Slashdot Mirror
Table Top Fusion Courtesy of Tiny Bubbles
Erik Baard writes: "The peer-reviewed journal Science is carrying a cover story about the possibility of table top fusion. Not cold fusion, mind you, but the apparatus might look that way to some. Oak Ridge and other labs say they have gotten the fingerprints of fusion (neutron production) from collapsing bubbles in liquid, a process that heats a local area to temperatures as hot as the surface of the sun, and releases photons.
The disputes are already here -- notably from Dr. Robert Park of the American Physical Society and from critical reviewers who say they haven't repeated the neutron production. But the authors say the critics didn't calibrate their equipment correctly. Articles regarding the discovery can be found on
Eureka Alert " CD: Looks legit, but Pons and Fleishman (and the University of Utah for that matter) talked a good game. I suppose I'll belive in tabletop fusion when a generator comes atached to my next laptop. The author of this post also has a longer article up at the Village Voice
Isn't this story about 28 days premature?
In this house we obey the laws of thermodynamics!
Ah...so that explains why soda pop explodes when placed next to my subwoofer. Now, I wonder which brand of soda will produce the highest nuclear yield. Talk about energy drinks...
RD
THE CRACKPOT INDEX by John Baez
A simple method for rating potentially revolutionary contributions to physics.
-5 point starting credit.
1 point for every statement that is widely agreed on to be false.
2 points for every statement that is clearly vacuous.
3 points for every statement that is logically inconsistent.
5 points for each such statement that is adhered to despite careful correction.
5 points for using a thought experiment that contradicts the results of a widely accepted real experiment.
5 points for each word in all capital letters (except for those with defective keyboards).
5 points for each mention of "Einstien", "Hawkins" or "Feynmann".
10 points for each claim that quantum mechanics is fundamentally misguided (without good evidence).
10 points for pointing out that you have gone to school, as if this were evidence of sanity.
10 points for beginning the description of your theory by saying how long you have been working on it.
10 points for mailing your theory to someone you don't know personally and asking them not to tell anyone else about it, for fear that your ideas will be stolen.
10 points for offering prize money to anyone who proves and/or finds any flaws in your theory.
10 points for each statement along the lines of "I'm not good at math, but my theory is conceptually right, so all I need is for someone to express it in terms of equations".
10 points for arguing that a current well-established theory is "only a theory", as if this were somehow a point against it.
10 points for arguing that while a current well-established theory predicts phenomena correctly, it doesn't explain "why" they occur, or fails to provide a "mechanism".
10 points for each favorable comparison of yourself to Einstein, or claim that special or general relativity are fundamentally misguided (without good evidence).
10 points for claiming that your work is on the cutting edge of a "paradigm shift".
20 points for suggesting that you deserve a Nobel prize.
20 points for each favorable comparison of yourself to Newton or claim that classical mechanics is fundamentally misguided (without good evidence).
20 points for every use of science fiction works or myths as if they were fact.
20 points for defending yourself by bringing up (real or imagined) ridicule accorded to your past theories.
20 points for each use of the phrase "hidebound reactionary".
20 points for each use of the phrase "self-appointed defender of the orthodoxy".
30 points for suggesting that a famous figure secretly disbelieved in a theory which he or she publicly supported. (E.g., that Feynman was a closet opponent of special relativity, as deduced by reading between the lines in his freshman physics textbooks.)
30 points for suggesting that Einstein, in his later years, was groping his way towards the ideas you now advocate.
30 points for claiming that your theories were developed by an extraterrestrial civilization (without good evidence).
40 points for comparing those who argue against your ideas to Nazis, stormtroopers, or brownshirts.
40 points for claiming that the "scientific establishment" is engaged in a "conspiracy" to prevent your work from gaining its well-deserved fame, or suchlike.
40 points for comparing yourself to Galileo, suggesting that a modern-day Inquisition is hard at work on your case, and so on.
40 points for claiming that when your theory is finally appreciated, present-day science will be seen for the sham it truly is. (30 more points for fantasizing about show trials in which scientists who mocked your theories will be forced to recant.)
50 points for claiming you have a revolutionary theory but giving no concrete testable predictions.
If you don't get a lot of those pesky neturons, it'd be fun to tinker with one of these in the garage. What's deuterium go for these days?
I'm trying to teach myself to set people on fire with my mind... Is it hot in here?
Sonoluminescence: an Introduction
Single Bubble Sonoluminescence HOWTO
Since sonoluminescence dosent seem to scale up (to my knowledge) this seems like a moot point. It is sort of cool to have a cheap way to study micro-fusion though.
