6.5 Electron Beamline System Description

Transcription

1 6.5 Electron Beamline System Description The electron beamline for the photoinjector consists of the rf gun (see Section 6.3), Linac 0 (L0, also commonly called the booster accelerator), and the Matching Section. The gun is surrounded by an emittance compensating solenoid. (See Section 6.1.3, Design Principles.) Linac 0 consists of two SLAC-type 3-m S-band accelerator sections, an associated solenoid, and necessary drift sections. It is followed by a Matching Section (MS), also known as "dog-leg" 1 (DL1), and associated instrumentation that brings the beam from the injector vault to Linac 1 (L1) in the accelerator housing. The optical design of the MS is described in Section 7.5.1, Low- Energy Dog-Leg. The design accomodates various diagnostics that are discussed in the following subsections. The overall layout of the photoinjector system in the context of the injector vault and accelerator housing is shown in Fig The figure shows the new radiation shielding separating the injector vault from the linac housing. The injector personnel protection system is designed to allow access to the injector when the linac is operating (including 50-GeV beams), but electron beams in the injector cannot be run when the main linac is in permitted access. (See Section 6.7, Radiation Protection Issues.) Note that it is possible to run injector beams into the injector beam dump just inside the linac housing even when the linac is operating with beams to the Research Yard (End Station A, etc.) since the bend deflecting the beam into the dump is pulsed. Chapter 6 Version 12/6/00 35 of 58

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3 A schematic layout showing only the principal beamline elements is shown in Fig It is not to scale. The diagnostics are discussed in Section 6.5.2, Standard Beamline Diagnostics. Because of the unique nature of the LCLS photoinjector, there is also the need for several stateof-the art diagnostics. These are described in Section 6.5.3, Slice Emittance, and 6.5.4, Temporal Pulse Shape. The rf distribution system is shown in Fig UnSLEDed klystrons will be used to improve the phase and amplitude stability. Since the accelerating sections will operate at about 25 MeV/m (see Section 6.6.1, PARMELA Simulations), they will each require their own 5045 klystron according to the relation for a standard SLAC 3-m section installed in the 3-km linac: E[ MeV] = 10 P[ MW], (6.5-1) where E is the electron beam energy increment in units of MeV and P is the klystron rf power at the klystron in units of MW. Thus three klystrons are required including one to power the gun. This arrangement will also allow the rf phase and amplitude for the rf gun and the two 3-m sections to be controlled independently using low-power controls. Figure Schematic layout (not to scale) showing only the principal beamline elements, the location of the diagnostics, and the rf distribution system. In the figure are shown the rf gun (G), the emittance compensating solenoid (S1), charge coupled devices (CCD), klystrons (K), the focusing solenoid (S2) around the first 3-m accelerating section (L0-1). The Matching Section (MS) begins with the end of the second accelerating section (L0-2). A bunch length monitor (BLM) and electro-optic device (EO) are located in the MS. Near the end of the MS is an injector tune up dump that also serves as a Faraday cup. The photoinjector, including the MS, ends at the entrance to the Linac 1 (L1). Chapter 6 Version 12/6/00 37 of 58

