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9.1 Commissioning and common usage diagnostics 9.1.1 Requirements The diagnostic requirements are derived from the User experimental requirements, based primarily in the LCLS Initial Experiments. (including FEL commissioning, and facility requirements (see section 9.3). Tables of requirements for each of the initial experiments have been compiled, those required for FEL commissioning, and those requested by multiple Users, are separated and described here (atomic physics, plasma physics, FEL physics, nanoscale dynamics in condensed matter, femtosecond chemistry, structural biology). Other experiment-specific diagnostics are dealt with in the sections describing each of the first experiments. Based on these tables, the diagnostics are considered as being required for 827 eV (1.5 nm) first harmonic operation and 8.28 keV first harmonic operation (0.15 nm), and allowing for measurement of both the coherent and spontaneous parts of the spectrum. Present day x-ray detection techniques and time resolutions are: inorganic scintillators (10 ns) and plastic scintillators (1 ns), semiconductor pn junctions (100 ps), semiconductor photoconductors (6 ps) and photocathodes (20 ps). Thus present day techniques have a best time resolution that is about the inter-pulse time, but not the pusle duration of ~200 fs, let alone the internal ~1 fs spikes. That is, only parameters totalled over an individual pulse can be obtained as long as the data is read out before the next pulse arrives. The technology nearest to that required is the semiconductor photocathodes. One solution to obtaining sub 100 fs resolution may be electronic or optical preprocessing of all or part of the beam, but then phase information will be lost. 9.1.2 Coherent radiation properties 9.1.2.1 Spectrum 9.1.2.1.1 low energy: grating spectrometers Roman Tatchyn is providing this section 9.1.2.1.2 high energy: crystal spectrometers Roman Tatchyn is providing this section 9.1.2.2 Total energy (including profile) 9.1.2.2.1 Total energy, accurate, intrusive: calorimetery John Arthur is providing this section 9.1.2.2.2 Total energy, every pulse, non intrusive: ionization chamber The total energy can be measured using an ionization chambers. The proportionality between ionization current and photon number breaks down at high incident photon flux, because space charge is important. The limiting factor is the very low ion mobility. The ions experience resonant charge exchange on the atoms, the cross-section is very large and, accordingly, the mobility is very low. So, in order to prevent a build-up of the positively charged column, one has to apply very high voltages. This, in turn, is limited by the Paschen breakdown constraint. Helium is better than the heavier gases because of a favorable (1/M) dependence of the mobility on the ion mass, a smaller charge exchange cross-section (which also leads to an increase of the mobility) and a higher breakdown limit. 9.1.2.2.3 Profile, intrusive and partially intrusive: x-ray CCD camera The LCLS X-FEL needs x-ray imaging systems for measuring the spatial distribution and divergence of the raw beam, for alignment and focusing of optical elements, and for pulse-to-pulse monitoring of the beam shape, centroid, and intensity. Unfortunately a single instrument cannot currently meet these requirements. The most useful instrument, in the short term, will be a high-resolution, CCD-like, camera for measuring spatial distributions and for alignment and focusing of optical elements. Traditional instruments have used phosphorus screens to convert x-rays to visible light that can be recorded by a CCD. Even with a microscope objective to magnify the screen, the spatial resolution is limited by the spatial resolution of the phosphorus that is typically in the range of 10 to 50 microns. Such resolutions are of marginal utility to the LCLS which has a beam diameter at 8 keV of 100 microns. Recently workers at the ESRF synchrotron facility have discovered a crystal that when used as a scintillator for x-rays achieves 0.8 micron resolution. The scintillator is a 5 micron thick Ce doped YAG crystal on a 100 micron YAG substrate. This camera will not work as a pulse-to-pulse beam monitor device at LCLS because of its slow readout speed, and the susceptibility of the YAG crystal to damage in the intense X-FEL