrating the magnetization process within the liner itself. This reduces complexity and potential points of failure associated with external coil systems.
  5. Multi-Stage Operation:
    • The AutoMag process consists of three stages: fuel magnetization, liner dielectric breakdown, and liner implosion. This structured approach allows for precise control over the conditions leading to the implosion, which is critical for achieving the desired thermonuclear conditions.

In summary, AutoMag is an innovative solution that enhances the efficiency and effectiveness of the MagLIF process by enabling stronger magnetic fields, improving current coupling, providing better diagnostic access, and simplifying the experimental setup. These advancements are essential for advancing research in magneto-inertial fusion and achieving successful fusion reactions.

AutoMag liner concept with an internal on-axis microBdot probe [1].

Preliminary nonimploding AutoMag experiments were conducted on the Mykonos accelerator [1] to help inform the hardware designs for a Z machine demonstration. Three aluminum hardware configurations were fielded with variations of Stycast insulator material application. The Mykonos AutoMag hardware’s internal axial magnetic field was diagnosed prior to breakdown (i.e., during pure helical current flow) with microBdots, the results of which match ANSYS Maxwell preshot transient magnetic simulations. These tests showed that AutoMag liners produce axial magnetic fields effectively.

Applied B-field on Mykonos (ABM) commissioning

The Applied B-Field on Mykonos (ABM) subsystem utilizes pulsed resistive electromagnets to generate current pulses that provide an axial magnetic field in the Mykonos load vacuum region.  A Helmholtz-like pair of coils is used to generate this field while providing radial diagnostic lines of sight into the target region for imaging and spectroscopy diagnostics.  The current coil assembly utilizes a 55-turn “bottom” coil and an 88-turn “top” coil to generate approximately 10-T at the axial midplane between the coils.  The current configuration of the coil pair includes approximately 23mm of axial access to the coaxial region with 8-way periodicity of 26-degree azimuthal window cutouts in the titanium middle plate.  These windows align with radial viewports in the Mykonos vacuum chamber to couple air-side optical measurements into the chamber. 

The Applied B on Mykonos coil system installed in the Mykonos vacuum chamber (left) with intermediate vacuum lid in place. A rendered half-section through the indicated line (right) shows a Helmholtz-like coil pair capable of magnetizing Mykonos targets within the ~40 cm3 load volume (dotted green box) to ~10 T.
The Applied B on Mykonos coil system installed in the Mykonos vacuum chamber (left) with intermediate vacuum lid in place. A rendered half-section through the indicated line (right) shows a Helmholtz-like coil pair capable of magnetizing Mykonos targets within the ~40 cm3 load volume (dotted green box) to ~10 T.

The ABM coil system is driven by a 1.6-mF ignitron-switched capacitor bank.  When charged to 8kV and driving the nominal 750-µH coil system shown in the figure below, the system delivers a ~9.4-kA current pulse with a peak current occurring at ~1.5ms.  This slow risetime is used to ensure sufficient time for the applied magnetic field to diffuse through the load hardware and achieve the desired flux distribution in the load region.  

The Mykonos power feed is extended into the center of the coil pair to place the target in the axial field region.  This cylindrical region is roughly 36mm in diameter, about 25mm tall, and is axially centered within the radial lines of sight in the coil middle plate.  The following figures show axial variation in magnetic field at peak current (peak field), and time evolution of the field at midplane and +/- 5mm. 

Measured axial magnetic field distribution of ABM coil pair when driven by 4mF ABZ capacitor banks (during coil commissioning and checkout).
Measured axial magnetic field distribution of ABM coil pair when driven by 4mF ABZ capacitor banks (during coil commissioning and checkout).
Axial B-field variability along the machine axis from the upper face of the Mykonos anode. The diagnostic midplane and ideal location of the target region of interest is about 10mm above this surface.
Axial B-field variability along the machine axis from the upper face of the Mykonos anode. The diagnostic midplane and ideal location of the target region of interest is about 10mm above this surface.

