Transcript PowerPoint Template

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
LOGO
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;
Microwave scattering is capable of resolving only wavelengths larger than half
the microwave wavelength (>1 mm);
λ
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 λl is a technique which is capable of resolving short-wavelength
plasma fluctuations without perturbing the plasma.
Background
the high-power CO2 laser：
λl =1.06 ×10-2mm
maximize the scattered power (which is proportional to λl2);
resolve the wavelengths of all collective phenomena.
Principle
Principle
The plasma is generated with an rf voltage applied to a probe,
which levels of modulation of the electron density as low as
6×105 cm-3, and plasma wavelengths between 2.0 and 0.75
mm.
Principle
For a maximum in scattered power, the angle θs between the incident
and scattered light is given by the Bragg condition θs= 2 sin -1(λl/2λp)
We assume a density fluctuation N produced by the driving probe of
the form:
Principle
Since the light mixing signal is proportional to Es , It is the factor
exp(iKx0) which accounts for the oscillation of the signal as a
function of magnetic field.
the scattered power Ps is found to be:
results
Solid curve, light-mixing signal, Ps1/2 × 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).
results
Figure shows the output of the lock-in amplifier as a
function of B for θs 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 λp <0. 1 cm. This is probably because the size of
the sheath around the driving probe is comparable to these
values of λp .
results
Light scattering data (closed circles) and probe data using an rf interferometer (open triangles). Solid
lines, theoretical dispersion relation of cyclotron-harmonic waves for (fp/fc) = 120, where fc is the
cyclotron frequency, Ris the cyclotron radius (kBTe/m)1/2 /2π fc ,and fp is the plasma frequency.
results
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 λp as defined
by θs.
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
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 6×105/cm3.
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.
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.
In particular, this range of frequencies and wavelengths is suitable for
studying ion sound turbulence in the Tokamak.
reference
1. C. M. Surko, R. E. Slusher, and D. R. Moler Bell Laboratories,
Murray Hill, ¹coJersey 07974
2. G. A. Wurden, M. Ono, and K. L. Wong Plasma Physics Laboratory,
Princeton University, Princeton, New Jersey 08544