Detailed Design Report

Transcription

1 Detailed Design Report Chapter 3 MAX IV 1.5 GeV Storage Ring 3.9. Diagnostics (RF and Vacuum not Included) MAX IV Facility

2 CHAPTER 3.9. DIAGNOSTICS (RF AND VACUUM NOT INCLUDED) 1(10) 3.9. Diagnostics (RF and Vacuum not Included) 3.9. Diagnostics (RF and Vacuum not Included) BPMs (Required Performance: Stored Beam and Single Beam Passage) Tune Measurement: Stripline Monitor and/or Button BPMs Current Measurement: DCCT and BPMs Emittance Monitor: Pi-Polarization Method Bunch Length Monitor Filling Pattern or Bunch Current Monitor Scrapers with Possible Function as Dedicated Aperture to Protect IDs Pinger Magnets Beam Loss Monitor System Temperature Sensors Beam Dynamics Measurements/ Control System Applications References...10

3 CHAPTER 3.9. DIAGNOSTICS (RF AND VACUUM NOT INCLUDED) 2(10) 3.9. Diagnostics (RF and Vacuum not Included) BPMs (Required Performance: Stored Beam and Single Beam Passage) Considering the commissioning phase, we need less resolution, but we can only sample a short pulse. The MAX IV ring injector will deliver a pc pulse with a pulse length of 1-2ns for commissioning. This will require the electronics to be able to detect and calculate the position from this short pulse. A ringing filter is a way to achieve this, which is used in one commercial unit. This unit reaches quite good position readings using a ringing filter during commissioning BPM Electronics System for the 1.5 GeV Storage Ring The 1.5 GeV MAX IV storage ring BPM electronics is required to measure the beam position accurately within the full specified storage ring current range. Position information must be stable and provided at rates up to 10 khz for Fast-Orbit-Feedback. The 1.5 GeV storage ring will use 36 BPM button pickups for the beam position monitors. The BPM electronics for the 1.5 GeV storage ring will be of the same type as the 3GeV MAX IV storage ring. The main route will therefore be to equip all 36 BPM pickups with Fast Digital BPM electronics. The electronics should have single-pass capabilities to help with commissioning. The cost-saving option of Hybrid Electronics is a secondary option. For more detailed information about the BPM electronics option, refer to the BPM sections in the Diagnostics chapters for the 3 GeV storage ring Tune Measurement: Stripline Monitor and/or Button BPMs One could either chose to base the tune measurement on a dedicated spectrum analyser with tracking generator, or one could chose a turn by turn mode read-out of one BPM head followed by a fast Fourier transform calculation. In both cases an excitation of the beam is necessary, and one should consider the excitation amplitude in the two cases. In principle, a smaller excitation would be sufficient if a traditional 15 cm quarter wave (fifth harmonic) strip-line detector is used instead of the button type BPM. This is because the tracking feature used by the spectrum analyzer is inherently more effective in detecting a

4 CHAPTER 3.9. DIAGNOSTICS (RF AND VACUUM NOT INCLUDED) 3(10) disturbance (synchronous detection). Actually a counter- example is the SLS, where only the turn by turn data from one BPM is used for tune measurements. In this case the tune measurement is limited to the injection moments in the top-up mode (the kicker excitation of the stored beam), since the necessary excitation between injections would disturb the users. For a strip-line detection system this will not be a limitation, because of the inherently more effective detection. In conclusion, one should study the possibility of a traditional excitation/detection with quarter wave strip-lines. Such a study would include; A mechanical design study of the strip-lines for vacuum chamber in the MAX IV 1.5 GeV ring. Estimation of the impedance contribution from the device. A cost estimation of a dedicated spectrum analyser together with power amplifiers. Considerations if there might be beneficial effects beside the tune measurement, for example a possibility for slightly larger, frequency controlled, excitation of the beam, which is needed when using the spin polarization method for beam energy calibration. Also the possibility of detection of lower currents than detectable with the button type BPMs should be taken into account. We need to consider where to install the two strip-line pairs (with diagonal pairs it is sufficient with one excitation pair and one detection pair), either in the injection straight (chamber aperture?), or in the RF straight Current Measurement: DCCT and BPMs Stored Beam A commercially available DC current transformer should be capable of measuring the stored beam current to a relative precision of at least 10E-4. This should be sufficient for both lifetime calculations and current stabilization in the top-up mode. Lifetime measurements may be improved by averaging a large number BPMs, where relative strength of the sum signal of all four buttons is being used Commissioning During the commissioning phase one should use the possibility of monitoring the sum signal from selected BPM heads. This should give a crude measure of the beam current during the first turn in the machine. Also the sum signal from a strip-line pair may be used for relative measurements. This signal should be stronger than the button type sum signals, and may therefore be crucial at commissioning.

