---
title: 'EXL-50U Spherical Tokamak: Advanced Fusion Device'
url: https://www.emergentmind.com/topics/exl-50u-spherical-tokamak
type: topic
---

# EXL-50U Spherical Tokamak: Advanced Fusion Device

The EXL-50U spherical tokamak is a compact, low-aspect-ratio fusion device designed for advanced plasma confinement, high-efficiency non-inductive current drive, and reactor-relevant hydrogen–boron fuel experiments. Upgraded from the original EXL-50, EXL-50U implements major engineering, diagnostic, and operational enhancements, targeting 1 MA-class plasma currents, hot-ion modes, and scalable empirical demonstrations supportive of next-generation spherical tokamak (ST) reactors. Research on EXL-50U provides a definitive platform for exploring multi-harmonic electron cyclotron wave (ECW) current drive, real-time equilibrium control, and advanced wall conditioning for low-Z, impurity-managed operation.

## 1. Device Geometry, Magnet System, and Core Parameters

EXL-50U features a major radius $R_0 = 0.6$–$0.8$ m, minor radius $a = 0.4$–$0.45$ m, and aspect ratio $A = R_0 / a \approx 1.5$–$1.85$ [2502.04745][2401.11338][2512.18941]. An all-metallic vacuum vessel, reinforced for mechanical stiffness, houses the full-metal center column with a 0.5–0.6 Vs-capable central solenoid (CS), water-cooled copper toroidal field (TF) coils (providing $B_T \leq 1.2$ T at $R_0$), and a flexible set of four to twelve poloidal field (PF) coils for shaping and stability. The device operates with both limiter and single-null divertor configurations, and incorporates actively cooled, tungsten first-wall components. Upgrades include expanded port access for Thomson scattering, bolometry, interferometry, and charge-exchange recombination spectroscopy (CXRS), as well as fast magnetic diagnostics.

Nominal plasma performance targets and achievements include:

| Parameter         | Value               | Comments                                             |
|-------------------|---------------------|------------------------------------------------------|
| $R_0$             | $0.6$–$0.8$ m       | Extended from EXL-50 for improved shaping            |
| $a$               | $0.4$–$0.45$ m      | Implies $A \sim 1.5$                                 |
| $B_T$             | $0.56$–$1.2$ T      | On-axis, tunable by TF current                       |
| $I_p$             | $\leq 1.03$ MA      | Limiter/divertor, non-inductive ramp-up achieved     |
| $\kappa$          | $1.5$–$2.0$         | Elongation at LCFS                                   |
| $\langle n_e \rangle$ | $(1$–$2)\times10^{19}$ m$^{-3}$ | Line-averaged, Thomson/interferometer                |
| $T_{e0}$          | $3$–$3.5$ keV       | Core, with 800 kW ECRH                               |
| $T_{i0}$          | Up to $6$ keV target| Goal for p–B campaign                                |
| $q_a$             | $\approx 4$         | Edge safety factor at $I_p=1$ MA, $B_\phi=1$ T       |

Confinement and equilibrium analyses employ the Grad–Shafranov formalism, multi-fluid models, and real-time plasma reconstruction [2601.12378].

## 2. Heating, Current Drive, and Non-Inductive Operation

EXL-50U's core innovation is non-inductive current start-up and ramp-up relying on multi-harmonic ECW in synergy with Ohmic assist from the CS. Six gyrotrons (three at 28 GHz, two at 50 GHz, up to 200 kW each, 2 s pulse duration) enable O/X-mode launching from the low-field-side [2512.18941][2505.08815]. By controlling $B_T$, the radial location and number of accessible EC resonance layers (n=1–5) are adjusted such that multiple harmonics lie within the plasma—a prerequisite for high-efficiency, non-linear multi-harmonic EC absorption and current drive [2109.04161].

Non-inductive start-up efficiency increases monotonically with the number of in-plasma harmonics: for 28 GHz at $R_\mathrm{out}=1.200$ m, raising from 1 to 3 layers increased non-inductive $I_p$ from 25 to 78 kA; similar scaling is shown for 50 GHz (Table below).

| EC Harmonics Accessible | Peak $I_p$ (kA, 28 GHz) | Peak $I_p$ (kA, 50 GHz) |
|------------------------|-------------------------|-------------------------|
| One layer              | 25                      | —                       |
| Two layers             | 45                      | —                       |
| Three layers           | 78                      | —                       |
| 2–4 layers (by limiter/BT scan)| —             | ≈96                     |

With multi-pass absorption (wall reflection and O$\rightarrow$X conversion), ECW-driven current-drive efficiency ($\eta_\mathrm{CD}$) reaches 0.06 MA MW$^{-1}$m$^{-1}$ for three or more harmonics, and net non-inductive fraction ($I_\mathrm{NI}/I_p$) can reach 70% at $I_p = 1$ MA (with ECW $+$ CS) [2512.18941].

Underlying mechanisms are:
- **Multi-harmonic heating**: Doppler-broadened cyclotron absorption at successive harmonic layers accelerates electrons to high energies.
- **Multiple reflections**: O-mode not absorbed reflects from the vessel wall, converts with probability $C_{OX}$ to X-mode, enabling further resonance.
- **Multi-pass absorption and mode conversion**: Post-reflection, X-mode or EBW is absorbed at the upper hybrid resonance (UHR), heating electrons to $>$100 keV [2109.04161][2512.18941].

Modeling with GENRAY+CQL3D and X-ray diagnostics validate that $>$60% of current is carried by this energetic electron population.

