Workshop on Polarized Electron Sources and Polarimeters

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transverse directions. With the axicon removed and the laser beam focused, one could scan the surface of the cathode using these translations and map the QE of the crystal. The following procedure was used during the measurements. The crystal was heatcleaned and activated. With the axicon removed, a very small diameter laser beam was scanned across the surface of the crystal, mapping the QE. The cathode was biased to only -64V during the mapping. Then the optical system was tuned to a certain configuration (small or large laser beam, ring-shaped beam etc.). The cathode was biased to -60kV, and the laser intensity was adjusted to produce a current of 120 µA. Each run continued for several days, and an integrated charge of 12-18 C was accumulated during each run. As the QE of the cathode decreased, the laser intensity was adjusted to keep the average current at about 100 µA. After the run the QE of the crystal was mapped again. The ratio of those two maps represents the change in the QE during the run. Figure 1 shows the results of the runs in four different configurations: a) small Gaussian laser beam (σ 0.15 mm) in the center of the cathode, b) large Gaussian beam (σ 2 mm), c) ring-shaped laser beam and d) small Gaussian beam parked in the corner of the cathode. FIGURE 1. Map of the QE degradation after the run with a) – small centered beam, b) – large beam, c) – ring-shaped beam and d) – small beam parked at the corner of the cathode.
The run with a small laser spot centered on the cathode produced a deep "hole" in the QE distribution. Naturally, the "`hole" was located in the center of the crystal; the ions damaged mostly the area where the electrons have been emitted from. However, in the case of the ring-shaped beam, most of the damage occurred in the center of the crystal again, although the laser intensity was negligibly low there. The ring-shaped area where the electrons were emitted from actually received less damage than the central part. The damage produced by a large Gaussian beam corresponds roughly to the beam shape, but the total amount of the damage is much higher. The Gaussian beam has tails, and significant amount of the electrons are emitted from the very edge of the cathode, where the transverse focusing forces are very large. Those electrons follow the extreme trajectories and get lost in the vicinity of the gun, causing out-gassing and increasing the ion-related damage. In contrast, an axicon-formed ring-shaped laser beam has very sharp edges with low tails and produces very few electrons with extreme trajectories. A small laser beam parked in the corner of the cathode produced a “hole” in the emitting spot, a groove of damage extending toward the center of the crystal, and the second “hole” located on this groove slightly over the cathode center. All these features can be understood by ion trajectory calculations. It should be noted that the anode hole acts as a defocusing element for the electrons, and a focusing element for ions. The ions produced in the vicinity of the cathode experience basically the same transverse forces as the electrons and follow approximately electron trajectories toward the cathode. The ions produced closer to the anode are affected by focusing fields of the anode at the part of the trajectory where their energy is still very low and the transverse forces are more effective. As a result, those ions tend to be focused toward the center of the cathode. The ions produced in the anode area are actually overfocused, but still damage mostly the central area of the crystal. These high energy ions are most likely responsible for the second “hole” on Fig.1d, giving a strong indication that although most ions are produced near cathode (due to high ionization cross section at low energy of the electrons), high energy ions produced near anode give a very significant contribution in ion-related damage. The measurements have been conducted with CW beam, and ion trapping must be considered. The ions produced in the beam line after the anode could be trapped in the electron beam, travel toward the gun, get accelerated in the cathode-anode gap and strike the cathode. The preliminary measurements at TJNAF [10] indicated that this process could be important and that it can be suppressed by biasing the anode. An additional run was conducted with the anode biased to +1kV and ring-shaped laser beam. A significant reduction in ion-related damage was measured, especially in the center area of the cathode, where most of the trapped ions are expected to be directed. SIMULATIONS AND CATHODE COOLER The simulations of the new gun and the beam line are under way. The simulations assume the current of up to 100 mA and gun voltage of 120 kV. The cathode diameter is 25 mm. The beam line includes two 90 dipole magnets, solenoidal lenses and a
Page 1 and 2: Workshop on Polari
Page 3 and 4: Surface Analysis of Damaged Superla
Page 5 and 6: High Brightness and High Polarizati
Page 7 and 8: FIGURE 1. Schematic drawing of the
Page 9 and 10: To be independent from possible ins
Page 11 and 12: vacuum gauge (AxTRAN X-11,ULVAC). B
Page 13 and 14: In order to research QE degradation
Page 15 and 16: Polarized Photocathode R&D for Futu
Page 17 and 18: charge limitation can be measured.
