Construction and Commissioning of PAL-XFEL Facility

applied sciences

Article Construction and Commissioning of PAL-XFEL Facility

In Soo Ko *, Heung-Sik Kang, Hoon Heo, Changbum Kim, Gyujin Kim, Chang-Ki Min, Haeryong Yang, Soung Youl Baek, Hyo-Jin Choi, Geonyeong Mun, Byoung Ryul Park, Young Jin Suh, Dong Cheol Shin, Jinyul Hu, Juho Hong, Seonghoon Jung, Sang-Hee Kim, KwangHoon Kim, Donghyun Na, Soung Soo Park, Yong Jung Park, Young Gyu Jung, Seong Hun Jeong, Hong Gi Lee, Sangbong Lee, Sojeong Lee, Bonggi Oh, Hyung Suck Suh, Jang-Hui Han, Min Ho Kim, Nam-Suk Jung, Young-Chan Kim, Mong-Soo Lee, Bong-Ho Lee, Chi-Won Sung, Ik-Su Mok, Jung-Moo Yang, Yong Woon Parc, Woul-Woo Lee, Chae-Soon Lee, Hocheol Shin, Ji Hwa Kim, Yongsam Kim, Jae Hyuk Lee, Sang-Youn Park, Jangwoo Kim, Jaeku Park, Intae Eom, Seungyu Rah, Sunam Kim, Ki Hyun Nam, Jaehyun Park, Jaehun Park, Sangsoo Kim, Soonnam Kwon, Ran An, Sang Han Park, Kyung Sook Kim, Hyojung Hyun, Seung Nam Kim, Seonghan Kim, Chung-Jong Yu, Bong-Soo Kim, Tai-Hee Kang, Kwang-Woo Kim, Seung-Hwan Kim, Hee-Seock Lee, Heung-Soo Lee, Ki-Hyeon Park, Tae-Yeong Koo, Dong-Eon Kim and Ki Bong Lee

Academic Editor: Kiyoshi Ueda Received: 23 March 2017; Accepted: 26 April 2017; Published: 17 May 2017

Abstract: The construction of Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL), a 0.1-nm hard X-ray free-electron laser (FEL) facility based on a 10-GeV S-band linear accelerator (LINAC), is achieved in Pohang, Korea by the end of 2016. The construction of the 1.11 km-long building was completed by the end of 2014, and the installation of the 10-GeV LINAC and undulators started in January 2015. The installation of the 10-GeV LINAC, together with the undulators and beamlines, was completed by the end of 2015. The commissioning began in April 2016, and the

Appl. Sci. 2017, 7, 479; doi:10.3390/app7050479 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 479 2 of 11

ﬁrst lasing of the hard X-ray FEL line was achieved on 14 June 2016. The progress of the PAL-XFEL construction and its commission are reported here.

Keywords: FEL; free electron laser; PAL; PAL-XFEL; construction; commissioning; LINAC; beamline

1. Introduction The Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL) project was started Appl. Sci. 2017, 7, 479 2 of 11 in 2011 for the generation of X-ray FEL radiation in a range of 0.1 to 10 nm for users. The Korean government launchedbeamlines, thewas completed project on by 1the April end of 2011 2015. The with commissioning a budget ofbegan 400 in billion April 2016, Won and (~400 the first million USD). lasing of the hard X‐ray FEL line was achieved on 14 June 2016. The progress of the PAL‐XFEL The facility hasconstruction the capacity and its for commission five undulator are reported lines here. in total; three hard X-ray (HX) undulator lines and two soft X-ray (SX) undulator lines. However, the budget was limited to two undulator lines; one for Keywords: FEL; free electron laser; PAL; PAL‐XFEL; construction; commissioning; LINAC; beamline a hard, and the other for a soft X-ray line. Since the PAL is the host institution carrying out the project, the project budget was able to avoid significant costs; for example, there was no need to purchase the land for the building site and a significant portion of the necessary infrastructures, such as power 1. Introduction transmission lines and substations, were already in place. A total of 75 members were involved during The Pohang Accelerator Laboratory X‐ray Free‐Electron Laser (PAL‐XFEL) project was started the project. Amongin 2011 for them, the generation there wereof X‐ray 35 FEL newly radiation hired in a range members of 0.1 to and 10 nm 39 for experienced users. The Korean members from an existing Pohanggovernment Light launched Source-II the project (PLS-II) on 1 April team. 2011 with A yearly a budget budget of 400 billion is shown Won (~400 in million Table USD).1. The facility has the capacity for five undulator lines in total; three hard X‐ray (HX) undulator lines and two soft X‐ray (SX) undulatorTable 1. lines.Budget However, of PAL-XFEL the budget Project.was limited to two undulator lines; one for a hard, and the other for a soft X‐ray line. Since the PAL is the host institution carrying out the project, the project budget was able to avoid significant costs; for example, there was no need to purchase the land for theYear building site and 2011 a significant 2012 portion 2013 of 2014 the necessary 2015 infrastructures, Total such as power transmissionBudget in billionlines and Won substations, 20 were 45 already 105 in place. 120 A 113.8 total of 403.8 75 members were involved during the project. Among them, there were 35 newly hired members and 39 experienced members from an existing Pohang Light Source‐II (PLS‐II) team. A yearly budget is shown in Table 1.