beam. Nevertheless the YAG crystal technology could be utilized for this purpose with slight modification as shown in figure 9.1.2.1. FEL Beam Objective Be Reflected beam YAG CCD Figure 9.1.2.1 Concept for a pulse-to-pulse beam monitoring camera. A small amount of the x-ray FEL beam is reflected by the Be foil onto a YAG scintillator whose emissions are recorded by a fast framing CCD camera. In this concept a thin foil of a low Z material such as Be acts as a beam splitter to partially reflect a portion of the beam onto the YAG crystal. The YAG crystal imaging system is the same as before except a fast framing CCD is substituted for the large format CCD used above. The major foreseeable problem with this concept is the background xray radiation impinging on the crystal due to Compton scattering of the FEL beam by the Be foil, and any fluorescence from an oxide layer on the foil surface. The existence of fast framing CCDs with suitable format and frame rate for the LCLS is not an issue, as suitable fast framing CCDs exist today. An alternate proposal is to utilize Compton scattering of the beam off of a low Z gas or solid, utilizing the crystal/fast framing camera. Gas s cattering cell FEL Beam Slits Compton s cattered x rays YAG Ob jective CCD Reflected b eam Figu re 1 APulse-to-pulse itor ing ofto beam pro file utilizwidth ing Comp tonCompton Figure 9.1.2.2 schematic ofmon a system measure beam using scattering from a gas cell 9.1.2.3 Centroid location 9.1.2.3.1 Ionization chambers The centroid location can be measured by multiple, suitably instrumented, ionization chambers, (also see the gas cell attenuator (9.2.2.1.3). Position detection is accomplished using segmented electrodes (K. Sato, Proceedings of the SPIE, detectors for cystallography and diffraction studies at synchrotron sources, Vol 3774 p 114 (1999)). Using 4 cm long electrodes, with a sawtooth base of 3 mm and height 10 mm, and low noise current read-out, the horizontal and vertical centroid can be measured to < 10 m. 9.1.2.3.2 crossed wire arrays Richard Bionta is providing this section 9.1.2.3.3 mirrors This measurement is performed by the ‘less intrusive profile diagnostic’, based on a CCD camera, see section 9.1.2.2.2. 9.1.2.4 Divergence This measurement is performed by multiple versions (at different locations) of the less intrusive profile diagnostic, based on a CCD camera, see section 9.1.2.2.2. 9.1.2.5 Temporal distribution One proposal to measure the overall pulse length is found in [B. Adams, Nucl. Instrum Meth., A459 339 (2001)]. An intense fs laser is used to depopulate the valence band of a semiconductor. Then incident x-rays with a particular nergy would be absorbed. Recording of a secondary process such as Auger electron emission, x-ray fluorescence, or resonant Raman scattering may allow the pulse duration to be measured. 9.1.2.6 Temporal Coherence: 9.1.2.6.1. Asymmetric Michelson interferometer Time-domain autocorrelation can be used to obtain information on the power spectral density of a temporal signal. Specifically, it can provide information on the temporal coherence length of a quasi-coherent source [1]. In the case of the LCLS, this is a critically important parameter directly related to the FEL-induced microbunch structure in the electron beam [2], and provisions for characterizing its statistics are presently under study by the LCLS X-Ray Optics R&D group [3]. With regard to interferometer design, a basic requirement is to minimize distortions in the temporal structure of the pulse so that valid autocorrelation spectra can be generated. A second requirement, arising from the extreme power density of the LCLS pulses is that the response of an optical element interacting with an LCLS pulse should remain sufficiently uninfluenced by the energy being absorbed during the interaction. A schematic of the proposed instrument is shown in Fig. 9.1.2.7.1. Although a transmission grating splitter is explicitly shown, techniques based on reflecting/transmitting foils [4] could also be considered. The split beams are reflected off mirrors M1 and M2 and recombine at the detector plane. With the mirrors parallel the path length difference between the two interferometer arms is 0. To induce a path length difference M1 is rotated counterclockwise through an angle . At the same time, the detector is rotated through the same angle and translated back a distance