Summary of the Workshop
Beam Dynamics meet Diagnostics
G. Franchetti (GSI)
The workshop "Beam Dynamics meets Diagnostics" pursued the mandate of EuCARD2 XBEAM Network to help breaking down barriers among groups of different expertise within the accelerator community. This workshop took place at the "Convitto della Calza" in Firenze from 4 to 6 November 2015. The 65 participants (see Fig. 1) mainly came from European countries, though a noticeable participation from non-European country such as USA and Japan has also been noted. Shown below is a breakdown of the participants by country and institute of origin.
* Austria: 1 (TU Wien / CIVIDEC);
* France: 2 (CEA, SOLEIL);
* Germany: 17 (ANKA-KIT, GSI, JFZ, DESY);
* Italy: 9 (LNF-INFN, LNL-INFN, U. Roma, U. of Sannio, U. Bologna);
* Japan: 5 (J-PARC/JAEA, KEK);
* Slovenia: 1 (Instrumentation Technologies);
* Spain: 4 (ALBA-CELLS, ESS-Bilbao);
* Sweden: 3 (ESS);
* Switzerland: 9 (CERN, PSI);
* Turkey: 1 (Middle East Technical University);
* UK: 7 (Diamond, ISIS, JAI, STFC);
* USA: 6 (BNL, Fermilab, ORNL/SNS);
The workshop was composed of the following sessions:
1. Experiences and beam commissioning: what worked? What didn't?
2. Challenges of future accelerators/upgrades;
3. Basic observables for beam dynamics and diagnostics;
4. Diagnostics and future trends;
5. Diagnostics and high intensity;
6. Storage rings & lepton machines;
7. News from Linacs.
The details of the program as well as a collection of all talks are available on the indico web site
https://indico.gsi.de/conferenceDisplay.py?ovw=True&confId=3509
Summary by session
Session 1: Experiences and beam commissioning: what worked? What didn't?
This session reviewed the experience from operation and commissioning of existing machines.
* Jorg Wenninger presented an overview of LHC run 2 through October 2015. The LHC re-commissioning in 2015 had the goal to achieve reliable operation with 25 ns bunch spacing. In June 2015 run 2 physics production started together with an electron-cloud "scrubbing" run. By October 2244 bunches per beam have been accelerated and collided in the LHC. The status of the machine is excellent, with one of the primary remaining concerns related to the further evolution, and scrubbing, of the e-cloud. At the maximum number of bunches, the heat load generated by the e-cloud is close to the cooling limit of the cryogenics; hence electron cloud is a main limitation for physics operation. In 2015, run 2 has reached a proton-proton peak luminosity of 5.1x10E33 cm-2s-1. The main challenges for beam instrumentation are electron-cloud diagnostics and instability diagnostics. The heat load on the cryogenic system is used as one of the e-cloud diagnostics tools; in addition, the energy loss per turn, as measured through the bunch-by-bunch synchronous phase shift, is used for e-cloud diagnostics. Measurements of beam emittances, though synchrotron-radiation monitors, are used to track blow up from beam-beam effects, e-cloud or impedance-related instabilities. After run 1 the instability diagnostics was improved. Diagnostics also is essential to understanding, and correcting, luminosity imbalances between experiments (~10%, ..., ~4%). Aperture restrictions can be mapped by using orbit bumps in conjunction with BLMs and BPMs. An earthquake in Chile excited oscillations of the LHC beam with 0.2 mm amplitude, indicating that the LHC could be used to study earthquakes.
* Michiko Minty reported the experiences and challenges of RHIC at Brookhaven. The standard RHIC beam-diagnostics instruments are BPMs and beam profile monitors. The RHIC challenges comprise measurements of beam emittances and of the linear optics. Significant efforts were invested to achieve better accuracy and resolution. RHIC emittances are measured indirectly via the luminosity monitors (ZDC) and with Ionization Profile Monitors (IPM). Measurement and control of tune, coupling, and chromaticity reached a better resolution, which has been exploited for use in feedback. Also a beam-based feedback has improved the RHIC accelerator performance. Future planned studies concern the demonstration of coherent electron cooling as well as bunched-beam electron cooling, experiments with upgraded physics detectors and the preparation for eRHIC.
