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Comment Re:Can't Carbon be nuclear? (Score 1) 253

Details matter. You are looking to build a fusion reactor, and this reaction is far more difficult than the DT reaction that the fusion community is focusing on.

They're also working on the substantially harder p-B reaction (which only has a trace of neutron output due to impurities/side reactions). That's substantially harder (and worth it!) but still not in the ballpark.

Comment Waste processing is solvable. (Score 5, Insightful) 253

The problem isn't the stuff that lasts 200,000 years. That is pretty low level. Its also not the highly radioactive stuff since it decays quickly. It's the stuff that lasts hundreds of years that is trouble. Luckily we are getting better at nuclear chemistry and our ability to separate the bad from the not bad, or even useful stuff is improving. If we hadn't had such a short sighted policy we would have moved even further.

Comment A field full of two layers of firefighters. (Score 1) 106

As mentioned previously, my mental model of semiconductors and the like is a fireman's water brigade, were either the majority of the line has buckets or empty hands.

It helps if, instead of a line, you think of a LOT them standing in a two-D array (like in the yard of the burning building, or a section of a parade that's stopped to do a little demo). It's really three-D, but we'll want to use up/down for something else in a bit...

For metallic electron conduction everybody has TWO buckets, one for each hand, and when a guy by the fire throws a buck of water on it (bucket and all) on the fire, a guy farther back immediately tosses him a bucket, the guy behind him essentially instantly throws HIM a bucket, andso on. Hands are effectively never empty.

For semiconductors, imagine two layers of these guys, the second standing on the firsts' shoulders or on a scaffold right above them, and about enough buckets for each of the guys on the ground to have two and the guys on the scaffold to have none. (There's actually many layers of scaffold, but the rest are so far up that it's hard to get a bucket to them, so they mostly just stand around.)

Usually nothing useful is happening. Everybody on the bottom layer has both hands full of buckets, and it's hard to hand a bucket up to the guys on the top.
  - Electron-hole pair creation: Somebody comes up with the energy to heave a bucket up to the guys on the upper layer, leaving a guy with one hand empty in the lower layer. (Maybe somebody (a photon, for instance) comes along with a lacrosse stick and whacks a bucket up to a guy in the top row - dying or becoming exhausted and much weaker from the effort.) Now you've got one guy with a free hand in the lower layer (a hole) and one bucket on the top layer (a free electron).
  - Electron conduction in a semiconductor is that bucket on the upper layer. The guys there can hand it around easily, or toss it along a diagonal until it would hit a guy - who catches it. They're all standing on accurately-spaced platforms so the bucket can go quite a way before somebody has to catch it. Suppose there's a slope to the yard, with the fire at the bottom. Then, if tossed too far, the bucket might pick up substantial speed and knock the guy who catches it out of place (electromigration), or fall down to the lower layer and knock another bucket out of somebody's hand and bounce, ending up with TWO buckets on the upper layer and an empty hand below (avalanche electron-hole creation).
  - Hole conduction is when you've got an empty hand on the bottom layer: Now it's easy for a guy with two buckets to hand a bucket to a guy with only one, exchanging a bucket for an empty hand. But now the guy whose hand had been empty has two buckets and nobody in the downhill/toward-fire direction to hand a bucket to, while the guy who handed it off has an empty hand and can grab a bucket from somebody farther uphill / closer to the water source - or beside him, or diagonally. So "empty-handedness" (a hole) can move around as a persistent entity while the individual buckets gradually work their way in the general direction of the fire, only making a bit of progress "when a hole comes by". Though the water makes progress toward the fire, the action is all where the holes are making progress away from the fire.
  - Electron-hole annihilation: Somebody has a bucket on the upper layer when a guy below him has an empty hand. So he drops the bucket. CLANG! Ouch! Now there's no "free bucket" on the upper layer, no free hand on the lower layer, and the energy of their separation went somewhere else (knocking the guy sideways so he bumps into his neighbor and generally making the guys vibrate, "creating a guy with a lacrosse stick who runs off to whack at buckets", etc.)
  - P-type doping: A guy in the bottom layer had a sore hand and only brought one bucket to the fire, thus having a free hand from the start. He can take a bucket when a neighbor pushes it at him (the hole moves away). But he'd like to hand it off and have his sore hand free again (so holes tend to stick around at his site). It's lots easier to "make a free hole" by convincing him to hold a bucket in his sore hand than by tossing a bucket up to the guys on the scaffold, but does take a little effort.
  - N-type doping: One of the guys on the upper level really likes to hold a bucket, so he brought one with him. The guy next to him can grab it from him, but if another comes along he'll try to hold on to it a bit until somebody shames him into letting go again or wrestles it from him. It's lots easier to get him to let you use his bucket for a while than to pull one up from the guys on the ground, but it does take a little effort.
  - Tunneling through a potential barrier: There's a ridge across the field. It's hard to hand buckets up to the guys on the ridge, so they don't flow across it very well (unless someone at the side of the field is pushing the buckets really hard...) Occasionally the guys on one side of the ridge hand a bucket through the legs of the guys standing on the ridge to the guys on the other side.
And so on. B-)