Information wants to be free like speech wants to be free, not like we want beer to be free.
PDF copies can be downloaded from here.
Ok, initial comment on this story has been very negative, but... The original Pons et. al. findings also claimed neutron production. So do those results all indicate experimental error or log-book-cooking that would make Michael Fastow weep with fatherly pride? Ever heard this one? ...whatever that means... and it's just the tech hasn't caught up to make an observation? Is it just possible that it's a matter of technology to produce tabletop/cold fusion? Heat treating the metal or something? High temp superconducting seems to still be a alot of hit or miss experimentation. Why would cold fusion be so different a technology from that?
Q: What does a neutrino detector actually detect?
A: The presence of funding.
Theorists were convinced that neutrinos would be observed jumping from tau to mu versions
Or am I just a clever troll?
ceci n'est pas un 'sig'
Don't be so quick to quickly discount cold fusion. When they first published they didn't release all of the details of their experiment, coupled with the fact that they were a couple of chemists taking one of the physics holy grails they were met with much animosity. Big energy companies also felt understandably threatened by the possibility of cold fusion and were very influential is "debunking" it. I've heard of various reputable scientists who have claimed to have achieved some extraneous heat production. I've also heard of one instance where scientists supposed to research it for the US government at one university (I don't know where) supposedly adjusted the baseline of their experiment to account for some extraneous heat production. Does this mean I believe in cold fusion? No, yet I do believe it is something that deserves some unbiased research, to allow political interests to dictate wether a phenomenon is idiocy and when contrasted to the potential benefits (or risks) is unthinkable.
I stole this Sig
- a peer-reviewed article appearing in a major (if not the major) scientific journal,
- reporting an experimental result (not a business plan),
- that we're hearing about because the article is going to press (not because it was planned or submitted; admittedly, we're hearing it a little early because of advance reports).
These are all good signs of good science. The better sign will be attempts to reproduce the experiment, with both successes and failures published in the same professional manner.It's an extraordinary claim, and will require extraordinary evidence. Yes, this is just a first step; but at least it's in the right direction.
Stupid job ads, weird spam, occasional insight at
You too can make sonoluminescence happen. Try it with some deuterium and see if you can get fusion. Sound complicated, just use this easy to follow guide. It will give you step by step instructions for reproducing that special kind of magic that is sonoluminescence. All you need is:
- sinus generator: (sounds a bit painful)any function generator working around 25kHz, adjustable to +/-1Hz
(+/-10Hz may work, too)
- amplifier: nearly any kind of audio amplifier will do. If you're not sure, measure
the saturation voltage: 40V peak-to-peak should be enough.
- 2-trace oscilloscope
- 2 piezoceramic Transducers (drivers):around d=16mm in diameter, h=8mm thick
- piezoceramic pill-transducer (microphone):around 3mm in diameter, 1mm thick
- three finger clamp
- laboratory stand
- flask:take a 100ml Pyrex/Duran spherical flask, diameter 65mm, with a small neck. An industrial one has poor optical quality, so better take a free blown one.
- coil(s): around 20mH, see text
- resistors: 1M, 10k, 1R
- coaxial cable
- quick-drying epoxy glue
- an eyedropper or a syringe (one of those little do-it-yourself subcutaneous is very good)
- degassed distilled water:
- Pyrex/Duran Erlenmeyer flask (0.5 or 1l) and airtight stopper with pipe, rubber hose and clamp to close it
- aluminium/highgrade steel drinking bottle (0.5 or 1l) with screw cap; one of those found in camping stores, a bare one without varnish
- a bubble
;-)
oh, and it is nice to have:or
- second oscilloscope
- vacuum pump
- high-pass filter
- laser
Go for it kids. By the way, my favourite part is this quote: "Increase the driving voltage until you hear a horrible screeching noise, which sounds like your flask is going to crack. Don't be surprised if it does".I have to fill in some more text here, becasue slashdot sais I have too few characters per line. Well its just a bloody list of things. Of course there won't be much to each line, what do you expect?
"Me and my girl named bimbo . . . limbo . . . spam" - Captain Beefheart.
The surface of the sun is at about 5700K, far below that required for fusion. I thought this meant the science was totally implausible, but it turns out to be an error in the Slashdot summary. ...".
