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The MAMI source of polarized electrons
Nuclear Instruments
and Methods in Physics Research A 391
( 1997) 498-506
-_ ll!B ELSJZVIER
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SecllonA
The MAMI source of polarized electrons K. Aulenbacher a,*, Ch. Nachtigallb, H.G. Andresen”, J. Bermutha, Th. Dombo”, P. Drescherb, H. Euteneuer”, H. F&herb, D.v. Harrach”, P. Hartmann”, J. Hoffmanna, P. Jenneweina, K.H. Kaiser a, S. Kiibis”, H.J. Kreidel”, J. Langbeinb, M. Petri”, S. Pliitzerb, E. Reichertb, M. Schemiesb, H.-J. Sch6peb, K.-H. Steffensa, M. Steigerwaldb, H. Trautnerb, Th. Weis”
Abstract The present work describes the source of polarized electrons that is run at the 855 MeV race track microtron MAMI at the Johannes Gutenberg Universitiit in Mainz. The source is based on photoelectron emission from (III-V)-semiconductors. Presently strained layer InGaP- or GaAsP-cathodes are used, which are processed to negative electron affinity by coverage of the surface with a submonolayer of caesium and oxygen, Electron beams spin-polarized up to a degree of P=55% at a quantum efficiency of QE = 2% or P = 75% at QE = 0.4%, respectively, are obtained. The well-known but hitherto unsolved problem of limited cathode lifetime has been sidestepped by the attachment of an UHV load lock system to the source electron gun. It allows quick replacement of cathodes without breaking the gun vacuum. Availabilities in excess of 85% are obtained regularly in beamtimes longer than 100 h. The source was mainly applied in measurements of nucleon form factors via the reactions ‘H(e’, e’a), ‘D(Z,e’p’), ‘D(Z, e’ii), and 3Ge( Z, e’n). More than 1600 h beamtime have been accomplished in physics experiments with polarized electron beams at MAMI up to now. PACS:
79.60
1. Introduction The Mainz race track microtron cascade MAMI delivers a C.W. electron beam of energies up to 855MeV [l]. The physics program at MAMI proposes a series of experiments with polarized electron beams. Examples are measurement of electromagnetic nuclear formfactors [24], test of the Gerasimov-Drell-Hearn sum rule in photon proton scattering [5], and determination of s-quark contributions to the Pauli form factors of the nucleon in a parity violation experiment [6]. The present paper describes the source of polarized electrons applied in these investigations. The MAMI source of polarized electrons is based on photoelectron emission of (III-V)-semiconductor crystals. This process was discovered in 1970s by Pierce and collaborators [7-91 and Lampel and Weisbuch [iO] and is today the standard way to produce spin-polarized electron beams. * Corresponding author. Tel.: f49 6131 398504, fax: f49 6131 392964: e-mail: aulenbac@,kph.uni-mainz.de. 0168-9002/97/$17.00 PII SO 168-9002(
Examples of sources of polarized electrons applying photoelectron emission from GaAs or one of its relatives are discussed, e.g. in Refs. [ 1 l-1 61 A review of the work done before 1993 has been given by Pierce [ 171. A collection of papers describing recent developments may be found in Ref. [18] The degree of polarization available in emission from bulk GaAs is limited to values below 50% [9]. Reduction of the degeneracy between heavy hole and light hole bands at the valence band edge of (III-V)-semiconductor cathodes may allow higher polarization values. Application of strain [19-261 to the emissive cathode layer or growth of a super-lattice structure consisting of two materials with different gap energies [27] proved to be the most effective method in lifting the heavy hole-light hole degeneracy. In praxi strained layers of GaAs are the only new generation cathodes installed in sources of polarized electrons applied in real physics experiments [ 16,281 up to now. Degrees of spin polarization in the P = 80% range are achieved at quantum efficiencies around QE = 0.2% [ 161. The present work successfully applied strained layer InGaP cathodes emitting
Copyright @ 1997 Elsevier Science B.V. All rights reserved 97)00528-7
K. Aulerlbacher
et al. I Nucl. hstr.
