Development of a multichannel dispersion interferometer at TEXTOR

Yasuhara

Review of Scientific Instruments

et al. 2014

Since a dispersion interferometer is insensitive to mechanical vibrations, a vibration compensation system is not necessary. The CO2 laser dispersion interferometer with phase modulations on the Large Helical Device utilizes the new phase extraction method which uses modulation amplitudes and can improve a disadvantage of the original dispersion interferometer: measurement errors caused by variations of detected intensities. The phase variation within ±2 × 10(17) m(-3) is obtained without vibration compensation system. The measured line averaged electron density with the dispersion interferometer shows good agreement with that with the existing far infrared laser interferometer. Fringe jump errors in high density ranging up to 1.5 × 10(20) m(-3) can be overcome by a sufficient sampling rate of about 100 kHz.

Section: Introductionmentioning

confidence: 99%

“…The other method is a dispersion interferometer (DI). [6][7][8][9][10][11][12][13] As shown below, a dispersion interferometer is also insensitive to the mechanical vibrations. Since the measurement errors caused by the vibrations are small, the phase shift due to a plasma less than one fringe would be acceptable.…”

Section: Introductionmentioning

confidence: 99%

Dispersion interferometer using modulation amplitudes on LHD (invited)

Yasuhara

Review of Scientific Instruments

et al. 2014

“…The phase-modulated dispersion interferometer was introduced by Bagryansky et al 9,10 and its system was operated on TEXTOR. Instead of an electro-optical modulator used in the dispersion interferometer on TEXTOR, a photoelastic modulator ͑PEM͒ is adopted for the phase modulation in this work.…”

Section: A Principle Of Phase Extraction Methodsmentioning

confidence: 99%

“…9,10 This is largely due to difficulty of the second harmonic generation for the small power density of continuous-wave ͑cw͒ laser light. It is also one of the disadvantages that the interference signal of the dispersion interferometer is the same as that of a homodyne interferometer.…”

Section: Introductionmentioning

confidence: 99%

Development of CO2 laser dispersion interferometer with photoelastic modulator

Review of Scientific Instruments

Okajima

et al. 2010

A dispersion interferometer is one of the promising methods of the electron density measurement on large and high density fusion devices. This paper describes development of a CO 2 laser dispersion interferometer with a photoelastic modulator for phase modulation. In order to make the dispersion interferometer free from variations of the detected intensity, a new phase extraction method is introduced: The phase shift is evaluated from a ratio of amplitudes of the fundamental and the second harmonics of the phase modulation frequency in the detected interference signal. The proof-of-principle experiments demonstrate the feasibility of this method.

“…In contrast, the DI has no issue even if the plasma term is smaller than 2π owing to its immunity to vibrations. Thus, the DI becomes free from fringe jump errors if an appropriate wavelength is selected.Techniques that improve the resolution of DIs are currently being introduced [2][3][4][5]. This paper reports improved resolution and long-time stability in a DI that adopts phase modulation [2] with a photoelastic modulator (PEM) and a new signal processing method that uses a modulation amplitude ratio [4,5].…”

mentioning

confidence: 99%

Improved Resolution and Stability in a Dispersion Interferometer Using a Modulation Amplitude Ratio

Plasma and Fusion Research

Okajima

et al. 2010

A dispersion interferometer is immune to mechanical vibrations, which is a great advantage for application to steady-state fusion reactors. This paper describes the performance of a phase-modulated dispersion interferometer with a new phase extraction method using a modulation amplitude ratio. On steady-state fusion reactors, reliable electron density measurement is required for fueling control. However, the signal-to-noise ratio (S/N) of conventional interferometers is affected by mechanical vibrations. In addition, fringe jump errors are possible at high densities. One possible approach to these problems is the use of a dispersion interferometer (DI) [1]. The DI is immune to mechanical vibrations, and hence does not need a vibration isolator. A two-color system is also not needed even if short-wavelength lasers (a CO 2 laser and a YAG laser, for example) are used. The fringe jump errors are attributed to 2π ambiguity in the phase shift. If short-wavelength laser light with a phase shift smaller than 2π is selected, the interferometer becomes free from fringe jump errors in principle. However, the S/N is significantly degraded because the phase shift due to vibrations (∝ 1/λ, λ: wavelength) increases and that due to a plasma (∝ λ) decreases for conventional interferometers. In contrast, the DI has no issue even if the plasma term is smaller than 2π owing to its immunity to vibrations. Thus, the DI becomes free from fringe jump errors if an appropriate wavelength is selected.Techniques that improve the resolution of DIs are currently being introduced [2][3][4][5]. This paper reports improved resolution and long-time stability in a DI that adopts phase modulation [2] with a photoelastic modulator (PEM) and a new signal processing method that uses a modulation amplitude ratio [4,5]. Figure 1 shows a schematic view of the present optical and electrical systems used for bench testing. The light source is a CO 2 laser with an output power and wavelength of 8 W and 10.6 µm, respectively. The focused laser light is injected into a nonlinear crystal of AgGaSe 2 to generate the frequency-doubled component (FD1). A mixed beam consisting of the fundamental and frequency-doubled components, whose polarizations are orthogonal, passes through a PEM with a drive frequency where c p = e 2 2ε 0 m e c, ω: angular frequency of the laser light, Δd: change of the optical path length due to vibrations, c: light speed, e: charge of the electron, ε 0 : dielectric constant, m e : mass of the electron, n e : line averaged electron density, L: path length in a plasma, and φ 1 , φ 2 : initial phase. The phase of the detected interference signal I is difference between phases above. Because the phase terms resulting from vibrations, 2ωΔd/c, are common, as shown