# Design and Diagnostics of High-Precision Accelerator Neutrino Beams
## 3 Extraction and Monitoring of Primary Protons
The quality of the extracted spill is another critical issue of slow extraction: for precision experiments, a deviation of the spill from a constant particle flux might represent an issue (e.g., higher counting uncertainties and pile-up). To remove the high-frequency components of the RF, the beam is often de-bunched. In this case, the dominant frequencies that survive in the extracted spill are typically the power supply harmonics and other noise coming from the magnet currents. Mitigation techniques to dump these parasitic frequencies have been recently developed at GSI [59], J-PARC [60], and the SPS [49]. To monitor the primary protons of a neutrino beam experiment, dedicated diagnostic devices are installed in the proton transfer line before the hadron production target. This solution typically constrains the total neutrino flux uncertainty derived from the primary proton beam to a maximum of 1-2% [61; 62; 30], which, in standard neutrino beamlines, represents a sub-leading contribution with respect to the dominant hadron production uncertainties. A great variety of diagnostic devices is available for particle accelerators [63; 64; 65], according to their main energy, intensity, extraction scheme, particle type, etc. The most relevant primary diagnostic instruments for neutrino beams are briefly described in the following. * The total proton intensity is typically measured with Beam Current Transformers (BCT), non-destructive inductive devices that provide an absolute measurement of the total beam charge. Many variants are available for different types of beams, as their performance is strongly dependent on the spill length and instantaneous beam current [66; 67; 68; 69]. * Another important class of devices is the Secondary Emission Monitors (SEM). They can be used for different applications and consist of thin, metallic foils that are crossed by the beam to measure the emitted charge. When their signal is time-sampled, they can be used to measure the beam intensity profile during the extraction (Figure 3). Figure 3: Comparison between a nominal slow extracted spill and a “burst-mode slow extraction” one. The spill profiles have been measured with a secondary emission monitor at the SPS, during dedicated machine tests. The same intensity is extracted in the two cases. SEM that are made up of several ribbons can be used to measure the transverse beam profile. These devices can provide a measurement of the absolute beam intensity, too. However, their absolute calibration is prone to subtle systematic effects like charge emission due to activation [70]. SEM are particularly useful with \(\mathcal{O}(\mathrm{s})\)-long slow extracted spills, which are difficult to measure with a BCT. Many recent developments have shown performance improvements and possible alternatives, as, for instance, the graphite-based profile monitor at J-PARC [71] and the Cherenkov detector for proton Flux Measurments (CpFM) tested at the SPS [72; 73; 74]. Other commonly used devices for beam profile measurements are the Optical Transition Radiation (OTR) screens. These are particularly convenient for ultra-relativistic beams, where the particles crossing a thin foil emit transition radiation, due to the change of medium, that can be recorded by light detectors. * Beam Position Monitors (BPM) are a class of devices used to measure the transverse position of the beam inside the vacuum chamber or in air [75]. They are non-destructive devices composed of a set of two or four electrodes, which act through capacitive pick-up. Because of this, they require short pulses or bunched beams: they are often replaced by SEM or OTR screens in slow-extracted beams [70], or paired to them for increased reliability [76]. * Beam Loss Monitors (BLM) are crucial machine-protection elements. They measure beam losses, i.e., showers originated from the beam hitting or scraping a beamline element, and provide feedback to the control system.