[annie123 (OP)]

I recently read this in an article about plasma...Before fusion can occur here on Earth, the plasma must be heated to more than 100 million degrees Celsius – about 10 times hotter than the centre of the Sun! But that’s not even the most complicated bit; we managed to reach those temperatures and beyond in the 1990s.

how can this temp. have been achieved on earth without destroying everything around the location? And what fuel was used to do this? And how was it measured?

[chiralSPO]

As far as I understand it, there are four different ways people have gotten temperatures like this:

1) Using a nuclear bomb: Fusion occurs in a hydrogen bomb, but it has to be kicked off by first detonating a plutonium- or uranium-fueled fission bomb--this is not a controlled process and definitely "destroys everything around the location" (that is the point of this approach!)
2) Using a particle accelerator to blast ions at each other at significant fractions of the speed of light: For this, I don't think the actual temperature (before the collision) is that high, but the effective temperature can be higher than any other method listed here. The key is that it only "heats" up a few ions at a time (maybe up to a few thousand ions at once?), which is so negligibly small, that a flea might not notice getting hit by the ions...
3) Oscillating (electro)magnetic fields heating a plasma: Gases are first heated to a few thousand degrees, where they become plasmas, which can then be confined by magnetic fields and heated to temperatures needed for fusion. The most famous instrument for doing this is the Tokomak ( https://en.wikipedia.org/wiki/Tokamak ) Because the plasma is kept in place by magnetic fields, it doesn't actually touch the sides of the instrument, so melting things is not a huge problem (but this power-hungry beast never produced more power than it needed).
4) Lasers! By focusing several VERY powerful lasers all on the same spot for a very brief burst of energy, the very small target can be heated to extreme temperatures. This is the approach used at the National Ignition Facility ( https://en.wikipedia.org/wiki/National_Ignition_Facility )

I don't know how they measure the temperatures.

[evan_au]

How can we reach temperatures sufficient for fusion without melting everything?

In addition to the magnetic fields keeping the plasma away from the walls (as mentioned by ChialSPO):
- They ensure the plasma is very pure (hydrogen+a tiny amount of helium), so that it doesn't emit much visible light
- The walls are often made of a highly reflective material, so visible radiation is reflected back into the plasma
- The walls have internal pipes for cooling water, and (hopefully, some time in the future), drive turbines to produce electricity.
- To make the challenge worse, the magnetic containment fields are produced by superconducting magnets, which must be cooled to almost -200C, in very close proximity to the seething hot bath of plasma.

I don't know how they measure the temperatures (of millions of degrees).

- A plasma emits black-body radiation; for temperatures in the millions of degrees, radiation would occur into the X-Ray part of the spectrum.
- When fusion starts, neutrons are emitted by the plasma. This gives an idea of how close the conditions have approached the onset of fusion.

See: https://en.wikipedia.org/wiki/ITER#Cooling_systems

[Jaaanosik]

These guys know some stuff about plasma:

... and many things mentioned in this thread are questionable. Big time!

1hr 25min ... towards the end of the video, you can see some plasma there.
1hr 26min ... double layers, very interesting.

Anybody interested in plasma should watch the video.
This is one way how plasma can be done, without big budget like Wendelstein 7-X: https://en.wikipedia.org/wiki/Wendelstein_7-X

[Professor Mega-Mind]

There are other ways to heat a very small  ( or not ) target to a very high temperature .  Both mechanical , and sacrificIal , approaches may apply .     P.M.