The article claims "simulations also indicate that temperatures inside the collapsing bubbles may reach up to 10 million degrees Kelvin, as hot as the center of the sun." and "Temperatures inside these bubbles can be as high as 5000-7000 degrees Kelvin, about as hot as the sun?s surface. But, recent experiments by a number of researchers suggest that bubble temperatures can reach even higher temperatures--closer to the heat needed for nuclear fusion
Deuterium 'burns' at much lower temperatures than the ordinary hydrogen burning that powers our sun (where reaction rates are so slow it will take billions of years to use up the fuel supply.)
Quattuor res in hoc mundo sanctae sunt: libri, liberi, libertas et liberalitas.
Warning: Do not fuse your dog to anything. If you do decide to fuse a cat, use a strong superglue or firm adhesive to ensure they don't escape and claw your head off. Because it doesn't take Einstein to tell you, Cats = Evil^2.
Here is a link Science Magazine is providing:
Science Magazine
It has a pdf version of the article in question. Here is the abstract.
The evidence for fusion-capable temperatures inside a sonoluminescing bubble lies in two main categories:
- You can examine the emission spectrum of the bubble. The spectrum is continuous, with a peak which depends on a variety of factors (noble gas content, temperature of the fluid, etc.), so you can try to figure out the temperature based on the emission expected from a blackbody of a similar temperature. The last I heard, the temperature was at least an order of magnitude less than what you would need.
- You can run simulations which make assumptions about the bubble collapse mechanism. If the bubble remains perfectly spherical during the collapse, then you may get the temperatures being quoted in the article. But there are other theories for the collapse, and requiring the bubble to remain perfectly spherical during a violent collapse doesn't seem intuitively obvious to me.
It's been a few years since I worked with this stuff, so take this with a grain of salt, but I'm not optimistic about this paper being validated."she says i'm lousy conversation. as if that's supposed to help."
The idea of fusion in sonoluminescence is nothing new. I sat through a talk on it by some computational hydrodynamics experts from Lawrence Livermore National Lab in 1997 at a the Gaseous Electronics Conference in Hawaii (if you really care, you can probably look up the conference proceedings at http://www.aps.org).
... [pause during the palpable bristling of the audience] ... it's either an implosion or an explosion.'
... yes the temperatures are very low compared to fusion. However, a minuscule amount of fusion (think in terms of one or two atoms per microsecond) would occur and thus there would be measureable neutron flux (in theory). However, in practice, the neutron flux would be so low that it would be nearly impossible to distinguish from the background noise.
... that happens in the core. In fact, it is hotter inside a flourescent light tube (~50K-100K) than at the surface (but the heat conduction is so low that it isn't a safety issue).
The talk was pretty good. Their models were able to explain most of the features reasonably well without having to resort to exotic physics (i.e. quantum electrodynamic weirdness). I mostly remember sitting at this talk because the presenter made a reasonable witty comment (remember, talks like this are usually dry and boring with many audience members nodding off because they are always scheduled after lunch): `Scientists at LLNL have an innately superior understanding of all physics
However, the talk did run into a credibility problem when the presenter said the next step was too look for fusion. Several people in the audience correctly pointed out that the temperatures were several orders of magnitude too low. The presenter's response was that the
Without seeing the paper from the ORNL people, I really can't say if they have upped the sophistication or not though.
By the way, the temperatures at the surface of the sun are only ~6K (except in the wispy corona). Not nearly hot enough for fusion
Kevin
A Dean drive generates reactionless force of a special type. To measure this force, we use a special unit, the Bathroom Scale pound. BS pounds are whatever my bathroom scale measures. My bathroom scale seems to be more sensitive than Dean's. When I stand still on it, I weigh about 195 BS pounds. If I shake my arms at the right speed, my weight drops to 175 BS pounds. That is better than .1 BS G thrust. I suspect a carefully tuned counter-weighted drill motor can do far better.
So when I finally get my device perfected and my paper published, some mean professor is going to explain that measurement equipment may produce incorrect readings in certain situations. That it isn't enough to get the reading you want, you actually have to show you got a valid reading.
For those who want to duplicate my experiments so far, get an aged Health-O-Meter spring scale. Other types of scale have some weird reality field around them that interferes with crack pot physics.
In the "nice to have" section, it mentions a laser.
Well, der, I think this is obvious. Its always nice to have a laser. You could put this on basicaly any list of "nice to haves" for anything:
"Me and my girl named bimbo . . . limbo . . . spam" - Captain Beefheart.
