You are asking a few different questions here. The biggest problem for fusion in general is probably the limited theoretical understanding of plasmas, since simulations are intractable. The biggest practical problem faced by any commercial-scale deuterium-tritium fusion plant is a lack of neutron-resistant materials. Other concepts are built around aneutronic fuels such as pB11, but given the much higher temperatures required, my personal feeling is that this is shooting for Mars when one hasn't even made it to the moon.
As for whether breakeven is even possible (not counting H-bombs), we have to get a little technical. 'Breakeven' is defined as when the input power is equal to output power. However to generate net power, the fusion output power needs to be converted to electricity and fed into the reactor, which means there will be thermodynamic losses. A typical ratio quoted for net power generation, also termed 'ignition', is 1:5. ITER is shooting for 1:10.
The current record holder is the JT-60 tokamak in Japan. It has achieved slightly better than breakeven, with a caveat: these are simulated numbers. You see, for safety and convenience, JT-60 does not use tritium, just deuterium, so the numbers are extrapolated from the D-D case to the D-T case. This does not strike me as a major issue since other tokamaks (JET) have run with tritium, but I am not an expert.
Tokamaks are the furthest along in terms of achieving ignition, but there are still a host of practical issues that need to be solved before the technology can be commercialized.
As for whether breakeven is even possible (not counting H-bombs), we have to get a little technical. 'Breakeven' is defined as when the input power is equal to output power. However to generate net power, the fusion output power needs to be converted to electricity and fed into the reactor, which means there will be thermodynamic losses. A typical ratio quoted for net power generation, also termed 'ignition', is 1:5. ITER is shooting for 1:10.
The current record holder is the JT-60 tokamak in Japan. It has achieved slightly better than breakeven, with a caveat: these are simulated numbers. You see, for safety and convenience, JT-60 does not use tritium, just deuterium, so the numbers are extrapolated from the D-D case to the D-T case. This does not strike me as a major issue since other tokamaks (JET) have run with tritium, but I am not an expert.
Tokamaks are the furthest along in terms of achieving ignition, but there are still a host of practical issues that need to be solved before the technology can be commercialized.
http://en.wikipedia.org/wiki/JT-60 http://en.wikipedia.org/wiki/Lawson_criterion