1.0 Introduction. 1.1 General

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2 1.0 Introduction The Linac Coherent Light Source (LCLS) project at the Stanford Linear Accelerator Center (SLAC) is intended to create an x-ray free electron laser source of unprecedented brightness and short-time pulse duration. At a point after the electron beam has been diverted from the path of the photon beam, a series of stoppers must be remotely inserted into the photon beam path to allow entry into the Front End Enclosure and down-beam areas. The stoppers must be capable of stopping both the photon and electron beams, in case the electron-diverting magnets fail. 1.1 General The photon/electron stoppers are part of the Personnel Protection System (PPS). The primary control of the stoppers will be through the PPS. Status of the stopper position and Burn-Through-Monitor (BTM) integrity may be provided to other controls monitoring systems, such as the Beam Containment System (BCS). 1.2 Scope The parameters required to design and fabricate the photon/electron stoppers must be established to meet requirements of the PPS and BCS systems. The design parameters described herein only apply to the LCLS photon/electron stoppers located in the electron beam dump area; ST1and ST Beam Line Device Two Photon/Electron Stoppers are to be located in the LCLS E-Beam Dump enclosure. One stopper is located between the main dump line electromagnet string and the safety dump line permanent magnet bend string. The second stopper is located between the safety magnet permanent magnet bend string and the Wall 1 Collimator. The primary function of the Photon/Electron stoppers is to stop photons emitted from the LCLS FEL Undulator during access to the FEE, while electrons continue to be delivered to the main dump. The stoppers must stop the spontaneous radiated photons, FEL photons, and Bremsstrahlung γ rays. 2 of 9 that this is the correct version prior to use.

3 Photon damage simulations and testing of materials has been performed by LLNL. The results indicate that 10mm thick Boron Carbide (B 4 C) on the front face of the photon/electron stopper will stop the FEL photons without experiencing damage itself, and will reduce the total transmitted pulse energy to non-damaging levels for any downstream component. The secondary function of the photon/electron stoppers is to stop electrons in case the electromagnet main dump bend magnets fail while the FEE is in access. (In the case of ST2, both the electromagnet main dump bend magnets and the permanent magnet safety dump bend strings must fail for ST2 to see electrons.) Safety system function information may be found in PRD LCLS Personnel Protection System Requirements. Many stoppers have been built at SLAC, and are still in use in every beam line. For ease of maintenance and reliability, the vertical actuator design originally built for the PEPII injection beamlines will be used. The stopper block however, will be re-designed for the specific needs in the photon line in the E-Dump enclosure. The actuator is shown in Figure 1. Figure 1. Stopper actuator mechanism 3 of 9 that this is the correct version prior to use.

4 The stopper block will be made in multiple segments and with two BTM's to meet the requirement to detect the unlikely event of a burn-through caused by either the photon beam or electron beam. The first absorber segment will be B 4 C, 10mm thick, to stop the FEL pulse. A thick titanium-alloy wafer will spoil the electron beam to decrease the density of energy deposition in the following copper segment. A tungsten segment will follow the copper segment to further attenuate radiation escaping the downstream face of the copper segment. (see Figure 1). The stopper block will be water-cooled. A photon BTM will be placed behind the B 4 C and titanium to detect failure of the photon stopper element. An electron BTM will be placed downstream of the copper segment, upstream of the tungsten segment. The maximum energy deposition is in the bulk of the copper. The electron beam must burn through the copper to cause failure of the electron BTM. The electron BTM must fail prior to appreciable melting of the tungsten element. Figure 1. Stopper Block Material Stack 4 of 9 that this is the correct version prior to use.

5 The BTM's will be pressurized with ultrahigh vacuum (UHV)-compatible, filtered, dry air. Upon failure, pressure loss in the BTM will be detected using an electronic pressure sensor. The stopper block will be water-cooled. The device will only see the electron beam in accidental exposure or low-power conditions for testing performed by RP; however, the stopper will be designed to accept full beam power. 3.0 Device Control The stopper is actuated via a solenoid valve and must have a pair each of in and out limit switches. It will be equipped with a panel-mount cable connector. Specific cable connector types must be negotiated between mechanical and controls engineering and recorded in the cable plant database. The control hardware should be the same as for other LCLS in/out air-actuated devices, with the addition of BTM-failure detection. Indications of in/out and BTM status must be available to the PPS. The pressurized air electron and photon BTM's will be instrumented with an electronic pressure gauge. The signal from the pressure gauge must be made available to the PPS System. In addition to pressure monitoring, BCS Protection Ion Chambers (PIC's) will be placed at each stopper, as required in the SLAC Radiation Safety Systems Technical Basis Document. The PIC's will shut-off the electron beam to protect the stoppers from damage and thereby protect personnel (a Machine Protection System [MPS]-like function performed by the BCS for PPS stoppers). 4.0 Design Parameters The basic design parameters of the stopper are shown in Tables 1 and 2. Electron beam parameters are taken from PRD Electron Beam Power Absorbing Device Performance Requirements, PRD Insertable Beam Dumps in the Linac and LTU, and PRD Electron Dump-Line Requirements. The maximum single-pulse temperature rise was calculated for ST1 by the Radiation Physics group using conservative input parameters of a φ20 µm, 2 kw beam (120 Hz). Scaling the RP data to φ40 µm and 5kW yields the values indicated in Table 1. Scaling was 5 of 9 that this is the correct version prior to use.

