The principle of SAMI and some results in MAST

1. Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, Anhui, 230021, China

2. Culham Centre for Fusion Energy, Oxfordshine, OX143DB, UK

S. Zhang1,2, V. F. Shevchenko2, S. Freethy2

SAMI

• SAMI = synthetic aperture microwave imaging

• Imaging EBW emission

Motivation

• Edge current density is much more difficult to measure (MSE helpful but limited resolution);

• Pedestal physics, heating and current drive;

Outline

• 1. The principle of SAMI

• 2. The design of SAMI system of MAST

• 3. Some experimental results for MAST

• 1. The principle of SAMI

• 2. The design of SAMI system of MAST

• 3. Some experimental results for MAST

The electromagnetic wave in cold plasma

• According to the Appleton-Hartree formula, we can get the dispersion relation in different conditions.– If B=0

– If k//B

– If k⊥B

Electron Berstein Wave

• If we consider the thermal effect and allow the electrons possess a finite Larmor radius, there are new wave mode (EBW) occurs in plasma. – Are electrostatic and longitudinal

– Generated at harmonics of the local cyclotron frequency

– Power spectrum depends on Te at birth location

The Mode conversion (B-sX-O)

The angle of mode conversion

• The mode conversion only occurs at optimum angle has been solved analytically. (O mode -> sX mode)

• The transmissivity is:

Imaging patten

The measurement principle of SAMI

• Phase difference between antenna pairs

• Cross-correlation between pairs of antennas gives spatial Fourier transform of emission pattern

• Number of pixels: N^2 (N = number of antennas)

• We can therefore inverse Fourier transform the cross-correlations to recover emission pattern.

• Baselines must be chosen carefully to provide good coverage of Fourier space.

• 1. The principle of SAMI

• 2. The design of SAMI system of MAST

• 3. Some experimental results for MAST

The SAMI system in MAST

The system parameters of SAMI in MAST

• LO at 16 different frequencies coves frequency range: 10-35 GHz (different radial positions)

• 8 antennas (28 baselines)

• Vector measurement: I & Q

• Synthesized signals at 8 MHz 10 MHz and 12 MHz for active probing.

• Passive imaging– Mode conversion physics

(study the EBW and current drive)

– Edge current density profile

• Active probing– 2D+1F MHD spectra

– 2D+1F velocity map of turbulence

– Density fluctuation and pedestal physics

Antipodal vivaldi antenna and aperture synthesis array

• Electric field lies in the plane of the antenna.

• By changing the alignment of the antenna to the magnetic field lines of the plasma different modes (O and X) can be selected.

• Wide frequency range

• How to design the array– designed to minimise the redundancy in these antenna pairs and

minimise the level of oscillations in the array beam pattern

Simon Freethy’s PHD Thesis, 2012

The local oscillator and quadrature down-conversion

• Local oscillator switches between 16 frequencies within 10-35 GHz range, and it is for the heterodyne measurements.

• I & Q components allow to separate USB and LSB and analyse them independlly.

Data acquisition and control unit system

• 16 channels (8 antennas);

• 14 bit sample depth (dynamic range of the plasma during ELM)

• 0.5 s total acquisition time (length of MAST shot)

• <350 ps cross-channel skew (error in cross-correlation)

• 8 Gbytes/s data rate

• 4 Gbytes of data per shotBilly Huang’s PHD Thesis, 2013

The calibration of SAMI system

• Off-vessel: calibration performed at every frequency to obtain complex coefficients for each baseline for both upper and lower sideband.

• Through-vessel: calibration used to confirm robustness of off-vessel calibration.– All available port were tested

– It was found that vacuum windows and flanges have very little effect on the image coordinates and shape.

– Deviations of reconstructed imagers were within 200 for frequencies in 10-18 GHz range.

Simon Freethy’s PHD Thesis, 2012

• 1. The principle of SAMI

• 2. The design of SAMI system of MAST

• 3. Some experimental results for MAST

V F Shevchenko, R G L Vann, S J Freethy and B K Huang, Journal of Instrumentation, volume 7, P10016, 2012

The comparison between simulation and experimental results

• 1D full-wave mode-coupling code complemented with 3D ray-tracing modeling has been conducted for the same parameters.

• The experimental image shows the excellent agreement with simulation results.

Ex-vessel test with active probing

• Rotating corner reflector was used in active probing ex-vessel tests.

• The measured frequency shift value from SAMI is consistent with the Doppler shift due to the reflector rotation.

Experimental results in MAST

• These figures show the velocity maps reconstructed during L-mode and H-mode phase.

Thanks for your attention!

Sinuous antenna