5. TIME EVOLUTION
5.2 Individual evolution of the signal of a BLM
In this section, we will study the evolution of the signal of one monitor considered independently, for different types of monitors.
5.2.1 Arc monitor — low signal
The distributions of all losses(cf.§5.1.2 and fig. 5.1)showed that the monitors from the arc have an extremely narrow distribution, with a standard deviation of∼2·10−8Gy/s
5.2 Individual evolution of the signal of a BLM
Figure 5.4: Top: Distributions of the signal of arbitrary arc monitors, from the 8 arcs of the LHC, for RS09. This figure shows how similar the distributions are, in shape and value, even for monitors situated far away from each other. The 10 pA electronics offset corresponds to 1.8·10−7Gy/s, which is within the variations of signal. The BLMs see no loss. Bottom: Separated distributions of the signal of arbitrary arc monitors. These figures give the values of average signal and standard deviation.
5. TIME EVOLUTION
(compared to the dynamic range of 108) for roughly 300.000 entries. The signal of these monitors was studied in detail.
The BLMs were arbitrarily selected and taken from the 24th cell on the left of each interaction point, which is far away from the interaction points. The figure 5.4 top shows that the distributions are similar in shape and in value. Most arc monitors behave the same way, and the selected ones are a representative sample as seen in fig. 5.1.
All the values of average and standard deviation of the different signal distributions, calculated numerically, are given byfig. 5.4 bottom: average is around 2.3·10−7 Gy/s, and standard deviation around 1.8·10−8 Gy/s for running sum 9.
The values of the average signal are very close to the value of the 10 pA current offset permanently sent in the tunnel electronics. It corresponds roughly to a value of 65 expressed in bit units, and 1.8·10−7Gy/s for running sum 9. The smallest step, one bit unit, is worth 2.76·10−9Gy/s.
Moreover, the signal of these monitors was checked during times when there was no beam in the machine, and the signal distributions showed the same average. This shows that the arc monitors record no signal coming from deposited charges, and the signal measured over the offset level is only due to noise. There are no detectable losses in the arcs during normal operation.
5.2.2 Variations of the signal of a monitor
After studying the distribution of the low-signal BLMs, the next step was to study the variations of these signals. It was done by calculating the distribution of the difference between the signal of one BLM at one second and the previous second (cf. fig. 5.6).
This allows to study the effect of the noise.
The first observation is that the distribution of variation is similar for all chosen monitors, and has an mean value of zero at the precision of the measurement of the loss: the size of one bin corresponds to the value of one electronics bit. Then, the distribution can be fitted with a Gaussian (cf. fig. 5.5). The values of the mean and standard deviation given by the fit agree with the numerical average and standard deviation of the distributions (cf. fig. 5.6) to a precision higher than one electronics bit. The quality of the fit, including the errors, is given by the value of χ2 over the number of degrees of freedom, and was calculated to be ≃0.82.
5.2 Individual evolution of the signal of a BLM
Figure 5.5: Example of a Gaussian fit of the distribution of the signal variation of an arc monitor. The values of theσandµparameters of the gaussian agree with the numerical average and standard deviation to a precision higher than one electronics bit (width of one histogram bin).
To the level of precision of the loss measurement, the variations of the signal are Gaussian.
BLM expert name Location
BLMEI.04R1.B1E10_TANAR.4R1 Right of IR1 BLMEI.04R1.B2I10_TCTVA.4R1.B2 Right of IR1 BLMEI.04R7.B1E10_TCSG.A4R7.B1 Right of IR7 BLMEI.03L5.B1I10_MQXA Left of IR5 BLMEI.03R1.B1E30_MQXA Right of IR1 BLMEI.04R7.B2I30_MQWA.A4R7 Right of IR7
Table 5.1: Lists of monitors showing a high average signal (around 10−4Gy/s) in the default loss profile. The names are given in the same order as infig. 5.7 top.
5.2.3 IP monitors — high signal
The same study was done for monitors showing a permanently high signal(cf. fig. 5.7).
BLMs having some of the highest average signals (around 10−4Gy/s) were selected.
5. TIME EVOLUTION
Figure 5.6: Distributions, for selected BLMs of the arcs, of the differences of the signal at one second and the signal at the previous second, during the stable beam of the 23dof July 2011. The difference is a discrete number of bits; each bin covers exactly one value of the difference, and is the size of one electronics bit: 2.76·10−9Gy/s.
5.2 Individual evolution of the signal of a BLM
Figure 5.7: Top: Distributions of the signals of monitors showing high losses during stable beams. The data were taken during the stable beam run of the 23rd of July 2011. Bottom:
Separated distributions of the signal of monitors with high signal. These plots give the values of average signal and standard deviation. The names of the monitors are given on the top figure, and intab. 5.1. The distributions are not Gaussian, and correspond to losses happening during operations which can’t be expressed analytically.
5. TIME EVOLUTION
Their names are given in (cf. tab. 5.1). They are found around the interaction points:
this is were the collimators and absorbers, which permanently create losses, are in- stalled. These losses correspond to luminosity induced losses in IR1 and IR5, and to cleaning in IR7. The luminosity losses are constant, with a narrow distribution; the collimation losses show more variations even during “stable beams”.
Figure 5.8: Norm of the difference with the previous second during the stable beam of the 30th of April 2011. Even if this is the “stable beam” period, the number of peaks decreases during the first 3 hours. The norm of the difference also decreases in the first 3 hours; this is shown in more detail infig. 5.9.