Why aren't fusion reactors using centrifuges? If fusing of nuclei in stars requires pressure, heat, and gravity, why not use a centrifuge to increase the gravity? We currently have the technology to make diamonds, and generate immense temperatures. Thus surely if you increase the effective gravity, that would reduce the required temperatures! After all, as I understand it, the temperatures only need to be higher than a star, due to the lack of pressure - gravity!
 A: The pressures required to force fusion to occur are so great that there simply is no material existing that is strong enough to apply such forces to a sample of matter to be fused.
Regarding the use of a centrifuge to apply compression to a fusion sample, the same comments apply: there is no material strong enough to make such a centrifuge out of.
In any case, note also that the pressure needed to trigger fusion has to be applied to the sample in all directions, something that centrifuges cannot do.
A: To add to @NielsNelsen's answer: it's not just that no solid material can withstand the needed pressure. The heat and heat capacity of the plasma are also problems. The plasma is high pressure primarily because of the high temperature, the density of the plasma is actually quite low (remember: $P = k \,(\mathrm{number\ density})\, T$). Because of this, if you allow the plasma to come in contact with the walls it would very quickly cool - low density means low heat capacity.
That's why just about every fusion reactor idea relies on either inertia or magnetic fields to confine the reaction. In inertial confinement, you use lasers (National Ignition Facility), explosions (think fusion bombs), or something similar to cause the temperature and pressure to suddenly reach insane levels faster than the hydrogen can fly apart. The reaction then proceeds until the density drops.
Magnetic field confinement relies on inventive and careful structuring of dynamic magnetic fields to hold the plasma away from the reactor walls. These are the tokamaks, stellerators, etc.
A: 
Thus surely if you increase the effective gravity, that would reduce the required temperatures!

It really doesn't.  Temperature is effectively a measure of particles speed and KE (across a distribution).  That tells you what fraction of them have sufficient energy to overcome the coulomb barrier.
Increased pressure or density might affect the rate of fusion, but it doesn't let you lower the temperature.

the temperatures only need to be higher than a star, due to the lack of pressure - gravity!

No, you need temperatures very similar to that of a star.  There are a couple of things that will modify that need:

*

*Materials.  D-T or even D-D fusion can happen at temperatures slightly cooler than necessary for proton fusion.

*Efficiency.  Fusion happens at a relatively slow pace in the sun.  If you want the energy to come out faster, then a higher temperature greatly increases the rate.

A: Simply because gravity is attractive, while the centrifugal force is repulsive. You need a force from all sides in order to do the fusion process.
A: The answers given elsewhere don't really answer the question! Bear in mind, fusion CAN currently be achieved, it merely takes more energy than is produced! Fusion occurs when temperature, pressure are sufficient to fuse 2 or more atoms together. Current fusion reactor technology generates temperatures way beyond those even at the core of the sun, and the force of gravity on the sun is 1,280G! We can generate forces way beyond 1,280g (Car crash sensors can easily be rated at 3000G - I've repaired them!). Thus claiming we cannot generate solar temperatures clearly is wrong! So please, if you ACTUALLY know why increasing the pressure / gravity won't reduce the temperature required on earth compared to current temps, please answer!
