Workshop on Polarized Electron Sources and Polarimeters

More documents

Recommendations

Info

inside of a nitrogen–filled transfer vessel without any contact to the laboratory atmosphere. After attachment of the transfer vessel to the loading chamber and after pumping it out, photocathodes are moved into the preparation chamber and mounted on a carousel capable of keeping four cathodes. The carousel can be rotated into different positions for heat cleaning and (Cs,O) activation to NEA – state. This scheme of the photocathode preparation reproducibly provides photocathodes with high quantum efficiencies (QE) of ~ 20%. The main drawback of the scheme is that only ~ 3-4 restorations of degraded photocathodes by heat-cleaning and reactivation are possible. During the following cycles of heating and activation the QE drops strongly. In addition, the effect of current limitation due to surface photovoltage is observed for multiply recleaned cathodes. Formerly it was required to remove cathodes from the vacuum chamber for the chemical cleaning in the glove-box to restore their emission properties. With the low-energy atomic hydrogen treatment, multiple refreshings of the photocathodes within the UHV set up became possible. Photocathode preparation chamber 4·10-12 4·10 mbar -12 mbar Manipulator 2·10-9 2·10 mbar -9 mbar Loading and Atomic Hydrogen chamber side view H 2 Pd filter W-capillary Photocathode Leakage valve Pirani gauge Photocathode Photocathode carousel carousel oven To turbopump 140 l/s O 2 Transfer vessel Laser Cs UV-window BA gauge Photoelectron gun chamber N 2 1.2·10-11 1.2·10 mbar -11 mbar Photocathode pumping HCl ISO Glove box FIGURE 2. Schematic view of the photocathode treatment setup.
Atomic Hydrogen Treatment Atomic hydrogen (AH), produced by thermal cracking of H2-molecules inside a hot tungsten capillary, was implemented for the cleaning of transmission-mode GaAs– photocathodes with high quantum efficiency in a UHV multi–chamber photoelectron gun of the electron target. In contrast to previous measurements, where AH from RF sources was used [17,18], the low-energetic hydrogen from the capillary source was proved to be an efficient tool for multiple in-situ recleanings of the high QE cathodes without generating a “fatal” concentration of recombination centers within the nearsurface region. The design of our source (see Fig. 2) follows ideas presented in the paper [19] where a long tungsten capillary served as a high efficiency thermal cracker. Molecular hydrogen is brought into the hydrogen chamber through a heated palladium filter attached to the inlet of the capillary. We used a 60-mm-long capillary of 0.6 mm inner diameter. A 15-mm-part of the capillary near its outlet was heated by 21 W electron bombardment resulting in a capillary temperature of (1610 ± 25) 0 C. This temperature was high enough to crack 3-5% of the hydrogen molecules passing through the capillary, and was still sufficiently low to avoid a pollution of the GaAs surface by tungsten vapor. The pressure of molecular hydrogen at the inlet of the capillary was varied with a leakage valve in the range of 0.005-0.75 mbar, while the corresponding change of the pressure in the loading chamber, pumped by a 140 l/s TPU 180 H Balzers turbopump, was 10 -7 -10 -5 mbar. The exposure time was varied in the range of 2-15 minutes. For the AH treatment the cathode was heated to 450 0 C. Afterwards the valve at the inlet of palladium filter was closed, and the heating filaments of the oven and of the capillary were turned off. The following activation of the cathode by (Cs,O) in the preparation chamber typically resulted in quantum efficiencies of 10% -15% at ħω = 1.86 eV. It was also found experimentally that additional thermal annealing of the AH–treated photocathodes at about 450 0 C during 30 min within the preparation chamber, increases the achievable QE to 15% - 20%, and up to 20-25% using twostep-activation (with a maximum value of 35%). After degradation during the cathode operation in the electron gun, the photocathodes were heat-cleaned and activated 3-4 times with the maximum QE typically being obtained for the second activation. After that, the AH cleaning was required to recover the emission properties. UV photoelectron spectroscopy was used to optimize and to control the efficiency of the atomic hydrogen and heat treatments. It was found that an atomic hydrogen exposure of 50-200 L is sufficient to remove the degraded activation layer