JP said…

“You obviously know what you are talking about regarding Tokamak research. What are your views on Inertial Confinement Fusion driven by lasers?”

I believe that Tokamak fusion could be brought online as a power source feeding the grid easily within ten years and perhaps as few as five if we were willing to make the national commitment (and I do believe that it is in our countries interest to do so).

I do not believe laser driven inertial confinement fusion is likely to be a viable method of generating electricity from nuclear fusion within the next three decades, however, I can’t rule out the unexpected breakthrough.

Here are the reasons for my belief. Plasma physics as they apply to Tokamak fusion reactors are sufficiently well understood that building a reactor that will confine a plasma at the necessary temperature, density, and for the necessary time is entirely doable today.

Until recently there were two large questions that needed to be answered. First, can superconductive magnets be manufactured that could create the necessary magnetic fields for long term plasma confinement?

The second question, what materials hold up to constant neutron and ion bombardment? In addition to enduring the ion and neutron bombardment, it is also desirable to utilize materials with low neutron activation potential to minimize radioactive waste from neutron activated materials when the reactor is retired.

Unlike a fission reactor in which neutron embrittlement of the reactor vessel will eventually necessitate retirement, it isn’t clear that this will ever be the case for a fusion reactor. The penalty for a failed reactor vessel is far less serious in the case of a fusion reactor owing to the fact that it’s not holding tons of radioactive fission products and actinides.

The first question, whether or not it is possible to create a strong enough field for containment using superconductive magnets was answered by the Chinese EAST reactor when it saw first plasma in September of 2006. The answer was an unequivocal “Yes” because the superconductive coils at EAST functioned as designed providing plasma confinement.

Existing test reactors, prior to China’s EAST reactor, used copper coils for confinement magnets. The resistance of the copper winding caused them to rapidly heat up and limited operations to no more than 60 seconds. In addition copper coils waste a huge amount of power. Commercial reactors will use superconductive magnets to allow for sustained operation and to avoid wasting power.

The second question won’t be answered until we build a commercial power level reactor. EAST will be able to do some testing but at a power level of approximately 16 megawatts where commercial reactors will operate at power levels of 500 megawatts or more. One can try to extrapolate but when you’re dealing with a difference in power levels of 30x you really need to be able to test at the higher power levels.

Contrast this to the situation with laser driven inertial confinement fusion. In my opinion, the state of laser driven inertial confinement fusion is approximately where Tokamak fusion was in 1970. There are several major problems relating to laser driven inertial confinement fusion.

The first problem is that because this method relies on the implosion of a fuel pellet, the fuel pellet must be extremely symmetrical, to within a few microns, and the illumination by lasers must be extremely uniform. In practice the necessary uniformity of illumination has not been achievable.

The second problem is that the delivery of energy to the fuel pellet is extremely inefficient. The lasers themselves have proven to be inefficient. The wavelengths that can be generated by lasers at the necessary power levels are not efficient for heating the target so some method of increasing the frequency (decreasing the wavelength) is necessary. Presently, this is achieved through the use of optical frequency tripplers. The end result is that around 1.5% of the electricity used to fire the lasers ends up as useful laser output. Terrawatts of power are used to implode a fuel pellet that would provide the equivalent of a barrel of oil’s energy if all the fuel were fused. The energy economics just aren’t there without massive improvements.

Shortening the laser pulse increases the overall efficiency by concentrating the power during the time it is required for rapid heating and compression of the fuel pellet. But electrical engineering realities make this problematic. Wire has inductance, inductance limits the rate that current can rise. The operation of a laser also requires the presence of an optical resonant cavity; but a resonant cavity limits the sidebands necessary for a rapid pulse rise and fall. A lower Q cavity that would allow the necessary sideband frequencies results in less efficient laser operations. The same problem also exists in the optical frequency trippling device which involves an optically non-linear material and optical resonances.

The output is not consistent from one firing to the next, nor from beam to beam, and this creates issues with the even illumination of the target.

In inertial confinement fusion, the reaction area is extremely small, consequently the heat-load in that small area is tremendous. Tokamak’s represent a much more diffuse energy production source making heat removal less problematic (though still a tricky engineering question). Given that the problem of efficient heat removal is still present in Tokamak’s, the fact that it is many orders of magnitude greater in inertial confinement leads me to believe that it may be an problem for which no solution exists.

Then there is the problem of fuel. The most promising fuel for both types of reactor initially is a mixture of tritium and deuterium because this particular mixture has the lowest energy requirement to fuse. Deuterium is not a problem, one out of two thousand hydrogen atoms in seawater is a deuterium atom. There is enough deuterium to provide for our energy needs for 15 billion years at present energy consumption levels. Granted, those levels will increase and so perhaps it will only last a billion years. But by then I’m sure we can tap extraterrestrial sources of deuterium or perhaps be able to create the necessary conditions for fusion of ordinary hydrogen which is 2000x more plentiful.

Tritium does not occur in nature in substantial quantities. A very small amount is created by the bombardment of atoms in the upper atmosphere by cosmic rays.