I'm keenly interested in finding more material to read up on the observed Hall effect measurements. Thanks again for your contribution to the discussion.

The wikipedia article on the hall effect has a section on the hall effect in semiconductors, but both it and the reference it uses start from treating the hole as a charge carrier with a fixed charge and a mobility different from a free electron, and just computes formulai from there.

If the hall effect on hole currents were fallout from the hall effect on the individual electron bucket-transfers, rather than the hole acting like a positive charge carrier in its own right, you'd think it would go the other way

Comment Another useful vacuum tube: Thermionic converter. (Score 1) 106

Another vacuum tube technology with current applications and substantial advantages over semiconductor approaches to the same problems is the Thermionic Converter. This is a vacuum-tube technology heat engine that turns temperature differences into electric power - by boiling electrons off a hot electrode and collecting them, at a somewhat more negative voltage (like 0.5 to 1 volt), at a cooler electrode.

Semiconductor approaches such as the Peltier Cell tend to be limited in operating temperature due to the materials involved, and lose a major fraction of the available power to non-power-producing heat conduction from the hot to the cold side of the device. Thermionic converters, by contrast are vacuum devices, and inherently insulating (with the heat conducted almost entirely by the working electrons, where it is doing the generation, or parasitic infrared radiation, which can be reflected rater than absorbed at the cold side.) They work very well at temperatures of a couple thousand degrees, a good match to combustion, point-focused solar, and nuclear thermal sources.

Thermionic converters have been the subject to recent improvements, such as graphine electrodes. The power density limitation of space charge has been solved, by using a "control grid" to encourage to charge to move along from the emitter to the collector and magnetic fields to guide it (so it doesn't discharge the control grid and waste the power used to charge it).

Current thermionic technology can convert better than 30% of the available thermal energy to electrical power and achieves power densities in the ballpark of a kilowatt per 100 square cm (i.e. a disk about 4 1/2 inches in diameter). That's a reasonably respectable carnot engine. This makes it very useful for things like topping cycles in steam plants: You run it with the flame against the hot side so it is at the combustion temperature, and the "cold" side at the temperature of the superheated steam for your steam cycle. Rather than wasting the energy of that temperature drop (as you would with a pure steam cycle) you collect about a third of it as electricity.

It also beats the efficiency of currently available solar cell technology (and the 33.4% Shockleyâ"Queisser theoretical limit for single-junction cells), if you don't mind mounting it on a sun-tracker. Not only that, but you can capture the "waste heat" at a useful temperature without substantial impairment to the electrical generation or heat collection, and thus use the same surface area for both generation and solar heating. (Doing this with semiconductor solar cells doesn't work well, because they become far less efficient when running a couple tens of degrees above room temparature.)

Comment Re:Many a young engineer.... (Score 2) 106

... every schematic drawn by every semiconductor engineer got the arrow backwards.

