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Transcript
9.0 Introduction Objectives This section concerns those elements between the undulator end and the User experiments. It is written based on the User and FEL physics requirements for the x-ray optics and x-ray diagnostics, and the facility requirements (i.e. the facility protocols and guidelines). Some of the User and FEL physics requirements are found in the document “LCLS First Experiments (September 2000)”; more details have been obtained by working with individual User group representatives. From the known requirements, the appropriate hardware can be determined. It is usually required to preserve the high transverse coherence, the time structure, and sometimes the polarization, of the radiation. Both the x-ray optical components, and the x-ray diagnostics, can be categorized as a) those systems that are needed for FEL commissioning, or are required by multiple User experiments b) those that are specific to a limited number of the User experiments. In the case of the diagnostics, those that are required for commissioning or are required by multiple users are described first. Those specific to a particular experiment are described in the section detailing the particular experimental layout and optical requirements. Experimental halls. Two experimental halls are provided, one close to the undulator exit (Hall A, starting ~ 50 m from the undulator end) and one downstream of the undulator (Hall B, starting ~ 400 m from the undulator end). The total experimental floor area is a requirement of the first 6 experiments planned (including FEL physics); the locations are determined by local access roads and topology. Optics in Hall B will experience a reduced power density, by about a factor 15 (see below); this should allow many standard solutions and materials to be applied to the optics. Hall A is necessary if maximum coherence is important. Hall A is also necessary because a number of the optical elements and instruments developed (at least conceptually) for the LCLS depend on close proximity to the LCLS undulator for optimal operation or parameter values. These elements include (and are discussed later): a) planar take-off mirrors (if these were 300 m farther downstream they would need to be substantially longer, up to 1 meter/facet); b) refractive lenses (whether solid state or gaseous, such lenses are efficiencylimited by their aperture diameter - even at distances of ~50 m from the undulator their operation will be marginal); c) chirped-multilayer compressor optics (these are also efficiency-limited by the beam diameter); d) multilayer-based transmission gratings (with present techniques it is doubtful that the required high-quality and efficient gratings with apertures greater than ~100 m could be built). An additional reason for a close hall (Hall A) involves the transmission of the spontaneous synchrotron radiation (SR) to experiments. Close to the undulator, a ~1 cm aperture through the vacuum system should transmit a usable fraction of this spectrum. This aperture is at the limit at which the differential pumping systems we have specified can operate effectively. If the same SR cone needed to be transmitted to the far hall, the vacuum aperture would more than double, necessitating a complete redesign of the vacuum components. These re-specified components would significantly more expensive. The large flight distance to Hall B, and associated beam wander, is not expected to be a problem (the specification of beam "wander" in the LCLS design study report is ~10% of the beam diameter, i.e. independent of path length). The angular acceptance for SASE saturation through the undulator is of the same order as the beam divergence, so any beam angle excursions larger than this value would probably quench any coherent output (a similar constraint applies to the position of the beam axis). In addition to this, an active monitor and feedback system coupled to positional and attitude controls on the mirrors (and possibly the undulator) are planned to help stabilize beam movement. Figure 9.0.1 illustrates one possible configuration of the halls, in which various optical elements are indicated (all described later in detail). An important flexibility for various experimental applications is the possibility of separating, or suppressing, the spontaneous radiation with respect to the coherent line spectrum. This spectral-angular filtering function can be performed by: a) the absorption cell b) take-off mirrors or crystals, and c) by the horizontally and vertically tunable x-ray slits. Front end Undulator exit fast close valve and seat differential vacuum pumping slits / collimators differential vacuum pumping collimators differential vacuum pumping monochromet er spool calorimeter attenuator / absorber calorimeter beam stop / burn thru' monitor differential vacuum pumping mirror chamber 1, 3 degree differential vacuum pumping beam stop / burn thru' monitor Hall A3 beam stop/ burn thru' monitor Optical differential vacuum pumping Hall A1 Atomic physics Hall A2 beam