Patent Number: 
Section: claims

1. A plasma diagnosis system using multiple-pass Thomson scattering, comprising:a laser which supplies a laser pulse having predetermined polarization state and wavelength;an optical system configured to make the laser pulse travel multiple roundtrips along a predetermined optical path and focus the laser pulse to a predetermined location in plasma, and rotate a plane of polarization by 90 degrees after each complete roundtrip;a collection optics which is configured with a lens or a combination of lenses, collects lights scattered from a focal point in plasma, whereas the collected light by the vertical polarization of the laser pulse is referred to as ‘first collected scattering’ and the collected light by the horizontal polarization of the laser pulse is referred to as ‘second collected scattering’;a polychromator which is consisted of multiple channels of band pass filters which filter the collected lights provided from the collection optics according to spectral characteristics and output the filtered lights;a computer which measures spectral characteristics of the first and second collected scatterings by using the filtered lights provided from the polychromator and outputs a Thomson scattering signal contaminated with a background noise and the background noise, respectively,wherein the background noise is generated by scattering in plasma due to stray lights and is obtained from the second collected scattering, and the Thomson scattering signal contaminated with the background noise is obtained from the first collected scattering. 2. The plasma diagnosis system according to claim 1, wherein the optical system includes:a polarizing beam splitter (PBS) which is inserted in the predetermined optical path and reflects or transmits an incident laser pulse according to a polarization state of the incident laser pulse;a first reflecting mirror which is inserted in the optical path and reflects the laser pulse exited from the PBS back into the optical path;a Faraday rotator which rotates a plane of polarization of the laser pulse passing through the PBS by 45 degrees;a focusing lens which focuses the laser pulse outputted from the Faraday rotator on the predetermined location in plasma; anda second reflecting mirror which reflects the laser pulse outputted from the focusing lens back along the incident laser pulse path and refocuses the laser pulse to the predetermined location in plasma. 3. The plasma diagnosis system according to claim 1, further comprising an optical isolator between the laser and the optical system, andwherein the optical isolator prevents any back reflected lights from feeding back into the laser. 4. The plasma diagnosis system according to claim 1, further comprising a trigger module which is configured with a photo detector and outputs a trigger signal when the photo detector detects a part of the laser pulse on a predetermined position,wherein a signal processing of the polychromator is synchronized by using the trigger signal. 5. The plasma diagnosis system according to claim 4, wherein the photo detector is placed between the laser and the optical system or at an arbitrary position where stray light reflected from or transmitted through an optical component is accessible and outputs the trigger signal when detecting that the laser pulse is supplied from the laser to the optical system, detecting that the laser pulse is supplied from the optical system to the plasma, or detecting that the laser pulse is supplied at an arbitrary position of the optical system. 6. Plasma diagnosis system according to claim 1, wherein the computer measures a pure Thomson scattering signal without the background noise by subtracting the background noise from the Thomson scattering signal contaminated with the background noise. 7. Plasma diagnosis system according to claim 1, wherein the computer measures temperature and density of electrons in plasma by analyzing the spectral characteristics obtained in the polychromator.