PAL-XFEL includes a 10-GeV S-bandTable 1. (2856Budget of MHz) PAL‐XFEL normal-conducting Project. LINAC, which is about 700 m long. The LINAC consists of a photocathode RF gun, 176 S-band accelerating structures with Year 2011 2012 2013 2014 2015 Total 50 klystrons and matchingBudget modulators, in billion Won one20 X-band 45 RF105 system120 113.8 for403.8 linearization, and three bunch compressors in the HX line and one more for the SX line [1]. We also chose out-vacuum/variable-gap PAL‐XFEL includes a 10‐GeV S‐band (2856 MHz) normal‐conducting LINAC, which is about undulators for700 the m long. easy The change LINAC consists of beam of a photocathode parameters, RF andgun, 176 the S‐ fastband developmentaccelerating structures and with manufacturing of undulators.50 klystrons Beyond and thematching 10-GeV modulators, LINAC, one X‐ aband 250-m RF system long for hard linearization, X-ray and undulator three bunch hall follows. An experimentalcompressors hall, which in the HX is line 60 and m long one more and for 16 the m SX wide, line [1]. is We located also chose at out the‐vacuum/variable end of the facility.‐gap The total undulators for the easy change of beam parameters, and the fast development and manufacturing of 2 length of theundulators. building Beyond is 1110 the m,10‐GeV and LINAC, the entire a 250‐m ﬂoor long ishard 36,764 X‐ray mundulator. The hall building follows. An can withstand a maximum windexperimental load ofhall, 63 which m/s is (US60 m buildinglong and 16 code) m wide, and is located a seismic at the end intensity of the facility. of 0.19-g The total [2]. The facility 2 suffered no damagelength of the from building the is earthquake 1110 m, and the of entire a 5.8 floor magnitude is 36,764 m on. The 12 building September can withstand 2016, a nor from the maximum wind load of 63 m/s (US building code) and a seismic intensity of 0.19‐g [2]. The facility typhoon Chavasuffered on 5 no October damage from 2016. the The earthquake facility of isa 5.8 shown magnitude in Figure on 12 September1. 2016, nor from the typhoon Chava on 5 October 2016. The facility is shown in Figure 1.

Figure 1. Overview of Pohang Accelerator Laboratory. The Pohang Light Source (PLS) is shown in Figure 1. Overviewthe middle of (circular Pohang building) Accelerator and PAL‐ Laboratory.XFEL is shown above The Pohang the PLS. Light Source (PLS) is shown in the middle (circular building) and PAL-XFEL is shown above the PLS. Appl. Sci. 2017, 7, 479 3 of 11

Appl.2. LINAC Sci. 2017 , 7, 479 3 of 11 The PAL‐XFEL LINAC is divided into four acceleration sections (L1, L2, L3, and L4), three bunch 2.compressors LINAC (BC1, BC2, BC3), and a dogleg transport line to the undulators, as shown in Figure 2. The L1 section consists of two RF stations, where both are comprised of one klystron and two S‐band structures,The PAL-XFEL while L2 LINAChas 10, L3 is dividedhas four, into and four L4 has acceleration 27 RF stations sections where (L1, each L2, L3,station and has L4), one three klystron, bunch compressorsfour accelerating (BC1, structures, BC2, BC3), and and one a dogleg energy transport doubler. line A laser to the heater undulators, to mitigate as shown micro in‐bunching Figure2. Theinstability L1 section is placed consists right of two after RF the stations, injector, where and bothan X are‐band comprised cavity for of onelinearization klystron and is placed two S-band right structures,before the BC1. while The L2 hasmajor 10, parameters L3 has four, of and PAL L4‐XFEL has 27 are RF summarized stations where in eachTable station 2. has one klystron, four accelerating structures, and one energy doubler. A laser heater to mitigate micro-bunching instability is placed right after theTable injector, 2. Major and parameters an X-band of cavity PAL‐XFEL. for linearization is placed right before the BC1. The major parameters of PAL-XFEL are summarized in Table2. LINAC FEL radiation wavelengthTable 2. Major parameters0.1 nm (Hard of PAL-XFEL. X‐ray)/1 nm (Soft X‐ray) Electron energy 10 GeV Slice emittanceLINAC 0.5 mm‐mrad Beam chargeFEL radiation wavelength 0.2 0.1nC nm (Hard X-ray)/1 nm (Soft X-ray) Peak currentElectron at undulator energy 3.0 10kA GeV Slice emittance 0.5 mm-mrad Pulse repetitionBeam charge rate 60 Hz 0.2 nC Electron sourcePeak current at undulatorPhoto 3.0‐ kAcathode RF‐gun LINAC structurePulse repetition rateS‐band 60 Hz normal conducting Electron source Photo-cathode RF-gun UndulatorLINAC structure S-band normal conducting Type Undulator out‐vacuum, variable gap Length Type5 m out-vacuum, variable gap UndulatorLength period 26 mm 5 m (HX)/35 mm (SX) UndulatorUndulator min. Gap period 8.3 26mm mm (HX)/9.0 (HX)/35 mmmm (SX) (SX) Undulator min. Gap 8.3 mm (HX)/9.0 mm (SX) K value K value1.9727 1.9727 (HX)/3.3209 (HX)/3.3209 (SX) Peak B (inPeak Tesla) B (in Tesla)0.8124 0.8124 (HX)/1.0159 (HX)/1.0159 (SX) Vacuum chamber dimension × 2 Vacuum chamber dimension 13.413.4 × 6.76.7 mm mm2

