Sub-ns Thomson Scattering with 4 Picos ICCD camera

E.R. Kieft, et al. at the Eindhoven University of Technology were able to enhance the performance of their time-resolved spectrometer so they can forget about an absolute sensitivity calibration for the determination of plasma parameter using Thomson scattering. The time-resolved spectrometer is equipped with a 4 Picos ultra high speed camera providing a gating time down to 200ps rectangular.

Increase the signal to background ratio and the time-resolution of Thomson scattering measurements in plasma by using a ICCD camera.

In contrast to Compton scattering, Thomson scattering is an elastic scattering of electromagnetic radiation by a free moving charged particle. The interaction of optical photons with a free moving electrons is Thomson scattering. The electrons are accelerated within the electromagnetic field which is earmarked by an incoming photon. Due to its oscillating movement, the electron acts like an antenna and emits a photon predominantly perpendicular to its movement. Since plasmas exist of free moving charge particles Thomson scattering appears if Laser pulses are shot into plasma.

Characterize plasma using Thomson scattering

Thomson scattering is a proven technique for measuring electron densities and temperatures in plasma. Therefore a monochromatic light source is fired into the plasma and the scattered light is recorded. The signal intensity distribution determines the electron density within the plasma. Furthermore the spectral distribution of the scattered light is a measure of the temperature of the plasma. If the electrons have Maxwellian energy distribution, the scattered spectrum will have a Gaussian shape due to the Doppler effect. The peak position of the Gaussian thus indicates the relative temperature of the plasma.

However, the presence of random fluctuations in the strong background radiation emitted by the plasma handicap the determination of plasma parameter. A sophisticated fitting procedure can not be applied if the Thomson scattering signal is eroded by strong background signal.

Improvement by short gated ICCD cameras

Rather than increasing the laser pulse energy, which could probable disturb the plasma, E.R. Kieft, et al. went for shorter laser pulses and a reduced exposure time to decrease the level of recorded background radiation. By using the 4 Picos ICCD camera they were able to reduce the exposure time to almost 200 picoseconds. Additionally, the low internal jitter of the 4 Picos camera allowed a triggering of the camera directly by the laser pulse. Therefore, a fraction of the laser pulse was split to a fast photodiode and its signal was used as trigger of the ultra high speed ICCD camera.

Using the sub-ns time-resolved spectrometer the Thomson scattering measurements could be performed as a function of time. In 10 nanosecond time steps, the Thomson spectrum was recorded as a function of horizontal position in the plasma. After binning and fitting of the theoretical spectrum the electron density and temperature within the plasma could be derived.

This example shows the convenience of time-resolved spectrometer equipped with the ultrafast ICCD cameras from Stanford Computer Optics.

Title: Subnanosecond Thomson scattering setup for space and time resolved measurements with reduced background signal
Author: E. R. Kieft, C. H. J. M. Groothuis, J. J. A. M. van der Mullen, and V. Banine
Institute: Eindhoven University of Technology, Eindhoven, The Netherlands