Well, most of these journals are run by not-for-profit organizations (like the American Physical Society, etc). One of the top journals in my field is run by a couple physicists here at Princeton. They certainly get paid for their editorial roles, but this is not their main gig.
So, I don't think profit is at all a motive in publishing these journals. They are just trying to recoup costs. Subscription fees aren't the only source of income, the authors of articles actually pay page charges to get their article published. These can be substantial, maybe several hundred dollars. So, perhaps the articles can be offered for free download, but maybe the page charges will have to go up to compensate for this potential loss in subscription fees.
The washers and other hardware on the space station are most likely stainless steel (316), which makes them non-magnetic. So, I don't think magnets would help....
The baloon analogy is a good one. Although the basic fact is that currents in the plasma (which is a good conductor) cause the magnetic bubble to expand, these currents are generated by the thermal pressure of the plasma. This pressure pushes out against the magnetic field and the magnetic field tries to hold it in. Think of the field as a baloon, expanding until the surface tension balances the pressure of the gas inside. In the case of this bubble propulsion idea, there is a third force -- the solar wind. So your bubble expands until the force of the magnetic field+solar wind balances the plasma pressure. This is why they claim that the "sail" will get bigger as you go further out -- the solar wind pressure drops, and the thing will therefore expand more. I heard about this idea a while ago and read the early white papers. Sounds very interesting and is on a sound physical basis, but I think engineering issues will be hard to overcome (lots of potential damage to sensitive components by RF/high density plasma bombardment, etc).
If you want a non-baloon explanation -- the driving force is a plasma pressure gradient. You put hot plasma on the field and so that it is hottest near the space craft and cool further away (hard not to do this!). This pressure gradient creates a current around the spacecraft called a "diamagnetic current" (due to gyration of particles about the field lines in concert with a density or temperature gradient). This current wants to expand outwards (all closed circuits experience the "hoop" force that make them want to expand), and hence your bubble expands. Couple that with the fact that the dipole field is "frozen" into the plasma, the dipole field is dragged outward to form your bubble.
I just read some of the netdev and linux-kernel mailing list archive. I think that pppox support is in the 2.3.99-pre3 kernel, in drivers/net/ppp_generic.c. It seems that specific implementations of pppox will be handled by plugins to pppd.
PPPoE is PPP over Ethernet. Seems like many DSL providers are using this instead of DHCP or static IP's. I personally think it sucks, but until there is a fast access alternative here in central jersey I am stuck with it (I don't live in comcast's area, and so I can't get 2-way cable modem). Here's a portion of RFC 2516 on PPPoE:
PPP over Ethernet (PPPoE) provides the ability to connect a network of hosts over a simple bridging access device to a remote Access Concentrator. With this model, each host utilizes it's own PPP stack and the user is presented with a familiar user interface. Access control, billing and type of service can be done on a per-user, rather than a per-site, basis.
I just perused the 2.3.99-pre3 source, and I didn't see any sign of a pppox.c driver (not that I necessarily know where to look, I checked in drivers/net...). Anyway, I had heard that 2.4 would include pppox support, but maybe it will show up in the later releases (hopefully before 2.6...).
I have Bell Atlantic DSL (which has recently switched to PPPoE and dynamic ip's), and I use pppoed and the pppoe kernel patch by Jamal Hadi Salim (2.2.14 patch and pppoed source available from: http://www.davin.ottawa.on.ca/pppoe). It works great for me. I have heard others have success with the Roaring Penguin pppoe userspace client.
This is not a fork. Nautilus is the REPLACEMENT for gmc. It will be part of GNOME 2.0. I actually had not heard of Eazel until today, but I knew of the existence of nautilus and that the Gnome project wanted to replace MC/GMC with a brand spanking new file manager.
Prof. Lederman, I am Ph.D. student in plasma physics, a subfield of physics that has many large scale (both in physical and budgetary size) experimental projects. High Energy experiments tend to be even bigger and require more man power. I see this having two negative effects: (1) More money means more politics, and (2) experimental physics seems much harder to do in a university campus setting. My question(s) -- what is your forecast of the political status of scientific research and where do you think government research money will go in the future.
Black holes radiate (sounds contradictory, doesn't it), and therefore lose energy (mass). There is some equilibrium size reached which balances this loss to mass inflow.
