16th International Toki Conferenc e

1. 16th International Toki Conference P5-14 Advanced Imaging and Plasma Diagnostics R.Pavlichenko, K.Kawahata, A.J.H.Donné(1) Design of the 48,57 mm PoloidalPolarimeter for ITER National Institute for Fusion Science, Toki, Gifu 509-5292, Japan (1)FOM-Institute for Plasma Physics Rijnhuizen, NL-3439, Nieuwegein, Netherlands Ceratopia Toki, Gifu, JAPAN December 5-8, 2006

2. Concept of interferometry-polarimetry (I) Faraday rotation Cotton–Mouton effect circular shaped plasma (idealization) The Faraday rotation is caused by the presence of a magnetic field parallel to the direction of propagation of probing beam. The state of polarization can be described by the Stokes vector s(z). The evolution along the line of sight (z-direction) is given by: the rotation angle (related to the Faraday effect) the ellipticity(related to the Cotton–Mouton effect) After: DeMarco F., Segre S. E. : Plasma Phys. 14 (1972) 245. Calculated Faraday rotation angles (double pass through the plasma) for a horizontal fan of chords (right top) and the corresponding ellipticity values (right center) ; with q-profile, pressure profile and electron density profile on the left. Very small ellipticity (Cotton-Mouton effect) Design of the 48,57 mmPoloidalPolarimeter for ITER P5-14

3. Concept of interferometry-polarimetry (II) • Control of the current density profile becomes a paramount issue for the modern tokamakexperiments. • Polarimetry can provide information on the density andmagnetic field distribution inside plasma(current profile), utilizing the Cotton–Mouton and the Faraday effects in a magnetized plasma. • In order to evaluate B|| from the rotation angle, the electron density is necessary. Both ne, B|| must be measured along same chord simultaneously. Design of the 48,57 mmPoloidalPolarimeter for ITER P5-14

4. Concept of interferometry-polarimetry (III) Two general approaches exist to evaluate plasma current profile Polarimeter - polarimeter Polarimeter - interferometer B|| B|| polarimeter (Faraday) polarimeter (Faraday) ne ne polarimeter (Cotton–Mouton) interferometer Advantages • Lots of application on major tokamaks (JT-60U, JET, TotaSupra, RTP,NSTX,MST…) • There is no fringe jumps in principal Drawbacks Shorter wavelength laser, with smaller refraction and two color interferometer resolve the problems • Complicated channeling and calibration due to coupling of Faraday rotation and Cotton-Mouton effect. • Despite promising theoretical and numerical results there is very limited experimental support. 1ch pure Cotton–Mouton polarimeter (W7-AS) • Fringe jumps • Mechanical vibrations • Small Faraday rotation in the core plasma region Design of the 48,57 mmPoloidalPolarimeter for ITER P5-14

5. Test of two color FIR interferometer SINGLE CHANNEL TEST FIR Laser 1.06 mm YAG laser Silicon B.S. • For each channel same detector simultaneously detects the beat signals of the 57- and 48-mm laser lines; • Each interference signal can be separated electrically – the 57.2 µm at 0.6 MHz and the 47.6 µm at 1.6 MHz. • Mechanical vibration can be compensated by two color interferometer Ge：Ga Detector 10 fringes/div. C.C. Mirror Optical path length was modulated by using an electro-mechanical vibrator. 10 ms/div. Design of the 48,57 mmPoloidalPolarimeter for ITER P5-14

6. Transmission of the Gaussian beams Design of the 48,57 mmPoloidalPolarimeter for ITER P5-14

7. Laser beam deviation at the CRR Beam aperture diameter at CRR mustobey: Plasma cut-off frequency: Beam bending angle: Preferable choice of probe beam wavelength: Limited by vacant space inside the BSM (blanket shield modules) Distance from plasma center to CRR (changed with the beam chord) Central electron density The optimum wavelength for the polarimeter: Design of the 48,57 mmPoloidalPolarimeter for ITER P5-14

8. Circular waveguides diameter optimization Gaussian beams in hollow circular dielectric waveguides The hollow circular oversized waveguides are commonly used in some infrared and millimeter wave devices such as waveguide lasers or transmission lines for infrared interferometers/polarimeters. The advantages of the HE11 mode and Gaussian beams for propagation in these waveguides are well known: low attenuation, linear polarization, azimuthal symmetry Waveguide transmission After: Crenn J.P. : Int.J.IR&MMW. V14,No10 (1993) 1947. p=0.5588 λ1 = 48 µm 0.5721 λ2 = 57 µm 0.6313 Gaussian beam have to matched into the 40id dielectric WG to avoid power loss and mode conversion λ3 = 118 µm Ø1 = 40 mm Ø2 = 40 mm Ø3 = 90 mm Design of the 48,57 mmPoloidalPolarimeter for ITER P5-14

9. Waveguides, windows, corner retroreflectors Miter bends conversion losses Up to 8 Miter bends per channel VV Port vacuum interface Dielectric WG Corner retroreflector modules at the HFS BSM Design of the 48,57 mmPoloidalPolarimeter for ITER P5-14

10. Window and Beam splitter materials Tested Silicon windows Measured properties of Crystal Quartz, CVD-Diamond and Si nontransparent After: K. Nakayama, S.Okajima et al. 29th Conf. IRMMW, 2004 Design of the 48,57 mmPoloidalPolarimeter for ITER P5-14

11. SUMMARY • Proposed poloidal polarimeter will operate at 48,47 mm • The output power of 57.2mm laser is estimated to be over 1.6 W and that of 47.6mm is ~0.8 W. • Two color beat signals are simultaneously detected by a Ge:Ga detector. • Preferable polarimeter-interferometer configuration • Well established techniques, a lot of experience . • Shorter wavelength laser will significantly improve refraction problems. • Small Cotton-Mouton effect • Waveguided transmission line / miter bends – better focusing and tuning as well as much siple further maintenance … • High power two color beat signals are simultaneously detected by a Ge:Ga detector; it is possible to suppress fringe jumps and mechanical vibrations • Problems • Under some plasma condition there is a possibility of coupling Faraday and Cotton-Mouton effects. • Small Faraday rotation angle along some chords. Design of the 48,57 mmPoloidalPolarimeter for ITER P5-14