r2. Figure 9.1.2.7.1 Schematic layout of an asymmetric Michelson autocorrelator based on a grating splitter. As is tuned, the path difference r between the two arms is given by sin( 1 )sec( 1 2 ) tan(1 ) r (r0 r2 ) r1 2r0 tan( 1 ) tan(1 2 ) and the stroke distance r2 of the scanning/rotating detector by (1) tan(1 ) tan(1 2 ) r2 r0 tan(1 ) tan(1 2 ) (2) To illustrate parameter dependence, expand r about = 0. This yields: r (2r0 1 ) (2r0 12 ) 2 ... (3) 2 2 2 41 r2 r0 1 1 ... ( ) 3 3 ( ) 1 1 (4) and To illustrate a typical parameter range, let r0 = 0.5m. In Fig. 9.1.2.7.2 the variables r2 and r are plotted as functions of for two values of 1. r2 [m] r [] (x100) 2 1.5 r2 (Theta 1 = 0.005r) r2 (Theta1 = 0.01r) Delta r (Theta 1 = 0.01r) Delta r (Theta 1 = 0.005r) 1 0.5 0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 [rad] Figure 9.1.2.7.2 Tuning curves for an asymmetric Michelson autocorrelator for two 1. r0=0.5m. The ordinate values for each curve scale linearly with r0. For a given starting value of 1 (viz., in the symmetric limit), the reflectivities of M1 and M2 are equal. As the device is tuned, the reflectivity of M1 will increase (assuming it is a specular reflector), inducing an asymmetric scaling in the relative amplitudes of the interferometer beams. In this regard, the maximal asymmetry, as well as the maximum power loading condition, for specular interferometer facets will be determined by the mirror material and 1. To illustrate the range of Z-dependent reflectivities accessible with specular mirror materials curves spanning the 1 keV - 8.5 keV range of the LCLS fundamental are plotted vs. 1 for gold and beryllium in Fig. 9.1.2.7.3. Corresponding to these curves, the energy loading [eV/atom] induced in the mirror material - assuming irradiation with the full non-attenuated LCLS beam - is plotted in Fig. 9.1.2.7.4. It is evident that in order to develop a value of r of the same order of length as the LCLS pulse without excessive values of r0 or r2 the preferred value of 1 will lie in the > 0.005 rad range. In order to operate in this range, it consequently follows that substantial attenuation of the LCLS beam will be desirable prior to reflection in the interferometer. In the present design this can be accomplished in part by the grating splitter, whose diffraction efficiency can be controlled by adjusting the parameters (primarily the thickness) of the transmission grating. Without upstream attenuation the grating itself is likely to be damaged; however, a new grating, or grating area, could be inserted between shots, which would also allow for spectral tuning. The basic requirements on the grating parameters is that the dispersion be small enough to preserve the temporal structure of the LCLS pulses and that the spectral bandwidth of the diffracted orders be substantially larger than the bandwidth of the LCLS radiation. 1 Absolute reflectivity Au 1 keV 0.8 Au 8.6 keV 0.6 Be 1 keV 0.4 Be 8.6 keV 0.2 10 -5 0.0001 0.001 0.01 0.1 Grazing Incidence Angle [rad] Figure 9.1.2.7.3 Absolute reflectivities of Au and Be vs. grazing incidence angle at photon energies of 1 and 8.6 keV. 100 A[eV/atom] 10 1 Au 1 keV Au 8.6 keV Be 1keV 0.1 Be 8.6 keV 0.01 0.001 0.0001 10 -5 -5 10 0.0001 0.001 Grazing Incidence Angle 0.01 0.1 Figure 9.1.2.7.4 Energy loading of Au and Be vs. grazing incidence angle at photon energies of 1 and 8.6 keV. Irradiation with the unattenuated LCLS beam is assumed. Issues include: a) Path length and resolution. The practically attainable path length difference of the instrument is of the order of 20 m, for interferometer lengths of 2 m or less. However, since the resolution is given by dn/n ~ (l/r), this still corresponds to a resolution of the order of 5x10-6 at 8.5 keV. For larger values of 1 or substantially longer arm path lengths, correspondingly longer path length differences (and resolutions) could be generated. b) Recording. As the path length difference is tuned, the detector need not record the detailed fringe pattern associated with a given pulse. A mask consisting of apertures smaller than ~ l/(8(1-)), and with a varying period equal to the varying fringe pattern period, followed by an intensity detector would be adequate, provided the contrast ratio of the interference pattern remains sufficiently high over the operating range of the instrument. A mask of this type could be fabricated as a