and Meth. in Phys. Rex A 391 (1997) 498-506
electron beams spin-polarized up to a degree of P = 55% at a quantum efficiency of QE = 2% 1261 and strained layer GaAsP cathodes with P = 75% at QE = 0.4% [25]. The figures of merit P’. QE of 6 x lop3 and 2.3 x 10P3, respectively, are at least as high as that of the best strained layer (III-V)-cathodes described in literature. The transverse emittance of the beam at injection must stay below 1~ mm mrad at 100 keV in order to reach the high beam quality of MAMI in its standard operation with a thermionic gun. The gun in the present work is designed with help of the well-known EGUN-code of Herrmannsfeldt [29]. Emittance of the beam may be conveniently restricted by choosing a small diameter of the light spot at the cathode. The lifetime of negative electron affinity (NEA) cathodes installed in sources of polarized electrons is limited. The submonolayer of caesium and oxygen or fluorine that is necessary for attaining NEA may be disturbed by ions produced by the beam and poisoned by molecules present in the residual gas even if UHV in the low IO-” mbar range is maintained. This is particularly true in cases where the gun is run with high beam currents. The rate of ion production in the residual gas increases with current as well as the rate of molecules produced via electron-stimulated desorption from chamber walls by beam scraping. In the MAMI source described below the problem of limited cathode lifetime has been sidestepped by the attachment of an UHV load lock system to the gun chamber. It allows quick replacement of cathodes without breaking the gun vacuum. A load lock system is successfully used already at the SLAC source of polarized electrons [ 161.
2. Electron gun and its accessories The electron gun of the MAMI source of polarized electrons has a triode configuration. It is designed with help of the EGUN-code of Herrmannsfeldt [29]. Fig. l(a) sketches the layout of the gun electrodes [30]. Beam emittance was determined by a scanning wire method described in Ref. 1311. It was adjusted by decreasing the size of the laser spot. The gun is operated with a spot diameter of 0.25 mm. In this case, 90% of the beam current was found in an emittance volume of 0.5xmmmrad at 100 keV. Voltage breakdown may be a problem because of caesium introduced into the gun chamber by the so-called recaesiation procedure discussed below. Therefore, some care has been taken to get electrode shapes that preserve low electric field strengths at cathode and intermediate electrode surfaces. At the edge of the cathode electrode the maximum E-field is 2.95 MV/m, at the cathode itself 0.89 MV/m. and at the intermediate electrode 3.3 MV/m in the design shown in Fig. l(a). It has been pointed out by Alley and co-workers [16] that currents due to field emission should be kept below say 100 nA. This criterion is easily met by the gun of the
499
Fig. 1. (a) Section through the electrode configuration of the 100 keV electron gun: (b) elevator in down position.
present source. Field emission current of electrons from the cathode or the intermediate electrode are smaller than 10 n4 at 100 kV gun voltage. High voltage processing is possible while there is no crystal in the gun, but this has in fact not been necessary so far. The innermost cylindrical part, labeled elevator in Fig. 1, contains the conical photo cathode holder fitted in a conical seat [32]. The elevator may be rotated and moved down via a vacuum feedthrough not shown (BR2 in Fig, 2). Rotation is used to change the emission spot. During first use of the gun it turned out that in particular the central part of the photo cathode is damaged by ion bombardment. Therefore, the gun is operated now with the emission spot off-axis. This in turn allows to change the emission site on the cathode by just rotating the elevator. Beam steering is not affected by cathode rotation. Down movement of the elevator is needed in case of cathode exchange. Fig. l(b) sketches the elevator in downposition. The cathode holder is brought to the elevator and removed from it respectively by a fork fastened to a magnetic linear movement feedthrough, that serves for transport of cathodes between the cathode mount and the preparation chamber seen in Fig. 2. Fig. 2 presents a very schematic drawing of the load lock, preparation, and gun chamber assembly 1321. A new crystal is brought to the load lock chamber in a transport vessel filled with dry nitrogen. The vessel is bolted to a port of the load lock and not opened before pumpdown of the chamber. In this way, a cathode may be moved around after chemical cleaning without contact to laboratory air. Such a precaution was introduced by Terekhov and his co-workers [23]. A sample is moved from the load lock to the preparation chamber by a magnetic linear movement feedthrough MM 1 as soon as the residual pressure in the load lock chamber is in the 1O-’ mbar range. Up to six cathodes plus holders may be placed on a storage wheel in the preparation chamber. A cathode to be installed in the gun is heat-cleaned from the rear by radiation emitted from a hot tungsten filament. The sample is held at 580°C for half an hour.
