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LOGO 10.6-um Laser Scattering from Cyclotron-Harmonic Waves in a Plasma 报告人 : 孙兆轩 组员 : 王兴立,曹骑佛,孙兆 轩,周凡,魏然,江堤.

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Presentation on theme: "LOGO 10.6-um Laser Scattering from Cyclotron-Harmonic Waves in a Plasma 报告人 : 孙兆轩 组员 : 王兴立,曹骑佛,孙兆 轩,周凡,魏然,江堤."— Presentation transcript:

1 LOGO 10.6-um Laser Scattering from Cyclotron-Harmonic Waves in a Plasma 报告人 : 孙兆轩 组员 : 王兴立,曹骑佛,孙兆 轩,周凡,魏然,江堤

2 Background In Short wavelength fluctuations in plasmas; Electrostatic probes disturb the plasma for a distance of the order of the sheath size and are impractical in high-temperature plasmas; Electrostatic probes disturb the plasma for a distance of the order of the sheath size and are impractical in high-temperature plasmas; Microwave scattering is capable of resolving only wavelengths larger than half the microwave wavelength (>1 mm); Microwave scattering is capable of resolving only wavelengths larger than half the microwave wavelength (>1 mm); However, in many controllable laboratory plasmas and in plasmas suitable for fusion where the Debye wavelength is considerably less than 1 mm ( in the Tokamak =0.05 mm). In principle, scattering with a laser of suitable wavelength λ l is a technique which is capable of resolving short-wavelength plasma fluctuations without perturbing the plasma. λ

3 Background the high-power CO 2 laser : λ l =1.06 ×10 -2 mm λ l =1.06 ×10 -2 mm maximize the scattered power (which is proportional to λ l 2 ); maximize the scattered power (which is proportional to λ l 2 ); resolve the wavelengths of all collective phenomena. resolve the wavelengths of all collective phenomena.

4 Principle

5 The plasma is generated with an rf voltage applied to a probe, which levels of modulation of the electron density as low as 6×10 5 cm -3, and plasma wavelengths between 2.0 and 0.75 mm. Principle

6 For a maximum in scattered power, the angle θ s between the incident and scattered light is given by the Bragg condition θ s = 2 sin -1 (λ l /2λ p ) We assume a density fluctuation N produced by the driving probe of the form: Principle

7 Since the light mixing signal is proportional to E s, It is the factor exp(iKx 0 ) which accounts for the oscillation of the signal as a function of magnetic field. the scattered power P s is found to be: Principle

8 results Solid curve, light-mixing signal, P s 1/2 × cos ψ, at the output of LI as a function of the magnetic field B (ψ The detector was positioned at the Bragg angle for 1.5-mm waves. Dashed curve, calculated signal.is the phase angle between the scattered and local-oscillator electric fields).

9 Figure shows the output of the lock-in amplifier as a function of B for θ s corresponding to 1.5-mm waves. The waves were driven with 2. 5 W of rf power, and the lock-in time constant was 3 sec. The intensity of scattered signal was observed to decrease rapidly for λ p <0. 1 cm. This is probably because the size of the sheath around the driving probe is comparable to these values of λ p. results

10 Light scattering data (closed circles) and probe data using an rf interferometer (open triangles). Solid lines, theoretical dispersion relation of cyclotron-harmonic waves for (f p /f c ) = 120, where f c is the cyclotron frequency, Ris the cyclotron radius (k B T e /m) 1/2 /2π f c, and f p is the plasma frequency. results

11 For the light-scattering data shown in Figure, the points are determined by the values of magnetic field at the maximum of the envelope of the scattered signal and λ p as defined by θ s. The probe and light-scattering measurements agree within ± 5%. The discrepancy between the data and the theoretical dispersion relation for a Maxwellian plasma has been observed previously. The origin of the discrepancy is probably due to deviations from a Maxwellian electron velocity distribution. results

12 The limiting noise in these experiments is due to photon statistical fluctuations of the local oscillator and thermal background radiation. The limiting noise in these experiments is due to photon statistical fluctuations of the local oscillator and thermal background radiation. the minimum detectable electron density modulation n in our experiment is approximately 6×10 5 /cm 3. the minimum detectable electron density modulation n in our experiment is approximately 6×10 5 /cm 3. the signal-to-noise ratio would be improved to the point where the noise is due primarily to fluctuations in the local oscillator and is independent of local-oscillator power. the signal-to-noise ratio would be improved to the point where the noise is due primarily to fluctuations in the local oscillator and is independent of local-oscillator power. results

13 summary Infrared lasers and light-beating spectroscopy should prove useful in measuring the frequencies, wavelengths, and levels of fluctuation of collection phenomena over a wide range of values previously inaccessible by other techniques. Infrared lasers and light-beating spectroscopy should prove useful in measuring the frequencies, wavelengths, and levels of fluctuation of collection phenomena over a wide range of values previously inaccessible by other techniques. In particular, this range of frequencies and wavelengths is suitable for studying ion sound turbulence in the Tokamak. In particular, this range of frequencies and wavelengths is suitable for studying ion sound turbulence in the Tokamak.

14 reference 1.C. M. Surko, R. E. Slusher, and D. R. Moler Bell Laboratories, Murray Hill, ¹coJersey G. A. Wurden, M. Ono, and K. L. Wong Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08544

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