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LCLS Magnet Damage Management

Strategies for beam loss monitoring and management of magnet damage in the LCLS facility. Includes review of irradiation tests, damage measurements, and plans for follow-up tests.

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LCLS Magnet Damage Management

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  1. LCLS Magnet Damage ManagementHeinz-Dieter Nuhn, SLAC / LCLSJune 19, 2008 • Present Strategies for LCLS Beam Loss Monitoring • Review of the Individual Magnet Irradiation Test T-493 • Results of Damage Measurements • Plans for follow-up Mini-Undulator Irradiation Test 1

  2. LCLS Beam Loss Monitors (BLMs) Strategies • Radiation protection of the permanent magnet blocks is very important. • Funds have been limited and efforts needed to be focused to minimize costs. • A Physics Requirement Document, PRD 1.4-005 exists, defining the minimum requirements for the Beam Loss Monitors. • The damage estimates are based on published measurement results and a in-house simulations. 2

  3. Estimated Radiation-Based Magnet Damage • The loss of magnetization caused by a given amount of deposited radiation has been estimated by Alderman et al. [i] in 2000. • Their results imply that a 0.01% loss in magnetization occurs after exposure to a fast-neutron fluence of 1011 n/cm2. • A more recent report by Sasaki et al. [ii] challenges fast neutron fluence as damaging factor and, instead, proposes photons and electrons but does not provide a relation between integrated dose and damage. [i] J. Alderman, et. A., Radiation Induced Demagnetization of Nd-Fe-B Permanent Magnets, Advanced Photon Source Report LS-290 (2001) [ii] S. Sasaki, et al, Radiation Damage to Advanced Photon Source Undulators, Proceedings PAC2005. 3

  4. Estimate of Neutron Fluences from LCLS e- Beam • The radiation deposited in the permanent magnets blocks of the LCLS undulator, when a single electron (e-) strikes a 100-µm carbon foil upstream of the first undulator, has been simulated by A. Fasso [iii]. • The simulations predict a peak total dose of 1.0×10-9 rad/e- including a neutron (n) fluence of 1.8×10-4 n/cm2/e-, which translates into 1.8×105 n/cm2 for each rad of absorbed energy. • These numbers are based on peak damage results and should therefore be considered as worst case estimates. [iii] A. Fasso, Dose Absorbed in LCLS Undulator Magnets, I. Effect of a 100 µm Diamond Profile Monitor, RP-05-05, May 2005. 4

  5. Simulated Neutron Fluences for LCLS e- Beam on C Foil Simulated neutron fluences in the LCLS undulator magnets (upper jaw) from a single electron hitting a 100-µm-thick carbon foil upstream of the first undulator. Maximum Level is 1.8×10-4 n/cm2/e- 5

  6. Total Dose from LCLS e- Beam on C Foil Corresponding maximum deposited dose. Maximum Level is 1.0×10-9 rad/e- 6

  7. Radiation Limit Estimates ~0.01 mrad/pulse @ 120 Hz; ~1 mrad/s 7

  8. Pole Center Line Vacuum Chamber First; K=3.5000; Dx=-4.0 mm Neutral; K=3.4881; Dx= 0.0 mm Neutral; K=3.4881; Dx= 0.0 mm Neutral; K=3.4881; Dx= 0.0 mm Roll-Out; K=0.0000; Dx=+80.0 mm Horizontal Slide Undulator Roll-Away and K Adjustment Function 8

  9. Maximum Estimated Radiation Dose from BFW Operation All Undulators Rolled-In Undulators on DS Girders Rolled-Out (1/100) The small amount of scans expected, can be ignored for damage purposes; but might require MPS exception. 9

  10. Radiation Sources Possible reasons for generating elevated levels of radiation are • Electron Beam Steering Errors • Will be caught and will lead to beam abort. • Unintentional Insertion of Material into Beam Path • Will be caught and will lead to beam abort. • Intentional Insertion of Material into Beam Path • BFW operation • Is expected to produce the highest levels. May only be allowable when all down-stream undulators are rolled-out and beam charge is reduced to minimum. • Screen insertion • May only be allowable when all undulators are rolled-out and beam charge is reduced to minimum. • Background Radiation from Upstream Sources including Tune-Up Dump • Expected to be sufficiently suppressed by PCMUON collimator. • Beam Halo • Expected to be sufficiently suppressed through upstream collimation system. • May require halo detection system. 10

  11. General Requirements NOT FULLY REALIZED • One BLM device will be mounted upstream of each Undulator Segment • The BLM will provide a digital value proportional to the amount of energy deposited in the device for each electron bunch. • The monitor shall be able to detect and measure (with a precision of better than 25%) radiation levels corresponding to magnet dose levels as low as 10 µrad/pulse [0.1 µGy/pulse] and up to the maximum expected level of 10 mrad/pulse [100 µGy/pulse]. • The monitor needs to be designed to withstand the highest expected radiation levels of 1 rad/pulse without damage. • The radiation level received from each individual electron bunch needs to be reported after the passage of that bunch to allow the MPS to trip the beam before the next bunch at 120 Hz. 11