500
up/down movement and rotation of storage wheel
chamber with storage-wheel Transport-vessel 5
BRl
I I/ I
G-gun
0ae
load-lock-c
ectron-beam
load-lock-sys
te
NFG-pump
Fig. 2. Assembly of electron gun, preparation chamber. and load lock chamber of the MAMI source of polarized electrons.
NEA-preparation is done at room temperature by evaporation of caesium to the cathode surface. Caesium source is a Cs-dispenser from S.A.E.S. GETTERS, Milano. During the process the cathode is irradiated by He-Ne-laser light and the photoelectron current emitted is registered. As soon as the emission current reaches a first maximum, oxygen is introduced into the preparation chamber via a leak valve. Oxygen flow is adjusted so that the rate at which the current increases is optimized. The procedure is stopped when the current goes through its maximum. Cathode exchange starts with transport of the old cathode from the gun (see Fig. l(b)) to a spare position in the storage wheel of the preparation chamber indicated in Fig. 2 and ends with placing the fresh cathode in the gun elevator. Movement between the two chambers is accomplished by a fork attached to the rod of a magnetic linear motion feedthrough MM2 sketched in Figs. l(b) and Fig. 2. Movement of the gun elevator is done with manipulator BR2. The whole process of cathode exchange takes 10 mins approximately. The vacuum system consisting of gun chamber. preparation chamber, and load lock is built in all-metal
UHV technique. Gun chamber and preparation chamber are evacuated by combinations of 110 I/s-iongetter + 500 l/s NEG pumps and 60 l/s iongetter + 300 l/s NEG pumps, respectively. Base pressures are 1 x lo-” and 3 x lo-” mbar, respectively, with main contribution coming from hydrogen. Both chambers are not opened to air again unless a vacuum break is forced by a component failure. Fig. 3 presents an overall view of the source together with the first part of the beamline between source and the injection point of the race track microtron. The preparation chamber and load lock are not drawn in this picture. The beam is guided by magnetic quadrupoles, a-magnets. solenoid lenses, and steering coils along the 25 m beamline [33,34]. A spin rotator incorporated in the line is used to adjust the orientation of the polarization vector with respect to the vector of momentum [33,34]. The transmission of the beam from the gun to a Faraday cup 2 m downstream is better than 99%. A high value of transmission not only saves current but is particularly important for avoiding electronstimulated desorption of molecules by beam scraping at the
K. Aukwbncher
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Meth. in Phys. Rex A 391 (1997) 498-506
501
V-electrode
to load-lock
n-chamber umatic
valve
u
Not bakeable lm
I
Fig. 3. Overall view of the MAMI source of polarized electrons.
vacuum chamber walls. The transmission between gun and the distant injection point is better than 96%. The gun vacuum is isolated from the line vacuum by two-stage differential pumping indicated in Fig. 3. The circularly polarized light beam is introduced into the vacuum system via a view port at the first 90”-bend of the beamline. Spin polarization of the beam produced may be analysed by a Mott detector integrated in the spin rotator [33,34].
3. Laser sources and light beam optics Up to now two different laser light sources have been used. With InGaP-cathodes having an optimum wavelength at 670nm a commercially available dye laser pumped by an Argon ion laser was employed. ’ In case of strained GaAsP installed in the gun the dye laser was substituted by a Ti : Sapphire laser [35], that was built following a design of Zimmermann and colleagues [36]. The optimum wavelength for emission of polarized electrons from GaAsP is around 830 nm. Fig. 4 sketches the Ti: Sapphire laser used. It is tunable in the wavelength range 780-860 nm by adjusting the indicated Lyot-filter. Power up to 1.2 W is emitted at
’ Spectra Physics 2030cavity dumper.
