Pulse magnet of 10 T for power laser experiments with x-ray free-electron laser diagnostics

This paper presents the first platform at SACLA, Japan, that integrates a high-power optical laser, an X-ray free-electron laser probe, and a 10 T pulsed magnet to enable the study of magnetized high-energy-density matter under extreme conditions.

The Gist

Imagine trying to understand how a storm behaves inside a tiny, super-hot cloud of gas (plasma). Scientists have long wanted to study these storms, especially when they are squeezed by powerful magnetic fields, because this happens in stars, in fusion reactors, and even in the deep vacuum of space.

However, there's a big problem: these clouds are so dense that you can't see inside them with regular cameras or even standard X-rays. It's like trying to see the details of a tornado through a thick fog.

This paper describes a brand-new "super-microscope" built at a giant facility in Japan called SACLA. Here is how it works, broken down into simple parts:

1. The Three Ingredients

To solve the visibility problem, the scientists combined three powerful tools into one machine:

• The Heater (High-Power Laser): Think of this as a giant, super-fast blowtorch. It hits a tiny target and instantly turns it into a super-hot, high-pressure plasma cloud.
• The Flashlight (XFEL): This is an X-ray Free-Electron Laser. Unlike a normal flashlight that makes a blurry beam, this is a "super-precise" X-ray beam. It's so sharp it can see details smaller than a single human hair (actually, much smaller—down to the size of a bacterium). It acts like a high-speed camera flash that can freeze motion happening in a fraction of a second.
• The Squeeze (The Magnet): This is the new star of the show. The team built a special, lightweight "pulse magnet" (called Pi-Mag). It's like a super-strong electromagnet that can be turned on and off in a split second. It creates a magnetic field 100,000 times stronger than the Earth's magnetic field.

2. The "Split" Magnet Design

The magnet is designed like a pair of open hands (a "split-pair" coil).

• Why split it? If the magnet were a solid ring, the scientists couldn't shine their lasers or X-rays through it. By splitting it, they created little "windows" or tunnels.
• The Result: They can shine the heating laser and the X-ray camera through these windows from different angles, all while the magnetic field is squeezing the plasma in the middle. It's like having a cage where you can still see the animal inside from every side.

3. The Timing Trick

The hardest part was making these three things happen at the exact same time.

• The magnet needs a huge burst of electricity (10,000 amps!) to work.
• The lasers need to fire in a tiny window of time.
• The scientists synchronized everything so that the magnetic field hits its peak strength at the exact moment the lasers fire.
• The Challenge: When the laser hits the target, it creates a messy plasma that can cause electrical sparks (short circuits) inside the vacuum chamber. The team had to wrap the magnet's wires in special electrical tape (like heavy-duty duct tape for electricity) to stop these sparks from ruining the experiment.

4. What They Found (The First Test)

The team didn't just build the machine; they used it to watch a "turbulent" plasma storm.

• Without the Magnet: When they let the plasma swirl without a magnetic field, the energy moved around in a specific, predictable way (like water swirling down a drain).
• With the Magnet: When they turned on the 10-Tesla magnet, the behavior changed. The "slope" of the energy movement shifted.
• The Analogy: Imagine a crowd of people running in a chaotic circle. Without a fence, they run everywhere. If you put a strong magnetic fence around them, they can't move as freely; they get "stretched" and their chaotic running slows down. The magnet acted like an invisible fence that stopped the energy from spreading out as quickly, changing how the turbulence behaved.

Why This Matters

This machine is the first of its kind to combine a high-power laser, a super-strong magnet, and an ultra-sharp X-ray camera. It allows scientists to finally "see" what happens inside magnetized plasma storms with incredible detail. This helps them understand the physics of stars, improve fusion energy research, and study how matter behaves under extreme pressure and magnetic force.

In short, they built a new kind of "time machine" that lets us freeze-frame and examine the invisible, chaotic dance of matter in the universe's most extreme environments.

Technical Summary

Technical Summary: Pulse Magnet of 10 T for Power Laser Experiments with X-ray Free-Electron Laser Diagnostics

Problem Statement
The investigation of magnetized plasmas and solids under extreme conditions is critical for advancing high-energy-density physics (HEDP), including applications in laboratory astrophysics and inertial confinement fusion. However, a significant diagnostic gap exists: conventional optical diagnostics cannot penetrate the high-density plasmas generated in these experiments, while existing X-ray sources often lack the necessary spatial resolution (typically limited to ~25 µm) to visualize fine structural details. Although X-ray free-electron lasers (XFELs) offer sub-micron resolution and high brilliance, these capabilities have not yet been fully exploited to study magnetized processes in HEDP, particularly where high magnetic fields are required to influence plasma dynamics.

