Question

Why is the controlled fusion so difficult to achieve?(I'm not asking this because I think it's easy, I just need straightforward answers)

Answer 1

At ordinary pressures, the temperature needs to be something like 50 million kelvins for the speed of the colliding protons to be sufficient to overcome their mutual electric repulsion, and get them close enough for the nuclear force to take over and fuse them.

Constructing a reactor that can (1) heat a plasma to 50 million degrees and (2) not melt itself in the process is quite technologically difficult.

The Sun doesn't need quite these kinds of temperatures because at its core the pressure is very high, billions of atmospheres, sufficient to compress the fusing hydrogen to densities of about 160 g/cm^3. The extremely rapid collision rate at those temperatures compensates for the low probability of success on each collision.

There are two approaches to fusion: in one, efforts are made to hold a plasma steady at a high enough temperature to get fusion steadily going. This has been going on for about 50 years, with very limited success. It's just very very hard to control a 50 million degree gas.

The other is so-called "inertial" confinement, in which you blast a pellet of hydrogen with extremely high-powered lasers, which compress the pellet to very high densities, so that, like the case of the Sun, more modest temperatures will get fusion going. No attempt is made to keep the fusion reaction going steadily. Essentially you have a string of exceedingly tiny H-bombs going off many times per second. The major problem with this approach is that lasers are incredibly inefficient at turning energy into light, so you need staggering amounts of energy to get this going.

Fission is easy because it's spontaneous. Get some U-235 or Pu-239 together, and it starts to fission. In fact, it does it all the time, which is why it's radioactive. There are even cases where natural accumulations of uranium have led to natural nuclear reactors in the Earth have run a few million years, until they depleted the fissionable isotopes of uranium.

Answer 2

s/temperatures/pressures/ in the last sentence of the 3rd paragraph above.

Answer 3

Okay thanks. Two more questions. I know that plasma is a state of matter that has reached very high temperatures, but can you explain what it is more specifically?
Also, when gamma radiation is emitted from atoms, is that just energy from the atom or is it mass turning into energy and then being emitted?

Answer 4

A plasma is just a gas of electrically charged particles. So, for example, if you heat hydrogen gas hot enough, first the H2 molecules will break into separate H atoms, and then the H atoms will break into H+ cations (protons) and free electrons. That last is a plasma. You get one whenever the amount of energy in the motion of the particles is higher on average than the energy needed to ionize atoms. Pretty hot, in other words.

It might interest you to now that you can condense a plasma into a solid without reforming separate molecules first. This is what a metal is. Hydrogen normally forms molecules first, then solid hydrogen, but under extremely high pressure you can form hydrogen metal.

There is no difference. Mass is the same thing as energy. Weighing something is exactly equivalent to measuring its energy, and vice versa. However, by far the biggest chunk of energy any system has is called its "rest mass" or "rest energy." In everyday experience, the energy of things rarely budges even a tiny sliver above the rest energy, or mass. So when we weigh things, we always get essentially the same answer: the rest mass. Our scales are not sensitive enough to pick up the tiny changes in mass when things emit photons, slow down, or otherwise change their energy. We can calculate the change in energy only by observing the motion and using Newton's laws (or more accurate kinematic equations from relativity).

In a few cases the change in energy is really big, and you can actually measure the change by just weighing things before and after. That might be the case when a single atom emits a gamma ray, because a gamma ray has a hellacious amount of energy in it.

Answer 5

Please note also that mass is NOT the same thing as matter, although we speak sloppily that way sometimes. A chunk of matter is not a mass, it HAS a mass, just like it has an energy. Both mass and energy are properties of matter. I hope you were not thinking "matter turns into energy." That's a thing from Star Trek, and it actually doesn't even make logical sense, because energy is a property, while matter is stuff. It would be like saying a cat turns into furriness, or an ice cream cone turns into sweetness.

So when we say mass is energy, we mean that there is one property of matter,
which we can call its mass or its energy, whichever is convenient.

Answer 6

What does it mean to ionize something?
And about matter not turning into energy, isn't that what E=mc^2 proved, that energy is matter times a constant squared. And isn't that what happens during nuclear fusion. Where some of the matter in the atoms turns into energy?

Answer 7

To ionize something is to make ions out of the atoms of which it's made. You make an ion out of an atom by removing one or more electrons, or by attaching another electron. The first leaves it with a positive charge, the second with a negative.

No, the "m" in Einstein's equation stands for MASS not matter, and, as I said, mass is a property of matter, not matter itself, just like the blue of your blue car is a property of the car, not the car itself. You can change the color of your car, and an object can change its mass. Changing its mass is exactly the same thing as changing its energy, because the two are one and the same property.

No, matter does not turn to energy in fusion. What happens is that two protons (say), widely separated, have a much higher potential energy than they do when they are tightly bound together by the strong nuclear force, in a helium nucleus. You might think of the strong force as a rubber band around the two protons. When they are close, it is slack. When they are far apart, it is stretched tight. So they have much higher energy.

When you allow the two protons to fall together and become a helium nucleus, their potential energy is tremendously reduced. (A great deal of it escapes as gamma rays, while some stays as kinetic energy.) The total energy is reduced by so much that it can actually be measured as a change in their mass -- a reduction.

But the same thing happens if principle if you allow an ordinary rubber band that is stretched to relax. It will weigh slightly more when it is stretched than when it is relaxed, because it has more potential energy. Of course, for a rubber band, the force is very very small, compared to the strong nuclear force, so the change in mass is incredibly difficult to measure. (You can easily calculate it using Einstein's formula, if you like: look up the energy in a stretched rubber band, dividie by c^2, convert to units of mass. That division by c^2 will make the result incredibly tiny.)

It's just the fact that we're so used to very small changes in energy, that our scales cannot detect, that makes us think mass is constant. But in fact every time an object changes its energy, its mass, the same thing, changes, too. It's just that in fusion the energy changes are so large we *can* measured them by weighing.

Answer 8

Okay thanks