  12. Monitor Requirements • Each BLM device will be able to measure the total amount of absorbed dose covering the full area in front of the undulator magnets. • Each BLM device will be calibrated based on the radiation generated by the interaction of a well known beam with the BFW devices. • The calibration geometry will be simulated using FLUKA and MARS to obtain the calibration factors, i.e., the ratio between the maximum estimated damage in a magnet and the voltage produced by each BLM device. NOT FULLY REALIZED 12

  13. Beam Loss Monitor Area Coverage NOT FULLY REALIZED • Main purpose of BLM is the protection of undulator magnet blocks. • Less damage expected when segments are rolled-out. • One BLM will be positioned in front of each segment. • Its active area will be able to cover the full horizontal width of the magnet blocks • Two options for BLM x positions will be implemented to be activated by a local hardware switch: • (a) The BLM will be moved with the segment to keep the active BLM area at a fixed relation to the magnet blocks. • (b) The BLM will stay centered on the beam axis to allow radiation level estimates in roll-out position. 13

  14. BLM Purpose • The BLM will be used for two purposes • A: Inhibit bunches following an “above-threshold” radiation event. • B: Keep track of the accumulated exposure of the magnets in each undulator. • Purpose A is of highest priority. It will be integrated into the Machine Protection System (MPS) and requires only limited dynamic range from the detectors. • Purpose B is desirable for understanding long-term magnet damage in combination with the undulator exchange program but requires a large dynamic range for the radiation detectors (order 106) and much more sophisticated diagnostics hard and software. 14

  15. ANL Beam Loss Monitor Design BLM Mounted on BFW in Front of Undulator Segment Beam A total of 5 BLM deviceswill be installed. Rendering of Detector Courtesy of W. Berg, ANL 15

  16. Plan View of Short Drift Undulators Segments Beam Direction BFW Beam Loss Monitor BPM Quadrupole 16

  17. Additional Loss Monitors • Other Radiation Monitoring Devices • Dosimeters • Located at each undulator. Routinely replaced and evaluated. • Segmented Long Ion Chambers • Investigated • (Quartz)-Fibers • Investigated • Non-Radiative Loss Detectors • Pair of Charge Monitors (Toroids) • One upstream and one downstream of the undulator line • Used in comparator arrangement to detect losses of a few percent • Electron Beam Position Monitors (BPMs) • Continuously calculate trajectory and detect out-of-range situations • Quadrupole Positions and Corrector Power Supply Readbacks • Use deviation from setpoints • Estimate accumulated kicks to backup calculations based on BPMs. 17

  18. LCLS Undulator Irradiation Experiment (T-493) The LCLS electron beam is stopped in a copper dump, and 9 samples of magnet material are positioned at different distances from the dump. The layout to achieve a range of doses is calculated using FLUKA. The radiation absorbed will be measured by dosimeters. Magnetization will be measured before and after exposure. The integrated beam current will be needed to be recorded to 10%. 18

  19. Injector Far Hall Linac Coherent Light Source SLAC LINAC T-493 Endstation A Undulator Tunnel Near Hall 19

  20. T-493 Components installed ESA Beamline with copper cylinder and magnet blocks. Copper target for 13.7 GeV e- Beam. Diameter: 4 inches Length: 10 inches Dosimeters positioned at in the vicinity of each block. [See presentation by Johannes Bauer] BEAM 20 Photo courtesy of J. Bauer

  21. Magnet Block Assembly Straight-ahead mounting fixture on work bench with four magnet blocks (viewed in the direction of the beam.) 21

  22. Mounted Magnet Block Next to Heat Shield Magnet block mounted next to heat shield. Mounting fixture with magnet for first forward position with heat shield. 22

  23. ANL Delivery of 12 LCLS Undulator Magnet Blocks Material: Ne2Fe14B Block Thickness: 9 mm Block Height: 56.5 mm Block Width: 66 mm Material Density: 7.4 g/cm3 Block Volume: 33.6 cm3 Block Mass: 248.4 g Curie Point: 310 °C 23 Photo courtesy of S. Anderson

  24. Pre-Irradiation Magnetic Moment Measurements • The table shows the results of the measurement of magnetic moments for one of the magnet blocks (Serial No. 00659) as an example. • The Magnetic Moments are measured with a Helmholtz-Coil. • All magnetic measurements have been carried out by Scott Anderson. 24

  25. Top View Magnet Blocks Copper Cylinder Heat Shield r Beam Direction z Magnet Block Assembly (Top View) M9 M8 4 Magnet blocks in forward direction5 Magnet blocks in transverse direction 3 Magnet blocks kept for reference M7 M6 M5 M1 M4 M2 M3 25