In a Tokamak, the reactor vessel will be lined with a lithium blanket which will absorb neutrons and breed tritium in the process, thus solving two problems at once, that of preventing neutron activation and embrittlement of the reactor vessel and producing the tritium necessary for the reactors operation. Tritium also is created by neutrons bombarding deuterium present in the cooling water of fission reactors. There are engineering obstacles to using a similar lithium blanket in an inertial confinement reactor because of the much higher energy densities and heat loads.

In a Tokamak, a burning plasma, that is a plasma in which energy created by fusion reactions is sufficient to sustain the reaction indefinitely directly, is the desirable state, but it’s not absolutely necessary. As long as the output energy from fusion is sufficiently greater than the energy input required to cause it to take place after all the losses are considered, it can be commercially viable even without a burning plasma.

By contrast, a laser driven inertial confinement reaction will absolutely need to achieve a burning plasma condition to get a significant fuel burn because it is just not possible to concentrate enough laser power in a short enough time frame at a high enough efficiency to have a prayer of reaching scientific breakeven much less commercial breakeven otherwise.

In my view inertial confinement fusion is and will only be useful as an inefficient neutron source, and for research for the development of nuclear fusion weaponry. In short, a toy of the war mongers and not a viable peaceful energy source.

On the other hand, Tokamak fusion I believe is very much ready to be brought online. As I mentioned previously, the issue of whether or not superconductive magnets of sufficient strength could be manufactured was answered by China’s EAST reactor.

A second big question was could a diverter, a device that skims helium waste and removes it, withstand the bombardment by ions and neutrons. The primary issue was the heat load that would be generated by ion bombardment which would largely be due to what are known as localized edge mode instabilities. Recently a new technique has been developed that addresses the localized edge mode instabilities.

So with these two major problems largely addressed, there really aren’t any other known major hurdles. There may be some that we won’t know about until we have a plasma operating for hours or days but those will only be discovered with the building and operation of a commercial power level reactor. However, I feel optimistic that the way is now relatively clear.

With all of this said; there are a number of new schemes for producing fusion power that may ultimately prove to be far more economical and flexible than Tokamak reactors. Tokamak’s are by necessity physically large. They are too large and too heavy to find applications in transportation. Perhaps eventually they might be small enough to power say an aircraft carrier, cruise liner, or merchant cargo ship, but not airlines, trucks, automobiles, or even trains.

Tokamaks also probably will never be able to operate with aneutronic fuels. This is because reaching the necessary energies for fusion to occur thermally results in too large of a loss for temperatures higher than what are necessary for D-T fusion.

Other new configurations may allow for a reactor sufficiently compact and lightweight to find applications in transportation. I don’t know if they’ll ever fit in a Delorean but an airliner or train is a possibility.

I believe one of the most promising new designs is the Bussard inertial electrostatic confinement fusion reactor. This reactor uses magnetic fields to confine electrons only, thus creating an electrostatic well. The electrostatic well then confines and accelerates the ions. The beauty of this design is that electrons being much lighter than nuclei can be confined by relatively whimpy magnetic fields. The electrostatic well created then confines and accelerates the atomic nuclei which then collide and fuse.

This unit causes all of the nuclei to have the same energy at the center of the electrostatic well so that it can be tuned to the energy spectrum required for fusion and then all of the nuclei will be within that range. This is much different than in a Tokamak where you can only achieve an average energy but many nuclei will have either too much or not enough energy to fuse and both of these represented wasted energy.

The Bussard design has the potential to achieve fusion with aneutronic fuels eliminating the need for massive shielding while simultaneously allowing electricity to be directly tapped rather than thermally generated. It also has the potential to be made small enough for large trucks, buses, airliners, etc. Unfortunately, it’s development is currently not being funded and I find this to be almost criminal.

Another interesting design is the levitated dipole. This device also has the potential for achieving the necessary energies for aneutronic fuels to be fused, however, I do not feel it has to potential to be compact enough for most transportation applications. It’s really too early to know how much potential it has. The Bussard design by contrast has already been through six generations of test reactors, each one showing substantial gains over the previous.

An alternative to laser driven inertial confinement fusion is particle beam driven inertial confinement fusion. The particles in questions most frequently being heavy ions. The advantage of this over lasers is that a larger percentage of the input energy can be delivered to the target, however, ions being charged particles, have a natural tendency to diverge and not remain in a focused beam and that is a problem.

The Farnsworth fusor uses two concentric grids to accelerate deuterium ions towards the center of the device where a portion of them collide and fuse. This readily produces fusion but, not at sufficient power levels to be of commercial interest. The limitation on power level is the grids which are heated by ions colliding with them. The Bussard device gets around this problem by creating a virtual grid by steering electrons with a magnetic field.

There are numerous other methods of creating fusion at low power levels. Injecting deuterium gas into a nickel cylinder under high pressure results in some “cold fusion” where ions are absorbed into the nickel and fuse. The fusion rate is extremely low, too low for power generation and only marginal as a neutron source.

There is a method of creating some fusion using pyroelectric crystals. Methods using collapsing bubbles in acetone created with ultrasound. All of these do not generate sufficient fusion to generate power. They are useful as a neutron source and to study the physics involved but little else.

Right now, I believe that Tokamak fusion is sufficiently mature to be brought online in the near term. I do not believe any other fusion technology is sufficiently mature to be brought online as soon, but ultimately some of the other methods may prove superior when sufficently developed.