On Friday, May 20, 2011 at 5:34:49 AM UTC+1, Lester Welch wrote:

I follow the LHC via their dashboard - http://lhcdashboard.web.cern.ch/lhcdashboard/
and understand the individual graphs except for the "tune" graphs.
Can someone explain what they are and their significance.

I have just started looking at the LHC dashboard and have very little understanding of the displays. Do you have a link to somewhere where I can find an explanation of the displays?

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On 9/18/15 9/18/15 12:38 PM, mkiederer@aol.com wrote:

On Friday, May 20, 2011 at 5:34:49 AM UTC+1, Lester Welch wrote:

I follow the LHC via their dashboard - http://lhcdashboard.web.cern.ch/lhcdashboard/
and understand the individual graphs except for the "tune" graphs.

The tune of a circular accelerator (like the LHC) is how many betatron oscillations occur during one turn of the beam around the ring. These oscillations are transverse, and occur because the ring uses strong focusing (i.e. uses quadrupole magnets to repeatedly focus the beam as it goes around the
ring). Like most accelerators, the LHC keeps vertical and horizontal betatron oscillations separate (uncoupled), so each beam has two tunes.

There are resonances in the accelerator lattice that would cause the beam to be lost if the tune ever hit them. Every integer is such a resonance [#]. There are
other resonances that make up the "dynamic aperture" shown in the plots on that webpage. They have subtracted the integer just below the tune. The two axes are horizontal and vertical tune, and the dot is where the beam is right now (one plot for each beam in the LHC). It looks like there is also some history plotted.

[#] Consider a place in the ring where a small disturbance occurs
that (for example) kicks the beam up a little bit; when it returns
on the next turn, with an integral number of betatron oscillations
it will be in phase with the previous turn, so the same disturbance
will again kick the beam up a little bit; over many turns that will
become a large bit and the beam will hit some magnet aperture and
be lost (and probably melt the magnet). With a fractional tune,
successive turns arrive at this place with different positions and
angles, and the small disturbance does not build up (because the
focusing keeps the beam in the apertures).


Tom Roberts

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On Monday, May 23, 2011 at 1:54:55 AM UTC-4, Ken S. Tucker wrote:

Hi Lester, nice to meet you.

On May 22, 8:04 pm, Lester Welch <lester.we...@gmail.com> wrote:

On May 22, 8:48 pm, "Ken S. Tucker" <dynam...@vianet.on.ca> wrote:

On May 21, 11:54 pm, Tom Roberts <tjroberts...@sbcglobal.net> wrote:

Lester Welch wrote:

When I look
at the size of the beams (~1 mm) and knowing the distance they travel it is a technical marvel that they can be steered to collide. But if one beam was off - say - ~1/10 mm then the # of collisions would be significantly reduced. How accurately do they know the concentricity of the beams? What graph shows that? Perhaps the calculation of luminosity takes all of that into account.

I'm not sure what you mean by "concentricity".

At collision, the LHC beams are on the order of 30 microns in diameter [#]. This
depends on the transverse emittance and the beta-star at the collision point --
the latter varies for each experiment, depending on their design and specific
operating mode. I believe they use automatic feedback to align the beams for
maximum collision rate (i.e. maximum luminosity).

[#] This is one sigma. The beams are approximately Gaussian out
to about 3 sigma, but are scraped at larger radius to remove the
tails (aka halo).

The calculation of luminosity certainly takes "all that" into account, plus a
whole lot more. They monitor it continuously. It is plotted in "Fill Luminosity", which has a separate line for each experiment.

Note the ILC is MUCH more challenging to align the beams to collide, as their
beams are less than a micron in size at the collision point. Ditto for CLIC.
Electron machines can achieve much lower emittances due to the damping available
from synchrotron radiation. And the lower momentum of these machines permits a
lower beta-star.
Tom Roberts

I'm still seriously impressed by the focus and sweep angle control
of a CRT monitor (TV) 3 electron beams,RGB.
Furthermore, such an accelerator was affordable to the average
consumer!
That's mass produced 'beam' control.
I think we could do much better than a micron.
Regards
Ken S. Tucker

What is the beta* parameter on the luminosity graph?

You can email LHC with a reasonable question, and they will
either answer or direct you to a site that helps.
(they're the experts about LHC).

As I understand, the Beta is v/c , Beta* is the combined
collision energy of v/c with a beam doing -v/c.
Some authors use a relativistic Beta, and can mean the
increased energy in the 'beams' colliding.
(thats the relativity of inertial mass).

Focusing a beam is a challenge.
(I 1st learned about that using old B&W CRT's, a pin-prick
dot burns off the florescence).

Did you want details?
Regards
Ken S. Tucker

Hi lester and Ken

"burns off the florescence" There was a problems with ions in the early B&W CRTs. A few negative ions would mix with the electron beam and be accelerated towards the phosphor. Being heavier the ions would be less deflected by a magnetic force mostly
hitting the central part of the TV screen. The end result was a brown fuzzy dot about 3 inches in diameter caused by ion hits. The solution was very clever. They pointed the electron gun off by 15 degrees then used what was called an ion trap magnet to
bend the electron beam back in the forward direction. The ions were too heavy to make this turn and ended up hitting the electron gun walls where no harm was done. At a lated date they coated the phosphor with a thin film of aluminum to prevent this
damage so the clever magnetic ion trap was lost to history. If you can find an early 1950s / late 1940s TV you can still see the old ion trap around the neck of the CRT and electron gun visibly pointed off 15 degrees.

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