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Comment: Some more plasma physics is needed in their model (Score 4, Informative) 179

The paper is a one pager of introductory plasma physics. It isn't a serious calculation and it wasn't meant to be. Anyway ...

Their model is as follows. A plasma will reflect all electromagnetic radiation below a certain frequency, determined by its density. The plasma exerts a pressure like a gas and they then assume confinement of the plasma with a magnetic field, balancing the plasma pressure with the 'pressure' that a magnetic field exerts on charged particles. They then say that we can make magnetic fields in the range up to 100 T and working back, estimate the plasma frequency, which turns out to be in the UV. So great, you can deflect lasers into the UV with a modest confining field.

You need to look at some of the other numbers though.
First, what sort of plasma density do you need to reflect UV ? The answer is something like 10^28 per cubic m. This is enormous - fusion plasmas are about a million times less dense). It's getting close to solid state density eg if a solid has atoms 0.2 nm apart this is 10^29 atoms per cubic m. That is not going to be easy .... The other problem is that at such a high density, the collision frequency is very high so that a magnetic field is not very effective at producing confinement. Probably useless in fact.

The other thing to look at is the required plasma temperature. They assume a temperature of 1000 K, Unfortunately, the density of a plasma at 1000 K at thermal equilibrium is extremely low unless the background pressure is huge. So it has to be a lot hotter, in particular, comparable with the ionization energy which is roughly 100 000 K. And really, we need a fully ionized plasma because the magnetic field is not going to confine the neutral gas that we are using to make the plasma so that means we need a 100 000 K plasma. This means that the required magnetic field goes up by a factor of 10.

Would somebody else like to estimate how much power you need to dump into the plasma ?

Comment: Re:And how do you start it? (Score 1) 127

by hyperfine transition (#46664853) Attached to: New US Atomic Clock Goes Live

Typically, there is no single clock that is "the reference". The international time standard, UTC, is an average of about 400 or so atomic clocks from all around the world. There are a number of caesium fountains currently contributing to UTC. Even in a single laboratory, where there a number of clocks, these clocks will usually be averaged and one clock then adjusted to keep to this average. There are various practical reasons for this. If you have a number of similar clocks, the average will have a more stable frequency than a single clock. Some clocks are better at very short averaging times, others better for the long term so you can make an average that takes this into account. State-of-the-art clocks seldom work continuously so you need something else as a flywheel between operating times.

The main use for very good clocks is as frequency references, or to measure the time interval between two events (in which case any offset cancels out) rather than time of day references. Sub-nanosecond synchronisation of distributed systems is only used in some very specialised scientific applications like Very Long Baseline Interferometric telescopes.

Comment: Re:So... (Score 1) 127

by hyperfine transition (#46659299) Attached to: New US Atomic Clock Goes Live

The receiver might cost $60 an antenna suitable for permanent outdoor installation might cost the same, but running cable to the roof, assuming that's possible, could easily cost a few thousand dollars to do properly. You'll want lightning protection too, if you're running the antenna cable down in to your server racks.

But for nearly everyone, as others point out, the 10 ms or so accuracy you will get from a nearby NTP server, is more than enough.

Comment: Re:Mod parent up (Score 1) 127

by hyperfine transition (#46659259) Attached to: New US Atomic Clock Goes Live

In short, "counting vibrations" is a poor description of what's going on. In the case of a caesium fountain, what you're doing is driving a resonant transition in the atom with external microwaves. When the microwaves are tuned to the transition, the Cs atom changes its state, which can be detected. To make the microwaves, you usually start with something like a very good 5 MHz crystal oscillator and multiply it up (plus add an offset, also derived from the crystal) to get the transition frequency ( for the transition in Cs which defines what one second is, this is an exact frequency) . You then adjust the frequency of the crystal (this is done electrically) until you're centred on the resonance in the Cs atom. So then you can just electrically count the oscillations of the crystal and 5000000 of these will be your definition of what one second is.

Comment: Re:Labview (Score 1) 876

by hyperfine transition (#46193703) Attached to: Ask Slashdot: Why Are We Still Writing Text-Based Code?

I use LabVIEW for FPGA programming, with other LabVIEW code running on a real-time controller interfaced to the FPGA, with top-level control, logging etc on a PC. LabVIEW is nice for the FPGA because the metaphor suits the parallelism of FPGAs. Part of why it works too is that you're only dealing with a very limited subset of LabVIEW's functionality and you can't do complicated things, at least if you want to clock at high speed.
But the code running on the real-time controller and PC ... yuck. I'm an experienced C-coder and LabVIEW can be painful when algorithms eg for a servo, get complex. The standard joke "LabVIEW makes easy things easy" has plenty of truth in it. What it means is that once you get beyond a certain level of complexity, LabVIEW doesn't offer any advantage over a text-based language.

Comment: Nobody seems to know what to do with a 3D printer (Score 2) 155

by hyperfine transition (#46084697) Attached to: $499 3-D Printer Drew Plenty of Attention at CES (Video)

From the article "Gary Shu, XYZprinting's market development division senior manager, said the 3D printer can quickly create objects that users may need in their homes, such as a plastic cup or a plastic spoon.". I hope he comes up with a few better ideas than that.

Actually, a 3D printer would be useful to me for hobby projects like cosplay props, although probably a bit expensive. But around the house ? I look around for things completely made out of plastic that it would be practical to print if they broke or I needed another one but it's a struggle.

I suppose what all of these 3D printer manufacturers want to convince themselves and their investors is that there is a mass market for their product. The cheap printers still look very much like a hobbyist tool to me though.

Comment: Re:Actually... (Score 1) 77

by hyperfine transition (#44996231) Attached to: Quantum Computers Check Each Other's Work

I think your summary is right. Skimming the article
http://arxiv.org/abs/1309.0005
I get the same impression. In the paper they say that you have to be careful to design your tests to catch all the errors that would affect the answer. The summary doesn't say it, but one significant aspect of the work is that it is the first experimental demonstration of verification of a quantum computation.

Comment: Re:No need for cameras. (Score 1) 732

by hyperfine transition (#44733169) Attached to: EU Proposes To Fit Cars With Speed Limiters

GNSS receivers don' t use change in position to measure speed. They use the Doppler shift of the GNSS carrier signal ie a frequency measurement and this can be done accurately. Typically, accuracies of about 0.1 km/h are specified by the manufacturer. The usual caveats about satellite visibility apply.

If you think the system is working, ask someone who's waiting for a prompt.

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