5 CHAPTER 3.9. DIAGNOSTICS (RF AND VACUUM NOT INCLUDED) 4(10) We need to consider where to install the DCCT, either in the injection straight or in the RF straight Emittance Monitor: Pi-Polarization Method A robust on-line measurement of the beam sizes at one location in the ring, is an extremely versatile tool for a machine operator. Often just the on-line two-dimensional image, even without absolute beam size values, helps in finding the source of different beam instabilities. It is of high importance that this diagnostic device is running already during the commissioning phase, when it is crucial to pinpoint those beam instabilities having a simple origin. The purpose of the beam size measurement is eventually to determine the transverse beam emittances. Usually the vertical emittance is the most interesting since it is given by the machine alignment, and thus difficult to predict. For the emittance determinations, separate determinations of beta values and dispersion values at the observation point are needed (cf. Section ). Also a beam energy spread measurement is in principle needed. However, usually one can by different means verify that the beam energy spread is close to the theoretical one, given by the lattice. For the MAX IV 1.5 GeV ring the horizontal/vertical emittance is expected to be in the order of 6/0.06 nm rad. However, we should strive to detect/determine a vertical emittance down to 0.1 % of the horizontal, or 6 pm. We foresee one beam size monitor to be observing the central point of one of the achromat dipoles. Here are the beta values approx m and 16m in x and y respectively. Dispersion is ca 40 mm. Thus, we want to measure a sigmax of 49 um and a sigmay of 10 um. Starting with the vertical beam size measurement it is in magnitude larger compared to the one already performed at the SLS, using the so called pi-polarisation method [4]. This method utilizes the vertically polarized UV-Vis synchrotron radiation (SR), and has turned out to have potential to determine vertical emittances in the few pmrad region. The horizontal beam size can quite safely be determined by use of either polarization of UV-Vis SR, as demonstrated at SLS. A second monitor may be positioned observing either the entrance of the first bend or the exit of the second bend in the achromat. In this way a point of almost zero dispersion is observed. Here are betax = 2 m and betay = 10.Thus, sigmax = 110 um and sigmay = 8 um, for 0.1 % coupling. Both values should be possible to verify utilizing UV-Vis SR and the Pi-polarization method. Two ports as described above. Large efforts should be made to extract wide enough opening angles of the light and to protect the extraction mirror with a thin horizontal absorber.

