Progress and New Results from the H-1NF Scanning

InterferometerScott Collis, George Warr,

John Howard

A Time Multiplexed Interferometer

• The System is basically described as two Michelson Interferometers with a time varying viewing path viewing

• The phase shift induced by the plasma is given by

• The signal is time multiplexed, which means we only need one detector per probe beam

dlnλr2Δφ ec

Beams are swept by a grating wheel

• Wheel speed can be varied from 0-6000rpm

• At 6000 rpm with a 6 partitioned wheel one scan of the plasma takes 1.8ms

• The wheel also Doppler shifts the beam:

dmRf /

Motor and Grating

Continuous Grating allows viewing of small structures

• The grating constant changes linearly across a sector

• Possibility of seeing small structures, of order 1cm (beam size) such as islands and transport barriers

• Data can aliased when frequency of plasma oscillations approaches the scan frequency of the interferometer

Fosc > Fscan

Aliased Data

Calibration

•We need a transform from temporal position in scan

to spatial position in the H-1NF tank

A 2.5cm absorber was placed at three positions on the bottom

mirror

At 40cm above the centre

In the centre of the mirror

And at 40cm below the centre

Top Diagonal Results

Beam width: 1.3cm

Bottom Diagonal Results

Beam width: 3.2cm

Calculated Beam Positions

High Density ICRH Shot

•Fundamental ICRH resonance of Hydrogen

•B=0.5T

•Peaked density profile

•Highest B2ne product observed in H-1NF

Possibilities

• With the wheel running at high speed it is possible to get temporally and spatially resolved information on the plasma decay

• From this it will be possible to solve the two dimensional diffusion equation

• With the new ICRH mode it is possible to study the physics of fully ionized plasmas in H-1NF

But first….

• To tomographically reconstruct density maps of the H-1NF plasma we need a greater coverage of the plasma

• We will be expanding the fan of the diagonal views and installing a central view

Using the interferometer to understand particle transport

• Using a gas puffer it is possible to modulate the source term in the continuity equation

• By modulating the gas flow and using the high temporal and spatial resolution of the interferometer it is possible to gain an insight into how particle transport occurs in H-1NF

• Information can be combined with other instruments to gain not only ne but Te Ti and ion velocity

Conclusion

• The H-1NF Scanning interferometer can make spatially and temporally resolved measurements of the plasma with resolutions of up to 1cm and 1.8ms

• As the system is upgraded the data it produces will provide physical insight into the behavior of flexible Heliac plasmas