4 Klystron 21-2 will not be needed for L1 since the corresponding accelerator sections will be permanently removed for Bunch Compressor 1 (BC1). The rf from this klystron will be rerouted to the rf gun. Klystrons 20-7 and 20-8 are downstream of the positron source and thus are not used by PEP-II. Their rf will be rerouted to sections L0-1 and -2 respectively. In the future, should these stations need to be restored to the 50-GeV linac, new stations will have to be installed for L0. Station 24-7, liberated by Beam Compressor 2 (BC2), can be moved closer to L0 with modest effort. A separate variable voltage substation (VVS) will be used to provide ac power for the 3 injector rf stations. The separate VVS also facilitates an independent Personnel Protection System (PPS) zone for the injector vault. See also Section 6.7, Radiation Protection Issues. The axial field from the emittance compensating solenoid, S1, starts just downstream of the cathode and extends for 15 cm. There is also a weak solenoid, S2, around the first accelerating section, which makes a modest improvement in the final emittance as discussed in Section 6.1.3, Design Principles Standard Beamline Diagnostics Standard beam transport and diagnostic devices are indicated in Fig The apparatus and instrumentation shown there is designed to assist in transport and shaping of the electron bunch, and for measurement of key beam parameters. Issues concerning beam transport and beam measurement are linked from a practical standpoint, and also in terms of lattice design: the selection, placement, and operation of focusing and bending magnets is in part determined by emittance and energy spread diagnostics. On the other hand, the relative placements of the gun, gun solenoid, and booster tanks are chosen for optimal beam quality, and subsequent selection of diagnostic devices is constrained by available space and beam energy. Note that Fig is schematic and not properly scaled. There is an approximately one-meter drift between the downstream edge of the gun solenoid (S1) and the start of the booster (L0-1). This section contains both a Faraday cup (interceptive) and a toroid (non-interceptive) for measurement of bunch charge. Transverse distribution of charge is monitored on a retractable viewing screen. Bunch position at the booster entrance is obtained from a BPM, and steering coils provide directional control. The drift between the booster sections L0-1 and -2 is one-half meter in extent; a toroid monitors transmission through the first section, and a BPM and steering coils monitor and maintain trajectory. Diagnostics and transport elements downstream from L0 are used to match the 150 MeV injector beam through the bend and into the main linac, starting with L1. Emittance and energy spread measurements are also accomplished here. During normal beam operation, the quadrupole lenses nearest L0 are designed to operate at settings that facilitate emittance measurement, accomplished by wire scan profiles of the beam at downstream locations. An additional wire scan in the dispersive bend measures energy spread. These run-time measurements do not interfere with beam transport. Because of their importance in affecting FEL performance, emittance and Chapter 6 Version 12/6/00 38 of 58

5 energy spread are measured at additional locations enroute to the undulator; see Sections 7.8.1, Transverse Emittance Diagnostics, and 7.8.3, Beam Energy Spread Diagnostics, for further discussion of these diagnostics. The BLM diagnostic in Fig is a bunch length monitor consisting of a ceramic gap and rf pickup. Also indicated are a streak camera station and an electric-optical (EO) device, each of which can be used to monitor axial bunch charge distribution. In addition, the streak camera performs as a slice emittance diagnostic. The EO and slice emittance diagnostics are the topics of the following sections. Near the end of the Matching Section is an injector tune up dump. The dump provides a convient place to park the beam when tuning up the injector. It is also a diagnostic for measuring beam energy, energy spread, and, since the dump itself is a Faraday cup, the beam current. Since the pickoff for the dump line is pulsed, it can be operated even when straight-ahead linac beams are running for End Station A or other Research Yard locations. During LCLS operation, the dump pickoff can be operated at low rate to use the monitoring capabilities if desired Slice Emittance The wire-scanning emittance diagnostic described in Section 7.8.1, Transverse Emittance Diagnostics, measures the transverse emittance of the full axial projection of the electron bunch. Also of interest is the so-called "slice emittance", the (transverse) emittance of electrons in axial slices that are only a fraction of the full bunch length, say 2-ps slices out of a 10-ps bunch. Multiparticle simulations of photoinjector beams indicate that within the bunch, the axial variation in the transverse space charge force causes a smooth, non-filamented dilution in trace space density, with concomitant full bunch emittance growth relative to slice values. An analytic account has been given by Serafini and Rosenzweig [7]. Strategies for minimizing or reversing in the final beam this correlated emittance growth have received considerable attention, as in Section 6.1.2, Emittance Compensation. Beyond the benchmarking of code and verification of theoretical models, slice emittance is important in that it may be the more relevant parameter affecting FEL performance. A slice emittance diagnostic using a streak camera in combination with the well established quadrupole scan procedure has been demonstrated at LANL [16]. A quad scan relies on a set of beamwidth measurements obtained as a quadrupole lens is scanned through a range of focal lengths, to derive the three parameters that characterize the region in trace space occupied by the beam. In a typical application, optical radiation emitted from a screen inserted in the beam path is imaged onto a light sensitive detector, and a beamwidth measurement derived from the spatial dependence of the image intensity. Metal screens are typically prompt (subpicosecond) emitters of OTR, making them ideal light sources for preserving within the emitted optical pulse the axial structure (intensity) of the incident electron bunch. An apparatus or procedure capable of resolving both axial and transverse variations in OTR intensity enables a slice emittance measurement. Streak cameras with temporal resolution better than two picoseconds have been Chapter 6 Version 12/6/00 39 of 58

