If you wish to replicate the Sydney experiment, you must first obtain a Langmuir Probe.
I have requested quotes for these commercial Langmuir Probes:
Automated Langmuir Probe (ALP) System
BTW, how do you pronounce Langmuir? I imagine it rhymes with “pang-doer” or maybe “ma-doer”. ANSWER: it rhymes with lang-meer.
Prometheus Fusion Perfection 
At the end of the day, a Langmuir probe is just an electrode which can withstand the temperature of the plasma, off which you measure the voltage.
You might want to think twice about pay $$$ for something that you can make yourself, unless there is specific precision you’re after.