Choosing Tungsten for Tokamak Divertor Tiles: Design, Performance, and Limits

Why Tungsten Is Chosen for Divertor Surfaces

Tungsten’s melting point near 3422 °C and low sputtering yield make it resistant to the intense, repetitive heat loads caused by edge‑localized modes (ELMs) in a tokamak divertor. Its thermal conductivity of roughly 170 W/(m*K) at room temperature spreads incoming power quickly, lowering peak surface temperatures and reducing the risk of thermal cracking.

Bonding Tungsten Tiles to a Copper‑Chrome‑Zirconium Heat Sink

Engineers attach tungsten monoblocks to a CuCrZr alloy heat sink using active brazing or diffusion bonding. The bond layer must accommodate different coefficients of thermal expansion while providing a continuous path for heat flow. Properly designed joints survive thousands of ELM cycles without delamination.

Worked Example: Estimating Tile Temperature During an ELM Pulse

A simple one‑dimensional steady‑state estimate gives a first‑order check on whether a tile will stay below a critical temperature.

1. Define the incident heat flux q (W/m²) from an ELM. For illustration we use q = 10 MW/m².
2. Select the tile thickness L (m). A typical tungsten monoblock is about 5 mm thick → L = 0.005 m.
3. Use the thermal conductivity k of tungsten (≈170 W/(m*K) at 300 K).
4. Compute the temperature rise ΔT = q·L/k.
5. Add the bulk coolant temperature T₀ (≈300 K) to obtain the surface temperature estimate Tₛ = T₀ + ΔT.

q = 10e6;   // W/m^2
L = 0.005;  // m
k = 170;    // W/(m*K)
ΔT = q * L / k;   // K
Tₛ = 300 + ΔT;    // K

Plugging the numbers:

ΔT = (10 000 000 × 0.005) / 170 ≈ 294 K
Tₛ ≈ 300 K + 294 K = 594 K (≈321 °C)

This estimate shows that, for the chosen parameters, the tungsten surface stays well below its melting point and under the ductile‑to‑brittle transition temperature (~1000 K). Real ELM pulses are transient, so peak temperatures can be higher; the calculation is meant as a sanity check, not a substitute for detailed transient heat‑transfer analysis.
Trade‑offs and Practical Verification Steps

Brittleness: Tungsten becomes brittle below roughly 0.3 Tm (≈1000 K). Tile geometry must avoid sharp corners and the bond layer must relieve thermal stresses.
Erosion and redeposition: Sputtered tungsten can form dust that contaminates the core plasma. Surface finishing to a low roughness and routine impurity monitoring (spectroscopy of W I lines) are required.
Verification: Post‑mortem examination of tiles after a campaign (e.g., SEM for crack detection, profilometry for erosion) and infrared thermography during operation provide empirical confirmation that the design assumptions hold.

Actionable Checklist for Engineers

Confirm that the expected ELM heat flux does not exceed the design limit used in the ΔT calculation.
Verify tile thickness and thermal conductivity values for the specific tungsten grade and operating temperature.
Inspect brazed joints for uniformity and adequate thickness before installation.
Plan post‑run diagnostics: infrared scans, laser profilometry, and core‑plasma tungsten spectroscopy.
Schedule regular reviews of dust accumulation and adjust magnetic divertor geometry if erosion rates rise.