Photodiode based multi-modal diagnostic for low-energy neutral beam injection in the LTX-$\beta$ spherical tokamak

This paper presents a compact, multi-modal photodiode diagnostic array installed in the LTX-$\beta$ spherical tokamak that simultaneously measures beam-induced soft-x-ray emission, neutral-hydrogen line radiation, and broadband emission to characterize low-energy neutral beam injection dynamics and constrain time-resolved models of beam heating and fueling.

The Gist

Imagine a tiny, super-fast soccer ball (a hydrogen atom) being shot into a giant, donut-shaped oven (the tokamak) to heat up a soup of super-hot gas (plasma). The goal is to keep these fast balls bouncing around inside the oven long enough to heat the soup, rather than bouncing straight out the door or getting stuck on the walls.

This paper describes a new, compact "camera system" built to watch exactly what happens to these soccer balls when they enter the oven of a specific machine called LTX-β.

Here is how the system works and what the scientists found, explained simply:

1. The "Three-Eyed" Camera

Instead of using one big camera, the scientists built a special strip of sensors (photodiodes) that acts like a camera with three different types of "eyes" looking at the same spot at the same time:

• The X-Ray Eye (SXR): This eye wears special sunglasses (filters) that only let through high-energy X-rays. It watches for the "glow" created when the fast soccer balls crash into the hot gas and heat it up.
• The "Hydrogen Glow" Eye (Lyman-α): This eye is tuned to a very specific color of light that only hydrogen atoms emit when they are bouncing around near the walls. It helps the scientists see how much gas is being recycled or bouncing off the walls.
• The "All-Purpose" Eye (AXUV): This eye has no sunglasses. It sees everything: X-rays, visible light, and even the fast soccer balls themselves if they manage to escape the oven and hit the sensor.

2. The "Li-Wall" Oven

The LTX-β oven is special because its inner walls are coated with lithium (a soft, silvery metal). Think of lithium like a super-absorbent sponge.

• Normal walls (like stainless steel) are like a bouncy castle; they make the gas bounce back and forth, creating a lot of "recycling" (gas bouncing off walls).
• Lithium walls are like a vacuum cleaner; they soak up the gas, keeping the edge of the oven hot and clean. This is supposed to make the oven work better.

3. What the Camera Saw

When the scientists shot the hydrogen beams into the lithium-coated oven, the camera system worked perfectly. Here is what they learned:

• The "Flash" and the "Fade": When the beam turned on, all three eyes saw a bright flash. When the beam turned off, the signal didn't disappear instantly. It took a few milliseconds (thousandths of a second) to fade away.
• The Mystery of the Slow Fade: The scientists expected the fast balls to slow down and stop very quickly (like a car hitting a wall). However, the signal faded much slower than expected.
  • The Analogy: Imagine throwing a ball into a room. If the room is empty, the ball stops fast. If the room is full of invisible fog (neutral gas), the ball hits the fog, slows down gradually, and bounces around longer.
  • The Finding: The slow fade told the scientists that there is still a significant amount of "fog" (neutral gas) inside the oven. The fast balls are hitting this fog and losing energy through a process called "charge exchange" (swapping electrons with the fog) rather than just slowing down by hitting the hot gas.

4. The "Sponge" Effect of Lithium

The scientists noticed something interesting about how the "fog" changed depending on how much lithium was on the walls:

• Fresh Lithium (Early in the campaign): When the lithium coating was just applied, the signal faded very quickly. This suggested the walls were "dirty" or not fully absorbing yet, and the fast balls were getting lost or hitting the walls too soon.
• Well-Conditioned Lithium (Later in the campaign): After the lithium had been used for a while (and the walls were "seasoned"), the signal lasted a bit longer before fading. This suggests the lithium sponge was working better, trapping the gas and keeping the fast balls inside the oven longer to do their heating job.

Summary

This paper is about building a smart, multi-sensor tool to watch how a new type of "sponge-walled" fusion oven handles fuel. The tool proved that:

1. It can see the heat, the bouncing gas, and the escaping particles all at once.
2. The fast fuel particles don't just stop instantly; they get slowed down by hitting invisible gas clouds inside the oven.
3. The condition of the lithium walls changes how long these particles stay inside, which is crucial for understanding how to make fusion energy work efficiently in small machines.

The paper does not claim this will cure diseases or power cities tomorrow; it simply provides the first clear "video footage" of how fuel behaves in this specific, lithium-coated fusion experiment.