Methodology
To address this, the authors developed a novel experimental platform at the EH5 beamline of SACLA (Japan) that integrates three distinct systems: a high-power optical laser, an XFEL probe, and an external pulsed magnetic field.

• Pulse Power System (Pi-Mag): A compact, 50 kg pulsed power system was designed, featuring a 2.4 mF capacitor bank charged to 2 kV, storing 4.8 kJ of energy. Triggered by a thyristor, it delivers a maximum discharge current of 10 kA. The system is remotely controlled via a Raspberry Pi and includes safety interlocks synchronized with the experimental hutch door.
• Split-Pair Coil: The magnetic field is generated by a lightweight (150 g) split-pair coil made of CuAg alloy wire (10% Ag). The coil consists of 10 turns and 10 layers per pair with a 12 mm split, allowing for optical access. It features eight 45°-spaced ports in the equatorial plane and two 90° ports in the polar plane, enabling multi-angle diagnostics.
• Synchronization and Environment: The system operates in a vacuum environment. The magnetic field pulse (1.3 ms duration) is synchronized with the optical laser and XFEL pulses. The authors noted that abnormal discharges occurred when the power laser struck the target due to plasma-induced insulation breakdown; this was mitigated by applying electrical insulation tape to the electrodes.
• Diagnostics: The setup utilizes a high-power nanosecond laser (532 nm, 15–20 J) to drive shock waves and a 7 keV XFEL beam (non-focused, 0.6 mm diameter) as a probe. Imaging is performed using a LiF crystal detector for high-resolution (approx. 0.6 µm) offline radiography and a CCD-based detector for online alignment and shot monitoring.

Key Contributions
The primary contribution of this work is the successful commissioning of the first platform at SACLA capable of combining a 10 T magnetic field with high-power laser-driven matter and XFEL diagnostics.

• Field Performance: The split-pair coil generates a homogeneous magnetic field of 10 T at 6 kA within a volume of 0.5 cm³ (magnetic pressure ~0.04 GPa). The field homogeneity is ±2 mm, sufficient for the diagnostic field of view (< 0.6 mm).
• Operational Capability: The platform supports a repetition rate of one pulse every 10 minutes (limited by coil heating) and allows for up to 10 shots per vacuum cycle.
• Diagnostic Integration: The system successfully synchronizes the magnetic field peak with the arrival of both the drive laser and the XFEL probe, enabling the study of transient, non-equilibrium phenomena in magnetized environments.

Preliminary Results
The authors conducted a preliminary experiment to investigate magnetized turbulence using a multi-layer target designed to generate Rayleigh-Taylor (RT) instabilities and subsequent turbulent flow.

• Experimental Setup: A target consisting of a parylene ablator, a modulated brominated plastic pusher, and a resorcinol formaldehyde foam was irradiated by the optical laser to drive a shock wave.
• Observation: X-ray radiography was performed with and without a 9.3 T magnetic field applied perpendicular to the shock propagation.
• Spectral Analysis: Analysis of the spatial power spectrum revealed a distinct change in the turbulence slope. In the absence of a magnetic field, the slope was approximately -1.5. In the presence of the 9.3 T field, the slope shifted to -1.1.
• Interpretation: This shift suggests that the magnetic field influences the dissipation of energy, likely by inhibiting vortex stretching and eddy formation via the Lorentz force, thereby reducing energy transfer to smaller scales.

Significance
The paper claims that this platform opens a new field of investigation for the experimental characterization of magnetized turbulence, a topic previously limited to observational, theoretical, and computational studies. By enabling the measurement of power spectra in magnetized flows with micron-scale resolution, the platform allows for the testing of theoretical models regarding weak and strong magnetohydrodynamic (MHD) turbulence. The authors emphasize that this capability is essential for understanding how magnetic fields modify equations of state, wave dynamics, and turbulent plasma behavior in extreme conditions. Future work aims to increase the field strength to 15–20 T and to characterize turbulence anisotropy when the magnetic field is parallel to the laser axis.