  26. View in Beam Direction y r Heat Shield Magnet Block Assembly (View in Beam Directions) Copper Cylinder M1-M4 M9 M8 M7 M6 M5 Magnet Blocks 26

  27. Experiment T-493 Shift Records Magnet Irradiation Experiment T-493 ran for 38 shifts from 7/27-8/09/2007 27

  28. Delivered Power Delivered power levels alternated between about 125 W during Day and Swing Shifts and 185 W during Owl Shifts. During Day and Swing Shifts the experiment ran parasitically with LCLS commissioning. 28

  29. Tunnel Temperature Profile The temperature in the ESA tunnel stayed between 23-24.6°C during the entire 12-day data collection period. The plot shows diurnal cycle fluctuations. 29

  30. Magnetic Moment Evaluations: Results Summary • Shown are parameters for the 9 irradiated magnets and the Cu target • the estimated neutron fluence and dose levels • peak power levels • temperature estimates • The last two columns contain the results of the magnets’ demagnetization measurements. 30

  31. Detailed FLUKA model of the experiment • 13.7 GeV electron beam impinging on the copper dump • Computation of total dose, electromagnetic dose, neutron energy spectra • Quantity scored using a binning identical to the one used for the mapping of the magnetization loss M4 M1 M2 M3 Beam M5 M6 M7 M8 M9 31 Courtesy of J. Vollaire, SLAC

  32. Damage Gradients FLUKA Simulations by J. Vollaire, SLAC M1 M1 M2 M2 M3 M4 M3 M4 Threshold Estimates for 0.01 % Damage 32

  33. Additional Evaluation: Field Map Measurements Grid Size: 26 x 31 Points = 806 Points; Point Spacing: 2 mm; Method: Hall Probe Reference Magnet SN16673 33

  34. Field Map Measurements for M1 Absolute Magnetic Field Amplitudes [T] Reference Magnet Fields subtracted [T] 34

  35. Field Map Measurements for M2 Absolute Magnetic Field Amplitudes [T] Reference Magnet Fields subtracted [T] 35

  36. Field Map Measurements for M3 Absolute Magnetic Field Amplitudes [T] Reference Magnet Fields subtracted [T] 36

  37. Field Map Measurements for M5 Absolute Magnetic Field Amplitudes [T] Reference Magnet Fields subtracted [T] 37

  38. Example of Dose Mapping for the Four Downstream Samples Fluence [cm-2] Total Dose [J cm-3] 38 Courtesy of J. Vollaire, SLAC

  39. Dose Profile versus Magnetization Loss Profile 39 Courtesy of J. Vollaire, SLAC

  40. Next Experiments • T-493 was a measurement of the demagnetization of stand-alone magnets with no significant demagnetizing fields present. • Inside an undulator, the magnet blocks will be tightly packaged next to one another and magnet blocks might experience the magnetic fields of the neighboring magnets. • This scenario will be covered by the “Mini – Undulator Irradiation Test”. • Ben Poling, SLAC, has designed and built a Mini-Undulator from spare LCLS Undulator magnet and pole pieces. A second Mini-Undulator (for reference) will be built before the first irradiation run. • The magnetization of individual magnet pieces as well as the on-axis magnetic field of the assembled Mini-Undulators will be measured before and after the irradiation processes. • Irradiation will be done similar to T-493: A radiation field will be generated by the LCLS electron beam hitting a copper target in ESA. • This time, irradiation will be done in phases. 40

  41. Mini-Undulator Design by Ben Poling Courtesy of B. Poling, SLAC 41

  42. Mini-Undulator Design by Ben Poling Made from spare LCLS undulator magnet blocks (2 x 2 x 3) and pole pieces (2 x 2 x 5). Total number of periods: 3. Gap height and period length identical to LCLS undulator. Courtesy of B. Poling, SLAC 42

  43. Schedule for Test Sequence MINI-UND RUN 1 MINI-UND RUN 3 MINI-UND RUN 2 MINI-UND RUN 4 CANCELED 43

  44. Summary • The plan for monitoring and protecting the LCLS undulators from radiation was presented. • Irradiation test at SLAC have been carried out in August 2007: • Nine of the spare Nd2Fe14B permanent magnet pieces for the LCLS undulators have been exposed to radiation fields of various intensities under conditions that can be precisely calculated by FLUKA simulations. • The total exposure time was 12.5 days during which a copper target was hit by the 13.7 GeV LCLS electron beam. The total energy of the 36.8x1015 electrons that hit the target was 80 MJ. • After a cool-down period, the magnetization levels of the magnets have been measured and compared with the pre-irradiation values. The difference is being compared to the (FLUKA) estimated radiation levels received. • In addition, Mini-Undulators (3 periods, each) have been prepared for testing. The magnetic moments of each of the magnets as well as the on-axis magnetic fields after assembly will be measured and recorded. The plan is to irradiate one of them in up to four periods. • The present plan to do the irradiation before the August shutdown will probably not work out. 44

  45. End of Presentation 45

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