Highest-Voltage Electron Gun
bnl.gov
bnl.gov
You don't want to be in the same room as this thing when it's running.
When electrons are accelerated in rings, it's usually because you want to do something with the radiation, and not the electrons.
Instead use a powerful laser. Light carries momentum without the need to preserve charge.
You just explained it :)
Fun thought experiment: what happens to a beta emitter in outer space?
This works, initially. Now, you have to balance the charge on your space craft, or else you won’t be able to eject any more electrons.
If you balance this charge by ejecting protons, you have a hydrogen thruster.
If you balance this charge by ejecting positrons, you have a photon thruster (in the far field).
The former is still “tank” limited. The latter requires high energy to get any useful impulse.
... the ejected propellant mass times its velocity is equal to the spacecraft mass times its change in velocity. [0]
Electrons have very little mass; do we eject lots more of them? Is there away to store that many electrons in a small enough volume? Or does their velocity make up for the lack fo mass? Also, if we must expend mass, I don't see how the 'no limit' idea works; I'm also suspicious of any claim of free, unlimited propellant, if that's what you mean.
There's a very good chance that is all my misunderstanding of something ...
[0] https://descanso.jpl.nasa.gov/SciTechBook/series1/Goebel__cm...
Basically, it's taken as a "fact" that the relative speed of light is the same for two observers regardless of one observer's velocity vs. the other observer's. So if something is going 0.8x the speed of light, but light is still somehow observed to be traveling at the same speed for both (speed of light relative to both the fast-moving observer and the slow-moving observer), this apparent inconsistency is solved by realizing that ("magically") lengths are contracted for the faster observer. So the light appears to go the same distance to both observers, because distance itself is different to the two observers.
The net effect is that from 0.5x to 0.99999x the speed of light, momentum increases asymptotically as an object approaches the speed of light. Theoretically, if your spaceship could (truly magically) capture the momentum from ejecting one single electron to 0.9999....(58 nines in total)... a 1,000kg spaceship could achieve escape velocity to leave the entire solar system just from that one electron!
0: https://library.fiveable.me/principles-of-physics-iv/unit-9/...
https://www.bnl.gov/newsroom/news.php?a=222117 ("High-Voltage Gun Accelerates Electrons from Zero to 80 … Percent the Speed of Light")
This one's the same article, but in excerpted form.
It's like writing an article about "the world's tallest tower" and just saying "it goes really high!" to convey its height.
That article (https://www.bnl.gov/newsroom/news.php?a=222117) has the answer:
“In commissioning tests in a basement lab at SBU, ramping up the voltage to the goal of 350 kilovolts took about 23 hours. Then the gun operated maintenance free for six months”
The goal isn’t to get a high voltage, though; it’s to get fast electrons. for that, the article does say what this machine does: “The powerful gun speeds up the velocity of electrons to 80 percent the speed of light”
By the way, 350kV does not look enormous compared to cathode ray tubes. https://en.wikipedia.org/wiki/Cathode-ray_tube#Body: “The glass formulation determines the highest possible anode voltage and hence the maximum possible CRT screen size. For color, maximum voltages are often 24–32 kV, while for monochrome it is usually 21 or 24.5 kV”.
(IANAP)
If you want 10x more voltage, you can just make the CRT 10x longer. Think about how deep a normal TV is. Now multiply that by 10. An instrument of that size comfortably fits in a normal lab room.
And that’s only for static fields. If you’re willing to play tricks with dynamic fields you can use smaller potentials.
So the quotation "glass formulation limits the voltage" is probably correct, but it is supposed to mean: "glass formulation determines how opaque to X-rays the tube is, and what acceleration voltage can be safely used while remaining in compliance with the health regulations".
I think this is what it is supposed to be. Lead based glass is so common in CRTs precisely because of this reason.
The LHC often boasts energies in the TeV, but hadrons are much heavier than electrons, which I assume accounts for why this machine is so much smaller and why the energies are something like 8 orders of magnitude different.
Can someone tell me if these are reasonable assumptions?
I know almost nothing about particle physics.
> An electron-volt is the amount of energy gained or lost by a single electron when it moves through an electric potential difference of one volt.
I feel like if it was actually an electron-volt-second, that would appear in the definition. So I'm thinking that once the electron has traveled from a place of higher voltage to a place of lower voltage, it has gained energy according to the potential difference, and it doesn't actually matter whether it took a second or a year to do so.
It's easy to think of voltage as something like field strength, to be held more or less constant as the particle accelerates, but really it's a difference between two points, start and end, so the trip length has already been accounted for in the voltage measurement, and doesn't need to be further accounted for by measuring the time it took? Unsure, but that's my feeling anyhow.
What I was getting at is that there's only so much energy a field can put into a particle. Either the voltage will drop because the machine can't "keep up," or the particle will reach a terminal velocity where the force applied by the field is insufficient to accelerate the particle any more. But, barring those two things, the particle will continue to accelerate.
The voltage is the gradient across which the electron moves and gains (or loses) momentum, similar to a ball rolling up- or downhill. Once the electron has moved to the positive side of the electric field ("to the bottom of the hill" so to say), it can only gain more energy from that same field by first losing the equivalent in energy by moving back to the negatively charged segment ("the top of the hill").
Again, I'm no expert.
There are two solutions to gain the energy again from "the same" field.
1) Lost the energy it won in the last pass.
2) With the field to avoid losing the energy in the return trip.
You have a ring around which charged particles can travel. The voltage at the start is V, and the voltage just behind the start (the end) is defined to be zero. A charged particle "falls" down the potential until it gets to right before where it started. But then it will momentarily feel a large force in the opposite direction as it "climbs" back up to V from zero.
I don't know how particle accelerators avoid this, but the wiki on cyclotrons refers to a "rapidly varying electric field."
The trick to undestand the Cyclotron is in the graphic: chttps://en.wikipedia.org/wiki/Cyclotron#/media/File:Cyclotro...
It has two semicircular parts A an B, that act like a big capacitor. The electrons travel in circles, sometime it's inside A and sometimes is in B.
If the voltage between A and B is constant, it accelerates and the decelerates and then accelerates and the decelerates and then accelerates and the decelerates and then ... that is quite boring.
The trick is that when it's inside A you make A negative and B positive, so it accelerates going from A to B. While the electron is inside B you switch the voltages and now B negative and A positive, so it accelerates going from B to A. Now, While the electron is inside A you switch the voltages and now A negative and B positive, so it accelerates going from A to B. ...
So the trick to gain energy in both directions is to change the voltages of the semicircular parts. This change cost energy, so there is no magic creation of energy.
Probably the article in Wikipedia explains it better.
https://en.wikipedia.org/wiki/Large_Electron%E2%80%93Positro...
But that was a ring five miles across, not a sphere roughly the size of a sumo wrestler.
In this case, what is interesting is not the voltage per se, but the fact that the electron beam is polarized. A typical electron beam does not have aligned spins. In the Brookhaven setup, the electrons are spin-aligned. The full article subtitle is:
>Scientists and engineers develop world's highest-voltage, highest-intensity polarized photocathode electron gun, a crucial component for the future Electron-Ion Collider
COTS X-ray systems use mostly thermal emission tungsten filament cathodes or, rarely, field-emission carbon nanotube cathodes. The carbon nanotubes sound fancy but are mostly known for fancy sales pitches and frequent repair tickets. Photocathodes are a specialized technique.