NIF Laser sets records for neutron yield

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mheslep

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http://www.physorg.com/news/2010-11-laser-neutron-yield-energy.html

The neutron yield record was set on Sunday, Oct. 31, when the NIF team fired 121 kilojoules of ultraviolet laser light into a glass target filled with deuterium and tritium (DT) gas. The shot produced approximately 3 x 10^14 (300 trillion) neutrons, the highest neutron yield to date by an inertial confinement fusion facility.
DT produces 17 MeV per fusion, so that's an output of about 800 joules; a factor of about 150X short of break even for energy applied to the target, and perhaps ~1000X short of the gain required for a practical fusion power reactor given laser losses and thermal to electric conversion losses. Anyone familiar with the scaling laws NIF expects? Do they expect, for instance, an exponential increase with either laser shot energy or perhaps size of the target?
 

mheslep

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I see now the shot last week was a neutron instrument calibration shot. Still, NIF says it expects MJ outputs this fiscal year on their web site, a long way from 800J.
https://lasers.llnl.gov/about/missions/energy_for_the_future/life/how_life_works.php [Broken]
The National Ignition Campaign began experiments in 2009, and ignition and fusion energy yields of 10 to 15 megajoules (MJ) are anticipated during fiscal years 2010 or 2011. Fusion yields of 20 to 35 MJ are expected soon thereafter. Ultimately fusion yields of 100 MJ are expected on NIF.
Tick-Tock
 
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Mech_Engineer

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They only fired 121 KJ into the D-T target (well short of the required energy to achieve ignition), so it makes sense that the energy yield was low. NIF's laser system is capable of delivering around 15x that much energy when operating at full power (max power is ~1.8MJ IIRC).
 

mheslep

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They only fired 121 KJ into the D-T target (well short of the required energy to achieve ignition), so it makes sense that the neutron yield was low. NIF's laser system is capable of delivering around 15x that much energy when operating at full power (max power is ~1.8MJ IIRC).
Yes I know, but the figure of interest is not absolute energy out but energy gain (loss), which is why I asked about scaling laws. If NIF expected the same 0.6% return (800J/121KJ) for any given laser input energy, then they would obtain an equally useless ~12KJ out from a 1.8MJ laser shot (for power reactor purposes).

But, I recognize my argument implies a completely driven system. This is perhaps no longer the case once 'ignition' is achieved, though I don't know quite how to define that for non-steady state shot-target system like this.
 

Mech_Engineer

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Obviously the return isn't linear, because NIF's goal is to attain fusion ignition, a chain reaction which fuses the pellet (or a large portion of it) into Helium. Apparently it is expected that the current hohlraum design will result in a 20 MJ fusion energy release.

Wikipedia.org said:
... 1.8 MJ is left after conversion to UV, and about half of the remainder is lost in the x-ray conversion in the hohlraum. Of the rest, perhaps 10 to 20% of the resulting X-rays will be absorbed by the outer layers of the target.[18] The shockwave created by this heating absorbs about 140 kJ, which is expected to compress the fuel in the center of the target to a density of about 1,000 g/mL (or 1,000,000 kg/m³);[19]. It is expected this will cause about 20 MJ of fusion energy to be released.[18]
http://en.wikipedia.org/wiki/National_Ignition_Facility#NIF_and_ICF
 
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mheslep

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Obviously the return isn't linear, because NIF's goal is to attain fusion ignition, a chain reaction which fuses the pellet (or a large portion of it) into Helium. Apparently it is expected that the current hohlraum design will result in a 20 MJ fusion energy release.
Well fusion would not be accurately described as a 'chain' reaction as fission is. I follow that ignition would occur when sufficient fusion product energy raises the temperature of the plasma so that fusion continues following the laser and x-ray shot. Yes I understand NIF's goal is a 20-30MJ release as I quoted above.


Ah, here's what I wanted:

A generalized scaling law for the ignition energy of inertial confinement fusion capsules, 2001
M.C. Herrmann, M. Tabak and J.D. Lindl
http://iopscience.iop.org/0029-5515/41/1/308
The minimum energy needed to ignite an inertial confinement fusion capsule is of considerable interest in the optimization of an inertial fusion driver. Recent computational work investigating this minimum energy has found that it depends on the capsule implosion history, in particular, on the capsule drive pressure. This dependence is examined using a series of LASNEX simulations to find ignited capsules which have different values of the implosion velocity, fuel adiabat and drive pressure. It is found that the main effect of varying the drive pressure is to alter the stagnation of the capsule, changing its stagnation adiabat, which, in turn, affects the energy required for ignition. To account for this effect a generalized scaling law has been devised for the ignition energy, Eign proportional to alphaif1.88±0.05v-5.89±0.12P-0.77±0.03. This generalized scaling law agrees with the results of previous work in the appropriate limits.
 