6 available for some time. In a slice emittance application, an image of a line segment on the OTR screen is made at the narrow (~20 micron) slit entrance to the streak camera. Preserved in the OTR pulse, the streak tube output displays on its horizontal axis the electron beam intensity along this line segment, and on its vertical axis the temporal dependence of this intensity. A CCD (charge coupled device) image of tube output is ideal for analyzing the beamwidth of different slices, represented by some number of adjacent pixel rows. The streak camera and OTR optical system indicated in Fig , downstream from the quadrupole system at the exit of L0, is intended for a slice emittance measurement of the 150 MeV beam injected into the main linac Temporal Pulse Shape Features of the photoinjector laser system described in Section 6.4, Laser System, that tailor it for reliable electron production also facilitate applications of the laser to novel electron beam diagnostics. Stable, unconverted laser light (infrared and visible) constitutes a diagnostic beamline (probe) for applying electro-optic sampling techniques to the measurement of the temporal shape of the electron bunch. Temporal resolution of sampling measurements is determined by the duration of the probe pulse and its timing jitter relative to the uv pulse (which is used for photoelectron production). Nanosecond delay times can be set with subpicosecond stability for picosecond probe pulses. Probe pulses of millijoule energy are available. The positive uniaxial crystals LiNbO 3 and LiTaO 3 are suitable candidates for linear electro-optic beam sampling. In the linear regime, bias fields do not alter the crystal anisotropy. In previous work with 16 MeV electrons, wakefield-induced phase retardation in LiTaO 3 has been demonstrated at signal levels of order 10-1 radians with wake field sensitivity of order 1 radian-m/mv [47]. The electron beam longitudinal distribution and bunch length can be monitored noninvasively using beam wakefield components as a Pockels-effect bias to induce accumulated phase retardation of a probe pulse as it propagates through the crystal. The wakefield-induced Pockels effect generates a linear response which is determined by wakefield dynamics. In a standard configuration using cross-polarized optics, a null signal is set for zero wakefield amplitude; i.e., when the probe waveform and beam wake field are not coincident. Incident and transmitted probe pulses are transported to and from the crystal location by polarizationpreserving optical fiber. In general, picosecond or nanosecond (i.e., uncompressed) probe durations can be used. In the picosecond case, signals can be scanned by varying the relative probe-beam timing. This scanning may not be necessary for the nanosecond case if a fast detector is available as discussed next. Coincidence of the probe and beam wakefield timing generates a transmitted probe signal proportional to its accumulated retardation phase. This signal can be detected with fast diodes (10 s of gigahertz bandwidths) and transient digitizers as well as with frequency-resolved opticalgating (FROG) [48]. FROG is an established ultrafast diagnostic which measures the amplitude Chapter 6 Version 12/6/00 40 of 58

7 and phase history of the transmitted probe waveform with subpicosecond resolution. It is best suited for signals of short (picosecond) and ultrashort (subpicosecond) duration. Known electrooptic coefficients can also be used to estimate wake field amplitudes from the probe signal. The noninvasive feature of this diagnostic method affords the use of multiple sampling sites of known spacing for improved distinction of electron beam effects. Chapter 6 Version 12/6/00 41 of 58