* Tadashi Koseki reported experience from the beam commissioning of the J-PARC Main Ring (MR). The challenges of MR beam commissioning included resonance correction. BPM data provide information on the closed-orbit distortion, bunch position, as well as intra-bunch position. The requested BPM resolution is 20-40 micro-m. Resonance compensation is necessary for mitigating space charge effects: the linear coupling resonance was compensated in the MR at injection and at 3 GeV, and beam transmission consequently improved from 81% to 95%. A 2nd harmonic RF increased the beam transmission 97.5% to 98.5% in the 360kW operation. Beam instabilities are suppressed via proper bunch-by-bunch feedback systems, which prove to be extremely effective. Also an intra-bunch feedback system is used in operation, and provides a good performance. Slow extraction is performed with a dynamic bump system: this system improves the extraction efficiency from 98.3% to 99.5%. In addition the spill structure of the extracted beam is controlled with a transverse RF employed to grow the beam amplitude, and using a feedback on fast response quadrupoles. OTR (Optical Transition Radiation) monitors combined with FL (Fluorescence) systems are used to monitor both beam profile and halo. Studies of the optimal working point have been conducted, with a final beam survival ratio of 99.5% for proper vertical tune setting. In conclusion it has been demonstrated that the MR could deliver 1 MW beam power, if operated at high repetition rate. For 2018-2019 the actual target power values are 750 kW for fast extraction, and 100 kW for slow extraction.
Section 2 Challenges of future accelerators/upgrades;
* The challenges of the FAIR project were discussed by Ralph Steinhagen. The FAIR accelerator challenges include diagnostics and achieving an ultrahigh beam vacuum at highest beam intensities, superconducting magnets, RF cavities, and beam cooling. The dynamic pressure must be carefully controlled to avoid vacuum instabilities. A NEG coating of the vacuum pipe combined with pumping/collimation is planned for SIS18. U29+ loss positions in SIS100 are peaked (by design) at the cryo-absorbers (collimators). The dynamic vacuum requires a huge pumping speed, which is achieved by the cryogenic vacuum chambers, together with NEG-coating of most of the warm vacuum chambers. Instrumentation to detect beam loss is indispensable as a tool for controlling dynamic pressure. Beam loss control is also mandatory for the multiturn injection, which requires BPMs and IPMs to maintain injection steering and verify beam profiles. The FAIR proton (antiproton) and Uranium scenario requires a good control of the machine optics, which translates into additional diagnostics requirements. Special beam dynamics scenarios are found for slow extraction, which requires a good control of machine optics including Q and Q', as well as a good control of the nonlinearities arising from magnet field errors (or their correction). The allocated beam loss budget necessitates the control of energy deposition in the magnet coils by means of IC-type BLM signals. All foreseen beam instrumentation and diagnostics tools are essential for the proper functioning of the FAIR accelerators, in particular good longitudinal diagnostics is also needed to control the RF gymnastics.
* The technical challenges of EDM measurements at FZ Juelich were discussed by Frank Rathmann. The measurement of the electric dipole moment is very demanding for the accelerator. A high-precision, electrostatic storage must meet tight tolerances for alignment, field stability, field homogeneity, and shielding from any perturbing magnetic fields. The beam intensity should be reasonably high, and the stored beam should consist of polarized hadrons. Beam positioning monitoring should reach 1 nm, and the electric field 17 MV/m for plates of 2 cm distance. In the talk it was envisioned to use COSY for the first direct p, and d EDM measurements through a resonance method with "magic" RF Wien filter.