Figure 2. Schematic layout of PAL‐XFEL. Figure 2. Schematic layout of PAL-XFEL. Appl. Sci. 2017, 7, 479 4 of 11 Appl. Sci. 2017, 7, 479 4 of 11 The total length of the LINAC tunnel is about 710 m. There are 176 S‐band accelerating structures and 42 energy doublers. The major high power devices of the 10‐GeV linear accelerator are the modulators,The total the length klystrons, of the the LINAC energy tunnel doublers is about (ED), 710 and m. There the accelerating are 176 S-band structures accelerating (AS). structures An energy doublerand 42 increases energy doublers. the peak Thepower major of the high RF power pulse devices by reducing of the the 10-GeV RF pulse linear length accelerator to increase are the the energymodulators, gain at the the klystrons, accelerating the energystructure. doublers The energy (ED), and doubler the accelerating was designed structures by PAL (AS). and An fabricated energy doubler increases the peak power of the RF pulse by reducing the RF pulse length to increase the by a domestic company. There are 50 modulators for the S‐band klystrons, and there is one modulator energy gain at the accelerating structure. The energy doubler was designed by PAL and fabricated by for the X‐band klystron that is used for linearizing the electron beam. The LINAC requires 46 S‐band a domestic company. There are 50 modulators for the S-band klystrons, and there is one modulator klystrons to obtain an electron beam energy of 10 GeV. One S‐band klystron is dedicated for the RF for the X-band klystron that is used for linearizing the electron beam. The LINAC requires 46 S-band gun, and three RF stations are designated for deflectors to measure the electron bunch length. klystrons to obtain an electron beam energy of 10 GeV. One S-band klystron is dedicated for the RF The klystron requires an RF drive signal at the level of a few hundred watts. A low‐level radio gun, and three RF stations are designated for deﬂectors to measure the electron bunch length. frequencyThe (LLRF) klystron and requires a solid an‐state RF drive amplifier signal (SSA) at the are level necessary of a few to hundred supply watts.the drive A low-levelsignal to a klystron.radio frequency To achieve (LLRF) beam and energy a solid-state stability ampliﬁer of below (SSA) 0.02% are and necessary an arrival to supply time thejitter drive of 20 signal‐fs for PALto‐ aXFEL, klystron. the LINAC To achieve RF parameters beam energy should stability be as of stable below as 0.02% 0.03 degrees and an arrivalfor the timeRF phase jitter and of 20-fs 0.02% forfor the PAL-XFEL, RF amplitude the LINACfor S‐band RF parametersRF systems, should and 0.1 be degree/0.04% as stable as 0.03 for the degrees X‐band for linearizer the RF phase RF system. and The0.02% pulse for‐to the‐pulse RF amplitude klystron forRF S-band stability RF is systems, determined and 0.1 by degree/0.04% the klystron for beam the X-band voltage linearizer driven RFby a modulator.system. The Therefore, pulse-to-pulse the klystron klystron modulator RF stability beam is determined voltage by should the klystron be as stable beam voltage as 50 ppm driven for by the 0.03a modulator.‐degree S‐band Therefore, RF and the 0.1 klystron‐degree modulator X‐band RF beam [3]. voltage should be as stable as 50 ppm for the 0.03-degreeAn LLRF S-band system RF consists and 0.1-degree of an SSA, X-band a phase RF [3 and]. amplitude detector (PAD), and a phase and amplitudeAn LLRFcontrol system (PAC) consists unit. The of an function SSA, a of phase the PAC and amplitude is to control detector the phase (PAD), and and amplitude a phase andof the RFamplitude drive signal control to a klystron, (PAC) unit. to Theprovide function a pulsed of the RF PAC signal, is to controland to reverse the phase the and RF amplitude phase of the of theklystron RF drivedrive signal signal in to the a klystron, middle toof providethe pulse a pulsed by turning RF signal, on the and Phase to reverse Shift theKey RF (PSK) phase 180 of‐ thedegree klystron phase shifter).drive signal The RF in pulse the middle length of of the the pulse drive by signal turning is on 4 μ thes and Phase the Shift PSK Key is on (PSK) after 180-degree 3.17 μs from phase the shifter). starting timeThe of RF the pulse pulse. length The of thePAL drive‐XFEL signal LINAC is 4 µ stunnel and the and PSK the is on klystron after 3.17 galleryµs from are the startingshown timein Figures of the 3 andpulse. 4, respectively. The PAL-XFEL LINAC tunnel and the klystron gallery are shown in Figures3 and4, respectively.