What really gets my goat is that the editors of Slashdot are apparently unaware of the position of the U.S. Navy's Naval Ocean Systems Center in favor of cold fusion, and their long-suffering and pioneering work on the particular kind known as codeposition fusion:
http://www.spawar.navy.mil/sti/publications/pubs/t r/1696/tr1696.pdf
I have copied that tech report, along with a diagram you can use to do cold fusion on your desktop for less than US$500, in this directory:
http://www.bovik.org/codeposition
Please mod me up; I am posting as AC due to time pressures and a different browser in use at the moment. Thanks in advance.
Sincerely,
James Salsman
james at bovik dot org
Here's the problem: Copyright violations. Believe me, I'd love to. But we can't because of that. People have asked how Google caches - frankly, I don't know how they legally do that. But because we're a content site, versus a search engine, we would be more liable for reprinting without permission which is a big No No in all print/editorial media.
Yeah, I'm that guy.
I just finished reading Park's book "Voodoo Science - The Road From Foolishness to Fraud" (ISBN 0-19-514710-3) - and a central theme throughout the book is his "annoyance" (ok, that is putting it kindly) with scientists and inventors who either get caught up in their experiments and go down a self-deception path (Pons/Fleishmann, Joseph Newmann) or those who outright deceive others for monetary gain.
The way this is sounding - it is sounding like so much "voodoo science", simply because of the irreproducibility of it (but, who knows? Maybe others will have success - may be too early to tell)...
Reason is the Path to God - Anon
"If the creation of nuclear technology became this simple, it not only gives terrorists an easier method to attain nuclear energy, but a way to actually create it themselves."
People like you are going to drive me prematurely bald...
Fusion reactors != fusion bombs
Fustion reactors are so damned far removed from fusion bombs that it's been about 50 years since we developed the second and we still haven't figured out the first. I'm willing to bet it will take at least another 50 years after the development of fusion reactors before we can make a fusion bomb that doesn't require a fission bomb to actually get the thing to go off.
"While it's true that a nuclear explosive based on this current method wouldn't spread as much harmful radiation as a uranium based explosive,"
You're right about that, but you have no idea how right you are.
First and foremost, the act of fission frees neutrons from their parent atoms. A lot of neutrons. Enough neutrons to set off the fission reaction. Fusion generates far fewer free neutrons (if at all, depending on your fuel) because it's busy trying to form atoms instead of breaking them apart.
Secondly, when people think of "radiation" from a bomb they think of the fallout (since the actual radiation from the explosion lasts as long as the actual explosion). Fusion in and of itself has no fallout. The fallout from modern hydrogen bombs is from the fission bomb that's used to set it off. No fission bomb, no fallout.
"it's potential damage far outweighs that of a dirty bomb."
Now here is where you need to lay off the crack pipe.
Getting a fission reaction to start is pretty easy: get a neutron-producer close to a clump of unstable atoms. Getting a fusion reaction to start, on the other hand, requires a LOT of input heat in the beginning in order to generate the plasma the reactions takes place in. So much heat that the pressure at the heart of Jupiter isn't enough to start a sustainable reaction. In the past 50 years the only way we've been able to pull it off is with a fission bomb.
But let's pretend that a pure fusion bomb is possible in the short term. Although it's possible to squeeze a fission bomb into something the size of a suitcase, your average 20 megaton device is more or less a cubic meter in size. But it's only that small because the heat generator is a tiny little suitcase-sized fission bomb. If we try to use a fusion reactor to generate the heat instead of a fission bomb, I don't see the device being small enough to fit into a cargo container (probably the largest possible size for a device to be useful to terrorists).
But what if they try to blow up a fusion power plant? Fission reactors are heavily shielded to keep the inside in. Probable fusion reactors would be heavily shielded to keep the outside out. If a tokomak loses magnetic containment, the plasma expands, cools, and reverts back to a gas. If it loses its physical containment, air gets in, conducts/convects away heat from the plasma, the plasma cools and reverts back to a gas. If you try to blow it up you just end up with a negligible amount of hot gas on top of the explosive.
Personally, I'd be a hell of a lot more frightened of an attack at a coal-fired plant. Have you ever seen what a spark can make coal dust do? Or what about popping off the fuel tanks at a natural gas power plant? And while I'm on the subject of boiling liquid-vapor explosions, oil refineries look awfully unprotected...
It also explains why 2.45 Mev neutrons, which Teleyarkhan claims are the byproduct of the fusion of Hydrogen-2 into Helium-3, are seen coming out of the flask. They are simply 14 Mev neutrons which have slowed down by bouncing off various nuclei.