from the GaAs surface. Since 2002 AH–cleaning is routinely used for the refreshing of GaAs photocathodes at the TSR electron target. Ten photocathodes from two groups were used, two of which were grown by liquid phase epitaxy (LPE) and the others by metal–organic chemical vapor deposition (MOCVD). Photocathodes from the same group behaved in a similar way with respect of multiple AH-cleaning, but there was a difference between the groups. The LPE grown photocathodes, after cleaning with HCl–isopropanol, heat-cleaning and subsequent activation showed a maximum QE of about 15 – 25% achieved with two-step activation. A serious of AH treatments, each with a typical dose of about 400 L, was used for the LPE grown photocathode with 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 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: However, it is emphasized here that
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
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
Page 101 and 102:
for emittance compensation solenoid
Page 103 and 104:
If one can make electrodes that do
Page 105 and 106:
The last important parameter is cat
Page 107 and 108:
Status of the ALICE Energy Recovery
Page 109 and 110:
TABLE 1. Booster & main linac gradi
Page 111 and 112:
• The beam was fully-characterise
Page 113 and 114:
photocathode. This limitation means
Page 115 and 116:
whilst focusing the heat onto the p
Page 117 and 118:
Development of an electron gun for
Page 119 and 120:
FIGURE 2. Preparation, loading, and
Page 121 and 122:
may help to avoid electric field co
Page 123 and 124:
from its neighbors. The core-to-win
Page 125 and 126:
series connection and the output cu
Page 127 and 128:
Low Emittance Electron Gun for XFEL
Page 129 and 130:
FIGURE 3. Copper cathode and corres
Page 131 and 132:
current in the secondary which oppo
Page 133 and 134:
Equation (2) shows that the effect
Page 135 and 136:
Generally, the equation for the rad
Page 137 and 138:
High-Fidelity RF Gun Simulations wi
Page 139 and 140:
Causal Moving Window The methods us
Page 141 and 142:
(a) y / mm y / mm Transverse Distri
Page 143 and 144:
studies like long term studies of p
Page 145 and 146:
ehavior of the curves is independen
Page 147 and 148:
In Fig. 4 the curve shows an ASTRA
Page 149 and 150:
TABLE 1. RF cavity parameters calcu
Page 151 and 152:
PASSBAND AND ON-AXIS FIELD DISTRIBU
Page 153 and 154:
If the frequency shift on crest is
Page 155 and 156:
In order to increase the quality fa
Page 157 and 158:
THE EXPERIMENT The experimental set
Page 159 and 160:
FIRST COOL-DOWN In the first cool-d
Page 161 and 162:
Non Evaporable Getter (NEG) Pumps:
Page 163 and 164:
Examples of Applications NEG pumps
Page 165 and 166:
isotherms in a controlled and repro
Page 167 and 168:
chemical gettering that results whe
Page 169 and 170:
inside the photogun remains very go
Page 171 and 172:
High Brightness and High Polarizati
Page 173 and 174:
focusing lens is mounted on a trans
Page 175 and 176:
of electron polarization obtained b
Page 177 and 178:
FIGURE 7. Reduced brightness depend
Page 179 and 180:
FIGURE 1. Mott polarimeter with two
Page 181 and 182:
� ��
Page 183 and 184:
Compton Polarimetry at ELSA Wolfgan
Page 185 and 186:
In order to be detected, the Compto
Page 187 and 188:
monitored after the interaction by
Page 189 and 190:
where n + and n - are the counting
Page 191 and 192:
Our test cavity (Figure 3.) consist
Page 193 and 194:
Polarimetry at the Superconducting
Page 195 and 196:
30-130 MEV MØLLER POLARIMETER A 30
Page 197 and 198:
Benchmark studies among own and com
Page 199 and 200:
d2z = ds2 d2x = ds2 eQ eQ 4πε0 m0
Page 201 and 202:
ments of his tracing cord developin
Page 203 and 204:
all requires a slot in the side for
Page 205 and 206:
simulations in GPT show that these
Page 207 and 208:
Polarized Positrons at Jefferson La
Page 209 and 210:
GEANT4 [11]. Using Stokes parameter
Page 211 and 212:
Ultimately, the shower of particles
Page 213 and 214:
It was gratifying to hear about the
Page 215 and 216:
gun/photocathode, and likely repres
Page 217 and 218:
induced emittance growth, especiall
Page 219 and 220:
PESP Workshop Prog
Page 221 and 222:
PESP Posters Posters (17 total) •
Page 223 and 224:
Clendenin, James SLAC National Acce
Page 225 and 226:
McCarter, James Jefferson Lab 12000
Page 227 and 228:
Surles-Law, Ken Jefferson Lab 12050
show all

Workshop on Polarized Electron Sources and Polarimeters

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?