Figure 3. LINAC tunnel of PAL‐XFEL. Figure 3. LINAC tunnel of PAL-XFEL. Appl. Sci. 2017, 7, 479 5 of 11 Appl. Sci. 2017, 7, 479 5 of 11

Appl. Sci. 2017, 7, 479 5 of 11

Figure 4. Klystron gallery of PAL‐XFEL. Figure 4. Klystron gallery of PAL-XFEL.

3. Undulator 3. Undulator Figure 4. Klystron gallery of PAL‐XFEL. The PAL‐XFEL undulator system consists of 20 planar undulators for the hard X‐ray line (HX) Theand3. Undulator seven PAL-XFEL planar undulatorundulators systemfor the soft consists X‐ray undulator of 20 planar line undulators (SX) [4]. The for HX the covers hard a wavelength X-ray line (HX) of λ = 0.1~0.6 nm using a 4 to 10‐GeV electron beam. They are all out‐vacuum undulators with and sevenThe planar PAL‐ undulatorsXFEL undulator for thesystem soft consists X-ray undulatorof 20 planar line undulators (SX) [4]. for The the HX hard covers X‐ray aline wavelength (HX) variable gaps. The SX covers a wavelength of λ = 1.0–4.5 nm using a 3.15‐GeV electron beam. The HX of λ =and 0.1~0.6 seven nm planarusing undulators a 4 to for 10-GeV the soft electron X‐ray undulator beam. Theyline (SX) are [4]. all The out-vacuum HX covers a undulators wavelength with undulators have a 26‐mm undulator period. The gap is controlled remotely within 1 μm repeatability variableof λ gaps.= 0.1~0.6 The nm SX coversusing a a 4 wavelength to 10‐GeV electron of λ = 1.0–4.5 beam. nmTheyusing are all a 3.15-GeVout‐vacuum electron undulators beam. with The HX and the minimum gap is 8.3 mm. The height is controlled remotely, too. The SX undulators have a undulatorsvariable have gaps. a The 26-mm SX covers undulator a wavelength period. of The λ = gap1.0–4.5 is controllednm using a remotely3.15‐GeV electron within 1beam.µm repeatabilityThe HX 35‐mm undulator period and a minimum gap of 9.0 mm. Both hard and soft X‐ray undulators are undulators have a 26‐mm undulator period. The gap is controlled remotely within 1 μm repeatability and theplanar minimum type, and gap have is the 8.3 same mm. structures The height except is controlledfor the magnets. remotely, A self‐ too.seeding The section SX undulators is prepared have and the minimum gap is 8.3 mm. The height is controlled remotely, too. The SX undulators have a a 35-mmin HX undulator undulator period line. Two and elliptically a minimum polarizing gap of undulators 9.0 mm. Both are planned hard and to softbe installed X-ray undulators at the SX are 35‐mm undulator period and a minimum gap of 9.0 mm. Both hard and soft X‐ray undulators are planarbeamline type, and in coming have the years. same The structures installed HX except undulators for the are magnets. shown in A Figure self-seeding 5. section is prepared planar type, and have the same structures except for the magnets. A self‐seeding section is prepared in HX undulator line. Two elliptically polarizing undulators are planned to be installed at the SX in HX undulator line. Two elliptically polarizing undulators are planned to be installed at the SX beamlinebeamline in coming in coming years. years. The The installed installed HX HX undulators undulators are are shown shown in in Figure Figure 5. 5.

Figure 5. Hard X‐ray undulator tunnel of PAL‐XFEL.

Figure 5. Hard X‐ray undulator tunnel of PAL‐XFEL. Figure 5. Hard X-ray undulator tunnel of PAL-XFEL. Appl. Sci. 2017, 7, 479 6 of 11

4. Diagnostic System For the operation of PAL-XFEL, electron beam parameters, such as beam positions, energy, charge, transverse beam size, bunch length, and arrival time, should be measured and monitored. A total of 209 beam position monitors (BPMs) are used for the electron beam position measurement [5]. Forty-nine of them are cavity type BPMs, which can measure the beam position with sub-micrometer resolution in the undulator beamline. Ten bunch charge monitors are installed for the bunch charge measurement from the gun as well as beam loss monitoring though the accelerator. The beam proﬁle is measured with 54 screen monitors with YAG and/or Optical Transition Radiation (OTR) screens. Six spectrometer dipole systems are located at the gun section, laser heater, BC1, soft X-ray branch, BC3S and hard X-ray LINAC end. At both beam dumps at the ends of the hard and soft X-ray beamlines, the beam energy can also be measured with the screens. Three S-band deﬂector systems after BC1, BC3H and BC3S are used to measure the bunch longitudinal phase space. Table3 summarizes the major components of beam diagnostics and their functions.