* Frank Zimmermann presented the FCC challenges. The next future circular collider, FCC, is conceived to reach 100 TeV c.m. for a ring circumference of 100 km. The CDR and cost review will be delivered by 2018 in time for the next update of the European Strategy for Particle Physics. The preliminary FCC-hh layout foresees two straight collimation lines, each 2.8 km long, and an unprecedented stored beam energy of 8 GJ. Issues of machine protection, beam abort, collimation system design, beam transfer and injection, therefore, are of high relevance. Noteworthy, in particular, is the need to continually control the transverse and longitudinal emittances in the presence of strong radiation damping using noise excitation, keeping either the beam-beam tune shift or the pile-up constant. The beam diagnostics for this emittance control comprises synchrotron-radiation based emittance measurements, tune-shift measurements, and bunch-by-bunch luminosity measurements. Achieving FCC-hh beam stability calls for fast feedback systems (5 ns bunch spacing), the use of Landau octupoles, etc. A critical design parameter for the collimation system is the physical size of the collimator gaps, as this determines the system length. Diagnostics should enable the required orbit and optics control at the collimators. Machine protection demands a safe injection, calling for adequate orbit and optics control in both transfer lines and collider rings. The acceptable loss level during ramp and in store can be confirmed by BLMs. Rapid beam abort to minimize losses relies on early failure detection. Good IR optics control for phase 2 could be realized by optics measurements with an AC dipole. For a high-luminosity lepton collider, installed in the same 100 km tunnel, FCC-ee, the BeDi challenges are obtaining small beam sizes and measuring these. Beam loss diagnostics at strategic locations is important for monitoring the radiative Bhabha scattering and beamstrahlung occurring at the collision point. Good optics control is necessary with BPMs and non-invasive profile monitors, e.g. ones based on synchrotron radiation.
Section 3: Basic observables for beam dynamics and diagnostics;
This section highlighted the main observables for accessing beam dynamics in hadron and electron machines, as well as dedicated studies of beam dynamics.
* Manfred Wendt reviewed selected uses of the beam position monitors: (1) Machine commissioning, trouble shooting, basic beam optics verification; (2) Measurement of injection oscillations, betatron and synchrotron tunes; (3) Beam injection / extraction optimization; (4) Chromaticity measurements; (5) Dispersion and beam energy measurements; (6) x-y coupling analysis; (7) Detailed beam optics studies, including mismatch and error location: magnet alignment and errors, non-linear field effects, etc.; (8) Beam phase and bunch arrival time measurements; (9) Orbit stabilization through BPM-based feedback systems. Alignment of the BPMs was scrutinized along with the correction of the pickup signal for nonlinear responses, and even the BPM wake-potential and impedances. In addition, signal processing, normalization, S/N ratio & BPM resolution were discussed. Long-term drift compensation, BPM resolution vs. beam current were among the other important topics addressed in this talk.
* Marcel Rosenthal presented the impact of orbit distortions on EDM measurements. The spin motion on the closed orbit was studied, and the effect of misalignments was examined. Critical is the role of the radial magnetic field which leads to a tilt of the spin closed orbit. The ORM method can be used for the orbit correction.
* James Zagel reviewed the use of residual gas ionization monitors at Fermilab. For all FNAL accelerator systems IPMs are used, for a multitude of applications: (1) turn by turn measurements; turns can be averaged to achieve, in principle, any accuracy desired; (2) injection tuning/matching; routinely used for the first 500 turns to monitor injection oscillations; (3) position and beam-size measurements anywhere in the cycle; collecting up to 65k samples per channel; (4) new at the Main Injector are High Speed Digitizers (80 MHz) providing multiple samples of each batch for better accuracy/sample, and allowing for digital filtering of signals on A/D; 96 channels are digitized; a control grid is used to gate off electrons for unmeasured batches, which should significantly increase the MCP lifetime; up to 65000 samples can be recorded at either for 1 batch with a sample per revolution, or spread across all batches for a number of turns.
* Robert Williamson discussed a non-destructive measurement for rings with strong space charge. Space charge widens the measured profile. The resulting measurement error was evaluated with the help of simulations, and a profile correction for the effect of space charge was introduced. The corrected profile was compared with the ideal profile. This correction of the IPM profile image is useful for IPM applications in operation and R&D studies.
