tokamak versus fusor efficiencies.

[Retric]

I would say 95% or more, having a few grams of plasma cool down inside the system has little impact of the magnetic field. The magnetic field is basically just electricity so recovering that energy has more to do with storing electricity and how efficiently you want to do that vs any complex issue. The only problem is to restart the system you can't have start over and rebuild the magnetic field so the best solution would be a high efficiency Flywheel which they are probably already using to reduce the grid demand on start up.

PS: When you start talking about losses on that level you need to have a lot of heat show up somewhere and dumping that much heat as soon as the system fails would melt stuff.

[Chris Bradley]

Wilfried Heil wrote:
> LHD produces 2.4x10^17 neutrons from D+D fusion and 4.3x10^15 neutrons from D+T fusion per 10s shot. The fusion fuel used is deuterium only, the DT neutrons come from the fusion of some of the tritium which is produced during the fusion run, hence the lower number.
>
> They normally use a mix of light hydrogen and deuterium, in order to keep the fusion rates low.

Wilfred,

When you posted this, my immediate reaction was to, simply, ask them the question. So this is 'FYI' wrt the discussion in hand:



>>>>>>>>>>

----- Original Message -----
From: "広報室" <nifs-kouhou@nifs.ac.jp>
To: <chris_macdonaldbradley@yahoo.co.uk>
Sent: Friday, January 09, 2009 12:32 AM
Subject: National Institute for Fusion Science


> Dear Chris Macdonald-Bradley,
>
> Hello. This is the Public Relations Office in the National Institute for
> Fusion Science.
> First of all, we thank you for your inquiry the other day.
> Attached is our answer to your question. We hope that they will be of your
> help.
>
> ___________________________________________________________________________
> Thank you for having interest in our experiment.
> Your question is ‘what is the highest number of neutrons ever emitted by
> LHD during an actual experimental pulse?’. Answer is that we have never
> emitted any neutron by LHD during actual experiments. The reason is that we
> have never used deuterium gas in LHD experiment. We only used hydrogen,
> helium, neon and argon gases in LHD from the start of LHD project. Paper
> which you found is described about the activation by neutron in future
> experiment in LHD. Now we are planning the experiments using deuterium gas
> in LHD to get more powerful plasma. In these future experiments, several
> times 10 to the 17th neutrons per pulse are expected.
> If you have further questions, please ask us without hesitation.
>
> Yours sincerely.
> National Institute for Fusion Science

>>>>>>>>>>>>


So the answer is that LHD cannot yet appear on Steven's table. It is, as yet, not a fusion experiment but a plasma containment experiment.

best regards,

Chris MB.

[Dan Tibbets]

I just read this thread. A couple of items.

I believe the record for the highest Fusor output was by by Robert Hirsch back in the late 1960's. Uing DT fuel he got ~ 1 trillion neutrons per second.
http://www.brian-mcdermott.com/fusionproof.htm

Do I understand the comments correctly. Once Tokamacks' reach ignition, the reaction becomes runaway- power increasing untill the magnets cannot contain the plasma, and the machine shuts down ( ie no longterm steady state possible, only runs limited by the reserve capacity of the magnets)? The power out cannot be throttable by fuel feed, magnet 'detuneing', etc?


Dan Tibbets

[Chris Bradley]

The discussion above didn't quite explain the actual issues/fields in a tokamak.

There are two magnetic fields generated.

Firstly, a poloidal field generated by the current being induced into the plasma. The plasma forms the secondary of a huge transformer. The core of that transformer is initially saturated one way. Then the power is reversed and it swings all the way to saturation in the opposite magnetic direction. During that ramp up of magnetic field, the plasma current is driven in one direction. This can, clearly, only last for a finite time - the time it takes the core to reach opposite saturation. If it were then reversed again (as it would in an AC transformer) the plasma current would also reverse and the whole thing would disintegrate in a puff of instability. So tokamak pulses to date are only as long as the induction pulse is ramping up the core to saturation in one direction.

The hope is that once the plasma initiates various thermal transportation within the torus so it will self-drive this current, so this saturation pluse will be just a 'starter motor'. Unfortunately, this starting action is as far as tokamaks have got to, the rest is still mostly theory.

