what are some of the practical problems encountered in modern fusion attempts?

bill nye scienceguy!

just curious.

Astronuc

There are two methods proposed for fusion - inertial confinement and magnetic confinement.

In inertial confinement, small pellets of frozen (DT) are dropped into a chamber and then blasted with lasers or electron beams. Lasers are preferred since they avoid using charges which are deflected by magnetic and electric fields. The main problems are the cryogenic storage, large power systems for lasers (or e-beams), reliably hitting the target, and then conversion of the thermonuclear energy into useful power.

In magnetic confinement, DT (or DD) gas is heated to plasma state (~50-100 keV, which is 580 million to 1.16 billion K), while confining the plasma in a magnetic (usually toroidal) field. The density is quite low (~10¹³-10¹⁴ particles/cc) because of the pressure limitations, which are a consequence of the limitations on maximum intensity of the magnetic field. Energy injection and energy losses are key problems.

Even so, there are significant materials issues related to the high energy neutrons from most fusion reactions. For example, the D+T reaction yields a 14.1 MeV neutron, while only 3.5 MeV goes to the alpha particle. The high energy neutrons interact with the surrounding structure (vacuum confinement system) embrittling and activating it over time. Energy recovery from the neutrons require that they are slowed and captured.

Aneutronic fusion reactions are possible, e.g. D + He³. However, even here, there is still the possibility of D+D, which produces neutrons in about half the reactions. One solution would be to have cool D plasma with injection of 'hot' He³ neutrals. However, He³ is not very abundant, and it is more difficult to accelerate He³ ions than D ions.

RaymondKennethPetry

The main problem is energy production as a factor of density and rate.

If positive-production were all that's needed you might do with a pair of D+ accelerators aimed through magnet-tubes almost-at each other inside a voltage-recovery sphere: Most D-D interactions would miss but as they are identical mass they'd exit to the sphere and return their eV, 100%.

But try to get a lot of density into that and even µA D-current is 10@13 D+/sec. departing from the accelerators about 1% c, spaced about 30µm, for density factor times a few watts.

Better might be to blast lithium bars with protons at 3MeV ... but gas recovery takes a lot of contraption energy.

Ray.

Morbius

Astronuc,

Beyond that - we haven't had a laser that was powerful enough to "ignite" a fusion
capsule.

The DT fusion reaction is:

D + T --> He4 + n + 17.6 MeV

The neutron "n" gets 14.1 MeV of the 17.6 MeV total, and the He4 gets 3.5 MeV.

In order to get "ignition" to keep the reaction going - one needs to trap one of the
byproducts to get the energy. One can forget about trapping the 14.1 Mev neutron
in a small fusion capsule - the mean free path of the high energy neutron is just too
long. One needs to trap the 3.5 MeV alpha [ He4 ].

However, up to now - the fusion pellets have been too small to even trap the alpha.
Larger pellets were impossible, because the lasers weren't powerful enough to
implode a large pellet.

That should change with NIF - the National Ignition Facility being built at
Lawrence Livermore National Laboratory. NIF will be able to put about a MegaJoule
of laser energy on the fusion target:

http://www.llnl.gov/nif/

The NIF building is complete and LLNL is in the process of bringing the 192 laser
chains online. Then we'll see.

Dr. Gregory Greenman
Physicist

vanesch

It was my (naive?) understanding that inertial confinement was not really a technique to commercially generate energy, but rather to study thermonuclear reactions and extreme matter states ? In other words, make tiny hydrogen bombs and study them in the lab. Is there a serious attempt, through this channel, to progress towards commercial power generation (in the style of ITER and its possible successor) ?

My question is an ignorant repetition of what my former professor of plasma physics told the class, now about 20 years ago.

Morbius

vanesch,

For decades now, Lawrence Livermore has studied how to make a commercial power
plant out of ICF. They have several conceptual plans that go by the name "HYLIFE":

http://www.llnl.gov/nif/library/ife.pdf

The first priority is, of course, to get fusion "ignition" - but the details of how to
take that breakthrough on to electric power production has not been ignored.

Dr. Gregory Greenman
Physicist

Astronuc

As Morbius indicated, ICF is considered as a possibility for producing electrical power. However, like magnetic confinement, there are some high technical hurdles to overcome.

IMO, unless a useful amount of eletrical energy can be produced from fusion, it's not worth the huge effort. There would be better investments of time and effort and money (capital).

Geoff St. Germaine

One of the major problems in magnetic confinement devices is the actual confinement of energy and particles. The energy confinement time is typically observed to be much less than that required to be able to have a fusion power gain greater than unity. This confinement time is much lower than what is predicted by classical theory and therefore there is some "anomalous transport" of particles and energy out of the hot plasma. The theory is that this anomalous transport is driven by various turbulence and instability modes in the plasma.

Operation schemes with improved confinement have been found (called H-modes, for high confinement modes) and the operating scheme for ITER is an H-mode. These H-modes are normally observed by substantially increasing the heating power to the plasma, commonly with neutral beam injection. My research field right now is related with Ohmic H-modes, which are H-modes produced on machines that (typically) are too small to have neutral beam heating and are induced using different schemes.

There are many other problems as well, including fuel breeding and energy extraction.