* Toshiyuki Mitsuhashi (KEK) presented the design of a novel coronagraph for beam-halo observations at the LHC. After explaining the underlying principles of the coronagraph, he proposed a concrete plan for performing halo observation using a coronagraph in the LHC. It consists of 2 phases. Phase1: test observation; design and construction of a coronagraph by modifying the optical design of a coronagraph built for KEK in 2005; aim would be a halo observation with 10E3 to 10E4 contrast to the beam core; the device would use the B2 SR monitor line. A diffraction analysis for the coronagraph at this phase includes the discussion on the Mie-scattering noise from the surface of the optical components. Phase2: design and construction of an optimum coronagraph for the LHC, aiming 10E5 to 10E6 contrast.
* Enrica Chiadroni discussed the longitudinal diagnostics for low-emittance electron beams. The characterization of both longitudinal and transverse phase spaces of the beam is a key ingredient for the verification and tuning of photo-injector parameters. It comprises measurements of mean energy, energy spread, beam arrival time, of time jitter and energy jitter. Ultra-short bunches require special methods, which are of two types: 1) time domain methods, based on, e.g., streak camera, RF diagnostics, or transversely deflecting RF cavity, and 2) frequency domain methods, which may involve the radiation from the electron bunch, coherent radiation effects, and autocorrelation measurements. Instruments for ultra-short bunches include time-resolved X-ray diagnostics, THz spectrometers, and plasma-based deflectors. Some conclusions from this presentation are that (1) RF deflectors are more suitable than electro-optical sampling techniques, and that (2) coherent radiation based diagnostics allows fs scale resolution.
* Alessandro Drago reviewed the main tune measurements techniques. This discussion covered the kickers used to excite the beam, and several types of beam position monitors such as stripline kickers. The beam position monitors are used to retrieve the frequency of the beam oscillations (used in DAFNE). Sum and difference operation for the individual position-monitor signals involve combiner and hybrid devices. The tunes are retrieved via a spectrum analyzer. Measurements of tunes with bunch-by-bunch feedback systems active were discussed for ELETTRA and SuperKEKB. BBQ techniques were also reviewed, and so was was the use of Schottky noise (Tevatron, LHC, RHIC). The tune feedback itself is also provides a bunch-by-bunch tune measurement, and in DAFNE this method is used to evaluate the strength of the e-cloud (whose effective density is proportional to the bunch tune shift). For example, the mitigation of the e-cloud calls for evaluating the effect of solenoids and of clearing electrodes, which can be based on the bunch betatron tune measurements using the bunch-by-bunch feedback system.
* Rodri Jones reviewed various techniques to measure chromaticity: tune change for different beam momenta; width of tune peak or damping time; amplitude ratio of synchrotron sidebands; width ratio of Schottky sidebands; bunch spectrum variations during betatron oscillations; and head-tail phase advance (in time domain). The main conclusion of this talk is that RF frequency modulation is most widely used as it is sufficient for the majority of machines. This method also offers the possibility for on-line measurement, if a sensitive, continuous tune measurement system is available. The main limitations of the RF frequency modulation arise from 1) the induced orbit change [prohibiting on-line measurement at synchrotron light sources & high intensity machines (e.g. LHC with physics production beams)], and 2) the relatively slow measurement rate. Schottky diagnostics is ideal for unbunched beams and for heavy ion machines. Other techniques could be used to measure chromaticity, such as a) fast RF phase modulation with tune spectrum demodulation, or b) low-excitation head-tail measurements.
* Rogelio Tomas reviewed the evolution of beta-beating measurements ranging from the ISR results of 1983, over LEAR 1988, LEP 1993, CESR 2000, HERA-p, SPS in 2000, PEP-II in 2006, to the first measurements in LHC in 2008, where a 100% beta-beating was corrected after identifying a single error source with the so-called segment-by-segment technique. AC dipoles are used to excite the beam in a non-dangerous fashion at many of the hadron storage rings (AGS, RHIC, SPS, Tevatron & LHC). The "N-BPM" method can be used to improve the beta measurements: in the LHC optics commissioning of 2015 the rms beta-beating was reduced to 2-3%. Beta-beating can also be measured using amplitude from BPM, but in this case the BPM calibration becomes important. This method was used at RHIC. Studies for HL-LHC show that beta-beating correction there seem to become ever more challenging (without correction numerical predictions show maximum beta-beat in the HL-LHC is ~ 200%!). A review of measurements for light sources shows that ORM and LOCO are the major techniques used at these machines presently. Improved BPM electronics and filters have allowed faster results from turn-by-turn techniques. Comparison of beta-beating after correction, applying various methods, is ongoing in several laboratories (SOLEIL, DIAMOND, ALBA, SLS, ESRF, PETRA III). A preliminary conclusion is that LOCO techniques yielded a beta beating 0.3% in SOLEIL and DIAMOND, while the corresponding results from turn-by-turn techniques are still missing.
