UCDavis PDG                                            

 

 

 

 

ECE Imaging on DIII-D

 

The DIII-D ECEI system is a dual-array configuration ECE Imaging on DIII-D ECE Imaging on DIII-D providing ECE Imaging on DIII-Dimages of the electron temperature at two distinct plasma locations. The images may be independently configured to capture adjacent regions or opposing sides of the magnetic axis. Additionally, the vertical and radial coverage of each image may be varied over a broad range.

 

HARDWARE

The dual-array ECEI installation on DIII-D [1] is the 3rd generation imaging system of its kind. It improves on the 1st and 2nd generation TEXTOR designs.

Some of its improved capabilities include:

  • Dual mini-lens array with internal beamsplitter - reduces channel spacing, maximizes LO coupling and eliminates need for LO beam dump
  • 600/900 MHz radial zoom control added to vertical zoom capability
  • Dual ECEI arrays for simultaneous low- and high-field imaging

 

 

 

 

 

 

 

Above: Laboratory setup of the DIII-D ECEI optics and detector arrays.

Above: ECEI system installed at the 270 degree midplane port on DIII-D.

 

Above: Frequency band detectable by ECEI on DIII-D (yellow). For this particular discharge, the low-field array covers the region indicated by the overlaid lower green band, and the high-field array covers the region of the upper green band. The 2nd harmonic ECE is plotted in blue and the right-handed X-mode cutoff is the dark green curve.

 

 

 

 

Above: Poloidal cross-section of DIII-D demonstrating the variation in ECEI coverage with vertical zoom and variable LO frequency.

 

Above: Mini-lens detector array. 24 vertical channels are accommodated in the same toroidal plane by using a beam splitter to apportion signal to adjacent channels.  Propagation directions of the LO and ECE signals are indicated.

 

 

 

 

Above: Schematic of mini-lens detector array.

 

 

 DATA

Perhaps the most impressive aspect of the dual-array ECEI system on DIII-D is the exciting data it has been collecting. Of the many experiments in which it has been employed, some of the highlights are described below.

 

          Sawtooth

 

The sawtooth instability in tokamak plasmas is observed to cause a rearrangement of the internal current profile as well as redistribution of particles and temperature. The MHD mode, ultimately leading to the crash, is a relatively large magnetic perturbation, characterized by toroidal/poloidal mode numbers n/m=1/1 and typically low frequency (≤10kHz). Nailing down a solid physics understanding of the crash event (which takes place on the order of 10s-100s of microseconds) has eluded physicists for decades since the instability’s discovery in the 1970s. Now, with imaging diagnostics, such as ECEI, we are able to capture the collapse of the electron temperature at the sawtooth crash with unsurpassed clarity. 2-dimensional data such as this provides an unambiguous measurement to which simulation models can compare.

 

Above: Select ECEI frames taken during the last precursor oscillation leading up to the sawtooth crash. The dashed white line in the first frame is the inversion radius.

 

 

          Alfvén eigenmodes

 

Unlike the sawtooth instability, Alfvén eigenmodes (AEs) are low-amplitude and typically characterized by medium to high toroidal mode number (n) and medium to high frequency. In many cases, multiple-n modes are simultaneously and/or sequentially destabilized in the plasma. The diagnosis of AEs in tokamaks is particularly important to the study of energetic ions. Sizable losses of energetic ions by AEs associated with mode-particle resonances have been well-documented on many tokamaks. An understanding of the stability limits and mode structure of AEs has become a high-priority research task in the fusion community. Imaging diagnostics, such as ECEI, provide a critical tool to the identification of such modes.

 

Above: (a) the symmetric reverse-shear AE (RSAE) structure predicted by ideal MHD is shown. TAEFL demonstrates that fast-ion contributions may alter the eigenmode structure, inducing symmetry breaking as shown in (b). Experimental measurement by ECEI, (c), confirms these predictions providing an opportunity for model validation [2]

 

[1] Tobias B.J., et al, “Commissioning of electron cyclotron emission imaging instrument on the DIII-D tokamak and first dataRev. Sci. Instrum. 81, 10D928 (2010).

[2] Tobias B.J., et al, “Fast Ion Induced Shearing of 2D Alfvén Eigenmodes Measured by Electron Cyclotron Emission ImagingPhys. Rev. Lett. 106, 075003 (2011).

 

 

UC Davis has fabricated and installed multichannel ECE Imaging systems on a number of fusion plasma tokamaks across the world. Follow the links below for a description of the systems involved

Description: Description: Description: Description: Description: Description: Description: *ECE Imaging on the EAST tokamak in China

Description: Description: Description: Description: Description: Description: Description: *ECE Imaging on the KSTAR tokamak in Korea

Description: Description: Description: Description: Description: Description: Description: Description: *ECE Imaging on the TEXTOR tokamak in Germany
Description: Description: Description: Description: Description: Description: Description: *ECE Imaging on the TEXT-U tokamak in the U.S.A. (Reference Only)

 


Description: Description: Description: Description: Description: Description: Description: email Comments to: Calvin Domier