Mech_Engineer

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Well fusion would not be accurately described as a 'chain' reaction as fission is. I follow that ignition would occur when sufficient fusion product energy raises the temperature of the plasma so that fusion continues following the laser and x-ray shot.

The fusion process can be considered a chain reaction given the correct initial conditions (e.g. enough temperature and pressure). While a fission chain reaction involves neutrons from fission events colliding with surrounding fuel causing more fission events, a fusion ignition (chain reaction) event involves released http://en.wikipedia.org/wiki/Alpha_particle" [Broken]. The fusion stage, once activated with a fission device which provides the necessary temperature and pressure, results in a devastating fusion energy chain reaction from a D-T mixture.

Fission : Neutrons :: D-T Fusion : Alpha Particles

Also from the Wikipedia article:
Wikipedia.org said:
The fusion reactions release high-energy particles, some of which (primarily alpha particles) collide with the high density fuel around it and slow down. This heats the fuel further, and can potentially cause that fuel to undergo fusion as well. Given the right overall conditions of the compressed fuel—high enough density and temperature—this heating process can result in a chain reaction, burning outward from the center where the shock wave started the reaction. This is a condition known as "ignition", which can lead to a significant portion of the fuel in the target undergoing fusion, and the release of significant amounts of energy.[6]
http://en.wikipedia.org/wiki/National_Ignition_Facility#Background
 
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Mech_Engineer

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Disambiguation-

Chain Reaction: "A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events."
http://en.wikipedia.org/wiki/Chain_reaction
 

mheslep

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The fusion process can be considered a chain reaction given the correct initial conditions (e.g. enough temperature and pressure). While a fission chain reaction involves neutrons from fission events colliding with surrounding fuel causing more fission events, a fusion ignition (chain reaction) event involves released http://en.wikipedia.org/wiki/Alpha_particle" [Broken] which heat surrounding fuel sustaining fusion burn.
More than alphas. Most of the energy is in the neutrons for DT fusion.
Edit: This similarity can also be seen in http://en.wikipedia.org/wiki/Fission_bomb#Pure_fission_weapons". The fusion stage, once activated with a fission device which provides the necessary temperature and pressure, results in a devastating fusion energy chain reaction from a D-T mixture.

Fission : Neutrons :: D-T Fusion : Alpha Particles
In fission primary, fusion secondary weapons it is the X-rays given off as a consequence of the fission reaction that heat and compress the DT fuel to the point of fusion ignition. Actually the difference between the two (fusion and fission weapons) might be illustrative. One of the reasons, as I understand it, that most modern weapons are fission primary, fusion secondary is that pure fission weapons are very difficult to build large (i.e. in the 100Kt and Mt range) exactly because of the chain reaction process. That is, as the chain reaction progresses stage by stage, another doubling of neutrons, another, another, etc, building energy, the fissionable material increasingly tends to disassociate and loose criticality, stopping the reaction, so that it is difficult to get beyond a certain total energy release regardless of the initial fissionable mass. None of that is true for the fusion part of weapon, which can therefore theoretically built to nearly any size.

In any case yes we both know how inertial fusion works, but if it is to be called a chain reaction then by the same logic chemical combustion must be called a 'chain reaction', with heat transfer after ignition prompting a self-sustaining reaction. As far as I know, chemists don't call combustion a chain reaction, but maybe it is simply a matter of viewing similar phenomenon from different perspectives.
 
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Mech_Engineer

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More than alphas. Most of the energy is in the neutrons for DT fusion.
As I understand it, the point is that the alphas contribute to heating of the fuel and sustaining the fusion burn, while the neutrons pass easily through the fuel and do not contribute to the heating.

In fission primary, fusion secondary weapons it is the X-rays given off as a consequence of the fission reaction that heat and compress the DT fuel to the point of fusion ignition.
Exactly, just as in NIF the UV lasers strike the hohlraum, creating X-rays which in-turn compress the D-T capsule.

In any case yes we both know how inertial fusion works, but if it is to be called a chain reaction then by the same logic chemical combustion must be called a 'chain reaction', with heat transfer after ignition prompting a self-sustaining reaction. As far as I know, chemists don't call combustion a chain reaction, but maybe it is simply a matter of viewing similar phenomenon from different perspectives.
In my opinion it is quite easy to view combustion as a chain-reaction. My point is that a chain-reaction is generally defined as a "positive feedback or self-amplifying chain of events," and can be applied to most any reaction or process which is self-sustaining or self-amplifying. With fission each neutron in can result in 2 neutrons out, in fusion each alpha heats the fuel and the hotter the fuel the more fusion events (alphas) per time.
 

mheslep

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As I understand it, the point is that the alphas contribute to heating of the fuel and sustaining the fusion burn, while the neutrons pass easily through the fuel and do not contribute to the heating.
Yes you must be right about that - certainly in the less dense plasmas in magnetic confinement. Maybe not so much in dense ones from implosion.
 

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