* Kazuhito Ohmi discussed space-charge studies based on beta-function measurements in the J-PARC MR. The beta function and phase advance, and x-y coupling have been measured in J-PARC MR with turn-by-turn monitors. These data have been used in a model of the beam dynamics, which includes the resonance excitation terms due to space charge and due to the lattice. Emittance growth is evaluated in combination with synchrotron motion. One aim of this study is to understand why the new MR working point (21.3, 21.4) is better than the working points found in beam experiments and simulations.
* Peter Forck reviewed the tune observation in SIS18 at high beam intensity. The motivation for this talk is the planned use of the SIS18 as a booster in the FAIR project. The precise control of beam parameters will allow emittance conservation and minimization of beam loss. Tune values are determined by beam excitation with a band-limited noise, and recording the beam motion over 4096 turns, which defines the possible resolution. High beam current shifts the tune spectrum, and understanding the relation of this spectral modification with the beam parameters is of great importance. Tune, orbit and beam position in SIS18 are obtained with the TOPOS system. Space charge is one important effect which modifies the tune spectra at high current. Theoretical models predict the shift of the spectral line consistently with observations for moderate space charge, but the measured spectra exhibit more complex features than the models. Quadrupolar oscillations have been measured as well. These measurements have confirmed that the space charge tune shift, DQsc, changes with beam intensity approximately as expected.
* The topic of impedances and pickups was covered by Mauro Migliorati. The coupling impedance of an accelerator or of accelerator components is measured with several methods. Bench measurements using wire or bead-pull techniques are often employed to measure the impedance of resonant structures. Beam based measurements of coupling impedances observe the effects which the impedance has on the beam dynamics, such as a tune shift or phase shifts with intensities. Also unwanted collective instabilities, like coupled bunch instabilities, contain important information on the machine coupling impedances. The real part of the longitudinal coupling impedance may be derived from the synchronous phase shift vs. beam current. The imaginary part of the longitudinal coupling impedance is obtained from the change of bunch length with beam current. The imaginary part of the global machine transverse impedance can be estimated from the tune shift with intensity. The local transverse impedance may be inferred from 1) impedance-induced orbit shift with intensity; 2) impedance-induced betatron phase beating with intensity, observed with a kick excitation or AC-dipole excitation.
* Bench measurements of small impedances were presented by Andrea Mostacci. Bench measurements are an important tool for estimating the coupling impedance of any particle accelerator device. The well-known technique based on the coaxial wire method allows exciting, in the device under test, a field similar to the one generated by an ultra-relativistic point charge. The coaxial wire method was reviewed along with the formulae widely used to convert measured scattering parameters to longitudinal and transverse impedance data. Coaxial resonator configurations suitable for measuring small impedances were discussed, and some typical measurement examples of interest for the CERN Large Hadron Collider were presented.
* Robert Voutta reviewed measurements of beam heat load. Superconducting magnet undulators are used because of their higher field for a given gap and period length, compared with electro-magnets, and their resulting increased brilliance and spectral range. However, for the cryogenic design of superconducting undulators the beam heat load to the cold vacuum chamber is a relevant issue. Sources of beam heat load include synchrotron radiation, resistive wall heating, RF effects (geometric impedance), and ion bombardment. The COLDDIAG device was installed in Diamond, and the beam heat load has been measured as function of bunch length. The measured heat load is higher than predicted by theory. The physical origin of the unexpected heat load remains an open issue.
