Hacker News Reader

Comment by Someone | Hacker News Reader

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”.

View on HN

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.

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).

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?

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.

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.

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.

> 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").

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

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.

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."

If all the other charges in the universe are fixed, then mattashii is right.

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.

FWIW the LEP collider accelerated electrons to 104.5 GeV:

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.

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.

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.

10x doesn't look enormous?

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)

terrestrial gamma-ray flashes in the upper atmosphere can have energies of up to 20 million electronvolts

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.

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.

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.

Reply on news.ycombinator.com