Workshop on Polarized Electron Sources and Polarimeters

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High Intensity Polarized Electron Gun Studies at MIT-Bates E. Tsentalovich MIT-Bates Linear Accelerator Center, Middleton, MA Abstract. A proposed new high-luminosity electron-ion collider requires a polarized electron source of extremely high intensity. An ion back bombardment of the cathode is the main factor limiting the intensity of modern polarized electron guns. This study examines the possibility of building a very high intensity polarized electron gun. Keywords: Polarized electron source, ion bombardment PACS: 29.25.Bx, 29.27.Hj INTRODUCTION Modern polarized electron sources [1-6] routinely produce average current of hundreds of µA with a polarization approaching 90%. This intensity satisfies the requirements of the existing accelerator facilities. New advances in nuclear physics are expected with the development of a high luminosity electron-ion collider (EIC). The concept of such a collider has been discussed in the nuclear physics communities around the world for the past decade. Several conceptual designs have been proposed. Brookhaven National Laboratory (BNL) develops two different versions of eRHIC [7], based on the existing Relativistic Heavy Ion Collider (RHIC) complex. ELIC project [8] is developed at Thomas Jefferson National Accelerator Facility (TJNAF). These EIC projects require a significantly higher intensity from a polarized electron source. The linac-ring version of eRHIC produces the most challenging requirements with an average current of up to 250 mA. MIT-Bates, in collaboration with BNL, is investigating the possibility of building a very high intensity polarized electron gun. MAJOR CHALLENGES Two main problems must be solved in order to build a polarized electron gun with a very high average current. The first challenge is the heat load on the cathode. Tens or even hundreds of Watts of laser power must be deposited on a high polarization (strained or superlattice) GaAs cathode in order to extract a current of hundreds of mA. Cathode holders in the existing guns can absorb less than 1W without temperature rise. Active cooling of the cathode is necessary to avoid overheating of the crystal. The cooling system must be compatible with the high voltage applied to the cathode and with UHV conditions of the polarized electron gun, and presents a formidable engineering challenge.
The most difficult task is to achieve the high average current with a reasonable lifetime. The main problem is ion back bombardment. An electron beam ionizes molecules of the residual gases in the vacuum chamber; positive ions accelerate in the cathode-anode gap and strike the cathode, damaging the surface of the crystal. The effect is proportional to the product of the average electron current and the pressure in the gun chamber. It is difficult to expect a significant improvement of the vacuum conditions over present state-of-the-art installations, and the effect limits the average current in existing guns to several hundreds of µA. Both problems could be addressed with increased cathode area. Heat transfer can be done more efficiently, and the ion damage will be distributed over a larger area, resulting in a higher lifetime of the cathode. Measurements conducted at TJNAF [9] with a green laser indicated that the lifetime increased by a factor of ~10 when the laser spot diameter was increased by factor of 4. In a simplified picture, the average current that could be extracted from the cathode, with the same lifetime, is proportional to the emitting area. In reality, the picture is more complicated. Negatively charged electrons and positive ions follow a somewhat different trajectory, and ions tend to damage mostly the central area of the cathode. Existing electron guns are using the Gaussian shaped laser beams, with the maximum beam intensity near the cathode center, where the ion damage is most significant. For a high-intensity gun, it could be beneficial to form a ring-shaped laser beam, so the majority of the laser power is applied to the peripheral area of the cathode. The major difficulty in this approach is to avoid beam losses in the gun and its vicinity. Very detailed simulations including space charge effects must be conducted to design a large area cathode with a ring-shaped emission. ION INDUCED DAMAGE STUDIES The measurements have been conducted with the existing test beam line described in [6]. This beam line was designed for cathode tests and certification at very low current. It includes Mott polarimeter with silicon detectors and Mylar-backed golden foils. This section of the beam line cannot be baked at high temperature, and thus the vacuum in the whole beam line is rather inferior. The beam line has narrow apertures, and beam losses of 5-10% are typical. These losses produce out-gassing at high current, and degrade vacuum conditions even farther, resulting in a relatively short lifetime of the cathode. However, the poor performance of the test beam line at high current was beneficial for our studies. The ion damage accumulated quickly, and the measurements could be conducted in a relatively short time. A fiber-coupled diode laser with a wavelength of λ=808nm was used for photoemission. The laser beam coming out of the fiber (0.4 mm diameter) has very large divergence (full open angle 25.4 ). The first focusing lens L1 produced an almost parallel laser beam. An axicon (conical lens) in conjunction with the second focusing lens L2 formed a ring-shaped laser beam in the location of the cathode. With the axicon removed from the system, lenses L1 and L2 formed a point-to-point focusing between the output of the laser fiber and the cathode, resulting in a very small laser spot. A longitudinal translation of L2 allowed varying the beam spot size. The L2 lens was mounted on a translation stage for a precise translation in both
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: SUMMARY AND PLANS It is challenging
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 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
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PHOTOCATHODE WITH NEA SURFACE USING
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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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