Data Acquisition and Real-Time Analysis

The quantities needed to carry out measurements (equations 45, 46) consist of the beam current and the beam lifetime together with a time stamp belonging to each of these values. The SLS control system provides such information by feeding these quantities to EPICS channels which can be read out and remotely archived. It is however necessary to collect this data, save it and evaluate it in real-time, in order to adapt the experimental procedure to current circumstances as well as to get experimental results during data acquisition.

An online tool has been provided which reads the data from the EPICS channels, transforms it to an appropriate format and units and then calculates other relevant properties. An example for such a calculation is determining the fraction of beam loss due to elastic gas scattering opposed to the loss through Touschek Scattering at a certain vacuum pressure. This ratio could be derived from the beam current. Recall equation 48:

$$\tau_{ts}=\frac{\tau\cdot\tau_{el}}{\tau_{el}-\tau}$$ (54)

and according to [14]

$$\tau_{el}~[h]=(112\pm10)\cdot\frac{(3.2\pm0.4)~[pbar\cdot s]}{P~[pbar]}$$ (55)

with the gas pressure related linearly to the current⁸:

$$P~[pbar]=5.11~[pbar]+0.01566~[pbar\cdot mA^{-1}]\cdot I_{beam}~[mA]$$ (56)

Once the necessary values are computed, they are written to a data file for off-line analysis and backup purposes. After a sufficient amount of data has been obtained, certain values of interest can be determined by fitting theoretical models to data. An example of this procedure is the calculation of the current polarization level which requires a polarization build-up to be fitted as depicted in figure 6:

$$a+b\left[1-\exp{\left(-\frac{x-c}{d}\right)}\right]$$ (57)

The important fit parameter is $d$, the characteristic equilibrium build-up time. In the fit seen in figure 6 $d=1261\pm 12$ which (according to equation 41) leads to $\tau_d=(3894\pm114)~s$ and therefore to $P_{eff}=(62.5\pm0.6)~\%$.

All these steps were implemented in a single program which allowed a complete online data analysis of polarization build-up in real-time. The actual program code is not complicated, but proved to be a very powerful tool during data acquisition.