This really should be kept in mind. There has been 50 years of tokamak development but it's like having spent 50 years building a car whose engine you can crank over but it doesn't actually start!!

Secondly there is a toroidal field. This is by a big solenoid all the way around the torus (which will be the bit which is super-conducting in ITER and is the case in a few other machines). This field 'confines' the ions to magnetic surfaces that are nested within the shape of the outer.

There are additional poloidal coils in most machines aswell, but these are mostly just to 'tune up' the poloidal fields that the inductive pulse generates and shape the plasma's cross-section to that desired.

The first induction field takes up a few MW to drive it (and the inductive 'pulse' lasts for 10 seconds or so). The second, the generation and creation of the toroidal field, takes 1GJ in JET and more in others. It is this toroidal field energy I have been toying with as something which really hammers the idea that these machines have got anywhere near over-unity efficiency before, though some folks would like you to believe that they have (as they don't count the energy put into these fields).

Once the thing runs continuously, sure, this is just a 'start up' investment of energy, but my thread here is trying to establish and compare current, today's, real genuine fusion machines' performances, warts'n'all, not just speculations on the future. (like - "I've built a really fast car, but it only cranks over at the moment" - this doesn't add up to an engineering reality to me and I can legitimately ask 'what is the fastest car today' and the answer cannot be a car due to be built or finished off tomorrow.)

[Wilfried Heil]

Chris -
the fusion data are _future_ operating parameters, from an engineering white paper which estimates the probable activation from those future runs.

Dan -
a Tokamak's plasma confinement time is limited, because the poloidal field (from axial coils in parallel with the plasma) can't be ramped up indefinitely. The resulting helical field stabilizes the plasma, but is induced only as long as the magnetic field changes. This is independent of whether the plasma shows fusion or not.

It may be possible to sustain this field externally in an inefficient way by "helicity injection" or by making use of thermal currents which would then draw a lot of power from the plasma.

So the Tokamak is a neat and simple design for short plasma experiments of a few seconds duration each, but may not be the best choice to begin with if continuous operation is desired.

[Richard Hull]

Chris Came up with a brilliant analogy that I will latch onto now as it is so majestic and yet simple in concept. This relates to the engine analogy of the last 50 years of fusion research. (it is actually 60 years if you include Project Sherwood, the rather secretive study of fusion reactor systems starting in the late 40's)

I would like to note that the first engines were one lungers (single cylinder jobs). All were different in concept as no one claimed to know how to make an engine. Stellarators, accelerators, mirror machines, torus types, etc.

When the one lungers wouldn't start, a tiny bit of money flooded in and they figured two cylinder machines might have a better chance of starting. They never did, of course.

By the late 60's, it was apparent that only hot fusion would work in a 4 or 6 cylinder multimillion dollar fusion motor. Still no go.

By the early 70's it was settled that only a tokamak or its derivative would work in a 8 or 10 cylinder fusion motor. At least 4 different 10 cylinder, near billion dollar each 10-15 year motor construction efforts dawdled into the late 90's. Some 2-3 second attempted starts turned over, but tended to blow the "mains" or "drop a valve" during the brief, abortive turnovers. Turbo's were added in the form of massive magnetic systems and, currently, the ultimate multi-billion dollar inverted V-16 turbo charged motor is now on the dynamometer as ITER and almost ready to attmept to be started, It has the best valves and roller bearing "mains". We are forewarned, however, that if this thing starts and runs for a decent test run, that "this is not and will not be a production engine." It is sort of a "motorama", auto show, eye-popper. We will have to wait many years for a real production motor, so get used to it. AND this assumes this V-16 starts and runs with no real issues like all the other motors that never started or started but crapped out just as, supposedly, useful RPM and horsepower developed shortly after crank over.

Fusion, it appears, never had a tin lizzie, ( model T ford), that actually did what a tin lizzie could do.... Actually supply motive power, transport goods,etc., albeit rather poorly. Fusion never even had a model A ford. Instead, Fusion appears to need to be born big, really big!

To many folks way of thinking, if you can't make a little tin lizzie, you should not be trying to make a Ferrari. It just seems reasonable.

I really hope I live to have to eat my words, but I fear I will not make it to fusion power on the mains. It will probably reside where it has always resided; in the minds.

Richard Hull