[HN Gopher] Highest-Voltage Electron Gun
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Highest-Voltage Electron Gun
Author : gmays
Score : 36 points
Date : 2024-10-23 14:42 UTC (4 days ago)
(HTM) web link (www.bnl.gov)
(TXT) w3m dump (www.bnl.gov)
| swayvil wrote:
| Do electrons get weird under high acceleration?
| analog31 wrote:
| Not particularly weird, but they radiate energy as they
| accelerate.
|
| You don't want to be in the same room as this thing when it's
| running.
| swayvil wrote:
| What kind of energy?
| mgc_mgc wrote:
| Photons. https://en.wikipedia.org/wiki/Bremsstrahlung
| analog31 wrote:
| Indeed, and this is a reason why, traditionally,
| electrons are studied using linear accelerators, rather
| than rings. It was the main purpose of Stanford Linear
| Accelerator (SLAC) as I understand it.
|
| When electrons are accelerated in rings, it's usually
| because you want to do something with the radiation, and
| not the electrons.
| Loughla wrote:
| What would that do to you? Sunburn?
| swayvil wrote:
| I'm figuring the photonic stuff covers all
| electromagnetic. So sunburn, microwave, cancer. Lots of
| options.
| klodolph wrote:
| Ordinary consumer equipment already accelerates electrons
| enough to produce X-rays. Or at least, it did, back in
| the CRT era. This accelerator is presumably more powerful
| than consumer entertainment hardware.
| guepe wrote:
| That reminds me of a question I was asking myself: instead of
| using rare gas for ion thrusters... why not ejecting very
| high velocity electrons ? Their mass is much, much smaller
| than any atom, but why wouldn't that lead to a thrust ?
| Possibly without limit except power - no limit on a "tank"?
| hansvm wrote:
| Where do the extra electrons come from?
| sbdhzjd wrote:
| The thruster would quickly die out once the static
| electricity on the vessel created an energy well that
| matches the energy of the electrons thrust.
|
| Instead use a powerful laser. Light carries momentum
| without the need to preserve charge.
| guepe wrote:
| Ah! That "well" is exactly what I couldn't understand
| from ion thrusters, where they shed electrons as they
| expulse ionized gas. All I could find was "to keep the
| same charge", but since there is no absolute ground,
| what's the problem?
|
| You just explained it :)
| addaon wrote:
| > why not ejecting very high velocity electrons ?
|
| 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.
| Workaccount2 wrote:
| How could you drop the ball so hard that you write an article
| titled "highest voltage electron gun" and not even state once
| what the voltage is in the article?
|
| It's like writing an article about "the world's tallest tower"
| and just saying "it goes really high!" to convey its height.
| Someone wrote:
| FTA: _"Read the complete story at the BNL website."_
|
| 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"_.
| MengerSponge wrote:
| 10x doesn't look enormous?
| fnordpiglet wrote:
| 350kv is definitely not enormous. Marx generators can make
| 100MV potentials and high energy particle accelerators can
| make fields in the gigavolt and teravolt ranges. The
| headline is just a bit ludicrous.
|
| (IANAP)
| sroussey wrote:
| terrestrial gamma-ray flashes in the upper atmosphere can
| have energies of up to 20 million electronvolts
| labcomputer wrote:
| The quote is wrong. It should say that the glass
| formulation limits the voltage _for a given acceleration
| distance_. In other words, the glass formulation limits the
| _field_ strength, not the _potential_.
|
| 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.
| generuso wrote:
| CRT tubes usually have stern safety labels warning to not
| exceed specified voltage, not because there will be an
| electric breakdown, but because this will generate vastly
| more X-rays.
|
| 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.
| ipdashc wrote:
| I think that was for "mundane" CRT tubes for old TVs and
| stuff. X-ray tubes for machines like baggage scanners can
| reach 150 kV or so, and it's nothing particularly special.
| Google seems to show papers for 400 kV tubes. 350 kV sounds
| high but not ludicrously so, so I'm probably missing
| something.
| __MatrixMan__ wrote:
| Since we're talking about electrons I assume that 350 kV
| implies that the energy of these particles is 350 keV?
|
| 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?
| MathMonkeyMan wrote:
| One thing that comes to mind is that the voltage difference
| determines the force on the charged particle, but in
| principle if the voltage can be maintained, then the same
| force could be used put an arbitrary amount of energy into
| the particle. So, at most 350 keV the first time around,
| but then at most 350 keV the second time around, and the
| third...
|
| I know almost nothing about particle physics.
| __MatrixMan__ wrote:
| I'm no specialist either, and initially I had the same
| thought, but wikipedia says this about an electron volt:
|
| > 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.
| MathMonkeyMan wrote:
| Strictly speaking, the energy gained by a charged
| particle along any path is the integral of the dot
| product between the electric field and the line element
| along the path (edit: times the charge). In the absence
| of changing magnetic fields, this is always the
| difference between the voltage at the starting and ending
| points.
|
| 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.
| mattashii wrote:
| > So, at most 350 keV the first time around, but then at
| most 350 keV the second time around, and the third...
|
| 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").
| MathMonkeyMan wrote:
| I don't think that this is true. The velocity of the
| particle entering the "high" side of the field does not
| affect the force applied on the particle by the field.
| Check out the wiki article on [cyclotrons][1]. I think
| the trick is turning the field on and off.
|
| Again, I'm no expert.
|
| [1]: https://en.wikipedia.org/wiki/Cyclotron
| gus_massa wrote:
| I tbink both of you agree!
|
| 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.
| MathMonkeyMan wrote:
| I interpreted mattashii's point as being this:
|
| 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."
| twic wrote:
| FWIW the LEP collider accelerated electrons to 104.5 GeV:
|
| https://en.wikipedia.org/wiki/Large_Electron%E2%80%93Positr
| o...
|
| But that was a ring five miles across, not a sphere roughly
| the size of a sumo wrestler.
| fecal_henge wrote:
| I think there are a few factors: 2 - beams going in two
| directions so double energy. 2000 - mass difference (feel
| like I'm going to get corrected om this one) n - the LHC is
| the last in a whole load of booster stages. It doesnt hit
| TeV from a standing start.
| scythe wrote:
| In a typical X-ray tube (which is also a cathode-ray tube),
| electrons (negative) are accelerated from the cathode (also
| negative) towards the anode (positive). A medical imaging
| X-ray operates at 150 kV, while a radiotherapy linear
| accelerator (LINAC) usually goes up to 22 MV. However, in
| these applications, the electrons are "pulled" by the anode
| as well as "pushed" by the cathode and RF cavities (RF only
| used in LINAC). In CRT televisions, the X-rays generated by
| the tube are shielded by a special glass screen containing
| strontium and barium.
|
| 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.
| 01100011 wrote:
| The point of the article isn't just the voltage. It's that
| they created a photocathode which converts laser pulses into
| electron bunches with a particular set of qualities(polarity,
| density). It's also how they diverged from traditional
| designs and placed the HV supply outside the chamber(it is
| difficult to get 350kV through a metal vacuum chamber wall
| for somewhat obvious reasons).
| wbl wrote:
| 10 times more energy. In fact 100 times more energy across
| any capacitance. The distances for safety are 10 times more.
| This thing is a beast.
| Luc wrote:
| https://www.bnl.gov/newsroom/news.php?a=222117
|
| 350 kV.
| dang wrote:
| (We've changed the URL since this was posted - see
| https://news.ycombinator.com/item?id=41965388)
| perihelions wrote:
| Primary source is
|
| 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.
| dang wrote:
| Changed from https://news.stonybrook.edu/university/sbu-
| helping-bnl-devel... above. Thanks!
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