I know this story is dead and this comment might never be seen, but I had to respond for the record. I'm sorry, but the "blue glow" in nuclear reactors is due to BETA DECAY not compton scattering. I am a Ph.D. candidate in physics and I do realize that photons have momentum. But a free electron and a free photon don't interact in such a way as to give the electron lots of momentum. Use your formula P = h_bar k and calculate the momentum carried by the photon. Now suppose that ALL of that momentum gets transferred to the electron. So divide that number by the mass of the electron and tell me the speed that comes out. it will be awfully tiny. Sorry, I don't mean to get personal, I just wanted to clear that up.
The electrons are mostly from beta decay associated with radioactive by-products of fission. These electrons come away with alot of energy. Compton scattering doesn't transfer too much energy to the electron (think about the mass difference -- photons have none, so it is very hard to exchange momentum and energy with a free electron -- although photons can kick electrons out of atomic orbit with some energy).
You can perfectly shield out RF if you have an unbroken perfect conductor surrounding your device. In practice, the conductor isn't perfect, so some of the RF penetrates, but the bigger problem is breaks in the faraday cage (for cabling into the device, cracks around doors, etc) -- RF can squirt through these. I think that shielding can get much better. It wouldn't be too expensive a proposition to shield computers from such an attack, I think...
Fatbrain.com has recently announced that it will offer an electronic publishing service, E-matter. What do you think about offering documents for download for a fee? Is this something that O'Reilly might be undertaking in the future?
Actually, the airstrip is gone now... There's just a field there, where someone plants some crops. I haven't been at PPPL long enough to have seen the airstrip -- I was told this by a tech who has been (maybe you know him - Jim Taylor).
I hate to tell you this, but your points are weak ones...
> Tritium doesn't exist in nature. So how do we get it? > From Fission reactors of course. So the fact of the >..... > MYTH I: Fusion Fuel is cheap and abundant, not so.
Fission reactors are not the only source of tritium! Yes, they provide neutrons, yes they are used now by the gubment to produce tritium for nuclear weapons. But you don't have to use neutrons from a fission reactor!!! Where, you ask do you get these neutrons -- from the FUSION reactor itself. Yep, like a fast breeder fission reactor, fusion plants will make tritium by employing a blanket containing lithium. This will produce plenty of tritium for the reactor to operate. I give you that to start the process up, you will have to have an alternative source of tritium, and a fission reactor may be the choice for that (BTW -- the half life of tritium is about 12 years).
> Now the problem of radiation. Fusion produces huge quantities of > Neutrons, even more than Fission produces. This means > that the materials in and around the reactor will be > constantly bombarded and they will be converted into > radioactive isotopes by the neutrons. This is ignoring > the problem of secondary radiation created by the > countless fission reactors required to produce the > tritium needed to run the fusion reactors. So fusion > isn't so clean. >... >MYTH II: Fusion power is clean. Busted!!!
Again, you don't quite have your facts straight... Fusion does produce neutrons, but nowhere near the amount present inside a fission reactor!! A fusion reactor will have a neutron power load on its walls that is about the same as the neutron power flux in a fission reactor, but the spectrum of neutrons is much different. The fusion neutrons (from D-T) are 14 MeV -- whereas the fission neutron spectrum is a thermal one, with the average energy about 0.0025 eV (well, a little bigger than room temperature, but many many orders of magnitude less than the fusion neutrons). Power flux is essentially energy times density. So the density of fusion neutrons is much much lower than that in a fission reactor (for the same power load on the walls).
The radiation problem in fusion reactors is fundamentally different than in fission plants. Yes, these fusion neutrons are absorbed by materials and thereby make them radioactive. But we have a CHOICE in what materials we put next to a fusion reactor (we have no choice but to use uranium or plutonium in a fission plant). So we can minimize the activation caused by these neutrons. Even using stainless steel, the half-life a majority of the activation products is very short -- on the order of years, not millenia. If we use specially engineered materials (like vanadium alloys and SiC ceramics) we can get the activation to a minimum. The neutrons are NOT the radiation problem in a fission reactor -- it is the horribly radioactive fission products. These are unavoidable in fission and live forever. We could even ELIMINATE activation entirely in a fusion plant if we use advanced fuels like p-B11 or D-He3 (which are harder to get to fuse than D-T, but have NO neutron emission).
