Workshop on Polarized Electron Sources and Polarimeters

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Optimization of Semiconductor Superlattice for Spin Polarized Electron Source L. G. Gerchikov a , Yu. A. Mamaev a , V. V. Kuz’michev a , Yu. P.Yashin a , V. S. Mikhrin b , A. P. Vasiliev b , A. E. Zhukov b a St. Petersburg State Polytechnic University, Russia b A. F. Ioffe Physicotechnical Institute RAS, Russia Abstract. Optimized AlInGaAs/AlGaAs superlattice with strained quantum wells has been developed, fabricated and studied. The choice of hetero-layer composition and thicknesses is based on calculations of Sl’s band energy spectrum, photoabsorption spectrum and transport properties. Electron emission from the developed photocathodes demonstrates maximal polarization above 90%. Keywords: Polarization, Photoemission, Superlattice. PACS: 72.25.Fe,73.21.Cd,79.60.-i INTRODUCTION The use of short-period superlattices (SL) opens up wide possibilities for band structure engineering of photoemitter working layer. The optimal choice of SL’s layers composition and thickness providing the high energy splitting of the valence band together with good transport properties for photoelectrons have made it possible to design photocathodes with electron polarization above 90% [1-3]. Photocathodes based on GaAs/GaAsP [1] and AlInGaAs/GaAlAs [2] SL structures with strained quantum wells (QW), GaAs/AlInGaAs-based SLs [3] with strained barriers, and socalled compensated SLs based on AlInGaAs/GaAsP SL structures with opposite strains in the wells and barriers [4] have been developed. These studies have also revealed several fundamental difficulties for achieving further progress. High electronic polarization, P, is achieved at the expense of quantum efficiency, QE. Indeed, the maximal spin orientation of photoelectrons takes place at the photoabsorption threshold where the light absorption coefficient is rather small. Maximal polarization of photoelectrons is limited by the mixture of light and heavy hole states which takes place even at the absorption edge due to the broadening of the valence band energy spectrum and smearing of the valence band edge. The mixture of light and heavy hole states can be reduced by increasing the energy splitting ∆Ehh–lh between the hh and lh subbands. However the splitting of SL valence band due to the deformation is limited by some critical value. Deformation beyond this level results in structural defects, smaller residual strain and lower polarization.
The valence band splitting ∆Ehh–lh in SL cannot exceed the valence band offsets. Therefore, to create a significant splitting ∆Ehh–lh ≈ 100 meV, SL with large band offsets at the heterointerfaces must be used. In this case, high barriers of hundreds of meV usually appear in the conduction band that complicates the transport of photoelectrons to the photocathode surface. Low electron mobility along the SL axis reduces the QE. This leads to a partial loss of spin orientation by electrons during their transport to the surface due to D’yakonov–Perel’ mechanism [5] for spin relaxation. Thus, choosing the optimal structure of a SL to provide large ∆Ehh–lh splitting in the absence of strain relaxation and in combination with good transport properties is a difficult problem. The solution requires a reliable method to calculate the energy spectrum of the SLs on the basis of ternary and quaternary III–V semiconductor compounds. In addition, the maximum value of photoelectron polarization depends crucially on the structural quality of the SLs, which imposes heightened requirements on the technology of their growth. VALENCE BAND SPLITTING The initial polarization of the photoelectrons can be increased by choosing structures with a high valence band splitting. The SL structures with strained QW layers where heavy and light hole minibands, in addition to the strain splitting, are moved aside due to different light and heavy-hole confinement energies in the QW, are the best for this purpose. However strain induced splitting and splitting caused by quantum confinement do not combine additively and the resulting energy splitting is a complicated function of SL composition and layer thickness. Deformation splitting of the QW’s valence band edge increases the QW depth for heavy holes and decreases it for the light holes. Thus the increase of deformation splitting reduces the quantum confinement contribution to ∆Ehh–lh. Fig. 1 illustrates the choice of optimal QW thickness for Al0.19In20Ga0.61As/Al0.4Ga0.6 As SL with strained QW and 2.1nm thick barriers. Here we show the positions of the highest hole minibands together with the energy splitting ∆Ehh–lh. calculated within multiband Kane model [6] as a function of the QW width a. For our structure we have chosen the QW width a = 5.4nm because the increase of ∆Ehh–lh at a > 5.4 nm saturates while the risk of strain relaxation increases. The deformation of QW layer and hence the deformation energy splitting of the valence band edge linearly varies with In concentration. The chosen for our structure In concentration 0.2 corresponds to 76meV of deformation splitting that together with quantum confinement contribution results in ∆Ehh–lh = 87eV. This valence band splitting is sufficient to provide the initial polarization of photoelectrons of up to 97% [2]. Aluminum concentration in the QW layer does not affect the deformation but changes the energy gap value. We have chosen it equal to 0.19 to adjust the photoabsorption threshold of SL and hence the position of the main polarization maximum to = 820nm. The Al concentration in the barrier layer 0.4 was chosen to achieve sufficiently high barriers in the SL’s valence band in order to provide large Ehh–lh. On the other hand the conduction band barriers should be transparent enough for effective electron transport. This was achieved by the choice of the barrier width b
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 and 22: The most difficult task is to achie
Page 23 and 24: The run with a small laser spot cen
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: the ring dispersion and from fluctu
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
Page 75 and 76: High Brightness and high polarizati
Page 77 and 78: Laser Substrate GaP 355 μm Zoom Bu
Page 79 and 80: process of NEA-PC with SL layers, o
Page 81 and 82: FIGURE 1. (a) Band-graded photocath
Page 83 and 84: Left (a): Homogeneous (HM) layer Ri
Page 85 and 86: K2CsSb Cathode Development John Sme
Page 87 and 88: for the potassium deposition. 40nm
Page 89 and 90: ias of 500V, with 60µA of emitted
Page 91 and 92:
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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