8 6.6 Photoinjector Performance PARMELA Simulations Emittance growth in the photoinjector from the rf gun through Linac 0 and the Matching Section has been studied using PARMELA simulations. The electric field map of the gun was obtained with SUPERFISH and directly used in PARMELA. SUPERFISH was also used to simulate the fields in the traveling-wave accelerating sections, and space harmonics were calculated to be used in PARMELA. rf fields were assumed to be cylindrically symmetric, which seems to be a reasonable assumption since, as discussed in Section 6.3.3, Symmetrization, the dipole rf fields which are normally dominant in an rf gun will be largely eliminated in the LCLS gun. A magnetic field map for the emittance compensating solenoid at the gun was produced using POISSON and passed to PARMELA. The air core solenoid around the first accelerating section was represented with single coils with appropriate strength to accurately represent the magnetic field from each. The magnetic and electric fields in the gun and accelerator region were adjusted to minimize the normalized emittance at the booster exit as discussed below. Charge as well as laser pulse radius and pulse length at the cathode have been extensively studied for 1.6-cell S-band rf guns as a function of solenoidal field and accelerating gradient in the gun and booster for several configurations using a variety of simulation codes; e.g., see Section 6.1.2, Emittance Compensation. The overall result is that to produce a beam of the highest possible brightness, a 1-mm radius and 10-ps bunch length is about optimum for 1 nc of charge. It is also clear that using spatial and temporal distributions that are uniform (flat top) rather than Gaussian will improve the resulting transverse emittance. As a practical matter, uniform distributions can only be approximated. Therefore the PARMELA simulations discussed here have usually assumed a risetime of 0.5 ps, which is within the capability of the laser system described in Section 6.4, Laser System. For the PARMELA simulations, the initial temporal uniform distribution was generated by stacking 9 Gaussian distributions with an rms width of 1 ps, each separated by 1.1 of S-band phase. The resulting bunch shape is shown in Fig The basic layout of Linac 0 (L0) and the Matching Section (MS) are shown in Fig The corresponding input parameters assumed for the PARMELA simulations are summarized in Table The peak electric field at the cathode is taken to be 120 MV/m. This is a compromise since the emittance appears to improve slightly for fields up to 140 MV/m. (See Fig ) However, as the field is increased above about 100 MV/m, not only does the dark current typically increase rather dramatically (quickly exceeding the photocurrent), but the frequency and intensity of rf breakdowns also increases. rf breakdowns tend to leave pits in the cathode surface that destroy the uniformity of the emissivity across the surface. With a Cu cathode, a field of 120 MV/m is chosen as sufficiently challenging with no significant loss of brightness. Chapter 6 Version 12/6/00 42 of 58

9 Table Parameter Input Parameters for PARMELA Simulation. [To be revised] Value Bunch charge at cathode Longitudinal charge distribution at cathode Transverse charge distribution at cathode Bunch length at cathode Bunch radius at cathode Peak rf field at cathode 1.0 nc Uniform with 0.5 or 1.0 ps risetime Uniform 2.9 ps rms 0.84 mm rms 120 MV/m Injection phase??? Emittance compensating solenoid: Axial field Physical length Location a of peak field (also center of physical solenoid) 3.3 kg 18.4 cm 19.1 cm Booster accelerator: Accelerating gradient Location a of input coupler for first section Drift distance between sections 24.1 MeV/m 1.4 m 0.5 m Linac focusing solenoid: Axial field Physical length Location a of start of physical solenoid Location a of exit of L0 Thermal emittance, ε n,th -1.5 kg 1.0 m 1.43 m 7.9 m m rms a With respect to position of cathode. Chapter 6 Version 12/6/00 43 of 58

10 Fig Temporal shape of electron pulse used as input for PARMELA simulations for the rf photoinjector. The scale for the abscissa is in picoseconds. The emittance compensating solenoid is physically 18.4 cm long. For the simulations here it is placed upstream against the rf gun, which results in the center of the solenoidal field being 19.1 cm from the cathode surface. The field map (Fig ) indicates that for this configuration the field at the cathode is essentially zero with the bucking coil field set to zero. Using only the gun, solenoid, and the immediately following drift space (i.e., no booster), the first emittance minimum after the solenoid was optimized by varying the solenoidal field and beam radius at the cathode. A value of B z = 3.3 kg and hard-edge radius of 1 mm was found to be optimum. An examination of Figs and -2 shows that this emittance minimum very nearly coincides with the new working point described in Section 6.1.2, Design Principles. A slightly larger value of B Z here is consistent with the solenoid being displaced somewhat downstream because of the physical interference of the gun structure. Next with both the booster (with an accelerating gradient of 25 MeV/m) and the focusing solenoid included, the emittance at the booster exit was minimized by varying the booster location (keeping the drift distance between the two sections fixed at 0.5 m) and the focusing solenoid field. The results are summarized in Fig The optimum position of the entrance to the first section was was found to be 1.4 m from the cathode and the solenoid field kg. Note that as the booster is moved toward the position for the minimum emittance, the emittance decreases more gently and and eventually monotonically, eventually approximating the shape shown in Fig Finally the emittance at the entrance to Linac 1 (L1) was minimized by varying the field and position of the focusing solenoid. An emittance minimum was found for a field of -1.5 kg and by positioning the start of the solenoid at 1.43 m with respect to the cathode. As a further refinement, a thermal emittance of m was added to the PARMELA deck [8]. Using these parameters, a final emittance of m was obtained for a 20K-particle run. See Fig Using the same parameters but substituting a 1.0 ps risetime for the input pulse increases the emittance by about 10%. Chapter 6 Version 12/6/00 44 of 58