> The third myth is that fusion power would be cheap, not > so. Compared to a Fusion reactor a Fision reactor is just a pile of > metal, and we all know how cheap they are. Sorry guys, Fusion will > be the most expensive energy on the planet. > MYTH III: Fusion Power is Cheap, nope.
You are right here, the fusion plant is going to cost a lot, based on present designs. But it is not so far off as you think-- fission plants are quite expensive due to licensing problems. Fusion will only become economical when the price of other fuels rises (or if we start to internalize the costs associated with pollution -- and hence raise the cost of fossil power).
> So what does this tell us? We could get an even better > deal using fast and slow breeder reactors using fission > technology. It would provide all the energy we could > conceivably need, and using breeders with reprocessing > it would not only produce almost no new waste, but it > could burn as fuel about 99% of the waste we have > now. If done right it could be fairly cheap as well, and > extremely safe. So get an almost limitless supply of > cheap and safe energy and at the same time get rid of > 99% of our current nuclear waste, yup, fast breeders > sound good to me too.
I agree that fission reactors, with fuel recycling, are part of the energy answer (+ breeding -- without breeding the uranium supply would run out on a 200 yr timescale I think). My undergraduate education was in nuclear engineering (and physics), and although my Ph.D. work is in plasmas/fusion, I am still a proponent of using fission.
BUT, only "MYTH III" has a leg left to stand on, I think.
Getting the ions moving quickly isn't the only problem -- you also need to have a lot of them reacting and you need to have them interacting for a long time. The cross section for simple binary collisions between ions is much larger than the fusion cross section, and so the ions must undergo many scattering collisions before fusing -- so you have to keep them around for a while. (This is why we don't simply fire two beams of D and T at one another -- which is the easiest way to get a fast collision between two ions). The solution to this is to let the ions collide and become thermal, but keep them trapped long enough to fuse (hence the tokamak, RFP, FRC, spheromak, etc...).
Using electrostatic forces to try to trap ions in fusion is problematic because you need huge potentials to trap a reasonable number of ions (you have to overcome the potential the ions themselves create). You need something like 10^13/cc density to get a reasonable fusion power, and electrostatic traps can get up to (I think) 10^8/cc if they are really clever.
There is an experiment similar to the one you mention at U. Wisconsin (at least it was there when I was visiting grad schools a few years back). They are more interested in using it as a thruster for spacecraft, though (no fusion involved here). I couldn't find a link for it on www.wisc.edu, though.
In fusion science circles "alternative" fusion research usually means studying thermonuclear devices that aren't tokamaks (for instance, NSTX, the spherical tokamak (they call it a torus now) here at PPPL). Other alternative research devices are Reverse Field Pinches (MST at Wisconsin), Stellerators (LHD in japan, there's also a big one in Germany and one under development here at Princeton), Field Reverse Configurations (U. Wash, my experiment here at PPPL can be run as an FRC)....
I am a graduate student, doing my thesis research at PPPL. If anyone is in the immediate area and is interested in seeing the tokamaks (and other plasma experiments) feel free to e-mail me and we can try to set up a tour....
I work on a much smaller experiment -- I came in to Princeton wanting to work on fusion, but I got really turned off by the politics involved...
I agree with you on your point that we need more gubment sponsored R&D for all energy research!
But "ban uranium"?? You must mean ban fission -- Uranium is already relatively abundant in the earth (we don't make it). Also -- Uranium in itself is not that much of a danger (yes, it has a long half-life, but it emits mainly wimpy alpha radiation - the biggest danger is in its toxicity as a heavy metal) -- fission products are the problem. I STILL think, however, that fission energy should be used in the near term as a non-polluting alternative to fossils. At least until other possibilities are available (fusion, renewables). The waste problem would be a lot easier to deal with if we didn't have to worry about proliferation...
>I'm not a physicist, but I know fision is a lot easier than fusion, right?
Fission is easier than fusion in general, but NOT in deuterium! (Uranium fission is a heck of a lot easier than deuterium fusion, basically because the U nucleus is large and rather unstable in the first place -- hit it with a neutron and it will break apart easy). It's pretty damn hard to just split a deuterium nucleus -- essentially impossible with a laser . Anyway, the neutrons come from fusion, and this is not at all surprising - no need to specifically look for He.
Slashdot Mirror
Well, most of these journals are run by not-for-profit organizations (like the American Physical Society, etc). One of the top journals in my field is run by a couple physicists here at Princeton. They certainly get paid for their editorial roles, but this is not their main gig.