Table 3. Major components of beam diagnostics and their functions.

Parameter Instruments Number Position Beam Energy Stripline Beam Position Monitor 160 Cavity BPM 49 Beam Charge Turbo Integrated Current Transformer (ICT) 10 Beam Size Screen Monitor 54 Wire Scanner 9 Bunch Length Coherent Radiation Monitor 4 Transverse Cavity 3 Arrival Time Arrival Time Monitor 10 Beam Loss Beam Loss Monitor 26

5. Beamlines There is one undulator line for the HX application and another for the SX application. However, there are several end-stations for each undulator line to support various requests from users. For hard X-ray application, there are two end-stations called HEH1 for the pump-probe experiment and HEH2 for the imaging [6]. These two stations are located in tandem. In order to provide HX FEL photons to HEH2, the entire HEH1 stage is able to move in the transverse direction by 1 m. When HX/SX FEL photons emerge from their corresponding undulator system, they pass through various components located in the undulator hall (UH) and the optics hall (OH). Both halls are isolated with proper concrete shielding. For the HX case, photons are then allowed to enter the experimental hall (EH) where HEH1 and HEH2 are located. Mirrors and a double crystal monochromator (DCM) are located in UH/OH as well as various collimators and safety shutter for radiation safety. In HEH1, beam position monitors (BPM) and profile intensity monitors (PIM) are installed to measure the position and the intensity of the HX FEL beam. An optical laser/X-ray correlator (OXC) is also installed to measure the offset of photon arrival times in fs accuracy. Two Be compound refractive lenses (Be-CRL) are installed along with three slits to define and optimize the XFEL beam. In the final stage, there is a hexapod diffractometer and 4-circle goniometer to adjust sample’s location. Finally, there is a robotic arm to control its position of the detector. The HEH1 beamline is shown in Figure6. Appl. Sci. 2017, 7, 479 7 of 11

Appl. Sci. 2017, 7, 479 7 of 11

Figure 6. Inside view of HEH1. Figure 6. Inside view of HEH1.

HEH2 is designed based on forwardFigure 6.scattering Inside view geometry, of HEH1. and will be used for the coherent X‐ HEH2ray imaging is designed (CXI) or based the serial on forwardfemtosecond scattering crystallography geometry, (SFX) and [7]. will The be XFEL used beam for the is coherentfocused to X-ray imagingabout (CXI)HEH2 2 μm or is by thedesigned the serial K‐B based femtosecondmirror. on A forward wire crystallography scanning scattering method geometry, (SFX) was [andused7]. The will to XFELmeasurebe used beam forthe the isfocusing focused coherent beam to X‐ about 2 µmrayprofiles. by imaging the K-BA tungsten (CXI) mirror. or wirethe A wireserial with scanning femtoseconda diameter method of crystallography 200 μ wasm was used placed (SFX) to measure at[7]. the The focal XFEL the point. focusing beam A isphoto focused beam diode proﬁles. to A tungstenaboutdetector 2 wire μ placedm withby thebehind a diameterK‐B mirror.the wire of A 200 waswireµm alsoscanning was used placed methodto atmeasure the was focal theused point. beam to measure Aintensity photo the diode during focusing detector the beam wire placed behindprofiles.scanning. the wire A There tungsten was are also alsowire used several with to measurea diagnosticdiameter the of devices beam 200 μ intensitym such was as placed Pop during‐in, at Quadrupolethe the focal wire point. scanning. BPM A (QBPM),photo There diode and are also photo‐diodes. The HEH2 beamline is shown in Figure 7. severaldetector diagnostic placed devices behind suchthe wire as Pop-in, was also Quadrupole used to measure BPM (QBPM),the beam and intensity photo-diodes. during the The wire HEH2 scanning. There are also several diagnostic devices such as Pop‐in, Quadrupole BPM (QBPM), and beamline is shown in Figure7. photo‐diodes. The HEH2 beamline is shown in Figure 7.

Figure 7. Inside view of HEH2.

Figure 7. Inside view of HEH2. Figure 7. Inside view of HEH2. Appl. Sci. 2017, 7, 479 8 of 11 Appl. Sci. 2017, 7, 479 8 of 11

ForFor thethe SXSX beamline,beamline, therethere areare alsoalso twotwo end-stations:end‐stations: one for coherent diffraction imaging (CDI) oror X-rayX‐ray emission/absorptionemission/absorption spectroscopy (XES/XAS), and and another another for for SX SX resonant resonant scattering. scattering.