11 Fig Variation of emittance as a function of entrance position of booster. The abscissa is in arbitrary units of distance, the ordinate in units of 10-6 m rms normalized emittance. [This figure will be re-parameterized in terms of distance to booster.] Fig Transverse normalized rms emittance as a function of distance from the cathode for 20,000 particles. A risetime of 1 ps is assumed. Thermal emittance is not included. [The 0.8x10-6 m curve will be used.] The transverse distribution of particles in the beam at the exit of L0 is shown in Fig The upper left plot is a normalized x-y scatter plot. The normalized x-x' phase space is shown in the upper right with the rms emittance ellipse given by the circle in the center. The density of particles in the center of the beam is not evident in these plots. The normalized rms slice emittance in x and y as a function of axial distance along the bunch is shown in the lower left. The projected value is shown by the horizontal line. On the lower right, The beta-mismatch parameter, ζ,, is shown as a function of z. ζ, is a measure of the mismatch between the beam Twiss parameters and the lattice. ζ, is defined in Section 7.8.1, Transverse Emittance Chapter 6 Version 12/6/00 45 of 58

12 Diagnostics. By Eq. (0.21), ζ, is normalized such that the values are always 1. A large variation in ζ, within the bunch will cause the emittance to grow regardless of the tuning of the lattice, i.e., there no matching will be perfect for the entire beam. Fig Normalized transverse phase space at the exit of L0. (1) Distribution of particles in the beam (upper plots). The scales are derived from the right-hand figure in which the rms emittance ellipse in the x-x' plane (only) is normalized to a circle having a radius of unity. (2) Transverse normalized slice emittances (lower left plot) in both planes and mismatch parameter, ζ, (lower right plot) along the bunch z-axis. The phase space plots for a series of 9 slices identified in the lower right plot of Fig are displayed in Fig The blue background in each case is the full projection in the x-x' plane, identical to the upper right plot of Fig ). The red represents the distribution in the x-x' plane of the particles in the particular slice. Note that the "halo" (particles outside the main core) occur primarily in the first and last slice. Chapter 6 Version 12/6/00 46 of 58

13 Fig Transverse distribution in the x-x' plane of particles in a slice (red) at the exit of L0 shown against a background of the full projection (blue). In each plot read left to right, the location of the slice along the z-axis of the bunch can be identified by the corresponding asterisk in the lower right plot of Fig The emittance values derived from the full distribution of particles is strongly influenced by the few particles outside the core. In Fig , the slice emittances are displayed for various cuts in the transverse tails. A 5% cut in the tails reduces the emittance for the central slices by about 40%. The effect is even more dramatic when the brightness of each slice is plotted, as in Fig The longitudinal distribution of particles at the exit of L0 is shown in Fig The energy as a function of axial position within the bunch is shown in the upper left with the corresponding particle distributions projected out from both planes shown in the upper right and lower left. The lower right distribution is the same as the lower left, but in terms of peak current instead of number of particles and time instead of axial position. The extremely low emittances predicted using HOMDYN as, for example, in Section 6.1.3, Design Principles, have been validated with respect to multi-particle tracking codes such as ITACA [11] and the UCLA version of PARMELA [49]. These results with configurations that are slightly different than the LCLS case indicate that even when finite risetimes and realistic thermal emittances are included, an integrated emittance lower than m should be Chapter 6 Version 12/6/00 47 of 58

14 attainable for the LCLS configuration [49]. Fine tuning for the LCLS injector using the standrad LANL PARMELA will continue. Fig Normalized slice emittance along the bunch for different cuts in the transverse distribution. When an increasing percentage of particles in the transverse tails are discarded, the emittance of the remaining core particles decreases rapidly. Fig Brightness along the bunch for different cuts in the transverse distribution. The brightness (equal to the peak current divided by the product of the normalized x and y emittances) increases rapidly towards the transverse core of the bunch. Chapter 6 Version 12/6/00 48 of 58