So, I don't think profit is at all a motive in publishing these journals. They are just trying to recoup costs. Subscription fees aren't the only source of income, the authors of articles actually pay page charges to get their article published. These can be substantial, maybe several hundred dollars. So, perhaps the articles can be offered for free download, but maybe the page charges will have to go up to compensate for this potential loss in subscription fees.
The washers and other hardware on the space station are most likely stainless steel (316), which makes them non-magnetic. So, I don't think magnets would help....
I thought I had seen this before....
m l
http://slashdot.org/articles/99/11/04/1148246.sht
Umm, maybe you should go read the link
The baloon analogy is a good one. Although the basic fact is that currents in the plasma (which is a good conductor) cause the magnetic bubble to expand, these currents are generated by the thermal pressure of the plasma. This pressure pushes out against the magnetic field and the magnetic field tries to hold it in. Think of the field as a baloon, expanding until the surface tension balances the pressure of the gas inside. In the case of this bubble propulsion idea, there is a third force -- the solar wind. So your bubble expands until the force of the magnetic field+solar wind balances the plasma pressure. This is why they claim that the "sail" will get bigger as you go further out -- the solar wind pressure drops, and the thing will therefore expand more. I heard about this idea a while ago and read the early white papers. Sounds very interesting and is on a sound physical basis, but I think engineering issues will be hard to overcome (lots of potential damage to sensitive components by RF/high density plasma bombardment, etc).
If you want a non-baloon explanation -- the driving force is a plasma pressure gradient. You put hot plasma on the field and so that it is hottest near the space craft and cool further away (hard not to do this!). This pressure gradient creates a current around the spacecraft called a "diamagnetic current" (due to gyration of particles about the field lines in concert with a density or temperature gradient). This current wants to expand outwards (all closed circuits experience the "hoop" force that make them want to expand), and hence your bubble expands. Couple that with the fact that the dipole field is "frozen" into the plasma, the dipole field is dragged outward to form your bubble.
I just read some of the netdev and linux-kernel mailing list archive. I think that pppox support is in the 2.3.99-pre3 kernel, in drivers/net/ppp_generic.c. It seems that specific implementations of pppox will be handled by plugins to pppd.
PPPoE is PPP over Ethernet. Seems like many DSL providers are using this instead of DHCP or static IP's. I personally think it sucks, but until there is a fast access alternative here in central jersey I am stuck with it (I don't live in comcast's area, and so I can't get 2-way cable modem). Here's a portion of RFC 2516 on PPPoE:
PPP over Ethernet (PPPoE) provides the ability to connect a network of hosts over a simple bridging access device to a remote Access Concentrator. With this model, each host utilizes it's own PPP stack and the user is presented with a familiar user interface. Access control, billing and type of service can be done on a per-user, rather than a per-site, basis.
I just perused the 2.3.99-pre3 source, and I didn't see any sign of a pppox.c driver (not that I necessarily know where to look, I checked in drivers/net...). Anyway, I had heard that 2.4 would include pppox support, but maybe it will show up in the later releases (hopefully before 2.6...).
I have Bell Atlantic DSL (which has recently switched to PPPoE and dynamic ip's), and I use
pppoed and the pppoe kernel patch by Jamal Hadi Salim (2.2.14 patch and pppoed source available from: http://www.davin.ottawa.on.ca/pppoe). It works great for me. I have heard others have success with the Roaring Penguin pppoe userspace client.
Try:
http://www.qubit.org
There are some good tutorials there.
This is not a fork. Nautilus is the REPLACEMENT for gmc. It will be part of GNOME 2.0. I actually had not heard of Eazel until today, but I knew of the existence of nautilus and that the Gnome project wanted to replace MC/GMC with a brand spanking new file manager.
Prof. Lederman, I am Ph.D. student in plasma physics, a subfield of physics that has many large scale (both in physical and budgetary size) experimental projects. High Energy experiments tend to be even bigger and require more man power. I see this having two negative effects: (1) More money means more politics, and (2) experimental physics seems much harder to do in a university campus setting. My question(s) -- what is your forecast of the political status of scientific research and where do you think government research money will go in the future.
Black holes radiate (sounds contradictory, doesn't it), and therefore lose energy (mass). There is some equilibrium size reached which balances this loss to mass inflow.
