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Particle Beam Diagnostics and Control Particle beam diagnostics and control G. Kube Deutsches Elektronen-Synchrotron DESY Notkestraße 85, 22607 Hamburg, Germany Summary. | Beam diagnostics and instrumentation are an essential part of any kind of accelerator. There is a large variety of parameters to be measured for obser- vation of particle beams with the precision required to tune, operate and improve the machine. Depending on the type of accelerator, for the same parameter the working principle of a monitor may strongly differ, and related to it also the requirements for accuracy. This report will mainly focus on electron beam diagnostic monitors presently in use at 4th generation light sources (single-pass Free Electron Lasers), and present the state-of-the-art diagnostic systems and concepts. 1. { Introduction Nowadays particle accelerators play an important role in a wide number of fields where a primary or secondary beam from an accelerator can be used for industrial or medical applications or for basic and applied research. The interaction of such beam with matter is exploited in order to analyze physical, chemical or biological samples, for a modification of physical, chemical or biological sample properties, or for fundamental research in basic subatomic physics. In order to cover such a wide range of applications different accelerator types are required. Cyclotrons are often used to produce medical isotopes for positron emission to- ⃝c Societ`aItaliana di Fisica 1 2 G. Kube mography (PET) and single photon emission computed tomography (SPECT). For elec- tron radiotherapy mainly linear accelerators (linacs) are in operation, while cyclotrons or synchrotrons are additionally used for proton therapy. Third generation synchrotron light sources are electron synchrotrons, while the new fourth generation light sources (free electron lasers) operating at short wavelengths are electron linac based accelera- tors. Neutrino beams for elementary particle physics are produced with large proton synchrotrons, and in linear or circular colliders different species of particles are brought into collision. As seen from this short compilation there exists a large number of accelerator types with different properties, and as consequence the demands on beam diagnostics and instrumentation varies depending on machine type and application. Being aware that such a wide field will not be summarized in a comprehensive way within a few pages, this report concentrates on the description of instrumentation and diagnostic concepts presently in use at 4th generation light sources, i.e. electron linac driven single-pass Free Electron Lasers (FELs). Examples from the VUV-FEL FLASH at DESY [1, 2] and the European XFEL (E-XFEL) [3, 4], currently under construction at DESY, will be given. Monitor concepts applied for particle beam diagnostics rely typically on one of the fol- lowing physical processes: (i) influence of the particle electromagnetic field, (ii) Coulomb interaction of charged particles penetrating matter, (iii) nuclear or elementary particle physics interactions, and (iv) interactions of particles with photon beams. However, there are fundamental differences in signal generation and underlying physical processes ap- plied for beam instrumentation between an electron machine and a hadron machine. In some cases this requires completely different monitor concepts even for the measurement of the same beam parameter. Therefore the emphasis in the following sections will be on diagnostics for single{pass FELs. The reader interested in general aspect of particle beam diagnostics will be referred to specific textbooks or lecture notes as in refs. [5, 6, 7, 8, 9] and the proceedings from the DIPAC and BIW conference series. Moreover, a detailed description of the sophisticated underlying monitor concepts is out of the focus of this report, only short summaries of the monitor working principles will be given together with references to the appropriate literature. This report is organized as follows: in the next section a short introduction to the instrumentation for beam current measurements will be given. Section 3 presents a brief overview over the instrumentation for beam position monitoring, while section 4 deals with transverse phase space diagnostics (i.e. emittance and transverse profile measure- ments). The last section is dedicated to beam instrumentation for the longitudinal phase space, i.e. bunch length diagnostics together with energy and energy spread monitors. In addition, a short introduction to timings systems at FELs will be given. 2. { Beam charge measurements One of the most important accelerator parameters is the electric beam current resp. the beam charge. There exist different methods to measure this value, which can roughly Particle beam diagnostics and control 3 be classified in two categories, intercepting and non{intercepting measurements. In the following a short description of the most common monitor concepts which are in use at linacs (AC measurements) will be given: the Faraday cup, the wall current monitor, the Alternating Current Transformer or Toroid, and as recent development the cavity{based dark current monitor. For more details the interested reader is referred to the tutorials about beam current measurements [10, 11, 12] and the recently published overview article [13]. 