Design and experimental tests of free electron laser wire scanners

SwissFEL is a x-rays free electron laser (FEL) driven by a 5.8 GeV linac under construction at Paul Scherrer Institut. In SwissFEL, wire scanners (WSCs) will be complementary to view-screens for emittance measurements and routinely used to monitor the transverse profile of the electron beam during FEL operations. The SwissFEL WSC is composed of an in-vacuum beam-probe—motorized by a stepper motor—and an out-vacuum pick-up of the wire signal. The mechanical stability of the WSC in-vacuum hardware has been characterized on a test bench. In particular, the motor induced vibrations of the wire have been measured and mapped for different motor speeds. Electron-beam tests of the entire WSC setup together with different wire materials have been carried out at the 250 MeV SwissFEL Injector Test Facility (SITF, Paul Scherrer Institut, CH) and at FERMI (Elettra-Sincrotrone Trieste, Italy). In particular, a comparative study of the relative measurement accuracy and the radiation-dose release of $\mathrm{Al}(99):\mathrm{Si}(1)$ and tungsten (W) wires has been carried out. On the basis of the outcome of the bench and electron-beam tests, the SwissFEL WSC can be qualified as a high resolution and machine-saving diagnostic tool in consideration of the mechanical stability of the scanning wire at the micrometer level and the choice of the wire material ensuring a drastic reduction of the radiation-dose release with respect to conventional metallic wires. The main aspects of the design, laboratory characterization and electron beam tests of the SwissFEL WSCs are presented.

Figure 1

View of the transverse section of the in-vacuum setup of the SwissFEL WSC: CF16 vacuum chamber, motorized UHV-Linear-Stage and wire fork. The SwissFEL wire-fork can be equipped with two triplets of wires. Thanks to a system of multiple pin slots, the horizontal and the vertical wires of each triplet can be set, respectively, at a distance from the center of the vacuum chamber of either 8 mm or 5.5 mm or 3 mm, see also Fig. 2. In order to outline this feature of the SwissFEL wire-fork—in both Figs. 1 and 2—all the three pin slots are virtually provided with wires. In the real wire-fork of SwissFEL, only one wire is fixed along the horizontal direction by means of one of the three possible pin slots (the same for the vertical direction).

Figure 2

Technical drawing of the SwissFEL wire-fork. Thanks to a system of multiple pin slots, the vertex of each wire-triplet can be set at three different distances (8,5.5 and 3 mm) from the center of the vacuum chamber. In order to outline such a flexibility feature of the design of the SwissFEL wire-fork—in the present technical drawing—all the three pin slots are shown to be provided with horizontal and vertical wires. In reality, only one of the three possible pin slots will be equipped with a wire so that each of the two wire triplets of the SwissFEL wire-fork will be composed of: one single horizontal wire; one single vertical wire; one single wire for XY coupling.

Figure 3

Image of a prototype of the SwissFEL WSC installed onto a girder together with the measurement setup of the stepper-motor induced vibrations of the wire.

Figure 4

Measured variations vs time—i.e., vs camera frame number—of the position of the wire centroid—$\mathrm{\Delta }$ centroid—with respect to the reference value as a function of the motor speed: (a) $0.1\mathrm{mm}/\mathrm{s}$; (b) $0.4\mathrm{mm}/\mathrm{s}$; (c) $0.5\mathrm{mm}/\mathrm{s}$; (d) $1.0\mathrm{mm}/\mathrm{s}$; (e) $2.0\mathrm{mm}/\mathrm{s}$; and (f) $3.0\mathrm{mm}/\mathrm{s}$. The camera frame rate is 565 fps, the camera exposure time is 1 ms.

Figure 5

Technical drawing of the SITF view-screen holder with the integrated WSC extension.

Figure 6

Comparison OTR against WSC of beam profile measurements. The relative accuracy of a WSC measurement against OTR is within 2%. Beam energy 230 MeV, beam charge 160 pC.

Figure 7

Energy distribution of the main components of the particle shower forward emitted in the polar angle $0-\pi /16\mathrm{rad}$ by an electron beam with energy 0.340 (a) and 5.2 (b) GeV colliding onto a $13\mu \mathrm{m}$ tungsten wire.

Figure 8

Number of electrons (a) and photons (b) forward emitted in the collision of an electron beam with a $13\mu \mathrm{m}$ tungsten wire as a function of the polar angle (rad) per primary incident particle for different beam energies: 0.340, 1.33, 2.99 and 5.2 GeV.

Figure 9

BLM readout as a function of the distance of the OTR screen intercepting the electron beam. Beam energy and charge: 245 MeV and 180 pC.