The ABM capability can feasibly be used for seeding an external magnetic field for electrothermal instability (ETI) research, low density plasma seeding and helical instability evolution, Field-Reversible Configuration (FRC) concepts for flux containment, magnetized power flow conditions, and any other ideas that visiting experimenters might bring to the Mykonos facility.  Different coil configurations are also possible, given development time.  Larger magnetized volumes are readily achievable with lower peak field values.

If considering using ABM for an experimental campaign, several considerations must be made for compatible target design within this operational paradigm.  Target geometries and supporting hardware should avoid using magnetically permeable materials or have high conductivity to avoid flux distribution, delayed field diffusion, and increased induced magnetic forces.  The coils can be reused (unlike ABZ coil pairs on Z, which are destroyed each shot) and require only 30 minutes to cool down from the ohmic heating load for 10-T operation.  Shot rates exceeding 3 shots a day are feasible with simple load hardware configurations that can be loaded quickly.

ElectroThermal Instability (ETI) research

Electrothermal instability (ETI) is a phenomenon that occurs in current-carrying conductors, particularly in the context of inertial confinement fusion (ICF). It is driven by Ohmic heating, which is the heating of a conductor due to the flow of electric current. ETI arises from the feedback loop between current density, electrical resistivity, and temperature. When there are inhomogeneities (such as impurities or voids) in the conductor, these can lead to non-uniform heating and expansion, resulting in complex behaviors such as the formation of plasma filaments.

Importance of ETI for Inertial Confinement Fusion (ICF):

  1. Understanding Plasma Formation:
    • ETI plays a critical role in the formation of plasmas during ICF experiments. The instability can lead to local overheating, which generates plasma filaments at lower currents than would be expected on uniform surfaces. This understanding is essential for predicting and controlling plasma behavior during fusion experiments.
  2. Impact on Current Density Perturbations:
    • Inhomogeneities in the conductor, such as engineered defects (ED), can create perturbations in current density that drive local overheating. These perturbations can seed the development of plasma, which is crucial for achieving the conditions necessary for fusion.
  3. Influence on Magneto-Rayleigh-Taylor (MRT) Instability:
    • ETI-generated density perturbations can lead to the magneto-Rayleigh-Taylor (MRT) instability, which can degrade the performance of ICF systems. A detailed understanding of ETI can help identify pathways to reduce MRT, thereby improving the efficiency and effectiveness of fusion experiments.
  4. Guiding Material Selection and Target Fabrication:
    • Insights gained from studying ETI can inform the selection of materials and the design of targets used in ICF. By understanding how different surface imperfections and material properties affect heating and plasma formation, researchers can optimize target designs to minimize unwanted instabilities.
  5. Validation of Computational Models:
    • ETI provides a framework for validating three-dimensional magnetohydrodynamic (3D-MHD) simulations. Experimental data on nonuniform heating and plasma formation can be compared with computational predictions, enhancing the reliability of models used to simulate ICF conditions.

In summary, ETI is a critical factor in understanding and controlling the dynamics of plasmas in inertial confinement fusion. By elucidating the mechanisms behind non-uniform heating and plasma formation, ETI research can lead to improved experimental outcomes, enhanced stability, and more effective fusion processes.

X-pinch radiography development for Z

The frequency, intensity, and spectral content of x-ray bursts generated from various Cr/Co/Ni alloy x-pinch configurations were characterized, with the aim of establishing a predictive relationship for burst timing and assess the suitability of spectral line strengths for radiography using spherical crystal imaging on the Z Machine. Using Ross-filtered silicon diodes in conjunction with TIXTL spectrometers, researchers successfully identified specific spectral lines associated with individual x-ray bursts in the diode time series. Additionally, a linear relationship was established between timing and wire mass/length for two wire alloys, MP35N and Stablohm.

References

[1] Shipley, G. A.. et al., “Megagauss-level magnetic field production in cm-scale auto-magnetizing helical liners pulsed to 500 kA in 125 ns“, Physics of Plasmas 25.5 (2018), doi: https://doi.org/10.1063/1.5028142.