## 3. Equilibrium Reconstruction and Real-Time Control

Real-time MHD equilibrium is achieved on EXL-50U using PTEFIT—a modular, GPU-accelerated solver mapping the Grad–Shafranov equation and a least-squares system to a PyTorch/TensorRT computational graph [2601.12378]. The algorithm reconstructs plasma flux and profiles at 129$\times$129 resolution, attaining $\langle 0.268$ ms time per slice, $>10^4\times$ faster than conventional offline EFIT, and sub-centimeter LCFS accuracy. The workflow incorporates Green’s function evaluation, Picard iteration, quadratic O/X-point search, and flux-surface averaging for safety factor and geometric indices.

Closed-loop feedback (PID for $R_\mathrm{max}$, multi-point “isoflux” for strike/X-point positioning) is realized at $\sim$1 ms cycle times, stabilizing the plasma to within 2 cm of target locations even in the presence of disturbances. The framework is immediately extensible to transport, stability, and kinetic solvers, and supports device portability through PyTorch–ONNX–TensorRT graph export.

## 4. Fueling, Impurity Control, and Wall Conditioning

EXL-50U implements integrated boronization for impurity control, wall conditioning, and real-time fueling [2505.08815]. Mixtures with 30% B$_2$H$_6$ and 70% H$_2$ yield core boron fractions $\gtrsim10\%$, stabilized by simultaneous boron powder injection during discharge. This reduces $Z_\text{eff}$, mitigates radiative loss, and improves ramp-up rates—evidenced by a 78% increase in $dI_p/dt$ for H–B shots versus hydrogen alone.

Wall boronization (gas, powder, pellet) is regularly applied to maintain low-Z plasma-facing surfaces, and dynamic gas-puffing with divertor cryopumping enables control over edge recycling and impurity influx. Impurity transport is characterized using spectroscopic and bolometric techniques, with modeling based on diffusive–pinch frameworks.

## 5. Diagnostics Suite and Turbulence Measurement

Comprehensive diagnostics installed on EXL-50U include multi-chord CO$_2$ and 140–330 GHz interferometry, Thomson scattering (profiled, $\sim$5% accuracy), CXRS for ion temperature, 32-channel bolometry, as well as high-frequency magnetic probes, Mirnov arrays, and hard X-ray spectrometry for energetic-electron studies [2505.08815][2601.12378]. 

For microturbulence, the Doppler backscattering (DBS) diagnostic concept [2509.18532] uses a U-band (40–60 GHz) beamline with toroidal and poloidal steering, optimized to match the high ($\sim$35$^{\circ}$) magnetic pitch angle at the outboard midplane. SCOTTY beam-tracing predicts accessible $k_\perp$ coverage of $2.5$–$9.5$ cm$^{-1}$, spanning $\rho=0.15$–$1.0$, with frequency tuning and angle matching to minimize signal attenuation due to mismatch.

## 6. Physics Results: Hydrogen–Boron Operation and Advanced Regimes

EXL-50U demonstrated the first 1 MA-class hydrogen–boron plasma for a ST at $B_\phi=1$ T, $R_0=0.6$ m, with non-inductive start-up and rapid ECRH+CS ramp-up [2505.08815]. Real-time boron fueling and wall conditioning yielded low impurity ($Z_\text{eff}$), improved $dI_p/dt$, and core $T_{e0}$ up to 3–3.5 keV. The operation regime achieved $\kappa\sim1.5$, $q_a\sim4$, and $\tau_E\sim0.07$ s (Ohmic), supporting $\gtrsim70\%$ non-inductive current fractions [2512.18941].

Physics insights:
- Low loop-voltage start-up with ECRH–CS synergy reduces stress on the CS system.
- Real-time boronization is an effective strategy for radiative and impurity control in metal-walled devices.
- Two-stage ramp-up profiles (fast initial, then slow to avoid plasma-wetting instability and disruptive expansion) are critical for device stability.
- Demonstrated $\eta_\mathrm{CD}\gtrsim1$ A W$^{-1}$ for multi-harmonic X-mode ECW, up to $\sim3$ A W$^{-1}$ in multi-pass scenarios [2109.04161][2502.04745].

EXL-50U thus validates the empirical scaling required for future reactor-class STs, especially EHL-2, targeting 3 MA, $R_0=1.05$ m, $T_{i0}=30$ keV, and advanced $T_i/T_e$ ratios [2401.11338].

## 7. Scientific Roadmap and Research Program

The EXL-50U program, operated by ENN Science and Technology and associated research teams, serves as the critical test bed for ST-based proton–boron fusion [2401.11338][2502.04745]. Key objectives and milestones include:

- Demonstrate $I_p>0.5$ MA non-inductively at high $\langle n_e \rangle$, $T_{i0}$>3 keV, $\tau_E>0.1$ s, and toroidal $\beta_t$>30%.
- Execute systematic boron fueling and wall-conditioning campaigns for impurity control and edge physics studies.
- Integrate and operate multi-MW heating and current-drive systems for D–T and p–$^{11}$B–relevant scenarios (including NBI, LHCD, ICRH upgrades).
- Deliver empirical guidance and validated models for the scaling, engineering, and physics integration of EHL-2—a 3 MA, $B_0=3$ T, hot-ion, advanced-fuel ST for the late 2020s [2401.11338].

EXL-50U is scheduled to undergo further upgrades, targeting $I_p>1.2$ MA, $B_\phi=1.2$ T, and higher elongation, with extended high-$\beta$ and pulse-length operation, advanced equilibrium feedback, and continuous impurity management.

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References:  
[2109.04161], [2104.14844], [2401.11338], [2502.04745], [2505.08815], [2512.18941], [2509.18532], [2601.12378]

Source: https://www.emergentmind.com/topics/exl-50u-spherical-tokamak