My coffee cup is clearly fused to the Ikea bookshelf beside my computer.
Trust those Nordic types to always be one step ahead! Next they're going to be inventing, like, operating systems, or something, on their tabletops!
- undoware.ca
Just finished reading the pdf of the manuscript. My biggest concern is the magnitude of the observed effect- a few standard deviations above background. The data acquisition runs lasted 7 or 12 hrs (or for several iterated 300s runs with another detector). The question raised here is, if you've got marginal statistics (particularly for an exceptional effect, if truly observed), then why not run the experiment longer, like a month? That should yield a strong enough signal that statistics are no longer an issue. With a small set of runs, there lies a risk of subconsicously self-selecting a fraction of the runs as the 'good' ones. I'm not saying that's what happened, but it can't be ruled out based on the data at hand.
Second concern is the accuracy of the shock hydrodynamic simulations, both the assumption of perfect spherical symmetry (which is crucial to a high concentration of energy at the very center) and the treatment of the complex interactions in the plasma during compression (Born-Mayer potentials, as used here, are outside their realm of validity when the substance ionizes, I suspect).
I'm not prepared to say "obviously wrong," (open mind = good) but there are red flags...
Curtains for windows?
I first heard about it when I spent a summer at Lawrence Livermore National Lab two years ago. An abstract of the Nature paper that group at Livermore published is available here
"It take 9 months to bear a child, no matter how many women you assign to the job."
I think this author is giving Pons and Fleischman a bit too much credit. While it is certainly true that both were well-respected chemists, their work on cold fusion was at best sloppy, and at worst, both inaccurate and deceptive.
:
Some facts in the case
1) They used heavy water (D_2O) in their experiments. Steven Koonin, a theoretical nuclear physicist, confronted them at a conference with a simple question : Had they done the simple test of using ordinary water? (Which wouldn't have produced fusion.) The answer was damning : No, they hadn't even thought of it.
2) Their work detecting neutrons (a certain biproduct of fusion, cold or not) from their experiment was presented in a most misleading fashion at conferences. They displayed figures without labels, and did not perform proper calibrations of their detection -- it was impossible to determine whether their "signals" were simply background. (Of course, their detections were orders of magnitude too small -- had the signal been commensurate with the heat produced, they would have been dead from the radioactivity.)
3) Moreover, when confronted with the the fact that their "signals" lacked a crucial feature known as the "Compton edge" (as any physics major has observed this in their labs classes) which must accompany any real signal, they further lopped off their plots so as to show only the spurious peak, making it impossible to realize that they were lacking the Compton edge.
4) They presented their research to the press prior to publication. This turned the scientific process into a media circus, impeding progress, and doing immense damage to the public conception of the scientfic process.
5) Rather than openly describing their methodology (a standard practice in any scientific discipline) to allow other researchers to reproduce their work, they kept their methods secret. I recall several groups were forced to set up their experiments using bits of video footage from the evening news.
6) Later claims by a number of researchers that some extraneous heat was being produced is quite a distinct issue from the original work of Pons and Fleischman. Pons and Fleischman's original claims were much bolder -- they claimed a very large extraneous heat output. It was later determined that they had simply done their calorimetry accounting wrong (a common error in calorimetry, but nonetheless surprising, because they were experts in calorimetry).
In sum, the way Pons and Fleischman conducted their work on cold fusion was a prime example of how science is not to be done. The image of Pons and Fleischman as two revolutionary figures taking on the physics establishment is simply not commensurate with the facts of the case -- they practiced very poor science, by the standards of any scientific discipline.
Bob
Science, like Nature, must also be tamed, with a view turned towards its preservation.
At the close of 1939, a woman sat on a snow covered log in a Swedish forest and re-read a letter from a chemist in Germany. The chemist had detected barium where he hadn't expected to find any. He wrote her because he couldn't figure out where the barium was coming from. The woman, Liese Mietner, figured out that the chemist, Otto Hahn, had split Uranium. Without Mietner's insight into the underlying physics, Hahn's observation might have been dismissed. So there might indeed be "some crazy chemistry..." taking place.
On the other hand, as soon as Mietner's nephew got back to England from his Christmas break, the British were reproducing Hahn's experiment. Without reproducible results, the results could just be background noise.
