The simplest coherence imaging system is constructed from the combination of these delay and shearing plates as shown in figure 3.12 a). The ray path in figure b) shows how each component affects the polarisation state.
The thickness of the shearing plate is usually chosen according to the desired density of interferometric fringes. The higher the fringe frequency in the image, the greater the spa- tial resolution of the measurement. The Nyquist sampling criterion requires the sampling rate to be equal or greater than twice the signal frequency [81]. In theory, this means a fringe could be sampled over a range of only a few pixels. In practice, high interferometric fringe-frequencies cause the instrumental fringe contrast to deteriorate due a number of ef- fects including the finite optical system resolution, birefringent plate inhomogeneities and averaging over the pixels leading to a consequent loss of dynamic range for temperature measurements. A carrier period of 10 pixels or more per fringe has been found to be an acceptable compromise between spatial resolution and dynamic range for this study.
§3.6 Snapshot coherence imaging 47
φ
polariser polariser filter + lens CCD Light source
waveplate
shearing plate
a)
b)
Figure 3.12: (a) Optical setup for a standard snapshot coherence imaging system. (b) diagram showing ray path through the optical system and depicts the wave polarisation state, the delay between the polarisation components due to the waveplate, and, the spatial separation of the polarisation components due to the shearing optic. This is shown for different ray incident angles.
Chapter 4
System design for MAGPIE
This chapter introduces the experimental setup and design considerations for coherence imaging, spectroscopy and Mach probe measurements on the MAGPIE linear plasma de- vice. The details of the MAGPIE device are presented followed by the optical design considerations for the coherence imaging system and the operational details of the mea- surement. The details of the Mach probe design and calibration are also included here.
4.1
The Magnetised Plasma Interaction Experiment
The MAGnetised Plasma Interaction Experiment (MAGPIE) is a linear helicon plasma chamber commissioned as part of the Australian Plasma Fusion Research Facility at the Australian National University. The device has been constructed to produce similar plasma conditions as observed in the divertor region in fusion devices with the funda- mental purpose of investigating the surface interactions between the high density helicon plasma and target samples under consideration for the divertor wall material. One of the current research studies on the device examine the production of negative ions in hydrogen for the formation of negative ion beams for applications in fusion heating as well as ex- amination into the properties of ammonia plasma as a boundary gas to trap heat entering the divertor region.
The setup of the MAGPIE solenoidal coils allows controlled variability in the axial mag- netic field. This along with the simple linear geometry appeals to the study of fundamen- tal plasma physics including energy transfer between the plasma and helicon wave, and, plasma chemistry and force balance of the equilibrium plasma. The large parameter space (magnetic field, pressure, power variation, geometry and fill gas) as well easy viewing ac- cess makes MAGPIE the ideal candidate for coherence imaging in order to characterise the physics underpinning ion flows and temperatures in helicon sources.
A schematic of MAGPIE is shown in figure 4.1. The plasma is contained within a cylin- drical column, 1.7 m in length and 10 cm in diameter. The column is divided into two sections. The source section consists of a 1 m Pyrex tube and accommodates the heating antenna. The target chamber (aptly named as this is where the sample targets are placed for material studies) is a 0.68 cm stainless steel vessel equipped with interchangeable
50 System design for MAGPIE
Figure 4.1: Diagram of the MAGPIE plasma chamber showing the placement of the coherence imaging system.
widows, blanking plates and entrant ports to allow for flexible diagnostic access.
Base pressures of 10−6Torr are achieved using a two-pump (turbonuclear pump and rotary pump) system located at the exit port of the source chamber. The gas enters via a butterfly valve located at the end plate of the target region. The gas flow is regulated by a flow meter and is generally set to provide pressures within the 1−10 mTorr range. A three- gauge (convectron→baratron→ion gauge) system is required to monitor the pressure in MAGPIE which ranges from atmospheric pressure (7.6×102Torr) to base pressure (∼10−6 Torr). The discharge gases include argon, hydrogen and helium.
The plasma is confined by an axial magnetic field produced by solenoidal coils (∼30 cm diameter) each positioned co-axially with the cylindrical vessel. Each coil contains 13.5 windings and are internally water-cooled. There are 12 coils in total, 5 are positioned at 16 cm intervals along the source region and another 5 are packed together with 1 cm spacing in the target region to produce a magnetically pinched plasma. The final 2 coils are also positioned with 1 cm spacing along the target chamber, with a 5 cm gap between the set of 5 target coils. These coils are usually inactive and have only been employed for studies requiring a magnetic nozzle configuration. The magnetic field is directed away from the target region towards the source (see figure 4.1).
The mirror and source coils are powered independently by separate 1000 A (20 V) variable DC power supplies. The maximum achievable magnetic field is ∼0.19 Tesla in the target region and 0.09 Tesla in the source. The magnetic field along the axis of MAGPIE for maximum field strength configuration is shown by the black solid curve in figure 4.2. In practice, the coils are usually operated with a maximum current of 800 A as higher currents cause the coils to heat too rapidly and limit measurement time to less than 1 minute, before an interlock system shuts off the system to allow for coil cooling. The reference magnetic field configuration for this work was taken as 400 A in the target coils (0.08 Tesla) and 50 A in the source coils (∼0.005 Tesla). The on axis magnetic field field profile for the reference configuration is shown by the red dashed plot in figure 4.2.