variable-period multilayer consisting of alternating high-Z/low-Z materials and operated in transmission. c) Photon flux. Estimates indicate that a detector that could record the fringe pattern would also be feasible, even at 1.5 Å. This is based on two factors. First, at small angles 1 of operation the wavelength of the interference pattern will be dilated by the factor (2(1-))-1. In practical terms, periods in the 50 -1500 Å range can be recorded. Second is that while recording materials that operate down to this level of resolution (e.g., PMMA, or Agdoped semiconductors) are known to require large amounts of energy per unit area (O(1 J/cm2)), such (single-shot) exposure requirements could easily be met by the LCLS, even far away from saturation. d) Beam splitting. It is estimated that in order to attain a 1 in the >0.005 rad range at 1.5 Å, grating periods of 300 Å or less will be required. Such structures, similarly to the masks described above, could be fabricated as multilayers and operated in transmission, a technique that has been developed in recent years at LLNL (11,12). Blurring of the LCLS temporal structure due to the splitter's dispersive effects could to a certain extent be mitigated by pinhole aperturing of the incoming light. Splitting methods based on homogeneous or perforated [4] foils operating in transmission/reflection could also be investigated, particularly if the practical performance of the grating splitter proves to be overly dispersion-limited. In this context, the development of broad-band, high-quality multilayers as alternatives to the specular reflectors assumed here could be pursued as a means of scaling down the length of the instrument, particularly for FTS or source-analysis applications for which the bandwidth reduction would be acceptable. e) Alignment. The most critical issue concerns the tolerances required on the alignments, positions, and motions of the interferometer components. Tolerance specifications will be determined to a large extent by the optical components utilized in the interferometer, the type of detector, and mode of operation of the instrument. For example, starting with the splitter, it is well- known that the far-field diffraction pattern of a normal-incidence transmission grating is invariant with respect to the transverse coordinates of the grating. Moreover, while it is sensitive to the inclination of the grating away from normal incidence, the dispersion angles vary with the cosine of the deviation angle, making the tolerance on the deviation of this parameter for controlling the lateral motion of the split beams to , say, the ~10-9 rad level fairly robust (~10-100 mrad). However the following elements, the mirrors, will require exceptionally stringent tolerances both on position and angle should, for example, maintenance of the interference pattern's lateral position on the detector plane to a fraction of the pattern's wavelength be required. This requirement would be necessary if operating with a mask followed by an intensity detector (as described above), and may well represent the limit on the lowest attainable wavelength that a practical device could operate at. However, these tolerances could be minimized if the detector was a resist that recorded the interference pattern of each shot. In this case the autocorrelation could be unfolded from the distribution and statistics of the interference pattern, and these would be substantially less sensitive to its lateral position. In the same context, it can be noted that the ability to control the interference patterns' lateral positions could be considerably enhanced - even for dispersion lengths of 1-2 meters - by replacing the mirrors with transmission grating splitters (the one corresponding to M1 being also rotatable), which would result in the same tolerance reduction as for the incoming beam splitter. Here again the copious flux of the LCLS would more than compensate for the substantially lower efficiency of the grating deflectors. Needless to say, the duration of the LCLS recording events will be so short that questions of tolerance on any component's motion during recording can be completely disregarded References [1] Fowles, G. R., Introduction to Modern Optics, New York: Holt, Rinehart and Winston, Inc., 1975, ch. 3, pp. 33-80. [2] Murphy, J. B., Pellegrini, C. "Introduction to the Physics of the Free Electron