High-K Scattering on NSTX (ETG Scattering)

This work is being carried out in collaboration with Drs. E. Mazzucato and H. Park of PPPL. For the NSTX ETG scattering system, the probe beam is launched on the toroidal midplane with the detector also located on the midplane. Observed fluctuation wave vectors are necessarily radial. If fluctuations were isotropic, the detector could be aimed anywhere along the probe beam and still detect a scattered signal as illustrated in Fig. 1. However, as noted by PPPL collaborator Dr. E. Mazzucato [1] with anisotropic fluctuations (drift wave turbulence is anisotropic with the fluctuation wave vector nearly perpendicular to the local magnetic field), the detector must be aimed at the particular region of the probe beam that satisfies the condition above (see Fig. 2). This imposes an additional constraint on the scattering process. Some regions common to the launch and receiving beams are, in essence, detuned because the magnetic field changes direction (see Fig. 3). Thus, the scattering volume is constricted and spatial resolution improves. If the detector is aimed at other regions of the probe beam, a scattered signal will not be observed. The instrument selectivity function, plotted in Fig. 4, illustrates the scattering volume constriction due to toroidal curvature. This beneficial effect is most pronounced at small scattering angles and low aspect ratio thereby making NSTX the ideal device on which to employ this arrangement.

Fig. 1. Illustration of the variation in scattered wavevector k_s as a function of scattering volume location.

Fig. 2. System of orthogonal coordinates. The u-axis is parallel to the tokamak equatorial plane and the t-axis is parallel to wave vector k_i.

Fig.3. Magnetic field (B₁and B₂) and wave vectors (k₁ and k₂) of detected fluctuations at two locations of a probing beam propagating perpendicular to the magnetic surfaces.

Fig. 4. The instrument selectivity function, plotted as a function of fluctuation wavenumber.

The initial illumination source is a 280 GHz (λ=1.1 mm), 200 mW Thomson CSF carcinotron or backward wave oscillator (BWO), powered by a high voltage Siemel carcinotron power supply. The tube, power supply, and recirculating cooler are located outside the NSTX test cell, with the tube output piped to the NSTX vacuum vessel using low loss corrugated waveguide. Optics placed at the other end of the corrugated waveguide transform the BWO output into a collimated beam which enters the plasma through an 8" diameter window on Bay H (see Figs. 5 and 6). This source beam may be aimed via a translatable/rotatable mirror to different plasma radii, with scattered radiation collected at both positive and negative scattering angles over a 20 degree range of angles. For example, operation at the innermost plasma radii might yield scattering angles of +10 degree to +30 degree, at the outermost plasma radii with angles of -30 degree to -10 degree, and at the plasma center with angles of -10 degree to +10 degree.

Fig. 5.Autocad layouts of high k scattering system ex-vessel optics on NSTX, showing the translatable mirror arrangement for launching the illumination beam on Bay H (left), and the collecting mirrors for the scattered beams on Bay K (right).

Fig. 6. Photographs of the Bay H (launch) port assembly.

At the other end of the vacuum vessel, on Bay K, a curved mirror is placed within a port extension box positioned below the plasma midplane beneath the FIReTIP entrance/exit windows as illustrated in Figs. 7 and 8. The large in-vacuum mirror will move to compensate for source aiming angle changes, and will send the scattered beam bundle through five small windows. An additional set of turning and telescoping mirrors located outside the exit window will compensate for the other two motions and direct the recollimated scattered beams down to the NSTX basement where the five channel detection system is placed.

Fig. 7. Photographs of the Bay K (receive) port assembly.

A photograph of the low noise receiver array is provided in Fig. 8. High sensitivity, low noise waveguide Schottky diode mixers are employed to receive the low power scattered signals. Each mixer is followed by a low noise preamplifier placed together inside an electrically shielded enclosure box. This results in an extremely low noise overall system temperature measured between 4500 degree and 6300 degree K (varies slightly by channel). The LO power to the mixers is provided by a solid state Gunn oscillator followed by a high efficiency frequency tripler which delivers 4.0 mW at 280 GHz. A biconical mixer serves as the reference detector, which samples a portion of the unscattered launch beam prior to propagation into the NSTX plasma, provides a ~900 MHz reference signal. The reference and plasma signals are then piped via low loss coaxial cables back to the diagnostic rack, where they are analyzed via I&Q demodulators.

Fig. 8. Photograph of the high-k collective scattering receiver array under test at UC Davis.

The system has a wavenumber coverage at 280 GHz that extends up to 20 cm^-1 as a function of scattering angle. By simultaneously adjusting the launch and collection mirror angles, the five channel system can be used to fill in the wavenumber gaps between channels on a shot-by-shot basis.

The key physics issues to be addressed lie in the detection of ETG turbulence and the study of electron thermal transport. Plotted in Figs. 9 and 10 are measured temperature and thermal diffusivity profiles during neutral beam injection (NBI) and high harmonic fast wave (HHFW) heated plasmas [2]. The NBI power is expected to primarily deposit on electrons, but T_i > T_e is experimentally observed (see Fig. 9). While the ion thermal transport is at or near neoclassical levels, the electron thermal transport remains poor with ETG turbulence suspected. The picture looks much different in high harmonic fast wave (HHFW) heated plasmas, in which strong core electron heating is observed. While electron thermal transport is still the dominant loss mechanism, ETG turbulence should be suppressed when T_e > T_i. This study focused on a conclusive determination of the presence or absence of ETG turbulence in NSTX is critical since the connection between ETG turbulence and electron thermal transport remains a controversial issue.

Fig. 9. Measured T_e and T_i profiles (left), and experimental thermal diffusivity profiles against major radius R (right) during high power NBI heating (from [2]).

Fig. 10. Measured T_e and T_i profiles (left), and experimental thermal diffusivity profiles against major radius R (right) during high power HHFW heating (from [2]).

The high-k scattering diagnostic system jointly under development by UC Davis and PPPL was successfully commissioned in August, 2005. The detection optics and electronics were completed by UC Davis and delivered to PPPL in early June. In parallel with this work, PPPL installed the Thomson-CSF carcinotron or backward wave oscillator (BWO) probe source, power supply, waveguide runs and transmission/collection optics with which to feed the five channel high-k scattering receiver. Physics studies are therefore anticipated during the next NSTX run period.

[1] E. Mazzucato, Localized Measurement of Turbulent Fluctuations in Tokamaks with Coherent Scattering of Electromagnetic Waves,” Physics of Plasmas 10, pp. 753-759, (2003).

[2] B.P. LeBlanc, et al., Confinement Studies of Auxiliary Heated NSTX Plasmas, Nuclear Fusion 44, pp. 513-23 (2004).