Problems with lower emittance and higher intensities?

[jmc2000]

What sort of problems occur when using lower emittance and higher intensities?

[adam_jeff]

As far as I know, lower emittance is quite hard to achieve but it doesn't really cause any problems. It does make the 'space charge' effect worse (this is the defocusing effect of having so many positive charges close to each other) but this is more a problem in the injectors than the LHC (space charge effect decreases with increased beam energy).

Higher intensity, however, can drive lots of non-linear instabilities in the beam: space charge (as above, mostly a problem in the injectors, and they already run with higher intensities so should be fine); electron cloud; RF cavity loading & higher-order modes; tune spread; and others.

[jmc2000]

As far as I know, lower emittance is quite hard to achieve but it doesn't really cause any problems. It does make the 'space charge' effect worse (this is the defocusing effect of having so many positive charges close to each other) but this is more a problem in the injectors than the LHC (space charge effect decreases with increased beam energy).

Higher intensity, however, can drive lots of non-linear instabilities in the beam: space charge (as above, mostly a problem in the injectors, and they already run with higher intensities so should be fine); electron cloud; RF cavity loading & higher-order modes; tune spread; and others.

Just curious, since they had to refilll 6 times with 840b before lowering the intensity and emittance. They're at 1380b with nominal intensity and emittance now, but with what looks like a new filling strategy: The beam size increases linearly from zero to around 0.5mm, until the end of the ramp, looking at the display dashboard. Or, it could be another way of measuring the beam size and an artifact of the measuring device, as you pointed out in an earlier thread.

[Kasuha]

I believe it's just lack of temporal resolution. They still (and always will) have to inject the beam in trains and for the train to get prepared and accelerated in SPS and then syncronized and injected to LHC it takes a bit of time but the technical minimum is about 20 seconds only while in past they were usually needing minutes for each.

[adam_jeff]

@ Kasuha: I think jmc is referring to beam size, that is diameter, not beam intensity. But you are quite right about the injections.

@ jmc2000: I see the ramp that I think you're referring to. In fact, the linear increase in beam size comes before the ramp, when there is no beam. In any case beam size = 0 can't be realistic. So I assume it's some kind of instrument error or, more likely, a database / plotting error in the graph: a lot of the instruments don't record data when there's no beam, or only record once an hour (no point filling the database with lots of zeros) and I gues that the dashboard graph has just interpolated a line between the last available data point and the first one of the new fill

[jmc2000]

@Adam the previous displays for the past 7 days don't have that ramp. There is an interesting paper:

Transverse Emittance Blow-Up due to the Operation of Wire Scanners, Analytical Predictions and Measurements : Particle Accelerator Conference, 2005. PAC 2005

So it could be a change in the operation of the beam size measuring instrument.

[adam_jeff]

Yes, there's definitely something funny about that particular ramp, but I still think it's software or database related.
The wire scanners are rarely used nowadays, since they are designed for low intensity. They can't scan them with more than 5 or 6 bunches in the machine, otherwise they burn the wire. I think that the main beam size measurement comes from the synchrotron light monitor now.

[RocketManKSC]

Yes this plot problem common with many COT and standard plot packages that want to plot drawing a line between points. This is not at all how a strip chart works. For Shuttle program we have special plotter code to handle this. For those using COTS programs we provide missing point retrieval software.