6.6. FELFEL CommissioningCommissioning AfterAfter thethe InjectorInjector TestTest FacilityFacility (ITF)(ITF) hadhad stoppedstopped itsits operationoperation byby thethe endend ofof SeptemberSeptember 2015, thethe photocathodephotocathode RFRF gungun andand thethe twotwo S-bandS‐band acceleratingaccelerating structuresstructures atat thethe ITFITF werewere movedmoved toto thethe mainmain PAL-XFELPAL‐XFEL LINAC. LINAC. The The installation installation of 51 of klystron 51 klystron modulators modulators in the LINACin the LINAC gallery wasgallery finished was asfinished of 30 November as of 30 November 2015. Twenty 2015. (20) Twenty HX undulators (20) HX undulators for the HX for line the were HX installed line were as installed of December as of 2015December in the 250-m2015 in long the 250 HX‐ undulatorm long HX tunnel. undulator Since tunnel. the PAL-XFEL Since the LINAC PAL‐XFEL has all LINAC new RF has components, all new RF ancomponents, RF aging or an conditioning RF aging or period conditioning is required. period This is RF required. conditioning This RF had conditioning started in November had started 2015, in andNovember continued 2015, until and the continued Korean Nuclear until the Safety Korean and Nuclear Security Safety Commission and Security (NSSC) Commission issued the operation (NSSC) permissionissued the operation on 12 April permission 2016. The actualon 12 beamApril commissioning2016. The actual was beam started commissioning on 14 April 2016.was started Since the on ITF14 April parts 2016. have alreadySince the been ITF conditioned parts have already during theirbeen useconditioned from 2012 during to 2015, their the use electron from beam 2012 emittedto 2015, fromthe electron the photocathode beam emitted gun from quickly the arrivedphotocathode at the first gun beam quickly analyzing arrived stationat the first (BAS0) beam on analyzing the same day.station By 25(BAS0) April, on the the 10-GeV same electron day. By beam 25 April, reached the BAS3, 10‐GeV and electron the commissioning beam reached of the BAS3, LINAC and was the completedcommissioning [8]. of the LINAC was completed [8]. EvenEven thoughthough thethe 10-GeV10‐GeV beambeam isis available,available, wewe havehave decideddecided toto reduce thethe electron beambeam energyenergy toto 4-GeV4‐GeV inin orderorder toto sendsend thethe electronelectron beambeam toto thethe mainmain dumpdump throughthrough thethe 6.7-mm6.7‐mm gapgap undulatorundulator chambers.chambers. Also,Also, thethe gapsgaps ofof allall undulatorsundulators werewere fullyfully opened.opened. TheseThese twotwo actionsactions werewere intentionallyintentionally chosenchosen toto minimizeminimize radiationradiation damagesdamages toto thethe permanentpermanent magnetsmagnets ofof thethe undulatorundulator system.system. AfterAfter thethe 4-GeV4‐GeV electronelectron beambeam reachedreached thethe mainmain dump,dump, anan efforteffort toto laselase thethe photonphoton beambeam waswas carriedcarried outout withwith severalseveral feedbackfeedback algorithmsalgorithms includingincluding thethe beam-basedbeam‐based alignmentalignment (BBA)(BBA) technique.technique. Finally, wewe lasedlased thethe photonsphotons atat 0.50.5 nmnm onon 06:0006:00 1414 AprilApril 2016.2016. TheThe thirdthird harmonicharmonic spectrumspectrum ofof 6.66.6 keVkeV waswas measuredmeasured withwith aa singlesingle shotshot spectrometerspectrometer located located in in the the HEH1, HEH1, as as shownshown in in FigureFigure8 [8 [9].9]. ItsIts widthwidth waswas 3030 eVeV oror 0.45%.0.45%. FigureFigure9 9 shows shows the the snapshot snapshot of of the the 0.2-nm 0.2‐nm lasing. lasing. A A bunch bunch length length of of 12.7-fs 12.7‐fs was was measured measured by by S-bandS‐band TransverseTransverse Deflecting Deflecting Cavity Cavity (TCAV)(TCAV) with with a a peak peak current current of of 3.7-kA. 3.7‐kA. MajorMajor achievementsachievements andand unexpectedunexpected interruptionsinterruptions duringduring thethe commissioning commissioning are are summarized summarized in in Table Table4 .4.