Figure 10

Vertical profiles of the electron beam—energy 245 MeV and charge 180 pC—resulting from the projection of an OTR image and from a WSC measurement of the beam: $\mathrm{sigma}(\mathrm{OTR},\mathrm{rms})=(0.308\pm 0.008)\mathrm{mm}$, $\mathrm{sigma}(\mathrm{WSC},\mathrm{rms})=(0.304\pm 0.014)\mathrm{mm}$. Motor-speed $0.2\mathrm{mm}/\mathrm{s}$.

Figure 11

Beam profile of a 10 pC beam—and energy 200 MeV—scanned by a $5\mu \mathrm{m}$ tungsten wire as reconstructed by the signal readout of (a) BLM1 and (b) BLM2 at a distance of 0.6 and 2.3 m from the WSC, respectively.

Figure 12

Beam profile of a 10 pC beam—and energy 200 MeV—scanned by a $5\mu \mathrm{m}$ tungsten wire as reconstructed by the signal readout of (a) BLM2 and (b) BLM3 at a distance of 2.3 and 3.5 m from the WSC, respectively.

Figure 13

Schematic layout of FERMI and of the SwissFEL WSC and BLM setup. In the picture, the WSC is indicated as WS while the BLM are represented as a red spiral. Quadrupoles $Q_j$ ($j=1,\dots ,4$) and view-screens $S{c}_i$ ($i=1,\dots ,3$) used at FERMI for emittance measurements are also sketched.

Figure 14

Picture of the installation of the SwissFEL WSC at FERMI. The WSC in-vacuum hardware is placed in the linac hall just after the dipole of the FERMI high energy spectrometer.

Figure 15

Horizontal—(a,c) and vertical—(b,d)- wire-scanned profiles of the FERMI electron beam (1.5 GeV, 300 pC). In each figure, signals from the three BLMs placed at different distances from the WSC are plotted: BLM(1) at 2.48 m, BLM(2) at 5.52 m and BLM(1) at 8.40 m. Experimental data in (a,b) results from a scan with $5\mu \mathrm{m}$ tungsten wire, while data (c,d) from a scan with a $13\mu \mathrm{m}$ tungsten wire. The scan was performed at a wire-fork speed of $0.20\mathrm{mm}/\mathrm{s}$ corresponding to a wire-step of 0.14 mm per rf shot at the machine repetition rate of 10 Hz. The measured beam size in the horizontal and vertical directions is $\sigma =53\mu \mathrm{m}$ and $\sigma =80\mu \mathrm{m}$ (rms), respectively, with a statistical error of about 2%.

Figure 16

Beam profile resulting from the scan of the FERMI electron beam with a $12.5\mu \mathrm{m}$ $\mathrm{Al}(99):\mathrm{Si}(1)$ wire (a) and a $5\mu \mathrm{m}$ W wire (b) as detected by a BLM placed at a distance of 2.48 m from the WSC. The scan was performed at a motor-speed of $0.2\mathrm{mm}/\mathrm{s}$ and at a beam energy of 1.325 GeV, charge 700 pC and 10 Hz repetition rate.

Figure 17

Beam size measurements resulting from a wire-scan of the FERMI electron beam performed with two homologous $12.5\mu \mathrm{m}$ $\mathrm{Al}(99):\mathrm{Si}(1)$ and a $5\mu \mathrm{m}$ W wires. During the measurement, the FERMI machine was set at stable and fixed beam conditions: 1.325 GeV, 700 pC and 10 Hz repetition rate. The WSC measurements of the beam profile were repeated for different motor-speeds: 0.1, 0.2 and $0.3\mathrm{mm}/\mathrm{s}$.

Figure 18

Average radiation-dose rate ($\mathrm{mGy}/\mathrm{h}$) measured by the 8 ionization chambers (I-C) placed in the FERMI FEL1 undulator chain when scanning with $12.5\mu \mathrm{m}$ $\mathrm{Al}(99):\mathrm{Si}(1)$ wire and a $5\mu \mathrm{m}$ tungsten wire—at a motor speed of $0.20\mathrm{mm}/\mathrm{s}$—an electron beam of 700 pC and energy of 1.325 GeV. The data in the plot result from the average—over 10 scans—of the dose-rate readout of the I-Cs integrated over 30 beam shots.

Figure 19

Average radiation-dose rate ($\mathrm{mGy}/\mathrm{h}$) measured by the 8 ionization chambers (I-C) placed in the FERMI FEL1 undulator chain when scanning—with (a) a $12.5\mu \mathrm{m}$ $\mathrm{Al}(99):\mathrm{Si}(1)$ wire and (b) a $5\mu \mathrm{m}$ tungsten wire—an electron beam of 700 pC and energy of 1.325 GeV at a motor speed of 0.10, 0.20 and $0.30\mathrm{mm}/\mathrm{s}$. The data in the plot are the result of the average of the I-C dose-rate readout over 10 scans.