Thomson Scattering

The gold-standard laser diagnostic for simultaneously measuring electron temperature and density in fusion plasmas

How Thomson Scattering Works

Thomson scattering is a diagnostic technique in which photons from a high-power pulsed laser scatter off free electrons in a plasma. Because the electrons are in thermal motion, the scattered light experiences a Doppler shift whose spectral width is proportional to the electron temperature T_e and whose total intensity is proportional to the electron density n_e. By collecting the scattered photons at a known angle and dispersing them through a polychromator, physicists extract both quantities from a single laser pulse.

The technique is named after J. J. Thomson, whose classical derivation of the scattering cross-section for a free charged particle underpins the measurement. The Thomson cross-section is extremely small—roughly 6.65 × 10⁻²⁹ m²—so the scattered signal is faint, demanding high-energy lasers (typically Nd:YAG at 1064 nm delivering 1–3 J per pulse) and sensitive detection optics.¹

Implementation on Fusion Devices

Modern tokamaks and stellarators deploy multi-point Thomson scattering systems that probe dozens of spatial locations along the laser beam path in a single shot. On ITER, the core and edge Thomson systems together will provide radial profiles of T_e and n_e with spatial resolution of roughly 5 cm in the core and better than 1 cm in the pedestal region, at repetition rates up to 100 Hz.²

Complementary Role in Plasma Control

Real-time Thomson scattering profiles feed directly into plasma control systems, enabling feedback on density and temperature for scenario optimization, disruption avoidance, and pedestal monitoring in H-mode plasmas. The diagnostic complements electron cyclotron emission (ECE) radiometry, which also measures T_e but can suffer cutoff effects at high density, and interferometry, which measures line-integrated density without temperature information.³

Because it is non-perturbative and spatially resolved, Thomson scattering remains the benchmark against which other electron diagnostics are cross-calibrated on virtually every major magnetic confinement experiment worldwide.⁴

Sources

1. Sheffield, J. et al., "Plasma Scattering of Electromagnetic Radiation," 2nd ed., Academic Press (2011)
2. Pasqualotto, R. et al., "High resolution Thomson scattering for Joint European Torus (JET)," Review of Scientific Instruments 75, 3891 (2004)
3. Carlstrom, T.N. et al., "Design and operation of the multipulse Thomson scattering diagnostic on DIII-D," Review of Scientific Instruments 63, 4901 (1992)
4. ITER Organization, "ITER Research Plan within the Staged Approach," ITR-18-003 (2018)

Related

Interferometry Electron Cyclotron Emission Plasma Diagnostics H-Mode