Edge-localized mode (ELM)#

Plasma instability#

Plasma instability refers to any disturbance or fluctuation in a plasma that grows over time and disrupts its equilibrium or ordered behavior. Instabilities can be driven by:

  • Current (e.g., too much current through the plasma column)

  • Pressure gradients (e.g., high pressure in the core vs. edge)

  • Magnetic field configuration (e.g., twisted field lines or bad confinement geometry)

  • Plasma rotation or flow (e.g., velocity shear)

L-mode (Low-confinement mode)#

  • Default state of plasma when it’s first heated and confined.

  • Energy and particles are lost quickly, especially at the edge of the plasma.

  • Lower plasma temperature and pressure.

  • Easier to maintain but not suitable for sustained fusion because of poor energy confinement.

H-mode (High-confinement mode)#

  • Discovered in 1982 at ASDEX (Germany).

  • Achieved when input power crosses a threshold.

  • Plasma forms a transport barrier at the edge, called the edge pedestal.

  • This barrier reduces turbulence, and confinement improves sharply.

  • Higher temperature, pressure, and density at the core. essential for efficient fusion.

L-H Transition#

  • If you keep increasing the heating power in L-mode, the plasma can transition into H-mode.

  • This is a nonlinear, sudden change (like a phase transition).

  • H-mode is more unstable, prone to ELMs (Edge Localized Modes), which are mini-explosions at the edge that can damage reactor walls.

L-H Transition Diagram

The top plot is Dα signal and bottom one is gradient plot.

profiles = xr.open_zarr(store, group='spectrometer_visible')
signal = profiles['filter_spectrometer_dalpha_voltage'].isel(dalpha_channel=1)

D-alpha refers to the spectral line emission from neutral deuterium atoms when electrons transition from the n = 3 → n = 2 energy level, probably, referring to Bohr’s model.

Where does it come from in a tokamak?#

  • Most of the plasma is ionized.

  • But at the edge, some neutral deuterium remains.

  • When hot electrons or ions from the plasma interact with these neutrals, they excite them.

  • When these neutrals relax (n=3 → n=2), they emit Dα photons.

What is shown in the plot?#

  • Left side of the top plot (up to ~700–800 index): This is L-mode — the edge is turbulent, but there’s no strong edge barrier and no large transient drops.

  • Somewhere between index 700–800: LH transition

  • Between ~800 and ~1200: The regular spikes in the Dα signal indicates ELM.

  • Back transition (HL transition): Around ~1200, both plots flatten out.

Edge-localized mode (ELM)#

  • mini explosion or burst of energy and particles at the edge of the plasma.

  • plasma instability that occurs at the edge of the plasma in H-mode.

  • It’s like the plasma periodically burping energy and particles from the edge.

  • Uncontrolled ELMs can:

    • Damage divertor plates

    • Reduce plasma confinement

    • Potentially trigger other instabilities, such as the resistive wall mode (RWM) or the neoclassical tearing mode (NTM).