Laser," in Frontiers of Particle Beams, M. Month, S. Turner, eds., Lecture Notes in Physics No. 296, H. Araki et al, eds., Springer-Verlag, Berlin, 1988, pp. 163-212. [3] Tatchyn, R., Arthur, J., Boyce, R., Cremer, T., Fasso, A., Montgomery, J., Vylet, V., Walz, D., Yotam, R., Freund, A. K., Howells, M. R., "X-ray Optics Design Studies for the 1.5-15 Å Linac Coherent Light Source (LCLS) at the Stanford Linear Accelerator Center (SLAC)," SPIE Proceedings 3154, 174-222 (1998). [4] Moler, E. J., Duarte, R. M., Howells, M. R., Hussain, Z., Oh, C., Spring, J., "First measurements using the ALS soft x-ray Fourier transform spectrometer," SPIE Proceedings 3154, 117-122 (1997). 9.1.2.7. Transverse coherence Young’s slits 9.1.3 Spontaneous radiation properties 9.1.3.1 Spectral structure of higher harmonics The reflectivity of perfect single crystals in the angstrom wavelength region is understood quantitatively. Thus a single crystal x-ray spectrometer is the ideal tool to study the intensity, wavelength and detailed spectral properties of the spontaneous radiation. Such a device is both simple in concept, a precision rotation axis and a perfect single crystal, preferably low Z to avoid damage and heating issues. The scattered intensity can be measured with high precision with any of several detectors that are generically ion chambers measuring the photon intensity in a current mode. The detailed structure of the spectrum is measured in an angular scan in the theta-two theta mode with a wide-open detector. In this way the integrated intensity as a function of photon energy is measured. From this the incident spectrum can be quantitatively recovered. By using the same spectrometer and measuring the angular positions of the harmonics at plus and minus Bragg angles one can measure within the precision of the rotary axis and the know ledge of the d-spacing of the crystal the wavelength of the radiation without the need for determining the zero of the rotation axis, but rather just needing the angular difference between the observed maxima at plus and minus angles. All of these techniques are well documented in the literature and have been in use for perhaps 50 years to characterize x-ray sources of various sorts 9.1.3.2 Energy of higher harmonics An ionization chamber (sometimes following a crystal spectrometer, as decribed in section 9.1.3.1, will be used. 9.1.3.3 Intensity of higher harmonics Jerry Hastings is providing this section 9.1.3.4 Angular distribution The measurement of the angular distribution of the harmonics of the spontaneous radiation requires an angular slit with micro-radian resolution. This is possible with two perfect crystals set to diffract in the so-called plus-plus arrangement. In this arrangement the crystal pair transmits radiation of a prescribed wavelength, determined by the crystal d-spacing and the angle between the normals to the diffracting planes of the two crystals. The angular acceptance of the pair is determined by the width of the diffraction profile of the perfect crystals used. The choice of this width is made by selecting the appropriate Bragg reflection. The mechanics required are straightforward. For the angular variation of the crystal pair a precise tangent arm mechanism with resolution on the microradian scale with perhaps a few milliradians of range is require. The relative settings of the two crystals must be established on the microradian scale and should be stable over the time of the experiment. Often it is possible to choose a monolithic two-reflection system that eliminates the need for the precise relative alignment stage with the restriction that it works at one wavelength. 9.1.3.5 Temporal distribution Jerry Hastings is providing this section 9.1.4 Other common usage diagnostics 9.1.4.1 Synchronization between FEL and pump or probe laser Independent pumps and probes will require measurement of synchronization, to better than 100 fs. It is likely that experiments will be performed in a statistical manner, utilizing the 120 Hz repetition rate, i.e. the synchronization timing between a pump and a probe beam will be measured after the experiments, and the data binned accordingly. Sub-picosecond timing measurements between the FEL pump or probe photon bunch, at > 0.8 keV, and a probe or pump that would likely be in the sub 2 eV photon energy range, are not available and must be developed.