Figure 8. The third harmonic spectrum of 6.6 keV was measured with a single shot spectrometer Figure 8. The third harmonic spectrum of 6.6 keV was measured with a single shot spectrometer located in the HEH1. located in the HEH1. Appl. Sci. 2017, 7, 479 9 of 11 Appl. Sci. 2017, 7, 479 9 of 11

Figure 9. Snapshot of the 0.2‐nm lasing. Figure 9. Snapshot of the 0.2-nm lasing. Table 4. Major achievements during the commissioning. Table 4. Major achievements during the commissioning. Date Energy (GeV) Remarks Permission issued by National Nuclear Safety and Security 4/12 (2016)Date Energy (GeV) Remarks Commission (NSSC) Permission issued by National Nuclear Safety and Security 4/124/14 (2016) 0.152 E‐Gun and BAS0 Commission (NSSC) 4/184/14 0.355 0.152 E-GunBAS1 and BAS0 4/194/18 0.355 0.355 Before BC2 (No BAS1 acceleration by L2) 4/204/19 2.545 0.355 Before BC2 (NoBAS2 acceleration by L2) 4/214/20 3.15 2.545 BAS3 (No acceleration BAS2 after BAS2) 04/25 (5:304/21 p.m.) 10 3.15 BAS3 (No accelerationBAS3 after BAS2) 04/255/19 (5:30 p.m.)10 10 Tuneup BAS3 dump 6/25/19 10 10 Passing the HX undulator Tuneup line, dump beam at the main dump 06/14 (6:006/2 a.m.) 4 10 PassingFirst the HX lasing undulator (0.5‐nm) line, observed beam at at the SCM36 main dump 06/14 (6:00 a.m.) 4 First lasing (0.5-nm) observed at SCM36 06/21 (3:00 a.m.) 4 Photon beam at Digital Current Monitor (DCM) in Optical Hutch 06/21 (3:00 a.m.) 4 Photon beam at Digital Current Monitor (DCM) in Optical Hutch July~August – Summer maintenance July~August – Summer maintenance 8/168/16 4 4 CommissioningCommissioning resumedresumed 8/308/30 4 4 RecoveredRecovered 0.5-nm0.5‐nm lasinglasing 9/99/9 4 4 HXHX BeamlineBeamline commissioningcommissioning startedstarted 09/1209/12 (8:30 (8:30 p.m.) p.m.) 4 4 EarthquakeEarthquake stoppedstopped commissioningcommissioning 9/299/29 – – DedicationDedication ceremonyceremony 10/310/3 4 4 CommissioningCommissioning resumedresumed 10/810/8 5.2 5.2 0.35-nm0.35‐nm lasinglasing 10/1610/16 6.7 6.7 0.2-nm0.2‐nm lasinglasing 11/27 8.04 0.144-nm lasing and saturation 11/27 8.04 0.144‐nm lasing and saturation 12/2 8.04 First experiments 12/2 8.04 First experiments 2/1 (2017) 3 1.5-nm SX lasing and saturation 2/1 (2017)3/16 3 9.78 1.5‐nm 0.1-nm SX lasing HX and lasing saturation 3/16 9.78 0.1‐nm HX lasing

7. Summary 7. Summary The PAL-XFEL project has been successfully constructed and commissioned by the end of 2016. The PAL‐XFEL project has been successfully constructed and commissioned by the end of 2016. It has now provided XFEL photons of 0.1 nm as designed. The Pohang Accelerator Laboratory has It has now provided XFEL photons of 0.1 nm as designed. The Pohang Accelerator Laboratory has already issued a call for proposals to potential (domestic and international) users on February 6, 2017. The successful user will use PAL‐XFEL’s first light in June 2017. Appl. Sci. 2017, 7, 479 10 of 11 already issued a call for proposals to potential (domestic and international) users on February 6, 2017. The successful user will use PAL-XFEL’s ﬁrst light in June 2017.