* Volker Hejny discussed the polarimetry for monitoring long coherent spin precession, and polarization based feedback. The goal of this study is to establish, maintain and monitor long coherent spin precession. With the EDDA detector the polarization and the spin coherence time can be measured. Also the dependence of the spin coherence time on the chromaticity was observed. Using the polarimetry a spin tune feedback system can be constructed. The spin tune is used to probe ring imperfections. In fact, EDM causes a tilt of the spin closed orbit, which can also be caused by the ring imperfections. Upcoming activities include the following: analyzing powers for pC and dC scattering; and the development of a dedicated polarimeter for EDM measurements.
Section 4: Diagnostics and future trends;
* Erich Griesmayer discussed the trends in diamond detectors for beam diagnostics purposes. Diamond is used for fast signal response (subnanosecond time scale). Single proton pulses may be detected, as demonstrated in 2003. In 2004 diamond detectors were used for the ATLAS detector at the LHC. Diamond detectors are also used as beam loss monitors (in operation at the LHC since 2010). In addition, diamond detectors are deployed in high radiation areas (HiRadMat). At the CERN n-TOF experiment diamond detectors are employed to detect neutrons. Use of these detectors is also found in research reactors, and for measuring 14 MeV neutrons obtained from a tritium target hit by a deuteron beam (PTB Braunschweig). An X-ray BPM has been realized using a diamond with a position resolution of 2 nm.
Section 5: Diagnostics and high intensity;
* Davide Reggiani (PSI) talked about the high intensity proton accelerator at PSI. Since many years the PSI 1.4 MW proton accelerator is an established and reliable user facility. The "production" beam current was gradually increased from 100 ?A (1974) to 2.2 mA (2009). Very low beam loss level (10E-4) is a mandatory condition, and to reach this goal beam diagnostics is essential! High current runs at 2.4 mA have been taking place during 2 shifts (16 hours) every 14 days. Application for an 2.4 mA operation license is under way. The flat top cavity was the main source of operational beam instabilities over the last years. Efforts are being made towards 3.0 mA beam intensity: 1) Target E (region) collimator redesign has been completed; its manufacturing is expected by 2017 ; 2) New injector-2 accelerating cavities are to be installed by 2018; 3) Studies are being carried out to refurbish the Flat Top cavity and/or replace it by a new design. ?
* Hiroyuki Harada discussed the injection and extraction for the J-PARC RCS. Horizontal, vertical, and longitudinal painting schemes were presented. The fast extraction was discussed including the issue of "ringing": The kicker field ringing changes the position of the two extracted beams. Ringing compensation is accomplished through an alternating time shift of half of the kickers. Measurement of the beam center has revealed a complex time structure. Systematic timing scans between all and every kicker, and other timing optimization studies were shown. Longitudinal studies were presented with comparisons of measurements with simulations, including predictions of the bunching factor. Measurements at extraction energy were presented, with studies of the intensity dependence of the rms beam width. It was concluded that: 1) the injection painting is well-controlled, mitigating space-charge effects and reducing the foil-hitting probability; 2) the extraction beam deviation caused by the kicker field ringing was corrected well by the applied timing optimization; 3) the beam loss except foil scattering was minimized well during the entire acceleration cycle, up to 1 MW-equivalent beam intensities; 4) beam loss due to foil scattering will be reduced by larger injection painting; and 5) the beam power for the user program will be increased to 1 MW in the near future.?
* William Blokland presented electron beam profile monitor measurements on the Montague Resonance in the SNS Ring. At the SNS it is found that beam profiles are coupled under certain conditions, and the setup to get right beam size on target then becomes more complicated. An electron scanner is used to retrieve the beam profile: the deflection of a straight sheet of electron is bent proportional to the local transverse beam density. The beam behavior close to the Montague resonance was investigated. However, it seems to be excluded that the emittance exchange measured is due to the Montague resonance because the observed coupling is stronger at lower beam intensities. Instead the observed behavior resembles the effect of a linear lattice coupling, although this coupling had been corrected by skew quadrupole prior to the experiment; further investigations are ongoing [at the SNS the beam dynamics meets the diagnostics every day drinking an espresso! (What else?)].