15 Fig Longitudinal distribution of particles in the beam at the exit of L0. The Matching Section between L0 and L1 was designed using the simulation code MAD. A plot of the TWISS parameters as a function of axial distance along the beamline is shown in Fig It is easily seen from the figure that the betas get very small, which could potentially result in undesirable emittance growth due to the high space charge density at this relatively low energy. To check for this possibility, the PARMELA simulation was extended through the Matching Section. Since the beam size aspect ratio reaches 12, cylindrical symmetry cannot be assumed. Instead a point-to-point calculation was launched. The resulting particle distribution for the same beam as displayed in Figs , -5, and -8 is shown in Figs to -11 for the end of the Matching Section (i.e., the beginning of L1). Note that there is no significant emittance growth in any of the 3 dimensions. Chapter 6 Version 12/6/00 49 of 58

16 [This figure in preparation.] Fig Transverse distribution of particles in the beam at the exit of MS. [This figure in preparation.] Fig Slice emittance at the exit of MS. [This figure in preparation.] Fig Longitudinal distribution of particles in the beam at the exit of MS. Table Parameter Bunch charge PARMELA Output Parameters at End of Linac 0. [To be revised.] Value 1.0 nc Injection phase??? Booster phase??? Pulse length 0.87 mm rms??? Energy 150 MeV Relative energy spread 0.13% rms??? Normalized transverse emittance: 1.0 ps risetime, ε n,th included, 20 K particles m rms 1.0 ps risetime, ε n,th not included, 20 K particles m rms 0.5 ps risetime, ε n,th included, 10 K particles, no optimization m rms 0.5 ps risetime, ε n,th included, 20 K particles m rms Gun Test Facility The Gun Test Facility (GTF) was commissioned at SLAC to develop and characterize an appropriate electron source for the LCLS. The rf gun chosen for initial study is the LCLS prototype gun. To allow temporal pulse shaping, the laser for driving the photocathode was required to have a rise time 1 ps. A Nd:glass laser was chosen over Ti:sapphire due to cost Chapter 6 Version 12/6/00 50 of 58

17 considerations. The primary goal of the GTF is to produce a beam with 1-nC beam with uniform spatial and temporal charge distribution at the cathode, to measure the emittance of this beam, and to determine the operating parameters necessary to produce the lowest possible emittance at 20 MeV. Secondary goals include characterizing the emittance as a function of charge and measuring the longitudinal emittance. Experimental Setup An LCLS prototype gun is installed at the GTF followed by an emittance compensating solenoid. No bucking solenoid is used since the steel flux returns used on the solenoid reduce the magnetic field at the cathode to only a few Gauss. After a short drift section following the gun and solenoid, a single 3-m SLAC linac is used to accelerate the beam to high energy in order to reduce the space charge force and freeze the transverse emittance. The accelerator is followed by a diagnostic section. The diagnostics include measurement of the emittance with either a quadrupole scan or two screen emittance measurement technique, spectrometer, toroids and Faraday Cup to measure the charge and an Optical Transition Radition (OTR) screen for bunch length measurement and possible slice emittance measurement utilzing a streak camera. In the drift between the gun and linac several screens and Faraday cups are also installed to diagnose the beam exiting the gun. In addition a 45 UV mirror is installed inside the vaccum system to direct the laser at near normal incidence onto the cathode. The grazing incidence ports on the 0.6 cell of the LCLS prototype also allow the laser to impinge at grazing incidence. The layout of the electron beam line is shown in Fig Laser The GTF drive laser system was specifically designed to be able to produce short UV pulses with adjustable width to optimize the electron beam emittance. A low cost laser with these capabilities has been built at the GTF using a Nd:glass regenerative amplifier incorporating chirped pulse amplification. The oscillator is a passively mode-locked diode-pumped glass laser operating at 1053 nm with approximately 300-fs transform-limited pulsewidth. The oscillator is mode-locked at 119 MHz, the 24th sub-harmonic of the accelerating rf. The pulse train from the oscillator is chirped to 300 ps/nm in a grating pair expander producing positive chirp. A single 1-nJ pulse is then selected at 2.5 Hz for amplification in the Nd:glass regenerative amplifier. After amplification, the 6-mJ pulse is spatially filtered and compressed to as short as 2 ps FWHM using an adjustable length negative chirp grating pair. One of the compressor gratings is mounted on an optical rail to allow for quick and repeatable pulse length adjustments by varying the grating separation and thus the compression ratio. The IR pulse is then frequency doubled and quadrupled in two type-i BBO crystals to generate approximately 400 µj of 263-nm light. Typical conversion efficiencies from the IR to the UV is 13% and up to 300 µj or energy can be delivered to the cathode. Chapter 6 Version 12/6/00 51 of 58