2 1. Intercepting measurements.{ Intercepting measurements are usually destructive to the beam. The generated monitor signal results from the absorption of a significant amount of the particle beam energy. Faraday cups are widely used, especially for the commissioning phase of a linear accelerator and as reference for cross-calibrations. In order to measure the beam charge, a cup made of conducting material is inserted in the beam path which is isolated from the beam pipe ground potential. When the beam hits the cup the charges are collected and integrated, delivering a signal which is proportional to the primary beam intensity. In case of lepton beams radiative losses may form an electromagnetic shower which has to be completely absorbed inside the cup material. To keep the monitor dimensions at reasonable values, Faraday cups are typically deployed at low particle beam energies, i.e. short behind the gun in a linac. Moreover, due to the heat{load problems they are usually used only for low current measurements down to the pA region. In ref. [14] the cup design for an electron beam up to 300 MeV is described. 2 2. Non{intercepting measurements.{ Non{intercepting measurements use the elec- tric or magnetic field coupling of the beam to the measuring instrument to determine the beam charge by typically integrating the beam current or Wall Image Current (WIC) coupled inductively or capacitively to the measurement device. The electric field of an ultra relativistic particle moving inside the vacuum chamber is effectively canceled out- side the conducting chamber by the WIC induced at the inner chamber diameter, while the magnetic part of the particle's field gets strongly attenuated in the non{magnetic chamber material. As consequence a high{resolution measuring device can only be in- stalled either in the vacuum chamber (as it is the case e.g. for beam position monitors), or outside the chamber if an alternative path for the WIC is provided. The latter method is widely used for beam charge measurements and realized such that a ceramic ring is soldered at both ends to the beam pipe to form a non{conducting gap through which the particle's field leaks out of the vacuum chamber. A Wall Current Monitor (WCM) is a device with rather high bandwidth up to 5 GHz and a lower cut–off frequency below hundreds of kHz which is sometimes also used for longitudinal bunch profile measurements, especially in hadron accelerators with bunch lengths in the nanosecond region. Here the non{conducting gap is bridged by a resistive network across the gap. The WCM acts as a current divider, providing separate paths for the high{frequency WIC component (through the load resistor) and the low{frequency one. The WCM lower cut–off frequency is proportional to the impedance ratio of the high{frequency and the low{frequency paths [13]. However, a WCM is prone to noise 4 G. Kube because leakage currents may flow directly through the resistors, and therefore a very good shielding is required. In addition, higher order modes (HOMs) leaking out of the gap have to be absorbed by ferrites. In case of high intensity beams care has to be taken that the heat generation due to HOM absorption in the ferrites is effectively dissipated. In an Alternating Current Transformers or Toroid the beam couples inductively to the measurement device. A particle bunch crossing the (ceramic) gap in the vacuum chamber induces a magnetic flux in a high permeability toroid surrounding the gap, i.e. it acts as a primary single turn winding in a classical transformer. The flux induces a secondary current in the transformer secondary windings. This current is a measure for the bunch current and can be detected as a voltage drop across a resistor. The bandwidth of a toroid ranges from a few Hz up to a GHz. The low{frequency cut–off is given by the winding inductance, the high{frequency cut–off by the capacitive coupling between the windings, stray and eddy currents, the energy loss in the core material, and the loss of permeability with high frequency [13]. Cavity monitors are also well suited for beam intensity monitoring. Here the ampli- tude of the monopole mode TM010 is a measure for the bunch current. Recently design and test of a monitor (originally designed as dark current monitor for the E-XFEL) with a sensitivity sufficient to resolve few-pC bunches was reported [15]. 3. { Beam position monitors Beam position monitors (BPMs) are the diagnostic devices which are most frequently used at nearly all types of accelerators like linacs, cyclotrons, synchrotrons operating with lepton, hadron or heavy{ion beams. They are essential during the phase of beam commissioning, for accelerator fault finding and trouble shooting, machine optics mea- surements, and accelerator optimization to achieve and keep the ultimate beam quality.
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