J-PARC High Level Physics Applications Magnets & Beams Day , Lecture Acce
Lenz, Sabine, Founder, Editor has reference to this Academic Journal, PHwiki organized this Journal J-PARC High Level Physics Applications Magnets & Beams Day , Lecture Accelerator View from the Physicist Why Use a Magnet to Guide a Charged Particle Beam For Charged particles, you can use Electric or Magnetic particles to modify a trajectory. As b increases, the required electric field to provide a comparable bend as a given magnetic field gets larger Practical limits on electric fields are 10 kV over mm, typical magnetic fields are 10 kG. Electric fields usually not used as long as b>~0.01 – homework: why Magnets In Accelerators The modern approach is to use separate function magnets: each magnet provides an independent multipole: Dipole steer Quadrupole – focus Sextupole correct chromaticity Offers independent control of different functions
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Magnet Nodes Magnets are the primary means on beam manipulation in the transverse plane in accelerators Some issues concerning magnet organization / class structure Permanent magnets, electro magnets Dipoles as long as bending, quadrupoles as long as focusing, sextupoles as long as chromaticity correction Main magnets, corrector or trim magnets Several magnets may be on a common power supply, magnets on a single power supply Magnet Nodes Many different types of magnets: Permanent Magnets Electro Magnets Dipoles, Quadrupoles, Sextupoles, Correctors dipole, quadrupole, See gov.sns.xal.smf.impl.magnet + other sub-classes Multiple magnet devices can exist at the same location: e.g. quadrupole with dipole corrector trim windings Some Magnet / Power Supply Properties / Interfaces As a beam physicist, controlling in addition to knowing the magnetic field is critical as long as an interface to a beam model Methods such as getField() in addition to setField() are necessary Need to know the effective magnetic length to get the effect of the field on the beam Need to know the parent power supply that controls it.
Independently Powered Magnets vs. Multiply Powered Magnets Individual power supply Expensive Common as long as lattice transitions where matching is required Power Supply B(I) is uniquely determined by the magnet properties Independently Powered Magnets vs. Multiply Powered Magnets Multiple magnets / power supply Common practice as long as long stretches of a lattice structure, often independent control as long as the horizontal in addition to vertical planes Power Supply B(I) is determined by the average of the involved magnet properties Setting the field of one magnet affects the field in others Power Supply / Magnet Control in XAL Magnet Interfaces Readback as long as each specific magnet: getField() Setting interface to the power supply, affects other magnets if its a multiple power supply Power Interfaces Readback provides the average field of all the magnets on this power supply Setpoint provides a setting as long as the average of all magnets on a power supply Note power supplies driving multiple magnets generally control magnets of the same type. The variations of B(I) from magnet to magnet are generally < 1% Magnetic Hysteresis Path dependence of the magnetic field Magnets are controlled by specifying the amount of current in the driving power supply Most accelerator magnets contain ferro-magentic material (iron) to increase the flux density in the region where the beam is The iron has a Atomic dipoles align themselves in addition to produce a magnetic field component themselves The magnitude of the magnetic field in the magnet as long as a fixed current depends how these dipole moments are lined up. This depends on the history of the current in the magnet Typical Accelerator Magnet Blue part is iron Note color code on the leads polarity counts too. The Hysteresis Loop The magnets are composed of conventional grain-oriented electrical steel. BR denotes the remanence in addition to HC is the coercive field. A hysteresis loop shows the relationship between the induced magnetic flux density (B) in addition to the magnetizing as long as ce (H(I)). B F(H(I)) result depends on the history. B saturates at high current Repeatability Studies We want to obtain a certain value of magnetic field (B) in addition to we can manipulate only the current (I). The solution is to use a defined, slow, repeatable procedure to set the current I0. By specifying the history, the magnetic flux density B will always be the same. Theoretically this is a complicated problem, but we have the accelerator as a gauge to estimate the reproducibility of the B-value. We can study this procedure experimentally. If we repeatedly get the same tuning state as long as the accelerator, we are satisfied. The accelerator state tuning characteristics: Losses (BLM signals) Beam trajectories (BPM signals) Brightness (light source) Magnet Cycling Approach (A. Shishlo) We have to choose parameters of cycling to provide the same final B value every time. Number of cycles, wait time, in addition to ramp rate Magnets Cycling Procedure Plan: Cycle using conservative ramp parameters Tweak to get a good tune Cycle again in addition to see that the tune is still good Move I up in addition to down to destroy the good tune Cycle again in addition to return to the good tune Change parameters to reduce the cycling time in addition to repeat 4,5 again, as long as the cycling is working. Example using the SNS HEBT Dipoles SNS HEBT Dipole Results (2) At 12 seconds, the cycling doesnt work too well. The losses are 10 times bigger after such fast cycling. SNS Main Ring Dipole Example Characterizing the Magnet Strength In accelerator physics, the magnet strength is often given by the field index kn: Where n=0 as long as dipole, n=1 as long as quad, This requires knowledge of the beam energy at the magnet to a priori convert a magnet measurement into a focusing strength. The field can be provided, in addition to focusing strength calculated as needed in a model configuration B vs. k, this is the question For an Accelerator Physics model, you need to characterize the field as either B or normalized field strength k B(I) is measured you must convert to k with the proper beam energy in as long as mation This is OK if the energy is known in addition to static at a given magnet Otherwise just use B in addition to calculate k internal to the model XAL is setup this way Where should the Field / Current Translation Occur Engineers deal with magnet current not field Typically Accelerator Physics provides the translation which is done in an IOC Advantage - physics units as long as magnets are available to the entire control system Disadvantage requires modification / reboots of IOC to update Could be done at a higher level (e.g. XAL) Advantage: direct control without IOC reboots etc. Disadvantage: not avaialable to all channel access clients Trans as long as mation from B-> I in addition to I->B should be consistent, or one can walk away from a desired setpoint Spline fits be careful Interpolation between measured points
Magnet Measurement Rotating Coils (harmonic coils) Provide in as long as mation on the magnitude of each pole of the field (dipole, quad, sextupole, ) in the normal in addition to skew directions. Magnetic Measurement Vibrating wire / taut wire Useful to find magnetic centers Hall Probe find the field at a point (size of the detector) Complex variants with 3-D measurements Flip Coil Useful as long as measuring the effective field along non-straight beam paths (e.g. dipoles) Magnets in Accelerators Prediction of the exact field a beam will feel is difficult Hysteresis effects Magnet mapping uncertainties Positioning uncertainties Differences in mapping power supplies vs. production Reproducibility is critical Use of beam measurements is needed to provide the actual field calibration (better than 1%)
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