Technical Summary

Technical Summary: Photodiode-based Multi-modal Diagnostic for Low-Energy Neutral Beam Injection in LTX-β

Problem Statement
In small tokamaks like LTX-β, low-energy neutral beam injection (NBI) faces a complex balance between penetration, shine-through, prompt orbit loss, and collisional slowing down. With beam energies of 12–20 keV, energetic particle confinement is strongly coupled to edge recycling and neutral-source physics. Previous experiments indicated prompt loss of nearly all beam ions in certain regimes, necessitating optimized beam geometry and higher currents to recover coupled power. However, interpreting these low-energy NBI regimes requires simultaneous diagnostics of beam deposition, edge neutral emission, and radiated power—measurements that were previously lacking in a single, compact system capable of operating within the unique, low-recycling lithium boundary of LTX-β.

Methodology and Instrumentation
The authors present a compact, in-vacuum diagnostic array designed to provide multi-modal, tangential views of the plasma, including the neutral beam path. The system utilizes windowless AXUV20ELM-style silicon photodiode arrays to avoid attenuation of VUV, EUV, and soft x-ray photons. The diagnostic consists of five vertically stacked rows mounted on a 6-inch ConFlat flange:

• Two Soft X-Ray (SXR) rows: 20 channels each, utilizing 5 µm beryllium filters to block lithium line emission while transmitting continuum bremsstrahlung and higher-energy impurity lines (C, O).
• Two Lyman-α rows: 20 channels each, using VUV/UV filters centered at 122 nm to isolate neutral-hydrogen line radiation associated with recycling and fueling.
• One Unfiltered AXUV row: 16 channels with a 50 µm air slit, designed to detect broadband emission, including direct impacts from fast neutrals.

The geometry features nearly overlapping, coincident tangential views from both the high-field and low-field sides. The system is mounted behind a pneumatically actuated gate valve to protect the optics during lithium evaporations and cleaning cycles. Signal conditioning is performed via air-side transimpedance and differential amplifiers, digitized at 250 kHz.

Key Results
Initial measurements from 12–20 keV hydrogen beam operations (specifically a campaign with re-aimed beam tangency at $r_{tan} \approx 33$ cm) yielded the following observations:

1. Beam-Synchronous Response: All three modalities (SXR, Lyman-α, AXUV) demonstrated clear, beam-correlated signals. The unfiltered AXUV signals showed the most significant excess relative to no-beam baselines, particularly on low-field-side chords.
2. SXR Interpretation: The filtered SXR signal scales with line-integrated $n_e^2 Z_{eff}$. Given the low impurity levels ($Z_{eff} \approx 1.2$) and the Be filter's transmission characteristics, the signal is primarily continuum bremsstrahlung, though small concentrations of C or O can disproportionately affect the reading.
3. AXUV Decay Dynamics: The unfiltered AXUV signals exhibited millisecond-scale rise and fall times (0.57–0.85 ms rise; 2.1–4.9 ms fall) that were significantly slower than the detector's intrinsic response. These decay times varied across sightlines, with shorter decays observed on high-tangency radius ($r_{tan}$) chords.
4. Lithium Conditioning Dependence: A comparison between early campaign shots (immediately following the first lithium evaporation after stainless-steel operation) and late campaign shots (with cumulative lithium deposition) revealed distinct differences. Early shots showed uniformly shorter fall times (~1.2 ms) compared to the later, more conditioned shots. This trend suggests that initial wall conditions, which may be "dirtier" and less effective at recycling control, result in higher effective neutral densities and faster charge-exchange losses.

Significance and Claims
The paper claims that this diagnostic successfully provides a simultaneous view of beam-heated radiation, neutral-hydrogen emission, and fast-neutral/bolometric responses in a compact form factor suitable for limited port access.

The primary significance lies in the ability to constrain forward models for small tokamaks. By comparing the measured AXUV decay times (1–5 ms) against classical slowing-down estimates (6–18 ms), the authors conclude that charge exchange with background neutrals contributes appreciably to the measured decay. The diagnostic allows for the estimation of time-resolved balances between beam heating and fueling. Specifically, the data suggests that lithium conditioning reduces recycling and increases beam capture (via electron-loss cross-sections) without necessarily increasing charge-exchange losses, thereby improving confinement.

The authors note that while the diagnostic provides robust experimental trends regarding lithium conditioning, isolating specific physical mechanisms requires future forward modeling that simultaneously fits fast-ion evolution, electron slowing-down, and neutral density profiles using the combined Lyman-α, AXUV, and SXR datasets.