6 CHAPTER 3.9. DIAGNOSTICS (RF AND VACUUM NOT INCLUDED) 5(10) Bunch Length Monitor A conventional choice is here a streak camera. However we should look into the possibility of using fast photodiodes, since the expected bunch rms length is around 170 ps. Photodiodes with rise-times (10-90%) around 80 ps are available for low price and are easy to operate (demonstrated at MAX II). A dedicated fast oscilloscope is of course necessary, but it should anyway be part of the diagnostic equipment, and could be used in various situations. A draw-back is of course the rise-time, but diodes with shorter rise-times may be used at the expense of the easiness of aligning them. We will need one, simple, extraction port of SR from a dipole for this device. Simple, in the meaning that no emphasis has to be put on the wavefront distortion of the extracted light. Most efficient would be to use the same extraction port as for the filling pattern monitor Filling Pattern or Bunch Current Monitor A filling pattern monitor will be a crucial device in order to keep the micron stability of the transverse beam position. The reason for this is that the front end electronics of the BPMs are likely to be dependent on the filling pattern. This is envisaged considering the fact that an un-even fill of the ring will result in bunches of different lengths, which in turn induces different signal levels in the BPMs. In order to assure the stability, a reasonably fixed filling pattern is needed. However, we will have a good possibility to keep a fixed wanted filling pattern, since we will fill individual buckets with the injector. A very effective filling pattern monitor, even used as a feed-back device to generate arbitrary filling patterns, has been realized for at SLS (where RF=500 MHZ). It is based on a photodiode detecting the light from individual bunches. This photodiode also gives superior linearity of the output signal to the bunch current, compared to either a sum signal taken from the BPMs, or from a stripline, to a fast oscilloscope (it is clear that these signals are bunch length dependent). Since we use only a 100 MHz RF, there will be good hope to successfully implement the photodiode solution for our filling pattern monitor and feed-back algorithm the for filling. We will need one, simple, extraction port of SR from a dipole for this device. Simple, in the meaning that no emphasis has to be put on the wavefront distortion of the extracted light. Most efficient would be to use the same extraction port as for bunch length measurements.

7 CHAPTER 3.9. DIAGNOSTICS (RF AND VACUUM NOT INCLUDED) 6(10) Scrapers with Possible Function as Dedicated Aperture to Protect IDs A vertical scraper will give valuable information on the average pressure in the machine, that is it will give the possibility to put numbers on elastic and inelastic (gas) lifetime and finally on the Touschek lifetime. Furthermore, with knowledge of the vertical beta function, the vertical acceptance of the ring can be determined. This is crucial information when commissioning different insertion devices. However, this diagnostic device could be designed also to be a dedicated protection of the IDs against radiation. Furthermore, this protection fulfils a third aim, and that is to concentrate all vertical losses from stored beam to a single area where radiation protection walls could be somewhat enhanced in order to cope with this radiation. Not only electrons that are scattered elastically against rest gas atoms, but also Touschek scattered electrons that eventually couple their horizontal motion to a vertical one will be caught in this bottle neck, instead of being lost in the IDs. A horizontal scraper does not necessarily show the horizontal physical acceptance of the machine. It could instead reveal the lattice dynamic energy acceptance, or the RF energy acceptance. A thorough investigation has to be done after the scraper scan is done. However, correctly interpreted the data should help in determining these important quantities. We should contemplate whether we want to place it at a dispersive or nondispersive location. The best place for the vertical scraper should be the injection straight, regarding the fact that the radiation shield anyhow here has to be slightly enforced. If possible one should allocate roughly 1m of the straight section to place this tapered bottle neck. Tapers are contemplated for reduction of the impedance, and the length gives a possibility for some radiation shielding (lead) on top and below the chamber. Furthermore one could think of a very simple device, where the entire unit is levelled to the position which just about shadows all the other straight sections and their IDs. Still it will require some kind of bellows at the ends, which probably should incorporate RF shields. However, regarding the fact that IDs are supposed to have gaps of only 10 mm, the movement of the whole device would not have to be more than roughly 6 mm. We would still be able to perform the entire scraper scan. The location of a traditional horizontal scraper should be determined. From radiation point of view it should also be in the injection straight where the radiation shielding is enforced, since it could also serve as a bottle neck. At the proper horizontal displacement it will shadow the rest of the ring from softly scattered Touschek particles Pinger Magnets Pinger magnets could require larger efforts to realize in the 1.5 GeV ring compared to the 3 GeV ring due to the shorter revolution time. The issue whether the nonlinear optics