stop / burn thru' monitor Hall A1 Plasma physics beam stop / burn thru' monitor Hall A2 lead FEL physics pinhole, calorimeter Hall A3 beam stop / burn thru' monitor lead mirror chamber1 degree differential vacuum pumping beam stop / burn thru' monitor Optical differential vacuum pumping Hall B1 Nanoscale dynamics collimators differential vacuum pumping Hall B2 beam stop / burn thru' monitor Hall B1 Femto chemistry Hall B2 Hall B3 beam stop / burn thru' monitor Biology Hall B3 Diagnostic beam stop / burn thru' monitor Diagnostic Diagnostic lead pinhole, calorimeter lead lead Figure 9.0.1 A schematic diagram of elements in the experimental halls. Photon-induced damage. In translating the User and facility requirements to hardware, the attributes of the FEL x-ray output must be considered. Detailed properties of both the coherent and spontaneous radiation are available. In particular, the photon energy, the energy per bunch, and the beam cross section, imply damage will be an important issue in component design; these parameters are shown in Table 9.0.1 for locations approximating the front of Hall A, and in Hall B. Only the coherent light poses a problem; the spontaneous emission is divergent and will usually be reduced by an upstream aperture. It is also mostly at larger energies than the fundamental, and the energy absorbed in optical components is low. Normal incidence The exact criteria for absorption and damage during pulses of such high intensity as those to be produced by the LCLS are not known, and will be part of the first atomic physics experiments, but photo absorption and possible subsequent melt are used here. Table 9.0.2 shows results for different materials used in Hall A1 (atomic physics experiments), for the worst case of 827 e.V. where absorption is largest [1]. Dose rates given here are for normal incidence, relevant for transmissive optics, calculated from photo-ionization cross sections ( phot oi onizati on), with the photon beam areal density phot on calculated for a propagated Gaussian beam: dose E phot on phot on phot oionization Ephot on is the photon energy. Comparing the dose predicted in eV/atom, and the dose required to melt, one finds Be, B4C, C and Si can be used without melt (although Si is already at > 0.5 the melt limit). Probably BeB, Li and LiH can also be used. Two other criteria must be considered for transmissive optics; first the material should be such that a phase change occurs in a reasonably large (> 5 m) distance, to allow construction. Second, the distance required for this phase change should be shorter than the penetration depth, so that a reasonable fraction of the light is transmitted. Table 9.0.1. Parameters of the FEL x-ray photon output that are involved in estimates of damage thresholds. Check divergence of SR FEL photon energy 0.828 keV (4.54 GeV electron beam) 8.27 keV (14.35 GeV electron beam) Coherent (fundamental) Spontaneous Coherent (fundamental) Spontaneous Energy per pulse (mJ) 3 1.4 2.5 22 Peak power (GW) 11 4.9 9 12 81 Photons/pulse 23x10 1.9x10 Divergence (fwhm, rad) 9 Spot (fwhm) at 50 m (m) Hall A 610 130 Spot (fwhm) at 400 m (m) Hall B 4400 570 Peak energy at 50 m (J. cm-2) Hall A 0.59 11.9 Peak energy at 400 m (J. cm-2) Hall B 0.01 0.57 310 12 1 100 Table 9.0. 2. Parameters relating to material absorption, damage, and phase change of 827 eV photons in Hall A1. w orst ca se FEL photon energy = 8 27 e V, Hall A1 front do se (EV/atom ) me lt (eV/ato m) ph ase depth (le ngth for a 1* p hase chang e) ( m) 5.2 pe netra tion de pth (um) Li Li H B BeB B4C C Si 0.001 0.001 0.005 0.005 0.14 0.22 0.38 Al 0.27 0.2 1.8 Cu po lystyrene 0.72 0.11 0.35 0.5 2.7 0.36 1.4 0.6 0.86 0.74 0.9 1.1 47 35 5.1 4.3 1.6 1 1.6 pe netra tion de pth / ph ase de pth (large is go od) 9.04 3.64 1.11 1.45 Grazing incidence Calculations for grazing incidence mirrors include the effect of energy density dilution through the angle (the footprint area increases), reflectivity, and deposition throughout an e-folding depth for the photons. The results demonstrate an interplay between atomic number, incidence angle, and photon energy. The absorbed energy density is [2]: A (eV / atom) Ppeak 2 i 1 R 2 q Dw p # Here Ppeak is the peak coherent power, is the standard deviation of the temporal pulse length, q is the electronic charge, Dw [cm] is the beam diameter at the optic, p [cm] is the 1/e penetration depth of the light into the material in a direction normal to the surface, # [cm-3] is the atomic density of the material, and R the reflection coefficient. Following conventional analysis [1,3], we show A vs. i in Fig. 9.0.2 for three candidate reflecting materials: Au (high-Z), Ni (medium-Z), and Be (low-Z), for three representative values of the LCLS's coherent fundamental and ~3rd harmonic lines (900 eV, 8500 eV, and 30000 eV). Selecting, e.g., A 0.01, a criterion suggested by earlier experimental work at SSRL [3], and safe with respect to melt, we may, for example, select an Au-coated mirror with I ~ 0.0001 for energies < 3 keV, and a Be-coated reflector with I ~ 0.0005 for all energies >3 