2) To Publish or Not to Publish: Publication is the right option. byt topics/b ubble/1793.pdf
Donald Kennedy, Editor
http://www.sciencemag.org/feature/data/ho
Every once in a while, we at Science receive a paper that causes us to
exercise particular care in handling, because it may be controversial or
because it is importantor both. The paper by Taleyarkhan et al. on p. 1868
of this issue is a case in point. It qualified for careful, responsible
treatment on both counts. And its history with us has exposed some of the
more unusual challenges that can arise in the publication process.
The paper reports experiments in which sonoluminescence is induced in
solutions of deuterated acetone subjected to sound waves and neutron
irradiation. These conditions cause bubbles to grow and then implode,
locally generating high pressures and temperatures and the emission of
sonoluminescent light. The authors present evidence for the production of
tritium in the solution, and for neutron emission coincident with the light
emission. They cautiously interpret these observations as evidence that
deuterium-deuterium fusion occurred in the imploding bubbles. That prospect
naturally encouraged us to treat the paper with care.
After the external review process had been completed, we scheduled the paper
for publication. Then we were contacted by senior science managers at Oak
Ridge National Laboratory (ORNL), who said that certain reservations had
developed
about the findings and their interpretation. In a series of telephone and
e-mail contacts, they urged that we delay the scheduled publication of the
paper. The authors participated in a series of meetings to discuss
objections raised by the ORNL managers, including some findings made by a
second group of scientists who had been asked to perform additional tests,
using the same apparatus but a different detector.
After some negotiation, a compromise was reached in which the authors
responded to criticisms and subsequently made some modifications in the text
to accommodate them. They also agreed to cite a short nonpeer-reviewed
communication in which the second group present measurements that disagree
in some respects with theirs, along with their own response to it. While
these agreements were being reached, Science received communications from
two distinguished scientists in this field, raising objections to the paper
and urging that we reconsider our plans to publish it. And the matter became
even more public on 1 March when Robert Park issued an airy, premature
dismissal from the American Physical Society. By this time, it had become
clear that a number of people didnt want us to publish this paper.
I have been asked, "Why are you going forward with a paper attached to so
much controversy?" Well, thats what we do; our mission is to put
interesting, potentially important science into public view after ensuring
its quality as best as we possibly can. After that, efforts at repetition
and reinterpretation can take place out in the open. Thats where it
belongs, not in an alternative universe in which anonymity prevails, rumor
leaks out, and facts stay inside. It goes without saying that we cannot
publish papers with a guarantee that every result is right. Were not that
smart. That is why we are prepared for occasional disappointment when our
internal judgments and our processes of external review turn out to be
wrong, and a provocative
result is not fully confirmed. What we ARE very sure of is that publication
is the right option, evenand perhaps especially
when there is some controversy.
A reporter also asked me whether this was the only time pressure has been
put on Science not to publish a paper. Although this case is exceptional, it
is not unique; we have been there before. The motivations for urging us not
to publish have varied from one case to another. Often they rest on serious
legitimate scientific differences of opinion, although sometimes that is not
so clear. In this instance, we see no good reason for abandoning our plans
to publish the paper, and we can see no merit whatsoever in the efforts to
discredit it in advance. Both the premature critics and those who believe in
the result would do well to wait for the scientific process to do its work.
I mostly agree to you, but I just have to be a little pedantic...:)
Regarding the number of neutrons produced by fusion and fission reactions, yes a D-T fusion reaction produces only one neutron while a U or Pu fission produces between about 3, depending on energy. But take into account that a single fusion reaction produces only 17.6 MeV while a single fission produces about 200 MeV. Add to that the fact that a large fraction of the neutrons in a fission reaction are used for inducing further fissions in the material. Remember neutron bombs? Tactical nukes meant to kill the crews of sovient tank hordes, while hopefully leaving the rest of West Germany relatively intact. They had very minimal amount of fission material in them, about 95% of the energy produced was by fusion. The reason was to have a as high as possible neutron flux, and also to minimize fallout. Most strategic warheads deployed today have only about 50% fusion output. The reason is that the casings are made of enriched uranium, the reason being that the Ulam-Teller staged radiation implosion type bomb needs a casing made of high-Z material for refllecting x-rays produced by the primary. So by additionally making the casing of fissionable material (it wont fission by itself, only fission induced by the fusion neutrons) you get better bang-for-weight.
And regarding detonating a fusion bomb without a fission primary, I read some rumors a while ago that the russians reportedly had some chemical explosive called "red mercury" capable of detonating a fusion bomb directly. As it IMHO sounds quite improbable, I'd guess it's just some rumor.