It feels like the Large Hadron Collider has just barely started doing science (and it's not even at full strength yet), but already plans are underway for its successor: the International Linear Collider, or ILC. The ILC will likely cost between $10 and $20 billion, and it's now looking like the host country will probably be Japan.
The LHC has done a stupendous job of colliding hadrons with each other. Hadrons, of course, are particles made up of other particles: a proton, for example, is made of three quarks (two up quarks and a down quark) held together with gluons. The LHC's circular design makes it possible to accelerate hadrons up to enormous energies and collide them efficiently.
To accelerate lightweight charged particles like electrons and anti-electrons, then, you have to work in straight lines, which means using a linear accelerator instead of a circular one. Unfortunately, the way straight lines work means that your particles just run through your accelerator once instead of a bunch of times, reducing the amount of energy that you can pack into them. In short, a linear accelerator can't get up to nearly the same amount of collision energy that a circular accelerator can: when upgraded in 2014, the LHC will reach collision energies of 14,000 GeV, while the ILC will top out at just 500 GeV, even with an accelerator track that's 31 kilometers long.
Okay, so why the heck would we want to spend $20 billion for a particle accelerator that's just 1/28th the strength of the one we've got right now? The answer is really quite straightforward: because we need it to accelerate electrons, and we want to smash electrons and anti-electrons together because electrons aren't hadrons. In other words, electrons aren't made of other things: they're fundamental particles, without any other crap inside them. Protons, being hadrons, are full of crap (like quarks and gluons), and when you smash them together, a bunch of energy gets gets transferred to said crap and then sorting out that crap (which you don't care about) from the crap that's actually interesting (like new particles) is an always difficult and sometimes impossible task.
So here's what the ILC's job will be: once the LHC has discovered something new (like the Higgs), the ILC will be able to make precision measurements of that new thing, since it'll be able to examine it without anything else getting in the way. The ILC should be able to determine the mass, spin, and interaction strengths of the Higgs, possibly investigate extra dimensions at higher energies, and perhaps even discover supersymmetric particles of dark matter, all of which would be too much of a jumbled, quarky mess to try and do in the LHC.
At the moment, Japan seems to be the likeliest candidate for the ILC simply because most of the U.S. and Europe are flat broke while Japan is willing to cover 50% of the cost itself. The final design has only just been presented, and we're still several years away from a site selection, so figuring in a seven to 10 year build time, the earliest we might see the ILC fire up would be 2025ish or so. That's not at all soon, but it's good to know that even with scientific funding losing priority in the west, the importance of fundamental science is still recognized somewhere in the world.
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