Davis Millimeter-Wave Research Center

Microwave/Millimeter Wave Technology

Plasma Diagnostics

 

 

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Plasma diagnostics

 

µ-wave vacuum electronics

 

µ-wave solid state technology

 

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Director:
Prof. N.C. Luhmann, Jr.

 

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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:

 

  • Utilize an optically driven FIR laser, operating at 604 GHz and 100 mW.
  • Design a steerable optics system to project the beam between ports G and L
  • Design a multi-channel array, heterodyne receiver system
  • Fabricate and install these systems on NSTX
  • Collect data to support ongoing transport and turbulence research

 

Enhancements over the former High-K scattering system include:

 

  • Increasing FIR frequency from 280 to 604 GHz for improved poloidal and radial wavenumber resolution and range
  • Increasing the beam power to 100 mW for improved signal-to-noise ratio
  • Eliminate solid state LO
  • Utilize advanced mixer designs
  • Benefit from new manufacturing technologies

 

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.

 

 

 

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Fig. 2. (a) Top views of two radial scattering schemes; (b) Side views (perpendicular to Bay L) of two poloidal scattering schemes for five k^'s (10 to 30 cm−1 every 5 cm−1).

 

 

 

 

 

Description: Description: ETG Wavenumber Spectrum

Fig. 3. 2D ETG wavenumber spectrum calculated by the nonlinear, global GYRO code. The typical measureable region accessible to the old high-k scattering system is enclosed by the black line. (Courtesy of W. Guttenfelder).

 

 

 

 

 

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.

 

 

 

 

 

 

 
 
 
 
 
 
              
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