I know this story is dead and this comment might never be seen, but I had to respond for the record. I'm sorry, but the "blue glow" in nuclear reactors is due to BETA DECAY not compton scattering. I am a Ph.D. candidate in physics and I do realize that photons have momentum. But a free electron and a free photon don't interact in such a way as to give the electron lots of momentum. Use your formula P = h_bar k and calculate the momentum carried by the photon. Now suppose that ALL of that momentum gets transferred to the electron. So divide that number by the mass of the electron and tell me the speed that comes out. it will be awfully tiny. Sorry, I don't mean to get personal, I just wanted to clear that up.
The electrons are mostly from beta decay associated with radioactive by-products of fission. These electrons come away with alot of energy. Compton scattering doesn't transfer too much energy to the electron (think about the mass difference -- photons have none, so it is very hard to exchange momentum and energy with a free electron -- although photons can kick electrons out of atomic orbit with some energy).
You can perfectly shield out RF if you have an unbroken perfect conductor surrounding your device. In practice, the conductor isn't perfect, so some of the RF penetrates, but the bigger problem is breaks in the faraday cage (for cabling into the device, cracks around doors, etc) -- RF can squirt through these. I think that shielding can get much better. It wouldn't be too expensive a proposition to shield computers from such an attack, I think...
Fatbrain.com has recently announced that it will offer an electronic publishing service, E-matter. What do you think about offering documents for download for a fee? Is this something that O'Reilly might be undertaking in the future?
Actually, the airstrip is gone now... There's just a field there, where someone plants some crops. I haven't been at PPPL long enough to have seen the airstrip -- I was told this by a tech who has been (maybe you know him - Jim Taylor).
I hate to tell you this, but your points are weak ones...
.....
> Tritium doesn't exist in nature. So how do we get it?
> From Fission reactors of course. So the fact of the
>
> MYTH I: Fusion Fuel is cheap and abundant, not so.
Fission reactors are not the only source of tritium! Yes, they
provide neutrons, yes they are used now by the gubment to produce
tritium for nuclear weapons. But you don't have to use neutrons from
a fission reactor!!! Where, you ask do you get these neutrons -- from
the FUSION reactor itself. Yep, like a fast breeder fission reactor,
fusion plants will make tritium by employing a blanket containing
lithium. This will produce plenty of tritium for the reactor to
operate. I give you that to start the process up, you will have to
have an alternative source of tritium, and a fission reactor may be
the choice for that (BTW -- the half life of tritium is about 12 years).
> Now the problem of radiation. Fusion produces huge quantities of
> Neutrons, even more than Fission produces. This means
> that the materials in and around the reactor will be
> constantly bombarded and they will be converted into
> radioactive isotopes by the neutrons. This is ignoring
> the problem of secondary radiation created by the
> countless fission reactors required to produce the
> tritium needed to run the fusion reactors. So fusion
> isn't so clean.
>...
>MYTH II: Fusion power is clean. Busted!!!
Again, you don't quite have your facts straight... Fusion does
produce neutrons, but nowhere near the amount present inside a fission
reactor!! A fusion reactor will have a neutron power load on its walls
that is about the same as the neutron power flux in a fission reactor,
but the spectrum of neutrons is much different. The fusion neutrons
(from D-T) are 14 MeV -- whereas the fission neutron spectrum is a
thermal one, with the average energy about 0.0025 eV (well, a little
bigger than room temperature, but many many orders of magnitude less
than the fusion neutrons). Power flux is essentially energy times
density. So the density of fusion neutrons is much much lower than
that in a fission reactor (for the same power load on the walls).
The radiation problem in fusion reactors is fundamentally different
than in fission plants. Yes, these fusion neutrons are absorbed by
materials and thereby make them radioactive. But we have a CHOICE in
what materials we put next to a fusion reactor (we have no choice but
to use uranium or plutonium in a fission plant). So we can minimize
the activation caused by these neutrons. Even using stainless steel,
the half-life a majority of the activation products is very short --
on the order of years, not millenia. If we use specially engineered
materials (like vanadium alloys and SiC ceramics) we can get the
activation to a minimum. The neutrons are NOT the radiation problem
in a fission reactor -- it is the horribly radioactive fission
products. These are unavoidable in fission and live forever. We
could even ELIMINATE activation entirely in a fusion plant if we use
advanced fuels like p-B11 or D-He3 (which are harder to get to fuse
than D-T, but have NO neutron emission).