keV. 100 A[eV/atom] 10 1 0.1 0.01 Au 1 keV Au 8.6 keV Au 30 keV Be 1keV Be 8.6 keV Ni 8.6 keV Ni 30 keV Ni 1 keV Be 30 keV 0.001 0.0001 10-5 10-6 -5 10 0.0001 0.001 0.01 0.1 Grazing Incidence Angle Figure 9.0.2. Peak power energy loading of candidate LCLS mirror materials vs. (TE) grazing incidence angle and LCLS energy. Diffraction Assuming a crystal to be cut to thicknesses equal to the extinction depth, and operated in a symmetric diffraction geometry, then a conservative estimate of power and energy loading can be obtained. Assume: 1) the angles of incidence of the radiation (with respect to the surface plane) are large enough to enforce negligible reflection; 2) on the average, the diffracted beams travels through approximately the same distance in the material as the 0th order beam; and 3) on average, the absorption depth, ta, for the diffracted beams is the same as for the incoming beam. Then the energy loading is: (eV / atom) Ppeak 2 qD2w #t a Selecting Si, C*, and Be as the candidate materials, we tabulate A for selected crystal plane orientations in Table 9.0.3. The lower absorptivities of the lower-Z materials result in substantially lower energy loading, in fact down to levels comparable to those that were presumed marginal for the mirror material. On the other hand, even at 1.5 Å, it is evident that carbon (i.e., diamond), ostensibly the material of choice, is beginning to experience substantial loading. At wavelengths > 2-3 Å, the corresponding numbers for all three materials would begin to warrant serious concern. Table 9.0.3. Selected crystal and energy loading parameters under LCLS beam conditions at 1.5 Å. Dw = 100 m. Assumed crystal thickness is ~te. Material Lattice Spacing dH [Å] 1st Order Resolution [Dl/l] A Diffraction (x10-6) [eV/atom] Angle [°] Be (002) 1.7916 24.1.75 22.8 0.014 Be (110) 1.1428 41.02 7.1 0.009 C* (111) 2.0593 21.36 59.7 0.069 C*(220) 1.2611 36.49 19.3 0.042 Si (111) 3.1354 13.84 135 2.011 Si(220) 1.9200 22.99 57.7 1.232 Table 9.0.4. Summary of suitable materials for optical components, without any FEL radiation attenuation transmi ssion grazing in cidence crystaldiff raction multi layers Front ha lls 0.8 keV 8 keV Margin ally B e, C, Li,B e, B,C, Si, Si andlo w-z compoundso f the above Au Be (ext reme incidence) none Be, C* Alllo w to mod erate z Alllo w to mod erate z 0.8 keV W M, Au OK Rear hall s 8 keV W,M, Au OK W,M, Au OK W,M, Au OK Alllo w to mod erate z W,M, Au OK Alllo w to mod erate z W,M, Au OK Table 9.0.4 summarizes the possible materials that can be used, if there is no FEL radiation attenuation. In Hall B the increased spot size reduced the energy density by a factor ~15, and more standard materials are possible at all photon energies. For example, W, M and Au have doses < 0.3 eV/atom for both 8 and 0.8 keV photon energies. Continued R&D into the topic of x-ray photon-material interaction and damage is imperative to reduce the risk of component damage associated with many of the User experiments. First, not all known physics has yet been included in the modeling described above; see section 9.6.0. Second, and more importantly, the remarkable photon areal densities available are expected to instigate new processes, which are the specific topic of the atomic physics experiments (see "LCLS First Experiments (September 2000)". The effects of the intensity spikes within a single FEL ~ 250 fs pulse, the spikes having characteristic times < 1 fs and intensities ~ 5 times the nominal value, are unknown. Absorbers and attenuators Gas, liquid or metal attenuators (see section 9.5.2.3, and 9.6.2.5) will be constructed to reduce the FEL beam intensity, both as an experimental control, and to allow the use of standard optical component and diagnostic designs. It is emphasized that, while a design for a gas attenuator has been provided, a basic question is how the actual LCLS pulses, whose intensity and degeneracy parameters lie well outside the regime of weak-field interactions, will interact with candidate absorbing media. We note the gas absorption cell [4] can be used for initial studies of scattering of the LCLS pulses by absorbing media. The chamber design, predicated on the initial use of xenon, can includes ports for line of sight fluorescence detection, as well for the introduction of external magnetic and electric fields. Due to its location inside the FFTB tunnel, provisions for a detector shielding enclosure have been included. References [1] R. M. Bionta, “Controlling Dose to Low Z Solids at LCLS”, LCLS note LCLS-TN-00-3 [2] R. Tatchyn, "LCLS Optics: Technological Issues and Scientific Opportunities," in Proceedings of the Workshop on Scientific Applications of Short Wavelength Coherent Light Sources, SLAC Report 414; SLAC-PUB 6064, March 1993 [3] R. Tatchyn, P. Csonka, H. Kilic, H. Watanabe, A. Fuller, M. Beck, A. Toor, J. Underwood, and R. Catura, "Focusing of undulator light at SPEAR with a lacquer-coated mirror to power densities of 109 watts/cm2," SPIE Proceedings No. 733, 368-376(1986) [4] D. Ryotov and A. Toor, "x-ray attenuation cell", LCLS TN-00-10 (2000)