* Markus Steck discussed how to detect a single particle by using a resonant cavity. This experiment was performed at the ESR, with the help of Schottky mass spectrometry. The precision of this method allows detecting the radioactive decay of a single particle. The Schottky resonator design and its main features were discussed. With this device even the measurement of electron capture decay is possible. Also an analysis of the cooling process can be carried out with this technique. In the FAIR collector ring isochronous mass measurements are planned for RIBs with energies 400-740 MeV/u.
Section 6: Storage rings & lepton machines
* Riccardo Bartolini reported the measurement of multibunch instabilities using the transverse multi bunch feedback (TMBF) data at Diamond. After the review of the multibunch instabilities, the experimental campaign at Diamond was described. The experiment was performed by artificially exciting a mode using a stripline driven at the frequency of that mode. Afterwards the excitation was stopped and free oscillations were measured, eventually running a feedback to damp unstable modes. The search for impedance sources is being carried out using this method. Although all coupled bunch modes were eventually stabilised by the TMBF system, it is clear that the machine impedance is changing with time in ways that are difficult to interpret and predict. The specific resonances observed at Diamond will be further investigated in order to identify possible causes beyond IDs, BPMs and RF cavities (e.g. collimators, etc.).
* The topic of analytical and numerical non-linear beam dynamics optimization was examined by Laurent S. Nadolski. This presentation covered the beam dynamics challenges for ultra-low emittance lepton rings. The tools and methods for optimization of the nonlinear dynamics are focused on single-particle transverse dynamics and consist of: 1) tracking codes for storage rings; 2) frequency map analysis; 3) determination of momentum aperture and Touschek lifetime; 4) resonance driving terms; and 5) new optimization methods based on genetic algorithms. The presentation concludes that nonlinear beam dynamics optimization is still a challenging task; the theory is mature and equipped with modern tools, allowing refined and accurate models (e.g., ones based on magnetic measurements, and alignment measurements). Beam-based experiments and a high quality of diagnostics (i.e. ease of use) are necessary, especially, to cross-check and benchmark the models against experiments. Online capability to explore nonlinear dynamics (tune shift, dynamic aperture, FMA, etc.) is highly desirable.
* Sara Casalbuoni discussed the diagnostics with undulator radiation. The constructive interference of the radiation emitted at each pole of the undulator gives rise to flux peaks at certain photon energies in the undulator spectrum: this can be used to qualify the undulator field quality as well as for electron beam characteristics such as measurements of emittance and energy spread. This talk presented the study of the undulator spectrum, in particular its sensitivity to energy spread, and emittance in terms of line width, and harmonic flux ratio. Examples from ESRF, and APS were shown, as well as for a laser wakefield accelerator.
* The FFAG diagnostics and challenges were presented by Suzie Sheehy. The Fixed Field Alternating Gradient accelerators (scaling and non-scaling) are characterized by the absence of a reference orbit, high repetition rate, and by the possibility of simultaneous acceleration of multiple bunches having different energies. The diagnostics needs include measurements of the beam size and beam position at all energies, the confirmation of the "k" value of the field index, and the monitoring of the "closed orbit" distortion. The talk reviewed the specific diagnostics installed at the KURRI FFAG: bunch monitor, probe intercept devices, triangle-plate bunch monitors, and radially movable BPMs. EMMA diagnostics was also presented. The experience from this machine indicates that the BPMs should provide beam position measurements with good precision over the entire aperture. Future challenges include beam-based alignment, measurement and control of high power beams, and the measurement of very large-emittance beams (muons) in a few turns.
Section 7: News from Linacs
* Andrei Shislo demonstrated how stripline BPMs can be used to measure the longitudinal Twiss parameters in the SNS Linac. It was also shown that longitudinal bunch distribution cannot be retrieved from these stripline BPMs. For short bunches the higher harmonics of the BPM electrode sum signal are not well measured. A new method for the longitudinal Twiss analysis was presented: Based on the knowledge of the bunch length at a specific location along the Linac, the initial Twiss parameters of the beam distribution can be retrieved. The RF cavities are used for this measurement. The method was applied for the SNS Linac, and produced results in good agreement with the XAL online model.
