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

2 P5-14 Design of the 48, 57  m Poloidal Polarimeter for ITER Concept of interferometry-polarimetry (I) The Faraday rotation is caused by the presence of a magnetic field parallel to the direction of propagation of probing beam. circular shaped plasma (idealization) 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) Faraday rotation the rotation angle (related to the Faraday effect) the ellipticity (related to the Cotton–Mouton effect) 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: After: DeMarco F., Segre S. E. : Plasma Phys. 14 (1972) 245. Cotton–Mouton effect

3 P5-14 Design of the 48, 57  m Poloidal Polarimeter for ITER Concept of interferometry-polarimetry (II) Control of the current density profile becomes a paramount issue for the modern tokamak experiments. Polarimetry can provide information on the density and magnetic 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 n e, B || must be measured along same chord simultaneously.

4 P5-14 Design of the 48, 57  m Poloidal Polarimeter for ITER Concept of interferometry-polarimetry (III) Two general approaches exist to evaluate plasma current profile Polarimeter - polarimeter Polarimeter - interferometer polarimeter (Faraday) polarimeter (Cotton–Mouton) B || nene interferometer B || nene Advantages There is no fringe jumps in principal Drawbacks Fringe jumps Mechanical vibrations Lots of application on major tokamaks (JT-60U, JET, TotaSupra, RTP,NSTX,MST…) 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) Shorter wavelength laser, with smaller refraction and two color interferometer resolve the problems polarimeter (Faraday) Small Faraday rotation in the core plasma region

5 P5-14 Design of the 48, 57  m Poloidal Polarimeter for ITER Test of two color FIR interferometer 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 1.06 mm YAG laser FIR Laser Silicon B.S. C.C. Mirror Optical path length was modulated by using an electro- mechanical vibrator. SINGLE CHANNEL TEST 10 fringes/div. 10 ms/div.

6 P5-14 Design of the 48, 57  m Poloidal Polarimeter for ITER Transmission of the Gaussian beams

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

8 P5-14 Design of the 48, 57  m Poloidal Polarimeter for ITER Circular waveguides diameter optimization Gaussian beams in hollow circular dielectric waveguides p= 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 HE 11 mode and Gaussian beams for propagation in these waveguides are well known: low attenuation, linear polarization, azimuthal symmetry λ 3 = 118 µm λ 2 = 57 µm λ 1 = 48 µm Ø 3 = 90 mm Ø 2 = 40 mm Ø 1 = 40 mm Waveguide transmission After: Crenn J.P. : Int.J.IR&MMW. V14,No10 (1993) Gaussian beam have to matched into the 40id dielectric WG to avoid power loss and mode conversion

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

10 P5-14 Design of the 48, 57  m Poloidal Polarimeter for ITER Window and Beam splitter materials Measured properties of Crystal Quartz, CVD-Diamond and Si Tested Silicon windows Refractive indexAbsorption coefficient (cm -1 ) 48 µm 57 µm 119 µm 48 µm 57 µm 119 µm Crystal quartz Silicon CVD-diamond － － nontransparent After: K. Nakayama, S.Okajima et al. 29 th Conf. IRMMW, 2004

11 P5-14 Design of the 48, 57  m Poloidal Polarimeter for ITER SUMMARY Proposed poloidal polarimeter will operate at 48,47  m The output power of 57.2  m laser is estimated to be over 1.6 W and that of 47.6  m 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.