Page 19 and 20: SUMMARY AND PLANS It is challenging
Page 21: The most difficult task is to achie
Page 25 and 26: Hydrogen Cleaning of Superlattice P
Page 27 and 28: HABS Prep Load UHV Vessel Ion Pump
Page 29 and 30: FIGURE 4. Comparison of results ach
Page 31 and 32: Heidelberg photoelectron target and
Page 33 and 34: However, it is emphasized here that
Page 35 and 36: Atomic Hydrogen Treatment Atomic hy
Page 37 and 38: Rydberg binding energy. In many cas
Page 39 and 40: the ring dispersion and from fluctu
Page 41 and 42: The valence band splitting ∆Ehh-l
Page 43 and 44: will be achieved by taking the last
Page 45 and 46: A Study of the Activated GaAs Surfa
Page 47 and 48: diffraction (LEED) and auger electr
Page 49 and 50: FIGURE 4. LEED pattern of the GaAs
Page 51 and 52: HIGH POLARIZATION PHOTOEMITTER IMMU
Page 53 and 54: QE (%) 10 9 8 7 6 5 4 3 2 1 650 Cs
Page 55 and 56: Superlattice Photocathode Damage An
Page 57 and 58: and reactivated between February 11
Page 59 and 60: Discussion of SIMS data and Compari
Page 61 and 62: MBE Growth of Graded Structures for
Page 63 and 64: conduction miniband is higher that
Page 65 and 66: GRADED ALUMINUM GALLIUM ARSENIDE SU
Page 67 and 68: Three wafers were grown with differ
Page 69 and 70: FIGURE 10. Polarization and QE as a
Page 71 and 72: PHOTOCATHODE WITH NEA SURFACE USING
Page 73 and 74:
FIGURE 2. Dependence of DOS on exci
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High Brightness and high polarizati
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Laser Substrate GaP 355 μm Zoom Bu
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process of NEA-PC with SL layers, o
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FIGURE 1. (a) Band-graded photocath
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Left (a): Homogeneous (HM) layer Ri
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K2CsSb Cathode Development John Sme
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for the potassium deposition. 40nm
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ias of 500V, with 60µA of emitted
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analyzing chamber are used for samp
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spectrum of the film was observed a
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Demo Free Electron Laser (FEL) [1]
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hours, or if the emitter returns at
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neon or argon would have helped pro
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for emittance compensation solenoid
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If one can make electrodes that do
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The last important parameter is cat
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Status of the ALICE Energy Recovery
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TABLE 1. Booster & main linac gradi
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• The beam was fully-characterise
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photocathode. This limitation means
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whilst focusing the heat onto the p
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Development of an electron gun for
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FIGURE 2. Preparation, loading, and
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may help to avoid electric field co
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from its neighbors. The core-to-win
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series connection and the output cu
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Low Emittance Electron Gun for XFEL
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FIGURE 3. Copper cathode and corres
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current in the secondary which oppo
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Equation (2) shows that the effect
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Generally, the equation for the rad
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High-Fidelity RF Gun Simulations wi
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Causal Moving Window The methods us
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(a) y / mm y / mm Transverse Distri
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studies like long term studies of p
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ehavior of the curves is independen
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In Fig. 4 the curve shows an ASTRA
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TABLE 1. RF cavity parameters calcu
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PASSBAND AND ON-AXIS FIELD DISTRIBU
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If the frequency shift on crest is
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In order to increase the quality fa
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THE EXPERIMENT The experimental set
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FIRST COOL-DOWN In the first cool-d
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Non Evaporable Getter (NEG) Pumps:
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Examples of Applications NEG pumps
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isotherms in a controlled and repro
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chemical gettering that results whe
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inside the photogun remains very go
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High Brightness and High Polarizati
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focusing lens is mounted on a trans
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of electron polarization obtained b
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FIGURE 7. Reduced brightness depend
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FIGURE 1. Mott polarimeter with two
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� ��
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Compton Polarimetry at ELSA Wolfgan
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In order to be detected, the Compto
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monitored after the interaction by
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where n + and n - are the counting
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Our test cavity (Figure 3.) consist
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Polarimetry at the Superconducting
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30-130 MEV MØLLER POLARIMETER A 30
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Benchmark studies among own and com
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d2z = ds2 d2x = ds2 eQ eQ 4πε0 m0
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ments of his tracing cord developin
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all requires a slot in the side for
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simulations in GPT show that these
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Polarized Positrons at Jefferson La
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GEANT4 [11]. Using Stokes parameter
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Ultimately, the shower of particles
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It was gratifying to hear about the
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gun/photocathode, and likely repres
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induced emittance growth, especiall
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PESP Workshop Prog
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PESP Posters Posters (17 total) •
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Clendenin, James SLAC National Acce
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McCarter, James Jefferson Lab 12000
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Surles-Law, Ken Jefferson Lab 12050
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Workshop on Polarized Electron Sources and Polarimeters

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