LCLS Magnet Damage Management Heinz-Dieter Nuhn, SLAC / LCLS June 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. - PowerPoint PPT Presentation
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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
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
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.
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.
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.
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.
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-
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.
Possible reasons for generating elevated levels of radiation areElectron 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 operationIs 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 insertionMay only be allowable when all undulators are rolled-out and beam charge is reduced to minimum.
Background Radiation from Upstream Sources including Tune-Up DumpExpected to be sufficiently suppressed by PCMUON collimator.
Beam HaloExpected to be sufficiently suppressed through upstream collimation system.May require halo detection system.
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.
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.
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 blocksTwo 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.
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 blocksTwo 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.
The BLM will be used for two purposesA: 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.
The BLM will be used for two purposesA: 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.
Magnet block mounted next to heat shield.Magnet block mounted next to heat shield.Mounting fixture with magnet for first forward position with heat shield.
Mounting fixture with magnet for first forward position with heat shield.
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.
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.
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.
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.