High-K Scattering System for NSTX-Upgrade
The upgrade of the National Spherical Torus Experiment will require many modifications to existing systems and diagnostics. These changes have provided an opportunity to greatly enhance the previous High-K scattering system. This project is underway, and is scheduled to be completed and installed during the 2012-2013 torus vent.
The High-K scattering system will measure density fluctuations using collective Thompson scattering. The data collected will be crucial for understanding transport and turbulence, which is a major barrier to sustained fusion reactions. This diagnostic will use a 600 GHz, 100 mW laser to probe the interior of NSTX. Turbulence oriented in poloidal and radial directions will scatter a small portion of the beam and a receiver will detect these scattered signals. By measuring the angle and intensity of the scattered photons we will better understand the source of turbulent behavior of plasmas in fusion reactors.
Figure 1. Proposed beam path for the High-K scattering system shown in red.
The goals of this project are the following:
Enhancements over the former High-K scattering system include:
The beam path from port G to port L offers increased coverage in poloidal and radial wavenumbers. Port L is relatively large at 13” tall and 5” wide which will allow a large spread of scattered angles. Furthermore, the increased beam frequency will diffract less, which will keep the scattered signals more closely spaced. The combined effect will offer k-poloidal up to 40 cm-1. Figure 2 shows several scattering arrangements for poloidal and radial measurements. One particular region of interest is where the spectral peak of ETG turbulence is predicted to occur in Fourier space (see Figure 3). The new system will be able to access this area, opposed to the coverage of the old system as seen in figure 3.
Low noise mixers are available at fundamental and subharmonic frequencies. If subharmonic mixers are used, then the current solid state LO can provide signals up to 25 mW at 300 GHz. Each mixer will require 3-8 mW of power, so this arrangement will limit the number channels that can be used. FIR lasers can operate at higher frequencies and powers and can expand the versatility of this diagnostic.
The optimum FIR source options, based on all of the aforementioned criteria, are to employ either CH3F (methyl fluoride) or HCOOH (formic acid) as the lasing medium and generate ~100 mW at 496.1 µm (604.3 GHz) or 513.0 µm (584.4 GHz), respectively.
The current status of this project is to optimize an FIR laser, and a CO2 pump laser. We are investigating the feasibility of using methyl fluoride or formic acid as a lasing medium. Once the lasers are fully characterized, the project can advance to the optic and receiver design stages.
As progress continues, this website will be updated.