6 achieved by linearly scaling the deposited energy per radial bin by 5 kw/2 kw and also scaling the deposited energy by the ratio of the areas of the increased radial bin to the original bin size. The average temperature rise in the stopper block materials at 120 Hz is a maximum of 4 C in the titanium. The titanium material is an alloy; Ti-6AL-4V. The tungsten material is a machinable, sintered tungsten alloy suitable for UHV applications. Electron Beam Parameter Value Units Horizontal RMS Beam Size 40 µm Vertical RMS Beam Size 40 µm Longitudinal RMS Beam Size 20 µm Nominal electron energy 4.3 to 14 GeV Bunch charge range 0.2 to 1 nc Maximum beam rate 120 Hz Max. average beam power 5 kw Max. reasonable insertion/extraction time 5 sec Table 1: Electron Beam Parameters for the Photon/Electron Stoppers. Stopper Stack Materials Value Units Photon Absorber Material B 4 C - Photon Absorber Length 10 mm Electron Absorber Materials Ti, W & Cu - Total Electron Absorber Length (Radiation Lengths) 25 X o Maximum Average Temperature Rise in Ti 4 C Maximum Single Pulse Temperature Rise in B4C 114 C Maximum Single Pulse Temperature Rise in Ti 176 C Maximum Single Pulse Temperature Rise in Cu 60 C Table 2: Stopper Block Material Parameters. 5.0 Procurement/Fabrication/Assembly The basis of estimate indicates that the stopper mechanical parts will be machined by an outside vendor and the final assembly will be performed at SLAC. Alternatives to the basis are allowed to reduce schedule and/or cost. The stopper controls hardware is anticipated to be identical to stoppers installed in the upstream areas of LCLS, with the addition of BTM failure- 6 of 9 that this is the correct version prior to use.

7 detection hardware. 5.1 Materials All parts to be installed inside the vacuum enclosure must conform to the SLAC vacuum standard for machined components, including materials requirements including materials source data and machining lubricant restrictions. See the latest revision of SLAC FP for details. All test results shall be recorded in the stopper traveler. 5.2 Machined Components All vacuum enclosure parts must be leak checked prior to assembly. All invacuum parts must be fabricated in accordance to SLAC vacuum standard SLAC FP All machined parts must be inspected for conformance to drawing dimensions and tolerances. Acceptance of parts is to be recorded on the component traveler. 5.2 Purchased Components All purchased components, including controls hardware, must conform to SLAC safety policies and practices. Controls hardware must be acceptablyrated by a nationally-recognized testing laboratory or pass internal SLAC electrical safety inspections. 5.3 Vacuum Assembly Assembly of in-vacuum parts must be performed in a SLAC Clean Room. Leak testing of components and subassemblies shall be performed in such a manner as to identify leaks at as early a time as possible. 5.3 Alignment Fiducials Included in the design of the stopper vacuum chambers are provisions for tooling ball sockets. Each stopper vacuum chamber must have the tooling ball sockets fiducialized on a CMM with respect to the vacuum chamber beam axis. 5.4 Identification 7 of 9 that this is the correct version prior to use.

8 Each stopper assembly shall have affixed to the outer surface on both sides The identifier LCLS Photon/Electron Stopper ST-<X> where <X> is the unit number. Total weight of the stopper assembly in pounds. Each stopper shall also have a white, painted arrow on each side, which points in the direction shown in the drawing, with Beam Direction lettered on the shaft of the arrow (with at least 1 cm high characters) as illustrated in Figure 3. Beam Direction Figure 3: Beam direction arrow. 6.0 Tests and Measurements Tests and measurements must be performed to assure that the installed stopper system will perform as designed. Tests and measurements shall include part and assembly qualification, electrical safety inspection, actuation testing, limit switch tests, BTM pressure tests, BTM pressure signal verification, leak checks, assembly fiducialization and vacuum qualification bake-out. Tests shall be performed in the laboratory to qualify the device prior to installation. Additional tests shall be performed after installation to qualify the system prior to delivery of electron or photon beams to the installed region. All tests and measurements shall be recorded on a device traveler which corresponds to the unique device serial number. 7.0 Installation Installation of the Photon/Electron Stoppers and control system shall conform to the Davis Bacon Act where required. Precision alignment of the beam line device is required; to be performed by the SLAC Metrology group. 8 of 9 that this is the correct version prior to use.

9 Interconnection and testing of controls shall be performed following vacuum interconnection. Additional field testing shall be performed by SLAC Radiation Physics. 8.0 Applicable Standards and Codes Design, fabrication, testing and acceptance must be in conformance to the following codes and SLAC standards. 1 SLAC SLAC Guidelines for Operations SLAC-I SLAC ES&H Manual 3 SLAC Technical Specification Fabrication of UHV Components FP NEMA National Electrical Manufactures Association: Standard 250 Enclosures for Electrical Equipment (1000 Volts Max.), section SLAC SLAC Radiation Safety Systems Technical Basis Document 9 of 9 that this is the correct version prior to use.