Workshop on Polarized Electron Sources and Polarimeters

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Recent Progress toward Robust Photocathodes G. A. Mulhollan and J. C. Bierman Saxet Surface Science, Austin, TX 78744, USA Abstract. RF photoinjectors for next generation spin-polarized electron accelerators require photocathodes capable of surviving RF gun operation. Free electron laser photoinjectors can benefit from more robust visible light excited photoemitters. A negative electron affinity gallium arsenide activation recipe has been found that diminishes its background gas susceptibility without any loss of near bandgap photoyield. The highest degree of immunity to carbon dioxide exposure was achieved with a combination of cesium and lithium. Activated amorphous silicon photocathodes evince advantageous properties for high current photoinjectors including low cost, substrate flexibility, visible light excitation and greatly reduced gas reactivity compared to gallium arsenide Keywords: Photocathode, NEA, GaAs, Immunization, Photoyield decay, Amorphous silicon, FEL PACS: 85.60.Ha, 29.25.Bx INTRODUCTION The motivations for robust photocathodes are manifold. Ideal photoemitters would perform well under very poor vacuum conditions. In practice, improved performance in operable vacuum is always desirable. It is well known that the environment in RF guns is still tough on photoemitters, though designs incorporating better pumping represent good progress. High electron spin-polarization photocathodes continue to demand ultra high vacuum (UHV) conditions for reliable operation. Better environmental immunity for these photocathodes is required for successful operation in RF guns, such as could be employed for the ILC. Photocathodes operated in high average current injectors manifest cumulative damage which limits operational lifetimes. Photocathode environmental interactions may be viewed as stemming from either chemical reactions or charged particle fluence. The principal source for chemical reaction participants is the background gas. Electron beam induced desorption can increase the quantity of these reactants. Several gas species, such as CO2,H2OandO2 are known to be harmful to the lifetime of negative electron affinity (NEA) photocathodes[1]. Detrimental charged particle fluences at photocathodes are primarily composed of ions. Low energy ions, most likely in RF guns, generate atomic displacements near the surface and can affect the activation layer as well as the near surface crystal structure. This damage can be annealed out and the photocathode activated again to an NEA state. High energy ions, such as found in DC guns can cause many displacements per particle if they impact the photocathode. The damage to the photocathode crystalline structure can be widespread and if the energy is large enough, the damage may not anneal out. Both high and low energy ion interactions can affect yield and the emitted electron spinpolarization.
HIGH POLARIZATION PHOTOEMITTER IMMUNIZATION Generation of high spin-polarization electrons from III-V and related semiconductors mandates the emitter be crystalline. Both single strained-layers and superlattices (strained and unstrained) have been employed in this role. The use of an activation layer to achieve NEA is also a requirement in common. The first goal of this work was to achieve a level of immunity against background gas chemical reactivity for such emitters. Several groups have studied the decay process of NEA photocathodes. While good progress toward understanding the chemical makeup of diminished yield photocathode surfaces has been made[2], both in process and susceptibility, this information has not provided insight into the prevention of the decay process, aside from straightforward improvements in vacuum base pressure leading to increased lifetimes. Our approach to the problem was to attempt to de-activate potential reaction sites. The overlayers on activated GaAs(100) are amorphous. This can be partly attributed to the large covalent radius of cesium, ∼2.3 Å, compared to the GaAs nearest neighbor distance. In a billiard ball model, large gaps are present between the cesium atoms. In fact, they are large enough for a much smaller covalent radius atom, e.g., oxygen, ∼0.73 Å, to fit through them and bond to the semiconductor surface and/or cesium. This argument suggests that if access to the underlying surface could be blocked during or following the activation process, then absorbed gas induced decay could be inhibited. The true picture must be formulated in terms of electron charge density with the gaps actually being regions of low charge density. An obvious choice for a blocking atom is another, smaller covalent radius alkali. While all alkali atoms lower the GaAs(100) electron affinity, the best performance is from cesium. This indicates that blocking the surface reactivity is likely to incurr a cost in the yield. An alkali source flange containing Cs, Rb, K, Na and Li getter channels was constructed to allow testing of each alkali’s effects on yield and sensitivity to reactive gas exposure. This flange was installed into an existing photocathode test system, described elsewhere[3]. Activations were first performed with each alkali individually to establish the proper operating curents. Standard activations were performed by depositing alkali until a peak in photocurrent was achieved. Following the peak, the oxidizer gas was introduced. The oxidizer partial pressure was adjusted for maximim photocurent increase (co-deposition) until the final peak value was achieved. In all cases, the activation oxidizer gas was NF3. Carbon dioxide was chosen as the archtype decay gas. It is a known strong reactant with NEA surfaces and is readily distinguishable in residual gas spectra. Dosing data were acquired under computer control. The cathode bias was disconnected between data points and the light source was shuttered when not in use. First, dark current values were acquired for each point, then the light source was unshuttered for the emission measurement. Data were recorded once a minute. Dosing consisted of three segments. In the first 30 minutes, the photocathode was exposed only to the normal background gas, base pressure 4x10 −11 Torr, primarily H2. In the second segment, the pressure, as measured by a nude ion gauge, was increased to 1.5x10 −10 Torr by introduction of the dosing gas, and in the final segment the pressure was increased to 5.0x10 −10 Torr. In keeping with the above argument, activations using two alkali types were initially attemped by introducing the second, non-Cs alkali after the first photocurrent peak was
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 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: FIGURE 4. LEED pattern of the GaAs
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
Page 93 and 94: spectrum of the film was observed a
Page 95 and 96: Demo Free Electron Laser (FEL) [1]
Page 97 and 98: hours, or if the emitter returns at
Page 99 and 100: 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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