Acknowledgments: In Soo Ko thanks to all members of the PAL-XFEL project (2011–2016) supported by Ministry of Science, ICT and Future Planning, Korea for their endeavor and dedications. Author Contributions: Contributions of authors are following: H.-S.K. Accelerator system design & commissioning; Hoo.H. Linac RF system design & commissioning; C.K. Diagnostics system design & commissioning; G.K. Diagnostics system design & commissioning; C.-K.M. Laser system design & commissioning; H.Y. Accelerator system design & commissioning; S.Y.B. Control system design & construction; H.-J.C. Control System design & construction; G.M. Control system design & construction; B.R.P. Machine interlock system design & construction; Y.J.S. Control system design & construction; D.C.S. Control system design & construction; J.Hu. LLRF system design & construction; J.Ho. RF-Gun design & construction; S.J. Laser System design & construction; S.-H.K. Klystron modulator system design & construction; K.K. Linac RF system design & commissioning; D.N. Vacuum system design & construction; S.S.P. Klystron modulator system design & construction; Y.J.P. Linac RF system design & construction; Y.G.J. Undulator system design & construction; S.H.J. Magnet power supply system design & construction; H.G.L. Undulator system design & construction; San.L. Undulator system design & construction; Soi.L. BPM system design & construction; B.O. Diagnostic system design & construction; H.S.S. Magnet system design & construction; J.-H.H. Injector system design and commissioning; M.H.K. Radiation safety system design and construction; N.-S.J. Radiation safety system design and construction; Y.-C.K. Building design and construction; M.-S.L. Building design and construction; B.-H.L. Utility system design and construction; C.-W.S. Utility system design and construction; I.-S.M. Building design and construction; J.-M.Y. Building design and construction; Y.W.P. Beam Dynamics; W.-W.L. Undulator system design & construction; C.-S.L. Beamline interlock system design & construction; H.S. Data center design & construction; J.H.K. Network system design & construction; Y.K. Beamline commissioning; J.H.L. Beamline commissioning; S.-Y.P. Control & DAQ system; J.K. X-ray optics commissioning; Jaeku P. Control & DAQ system; I.E. Optical laser system design, construction & commissioning; S.R. X-ray optics; Sun.K. Beamline design & commissioning; K.H.N. Serial femtosecond crystallography instrumentation design & commissioning; Jaehyun P. Serial femtosecond crystallography instrumentation design & commissioning; Jaehun P. Optical laser system commissioning; San.K. Coherent X-ray imaging instrumentation design & commissioning; Soo.K. Soft X-ray beamline design & commissioning; R.A. Soft X-ray beamline design & commissioning; S.H.P. Soft X-ray beamline design & commissioning; K.S.K. X-ray detector; Hyo.H. X-ray detector; S.N.K. Vacuum & beamline construction; Seo.K. Mechanical design & beamline construction; C.-J.Y. Beamline planning & management; B.-S.K. Beamline planning & management; T.-H.K. Beamline planning & management; K.-W.K. Beamline planning & management; S.H.K. Building design and construction; Hee-S.L. Radiation safety system design and construction; Heung.-S.L. RF system design and commissioning; K.-H.P. Magnet power supply system design & commissioning; T.-Y.K. Beamline commissioning; D.-E.K. Undulator system design & commissioning; K.B.L. Beamline commissioning. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Nam, S.H.; Kang, H.S.; Ko, I.S.; Cho, M. Upgrade of Pohang Light Source (PLS-II) and Challenge to PAL-XFEL. Synchrotron Radiat. News 2013, 26, 24–31. [CrossRef] 2. Ko, I.S. Status of PAL-XFEL Construction. Bull. AAPPS 2016, 26, 25–31. 3. Lee, H.S.; Park, S.S.; Kim, S.H.; Park, Y.J.; Heo, H.; Heo, I.; Kim, K.H.; Kang, H.S.; Kim, K.W.; Ko, I.S.; et al. PAL-XFEL Linac RF System. In Proceedings of the 7th International Particle Accelerator Conference, Busan, Korea, 8–13 May 2016; Petit-Jean-Genaz, C., Ed.; JACOW: Geneva, Switzerland, 2016; pp. 3192–3194. 4. Kim, D.E.; Jung, Y.G.; Lee, W.W.; Kang, H.S.; Ko, I.S.; Lee, H.G.; Lee, S.B.; Oh, B.G.; Suh, H.S.; Park, K.H.; et al. Development of PAL-XFEL Undulator System. In Proceedings of the 7th International Particle Accelerator Conference, Busan, Korea, 8–13 May 2016; Petit-Jean-Genaz, C., Ed.; JACOW: Geneva, Switzerland, 2016; pp. 4044–4046. 5. Kim, C.; Lee, S.; Kim, G.; Oh, B.; Yang, H.; Hong, J.; Choi, H.J.; Mun, G.; Baek, S.; Shin, D.; et al. Diagnostic System of the PAL-XFEL. In Proceedings of the 7th International Particle Accelerator Conference, Busan, Korea, 8–13 May 2016; Petit-Jean-Genaz, C., Ed.; JACOW: Geneva, Switzerland, 2016; pp. 2091–2094. 6. Park, J.; Eom, I.; Kang, T.-H.; Rah, S.; Nam, K.H.; Park, J.; Kim, S.; Kwon, S.; Park, S.H.; Kim, K.S.; et al. Design of a hard X-ray beamline and end-station for pump and probe experiments at Pohang Accelerator Laboratory X-ray Free Electron Laser Facility. Nucl. Instrum. Methods Phys. A 2016, 810, 74–79. [CrossRef] 7. Park, J.; Kim, S.; Nam, K.-H.; Kim, B.S.; Ko, I.S. Current Status of the CXI Beamline at the PAL-XFEL. J. Korean Phys. Soc. 2016, 69, 1089–1093. [CrossRef] Appl. Sci. 2017, 7, 479 11 of 11

8. Han, J.H. Beam Commissioning of PAL-XFEL. In Proceedings of the seventh International Particle Accelerator Conference, Busan, Korea, 8–13 May 2016; Petit-Jean-Genaz, C., Ed.; JACOW: Geneva, Switzerland, 2016; pp. 6–10. 9. Zhu, D.; Cammarata, M.; Feldkamp, J.M.; Fritz, D.M.; Hastings, J.B.; Lee, S.; Lemke, H.T.; Robert, A.; Turner, J.L.; Feng, Y. A single-shot transmissive spectrometer for hard X-ray free electron laser. Appl. Phys. Lett. 2012, 101, 034103. [CrossRef]

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