18 Faraday Cup Spectrometer Magnet Phosphor Screen & Energy Filter YAG Screen & OTR Screen Phosphor Screen, & Bunch Length Monitor Toroids Quadrupole Doublet Phosphor Screens & Faraday Cups Solenoid PCRF Gun 3m S-Band SLAC Linac Fig Side view of the GTF electron beamline. The UV pulse is typically imaged onto the cathode at near normal incidence using an 8-m long 1:1 telescope. Grazing incidence is also possible and transverse shaping can be accomplished by imaging an aperture, after the quadrupling crystal, onto the cathode. The laser energy and transverse pulse shape incident on the cathode are measured using a window less than 1 m from the cathode to pick-off two beams containing <3% of the total beam energy. One pulse is sent to an energy meter and second to a UV camera placed at the same distance from the beam pick-off as the cathode to allow transverse laser pulse shape measurements on a shot to shot basis. The 263-nm laser pulse is incident at near normal incidence on the cathode from a 45 Al (90% reflectivity) mirror mounted in the vacuum system. The laser timing jitter relative to the master rf clock is less than 2 ps rms [1]. Laser Temporal Pulse Shaping Since the laser pulse temporal profile is close to a Gaussian distribution, some form of pulse shaping is required in order to produce the minimum expected emittance according to simulations. Two distinct methods for temporal pulse shaping are available at the GTF taking advantage of the drive laser s unique short pulse and adjustable width capabilities. The first method is a time domain technique based on a Michelson interferometer similar to the pulse stacker reported elsewhere [50]. This method utilizes a polarizing beam splitter inside a Michelson interferometer to delay and stack up to four individual short UV laser pulses to produce a nearly flat-top output pulse. The amplitude and delay of each of the four sub pulses can be individually controlled to produce an adjustable shape output pulse. In addition the input pulse width can be varied to control the output pulse width and rise time. Adjacent pulses will be orthogonally polarized to avoid interference effects and separated by approximately the rms pulse duration to produce a flat top pulse. This method will effectively allow one to sum the pulse Chapter 6 Version 12/6/00 52 of 58

19 intensities instead of the field since the interference between each identically polarized beam will be small due to the long delay between identically polarized sub pulses. Fig shows typical calculated pulse intensities and the two orthogonally polarized components assuming a 3-ps FWHM Gaussian input beam and including measured values for loss and the polarization extinction ratio in the polarizing beam splitter. Under these conditions the output beam is roughly 10 ps FWHM with less than 3% ripple on the flat top and an approximately 2 ps risetime (10 to 90% risetime). Due to the polarization dependence of the reflectance of light at non-normal incidence on the cathode, the output pulse must be incident at near normal incidence to produce a near flat-top electron pulse. Input/4 Output Output - P Output - S Laser Amplitude Time (ps) Fig A 3-ps FWHM (σ = 1.3 ps) input pulse is converted into near flat-top laser pulse using a polarizing beam splitter inside a Michelson interferometer. The nearly flat output pulse in red is 10-ps FWHM (σ = 3.3 ps). The output pulse consists of an S and P polarization component shown in yellow and blue respectively. The second technique used for temporal pulse shaping is a frequency domain technique. A frequency mask is placed inside the IR optical pulse compressor at the point where the frequency domain has been mapped to physical space. Hard edged bandwidth filters chopping off the wings of the frequency spectrum will achieve initial frequency domain pulse shaping. Only a small amount of laser energy need be clipped when using a highly chirped beam to produce a significantly flatter pulse with less than 5% ripple as shown in Fig Only 10% of the energy was removed to steepen the rise time and flatten the top of the pulse when the input pulse is frequency chirped such that the pulse width is 10 times the transform limited pulse length. This pulse shaping would be performed in the IR so the effects of the doubling and quadrupling in the non-linear second harmonic generating crystals still needs to be considered. Depending upon the choice of crystals and the exact input pulse length, the ripple could get larger or smaller. Chapter 6 Version 12/6/00 53 of 58