* Thomas Shea reported the lessons learned about the minimum diagnostics requirements for beam commissioning and characterization. This talk focused on the SNS linac commissioning. Diagnostics requirements for an SNS-like linac, and its commissioning only, include: 1) BPM signal with phase and calibrated amplitude, co-designed with LLRF; 2) BLM (with background mitigation) and beam-current monitor (BCM) difference integrated into the machine protection system; 3) BCM for current control and target/dump protection; 4) target/dump imaging and halo monitoring, capable of verifying interface requirements: 5) a distribution of profile monitors (emittance for the front end); 6) full integration of all diagnostics with high level physics applications; performance verification with "safe" beam pulses; 7) dedicated diagnostics shifts for validation and calibration. If commissioning and/or performance ramp up follows an aggressive schedule the following additional tools are needed or helpful: 1) more beam distribution measurements (emittance, longitudinal and transverse profile, with extended dynamic range); measurements to support multi-particle model verification; 2) diversity and redundancy for virtually all measurements, with focus on machine protection and preference for non-invasive measurements; and 3) a suite of automated verification/tuning applications, and on-line data analysis tools.
* The measurement of the 6D beam distribution at low energy was discussed by Alexander Alexandrov. After pointing out that a 6D distribution cannot normally be derived from three 2D distributions like 2D x 2D x 2D, e.g. in view of correlations, he presented the slit-slit technique for measuring a 6D distribution. This method, which requires a magnetic spectrometer for energy determination, is not easily implemented because of: 1) the required long measurement time, estimated in tens of hours; and 2) a weak signal strength, 10 million times lower than the initial beam intensity. Notwithstanding, a dedicated Integrated Test Stand Facility for 6D beam distribution measurement is under construction at SNS. The present research goals were: 1) optimizing the 6D phase space measuring system for maximum resolution and maximum dynamic range; 2) developing an algorithm to generate particle distributions for use in PIC codes; 3) searching for high-dimensional correlations in the measured distributions; 4) developing and verifying methods for generating 6D distributions from low-dimensional projections; and 5) repeating the LEDA beam dynamics experiment with newly developed diagnostics.
* The minimum diagnostics requirements for beam commissioning and beam characterisation at future cavities were addressed by Marc Munoz for the example of the ESS. The commissioning challenges were: 1) decreasing time duration allocated for commissioning duration, requiring a better preparation; 2) significant in-kind contributions to projects; 3) beam instrumentation being one of the first targets for budget cuts; 4) increasing demands from nuclear safety authorities; 5) growing complexity of future particle accelerators. It was concluded that: 1) both the time and the diagnostic devices available for beam commissioning are limited; 2) planning ahead of time is essential; 3) "virtual commissioning" and "dry runs" should be used to detect faults and to test the various accelerator systems prior to beam operation (actual beam time is too valuable to debug software or to fix trivial faults); and 4) learning from the experience of similar labs is important.
Global Summary
The workshop "beam dynamics meets diagnostics" has brought together the two pertinent communities in the city of Florence, to discuss common issues and possibly to improve the present levels of synergy and communications. From the discussions a complex situation emerged, that is different for different types of machines, as well as for different laboratories and continents. For example, in Japan a distinction between beam dynamics and diagnostics does not seem to exist, and a similar situation is found in the Frascati laboratory. However, in laboratories like CERN or GSI, a clear distinction of roles does exist. The discussions at the workshop have resulted in the recommendation that the beam-dynamics colleagues help "specify" the target parameters of the diagnostics devices, and that they are also (partly) involved in the operational use of these devices.
The workshop participants did not recognize any particular mismatch in the communication between the two communities. A continuation of this inter-community workshop was discussed in the final discussion, where the participants reached an agreement to repeat this event in about two yearsﾕ time. The beam-instrumentation experts have also expressed the wish for a larger participation from the beam dynamics side. A wider circulation of the announcement of future similar events has been encouraged.
Figure1: Some participants of the BeDi2015 workshop in a group photo taken in the "chiostro" of the "Convitto della Calza".