8 CHAPTER 3.9. DIAGNOSTICS (RF AND VACUUM NOT INCLUDED) 7(10) characterisation is needed in the low energy ring is under investigation (c.f. Section ). They should be designed with thought on impedance contribution Beam Loss Monitor System During commissioning of the storage ring as well as during later machine studies (momentum acceptance studies, energy calibration, ID commissioning, etc.) it will be of interest to identify locations of elevated beam loss. A simple and inexpensive way to achieve this is the use of optical fibers running along the vacuum chamber. Such a system has already successfully been used at the MAX-FEL experiment. Four fibers run along the vacuum chamber; each fiber covers a 90 degree segment of the chamber. The fibers are taped directly onto the chamber. Since the fibers are very small they can be threaded through magnet apertures or ID gaps. Bremsstrahlung generated at beam loss creates visible radiation in the fiber at a location close to the loss area. This radiation can be transported by the fiber all the way to one end where the intensity is measured. If this measurement is synchronized with the beam revolution, the time of flight creates a relation between delay of a measured pulse and loss location along the fiber. Therefore if an intensity burst is measured, the delay of this burst with respect to the revolution trigger reveals the location of burst source along the fiber. Since four such fibers are installed with each in a different segment of the chamber, losses above and below the machine midplane can be distinguished from each other as well as losses to the inside and outside of the ring. The latter (if measured in coincidence) can be used to distinguish Touschek losses from gas scattering loses. If beam loss is to be measured specifically for the purpose of energy calibration (cf. section ), a measurement that can distinguish Touschek loss pairs from other scattering losses is of advantage. Such a system can be implemented by setting up two scintillators downstream of a dipole magnet on the inside and outside of the storage ring. Or alternatively, downstream of a vertical aperture limitation (for example an in-vacuum ID with closed gap) above and below the vacuum chamber. If the read-out from these scintillators is fed to a comparator and the generated coincidence signal is monitored, Touschek losses can easily be identified Temperature Sensors Considering the fact that the vacuum system should cope with the entire heat load, without any separated absorbers, it could be a great advantage to plan for a set of permanent

9 CHAPTER 3.9. DIAGNOSTICS (RF AND VACUUM NOT INCLUDED) 8(10) temperature sensors at crucial points around the ring fastened onto the vacuum chamber. Most often storage rings have during (and after) commissioning suffered from unexpected hot spots that severely affect the lifetime, and maybe other features of the beam. If temperature sensors will be used at for example front ends and/or beamlines we should strive to standardize them Beam Dynamics Measurements/ Control System Applications We list here beam dynamics measurements that need to be automatized Linear Optics Characterization The MATLAB Middle Layer (MML) software [5] will be used for the MAX IV storage rings. It contains a number of routines and applications for accelerator control, simulation and machine measurements including but not limited to: Accelerator Toolbox for machine physics simulations Orbit correction application Beam Position Monitor (BPM) offset calibration Chromaticity measurement Dispersion function measurement Beta function measurement Corrector magnet hysteresis measurement Response matrix measurement As MML is in use at a number of light sources including Diamond, SOLEIL and SSRF, the component routines have been tested extensively for a number of different machines. Setting up the MML software for controlling the MAX IV storage rings during characterization measurements will require a TANGO interface, rather than the EPICS interface. This had been previously done at SOLEIL. Furthermore, Linear Optics from Closed Orbits (LOCO) [6] is integrated in the MML package and will be used extensively for characterization of the linear optics, gain calibration of BPMs, et.c. This will require setting up an accurate Accelerator Toolbox (AT) lattice model for the 1.5 GeV storage ring Non-Linear Optics Characterization The issue whether the nonlinear optics characterisation is needed in the low energy ring is under investigation.