20 1 0.8 Intensity Filtered ouput pulse Chirped input pulse Fig Time (t/τ FWHM ) Laser pulse intensity vs. normalized time is plotted for a chirped pulse with a pulse length 10 times transform limited and the same pulse frequency filtered at ± 0.7 ν. The effectiveness of both methods in producing flat-top laser pulses as well as the resulting emittance of the electron beam will be measured at the GTF. A streak camera with less than 1-ps resolution at UV wavelengths is available to characterize the laser pulse width and shape at the GTF. This camera can also be used with the visible light emitted from the OTR screen downstream of the linac to characterize the electron beam bunch length. Experimental Program As mentioned earlier the primary goal of the GTF is to produce a beam with 1 nc of charge and a normalized rms emittance m. Since simulations indicate this can only be accomplished using near flat top pulses, the penultimate experiment at the GTF is to measure the emittance at 1 nc of charge with a near flat-top laser pulse. We have previously measured a 25% decrease in emittance by increasing the Gaussian pulse length from 5 ps FWHM to 8 ps. In addition to transverse emittance measurements, longitudinal emittance measurements are planned as well as measuring the emittance as a function of charge. Based on the new LCLS photoinjector working point, a measurement of the emittance directly exiting the gun as a function of longitudinal position is planned. According to simulation the minimum emittance is achieved when the first booster section is located at the emittance maximum in the drift region downstream of the gun (see Section 6.1.3, Design Principles). This measurement will be the first measurement of its kind to determine the existence and exact position of the emittance oscillation. This measurement is critical to the final design of the LCLS Chapter 6 Version 12/6/00 54 of 58

21 photoinjector since the final emittance is strongly dependent on the proper positioning of the booster section relative to the gun. 6.7 Radiation Protection Issues Access to the LCLS-Injector tunnel during operation of the Linac will be allowed with additional shielding against secondary radiation induced by beam losses in the Linac. These losses can be normal beam transport losses, occasional mis-steering beam losses, or rare accidentcase beam losses due to a Beam Containment System (BCS) failure. It can be shown that the accident case situation imposes the most stringent requirements on the shielding. In this case, the total dose equivalent must remain below 25 rem/h anywhere in the LCLS-injector tunnel. Assuming a Linac beam with a maximum power of 1.2 MW is lost at the entrance to the LCLS-injector tunnel, a shield thickness of at least 9 feet of concrete is necessary [51]. Since a shield of that thickness is not feasible at all places, in particular around the LCLSinjector beam line shielding penetrations, additional local lead shielding will be used to shadow the injector tunnel from potential radiation sources [51]. Access to the LCLS Injector beamline tunnel will be provided with a new Personnel Protection System (PPS) controlled entry module, that serves as an access control system, thus not allowing personnel to access the injector tunnel during injector operation. There will be two PPS stoppers to prevent radiation due to Linac beam losses from entering the injector tunnel through the beam pipe penetration in the shielding wall. Access to the LCLS-Injector alcove, located at Sector 20 of the Klystron Gallery above the injector and containing control and laser rooms, will be required during operation of the injector. The radiation protection requirements are met by standard measures, such as local shielding, limits on the gun current, or by BCS devices. Access to the Linac tunnel while the LCLS-injector is operating is not planned. The LCLS injector must be turned off (made safe), thus no prompt radiation will be present when access to the Linac is desired. Chapter 6 Version 12/6/00 55 of 58

22 References [47]M.J. Fitch et al., "Electro-optic measurement of the wake fields of 16 MeV electron bunches," UR- 1585/FERMILAB-TM-2096 (1999). [48]D.J. Kane and R. Trebino, Opt. Lett. 18 (1993) 823. [49] M. Ferrario et al., "New design study and related experimental program for the LCLS RF photoinjector," to be published in the proceedings of the 7th European Particle Accelerator Conference, Vienna, Austria, June 26-30, [50] C.W. Siders and A.J. Taylor, High-energy pulse train and shaped pulse generator using a 100% throughput 2 n -pulse Michelson interferometer, in Ultrafast Optics Conference, Monterey, CA, [51] S. Roesler, S. Mao, and W.R. Nelson, "Preliminary design for the LCLS-injector shielding wall," SLAC Memorandum (April 24, 2000). Chapter 6 Version 12/6/00 56 of 58