As I heard it, The arrow is "backward" because Benjamin Franklin, when doing his work unifying "vitreous" and "resinous" electricity as surplus and deficit of a single charge carrier (and identifying the "electrical pressure" later named "voltage"), took a guess at which corresponded to a surplus of a movable charge carrier. He had a 50% chance to assign "positive" to the TYPICAL moving charge carrier in the situations being experimented with (charge transfer by friction between different substances, currents in metallic conductors, and high voltage discharges in air and water-in-air aerosols) and happened to guess "wrong".

Thus we say electrons have a negative charge, "classical current" corresponds to the sum of the flow of moving positive charge minus the flow of negative charge (i.e. the negative of the electron current, which is all there is in normal-matter metallic conductors), the arrowhead on diodes (and junction transistors) points in the direction of classical current across a junction, and so on.

But though it's the charge carrier in metallic conduction and (hard) vacuum tubes, the electron ISN'T the only charge carrier. Even in the above list of phenomena, positive ion flow is a substantial part of electrical discharge currents in air - static sparks and lightning. Positive moving charge carriers are substantial contributors to current as you get to other plasma phenomena and technologies - gas-filled "vacuum" tubes (such as thyratons), gas an LIQUID filled "vacuum" tubes (ignatrons), gas discharge lighting, arc lighting, arc welding, prototype nuclear fusion reactors, ...

Move on to electrochemistry and ALL the charge carriers are ions - atoms or molecular groups with an unequal electron and proton count, and thus a net charge - which may be either positive or negative (and you're usually working wit a mix of both).

And then there's semiconductors, where you have both electrons and "holes" participating in metallic conduction. Yes, you can argue that hole propagation is actually electron movement. But holes act like a coherent physical entity in SO many ways that it's easier to treat them as charge carriers in their own right, with their own properties, than to drill down to the electron hops that underlie them. For starters, they're the only entity in "hole current" that maintains a long-term association with the movement of a bit of charge - any given electron is only involved in a single hop, while the hole exists from its creation (by an electron being ejected from a place in the semiconductor that an electron should be, by doping or excitation, leaving a hole) to their destruction (by a free electron falling into them and releasing the energy of electron-hole-pair separation). They move around - like a charge carrier with a very short (like usually just to the next atom of the solid material) mean free path.

For me the big tell is that they participate in the Hall Effect just as if they were a positive charge carrier being deflected by a magnetic field. The hall voltage tells you the difference between the fraction of the current carried by electrons excited into a conduction band and that carried by holes - whether you think of them as actual moving positive charge carriers or a coordinated hopping phenomenon among electrons that are still in a lower energy state. Further, much of interesting semiconductor behavior is mediated by whether electrons or holes are the "majority carrier" in a given region - exactly what the hall effect tells you about it.

So, as with many engineering phenomena, the sign for charge and current is arbitrary, and there are both real and virtual current carriers with positive charge. Saying "they got it wrong" when classical current is the reverse of electron current is just metallic/thermionic conduction chauvinism. B

Comment Re:Vacuum tubes handle EMP's better (Score 1) 106

"No point progressing since the bombs are gonna fall any day now. Then where will your fancy silicon highways and databases be?"

Given that the Internet Protocol and much of the rest of the networking technology that still underpins the Internet were developed as part of a cold-war program to create a communication system that could survive a nuclear attack that destroyed most of it, and still reorganize itself to pass messages quickly, efficiently, and automatically among any nodes that still had SOME path between them, your post seems to come from some alternate universe to the one I inhabit.

Comment Re:His basic thesis is probably correct (Score 1) 345

I think bottom line is that it is too expensive and therefore not sustainable to settle or explore space much further either funded through taxation, private fundraising or speculative investment. If launch costs come down and other technology makes it easier then either public or private efforts can do it. In terms of speculative investment... I agree there is no prospect for financial returns on any reasonable time frame so things like asteroid mining are not going to attract large investment until the math checks. And there is no business case to be made for investors to fund the settling the moon or mars or anywhere else. But neither is there a real case to be made to the public. Space exploration was the first thing to get axed in the 1970s when the US couldn't afford it. And the likelihood is that there will be additional times where funding for space exploration from the government coffers just won't be viable. Space exploration as a point of national pride only works for missions that can begin and end in a decade or so.

The trouble with being poor is that it takes up all your time.