10 CHAPTER 3.9. DIAGNOSTICS (RF AND VACUUM NOT INCLUDED) 9(10) Beam Energy Calibration In order to gain a detailed understanding of the storage ring (e.g. nonlinear momentum compaction) as well as to calibrate ID spectra, precise knowledge of the storage ring energy is required. In order to go beyond the 10^-3 accuracy achieved with Hall probe measurements of the dipole field, resonant spin depolarization can be used. This method delivers an energy calibration on the 10^-5 level and has successfully been used at several storage rings (cf. e.g. [8]). Basically, an undisturbed electron beam in the ideal storage ring polarizes anti-parallel to the guiding dipole field after injection through the emission of spin-flip radiation. Once the beam has polarized, excitation of the beam at a specific frequency can be used to depolarize it. The frequency at which the beam depolarizes is directly proportional to the beam energy. If the frequency of the excitation is swiped across an interval and the degree of polarization is monitored, the drop of polarization upon hitting the depolarizing resonance can be linked to the exact beam energy. This measurement does not necessarily require an expensive Compton polarimeter setup, however. Touschek lifetime is polarization-dependent (the higher the polarization the lower the Touschek-scattering cross section and hence the higher the Touschek lifetime) so that monitoring Touschek lifetime can reveal the degree of polarization of the stored beam in the machine. Similarly, the depolarizing resonance can be identified by a sudden drop of Touschek lifetime and/or a sudden increase in Touschek-scattered electron pairs. All that is then needed is a fast kicker (injection kicker, pinger magnet) with a tunable source (e.g. sine generator with settings to sweep the frequency over a predefined range). However, certain conditions should be met. Firstly, the lifetime has to be Touschekdominated in order for lifetime measurements to reveal polarization-dependence. Secondly, high levels of polarization need to be achieved before depolarization can take place and be detected. The first condition should be fulfilled for the vacuum conditioned MAX IV 1.5 GeV storage ring. In the base line design Touschek lifetime is dominating if Landau cavities are not operated (see Section ). The second condition is the more serious obstacle: high degrees of polarization in the stored beam require an excellent machine alignment with a well-corrected orbit. In addition the machine must be left quiet/stable for extended periods of time without orbit feedback or injection in order for polarization to build up. The build-up time in the MAX IV 1.5 GeV ring is 48 minutes. This means that the machine must be operated in a completely quiet and stable way without any corrections, top-up injection shots, or ID gap changes for at least three to five hours in order to successfully measure polarization build-up and find the depolarizing resonance. Finally, we note that the spin tune of the MAX IV 3 GeV storage ring should be around (corresponding to a depolarizing resonance around MHz or MHz). The fractional spin tune has a mirror at Both are fairly separated from the fractional horizontal and vertical betatron tunes (0.22/0.14). Thus, this should not prevent from reaching higher levels of polarization in the stored beam.

11 CHAPTER 3.9. DIAGNOSTICS (RF AND VACUUM NOT INCLUDED) 10(10) References [1] A. Olmos, F. Pérez, ALBA-CELLS, Cerdanyola, Barcelona, Spain, G. Rehm, Diamond Light Source, Oxfordshire, U.K. Matlab code for bpm button geometry computation, DIPAC07. [2] A. Stella, Analysis of the DAFNE Beam Position Monitor with a Boundary Element Method, INFNLNF, Accelerator Division, Frascati, December [3] F. Marcellini, DAFNE broad-band button electrodes, 1997 [4] Å. Andersson, M. Böge, A. Lüdeke, V. Schlott, A. Streun, Determination of a small vertical electron beam profile and emittance at the Swiss Light Source, Nuclear Instruments and Methods, A 591 (2008) [5] G J Portmann, Jeff Corbett, and Andrei Terebilo. An accelerator control middle layer using matlab. [6] J Safranek, G Portmann, A Terebilo, and C Steier. Matlab-based loco [7] R. Bartolini, I.P.S. Martin, J.H. Rowland, P. Kuske, F. Schmidt, Phys. Rev. ST Accel. Beams, 11, , [8] S.C. Leemann, M. Böge, M. Dehler, V. Schlott, Proceedings of EPAC 2002, Paris, France, pp [9] Libera Brilliance Specifications, Instrumentation Technologies