> The third myth is that fusion power would be cheap, not
> so. Compared to a Fusion reactor a Fision reactor is just a pile of
> metal, and we all know how cheap they are. Sorry guys, Fusion will
> be the most expensive energy on the planet.
> MYTH III: Fusion Power is Cheap, nope.
You are right here, the fusion plant is going to cost a lot, based on
present designs. But it is not so far off as you think-- fission
plants are quite expensive due to licensing problems. Fusion will
only become economical when the price of other fuels rises (or if we
start to internalize the costs associated with pollution -- and hence
raise the cost of fossil power).
> So what does this tell us? We could get an even better
> deal using fast and slow breeder reactors using fission
> technology. It would provide all the energy we could
> conceivably need, and using breeders with reprocessing
> it would not only produce almost no new waste, but it
> could burn as fuel about 99% of the waste we have
> now. If done right it could be fairly cheap as well, and
> extremely safe. So get an almost limitless supply of
> cheap and safe energy and at the same time get rid of
> 99% of our current nuclear waste, yup, fast breeders
> sound good to me too.
I agree that fission reactors, with fuel recycling, are part of the
energy answer (+ breeding -- without breeding the uranium supply would
run out on a 200 yr timescale I think). My undergraduate education
was in nuclear engineering (and physics), and although my Ph.D. work
is in plasmas/fusion, I am still a proponent of using fission.
BUT, only "MYTH III" has a leg left to stand on, I think.
Getting the ions moving quickly isn't the only problem -- you also need to have a lot of them reacting and you need to have them interacting for a long time. The cross section for simple binary collisions between ions is much larger than the fusion cross section, and so the ions must undergo many scattering collisions before fusing -- so you have to keep them around for a while. (This is why we don't simply fire two beams of D and T at one another -- which is the easiest way to get a fast collision between two ions). The solution to this is to let the ions collide and become thermal, but keep them trapped long enough to fuse (hence the tokamak, RFP, FRC, spheromak, etc...).
/cc density to get a reasonable fusion power, and electrostatic traps can get up to (I think) 10^8 /cc if they are really clever.
Using electrostatic forces to try to trap ions in fusion is problematic because you need huge potentials to trap a reasonable number of ions (you have to overcome the potential the ions themselves create). You need something like 10^13
There is an experiment similar to the one you mention at U. Wisconsin (at least it was there when I was visiting grad schools a few years back). They are more interested in using it as a thruster for spacecraft, though (no fusion involved here). I couldn't find a link for it on www.wisc.edu, though.
In fusion science circles "alternative" fusion research usually means studying thermonuclear devices that aren't tokamaks (for instance, NSTX, the spherical tokamak (they call it a torus now) here at PPPL). Other alternative research devices are Reverse Field Pinches (MST at Wisconsin), Stellerators (LHD in japan, there's also a big one in Germany and one under development here at Princeton), Field Reverse Configurations (U. Wash, my experiment here at PPPL can be run as an FRC)....
I am a graduate student, doing my thesis research at PPPL. If anyone is in the immediate area and is interested in seeing the tokamaks (and other plasma experiments) feel free to e-mail me and we can try to set up a tour....
I work on a much smaller experiment -- I came in to Princeton wanting to work on fusion, but I got really turned off by the politics involved...
I agree with you on your point that we need more gubment sponsored R&D for all energy research!
But "ban uranium"?? You must mean ban fission -- Uranium is already relatively abundant in the earth (we don't make it). Also --
Uranium in itself is not that much of a danger (yes, it has a long half-life, but it emits mainly wimpy alpha radiation - the biggest danger is in its toxicity as a heavy metal) -- fission products are the problem. I STILL think, however, that fission energy should be used in the near term as a non-polluting alternative to fossils. At least until other possibilities are available (fusion, renewables). The waste problem would be a lot easier to deal with if we didn't have to
worry about proliferation...
>I'm not a physicist, but I know fision is a lot easier than fusion, right?
Fission is easier than fusion in general, but NOT in deuterium! (Uranium fission is a heck of a lot easier than deuterium fusion, basically because the U nucleus is large and rather unstable in the first place -- hit it with a neutron and it will break apart easy). It's pretty damn hard to just split a deuterium nucleus -- essentially impossible with a laser . Anyway, the neutrons come from fusion, and this is not at all surprising - no need to specifically look for He.