ALICE was set up with AEMITR machine optics using the usual quadrupole scan method. Steering through the two apertures in Straight 2 was completed and terahertz generation was set up at 45kV antenna voltage (the maximum available at the time). The manually adjustable lens (12) was left at a position which would nominally focus the terahertz pulses at the interaction point in Straight 2. The data acquisition program was set up to capture the image of the beam in the chicane on screen ST2-DIA-YAG-01 at each step of the terahertz pump beam optical delay translation stage.
Synchronization between the probe laser and the electron beam was established as described in Chapter 4, using a photodiode pick up for the laser and a BPM pick up for the electrons; it had been calculated earlier that the signal from the laser arrives at the oscilloscope approximately 1.2 ns after the signal from the electrons, if they are perfectly synchronized inside Straight 2 of the accelerator. This point corresponds to a Coherent Synchrolock-AP fundamental phase value of 34900. If they are aligned in time on the oscilloscope, then the terahertz (laser) is 1.2 ns before the electrons in Straight 2. This point corresponds to a Synchrolock fundamental phase value of 37400.
During measurements of the terahertz electric field strength, the optical delay line allows the timing of the pump laser beam (which is converted to terahertz) to be varied relative to the probe laser beam. Because the ALICE laser systems (both the photo- cathode laser and the Ti:S) are synchronized to the ALICE RF system, the optical delay line can therefore also be used to vary the time of arrival of the terahertz pulses relative to the electron bunches. The temporal overlap of the terahertz and pump beams at the interaction point was determined previously, with the ZnTe crystal in the accelerator, to be at an optical delay line stage position of 4.65 mm. As the maximum range of this stage is±100 mm, the maximum time window available from the operation of the optical delay line is 1,333 ps. The measurements reported here were made over an optical delay line translation stage range of +90 to -90 mm, corresponding to a time window of 1,200 ps.
The experimental parameters for a selection of data sets are listed in table 5.7, along with the corresponding figure numbers. The data displayed is the horizontal and vertical sizes (σx and σy) of the beam image recorded on the chicane YAG screen (ST2-DIA- YAG-01). In order to try and reduce the noise in the measurements (due to train-to- train instability) each data point in the figures is a rolling average of five consecutive measurements, divided by the overall average. The Synchrolock-AP phase provided a coarse adjustment of the timing of the electron bunches and the terahertz pulses. This was then combined with the range and fine step control provided by the optical delay line to explore in detail a larger time window than could have been reached by either mechanism independently. Obviously, care was taken to ensure that there were no gaps in the range of timings explored. These data should be compared to figure 1.21, which shows what we would expect to see under the best possible circumstances, with all beam parameters the same as figure 1.19.
There is no evidence of a change in the electron beam size in figure 5.35, where it would be expected to be seen if there had been any interaction. Detailed analysis of the data, to improve the signal-to-noise ratio, did not reveal anything further. A number of further sets of data were collected, after making step changes in the Synchrolock phase, chosen so that for adjacent sets of data there was always some overlap of the time window. No evidence of interaction was detected in these additional measurements.
Part way through the collection of one of these sets of data a large global accelerator phase shift disrupted the excellent machine set-up and had to be recovered before further potential interaction data could be recorded. Figure 5.31 shows the effect of this sudden phase shift on the horizontal and vertical size of the beam image at the chicane YAG screen.
Figure 5.31: The effect of a sudden accelerator global phase shift on the horizontal
and vertical beam sizes (σxandσy) measured on the chicane YAG screen, during a scan of the relative timing of the electron bunch and the terahertz pulse. Each data point is a rolling average of five consecutive measurements, divided by the overall average.
Figure Synchrolock phase photodiode signal timing (ps) terahertz pulse timing (ps)
5.32 28,800 -3540 -2370 5.33 30,850 -2770 -1600 5.34 33,410 -2000 -830 5.35 34,842 -1230 <60 5.36 36,500 -460 710 5.37 38,000 310 1480
Table 5.7: Experimental data sets recorded on 6/11/11. A positive value for the photodiode signal timing indicates that it is measured on the ’scope to be before the electron BPM signal; a positive value for the terahertz pulse timing indicates that the
Figure 5.32: The horizontal (black) and vertical (red) beam sizes (σx andσy) on the chicane YAG screen, as a function of the relative timing of the electron bunch and the terahertz pulse. Each data point is a rolling average of five consecutive measurements, divided by the overall average. The terahertz pulse is 2370 ps after the electron bunch
Figure 5.33: The horizontal (black) and vertical (red) beam sizes (σx andσy) on the
chicane YAG screen, as a function of the relative timing of the electron bunch and the terahertz pulse. Each data point is a rolling average of five consecutive measurements, divided by the overall average. The terahertz pulse is 1600 ps after the electron bunch
Figure 5.34: The horizontal (black) and vertical (red) beam sizes (σx andσy) on the chicane YAG screen, as a function of the relative timing of the electron bunch and the terahertz pulse. Each data point is a rolling average of five consecutive measurements, divided by the overall average. The terahertz pulse is 830 ps after the electron bunch
Figure 5.35: The horizontal (black) and vertical (red) beam sizes (σx andσy) on the
chicane YAG screen, as a function of the relative timing of the electron bunch and the terahertz pulse. Each data point is a rolling average of five consecutive measurements, divided by the overall average. The time window includes temporal overlap of the terahertz pulses and electron bunches in Straight 2 and is thus when we would have
Figure 5.36: The horizontal (black) and vertical (red) beam sizes (σx andσy) mea- sured on the chicane YAG screen, as a function of the relative timing of the electron bunch and the terahertz pulse. Each data point is a rolling average of five consecutive measurements, divided by the overall average. The terahertz pulse is 710 ps before the
Figure 5.37: The horizontal (black) and vertical (red) beam sizes (σx andσy) mea-
sured on the chicane YAG screen, as a function of the relative timing of the electron bunch and the terahertz pulse. Each data point is a rolling average of five consecutive measurements, divided by the overall average. The terahertz pulse is 1480 ps before