15 Ar+-laser
+ 375B dye-laser
+ 3448
830nm with 5 W argon-ion pumplight. The linearly polarized laser light is transported via a 20 m mono-mode fiber to the source. Fig. 5 shows the elements arranged in an optical bench that guides the light to the cathode surface. The light emitted by the fiber exit is formed in a parallel beam by a microscope lens. A half-wave plate is used to adjust the orientation of the plane of linear polarization. A small fraction of the beam is reflected to a photodiode, whose output gives a measure of light power. The Glan prism produces a clean linear polarization of the light beam while the Pockels cell acts as a quarter wave plate and transforms the linear polarization into circular polarization. The degree of circular polarization achieved is better than 99.5%. The helicity of the light may be quickly switched from positive to negative sign by switching the polarity of the pockels-cell voltage. The switching is done at a frequency of 1 Hz in most experiments, the pockels-cell driver may be triggered externally at choice of the experiment to be served. Helicity switching is essential for elimination of instrumental asymmetries in an experiment. The beam expander widens the beam and focusses it on the cathode surface to a spot size of 0.25 mm. With the help of two totally reflecting prisms the spot position on the cathode may be adjusted. The two reflection planes are at right angles to each other so that the circular polarization is conserved.
502
LyotFilter
Fig. 4. C.w.-Ti : Sapphire-laser, dl = 35 mm, dl = 55 mm. Tuning range is 780-860 is emitted at 830 nm with 5 W argon-ion pump light.
fiber
,
nm by adjustment
of a Lyot-filter.
Power up to
I .ZW
beam expander lens
/’
i., /
glan
2~52 Al-l\
6 15)
I
Inr
,* *’
;/’
glassplate :
I
-
1 I
+-
pockels-cell
shutter
prism 2 (deflects the beam out of the plane of drawing)
-+I--
by moving the prisms the light spot at the cathode is adjusted
photo diode (monitors light power) Fig. 5. Optical elements for adjusting
beam spot at cathode and attaining
4. Strained layer cathodes Two different types of strained layer semiconductor photocathodes have been employed so far. One is InGaP the other is GaAsP. InGaP is produced by liquid-phase epitaxy at the Institute of Semiconductor Physics in Novosibirsk by the group of Bolkhovityanov and Terekhov [23]. In the present work, samples with composition Ino&ao.sP and layer thickness of 0.6 urn are used [26]. The installation procedure follows the closed cycle recipe proposed by Terekhov [23]. The main idea of the cycle is to avoid contact of cleaned sample surfaces with possibly poisonous laboratory atmosphere: in a first step the samples are introduced into a glove bag filled with dry nitrogen. The transport vessel mentioned above is also attached to the bag in this step. InGaP cathodes are chemically cleaned in the bag in a 1: 5 mixture of HCl and 2-propanol. After cleaning the cathodes are mounted in the transport vessel. The vessel is tightly closed, is detached from the bag, and bolted to a port of the load lock seen in Fig. 2. The load lock is pumped down then overnight by a turbo pump. The transport vessel is opened again when a residual pressure around 1O-* mbar is reached. The samples are transported to the preparation chamber and prepared to NEA as described above.
circular light polarization
In Fig. 6 spin polarization of emitted electrons and the quantum efficiency is shown as a function of photon energy of irradiating light. The data have been measured in a separate investigation [26]. The highest degree of spin polarization is observed at a wavelength of 670 nm. In the example of Fig. 7 the maximum value is P = 52%. We tested several samples, the mean of all maximum polarization values is 55%. The qlmnhlm efficiency is high compared to other strained cathodes described in literature In all cases tested we got quantum efficiencies in excess of 1% at 670 nm, the mean value being 2%. The figure ofmerit P’. QE = 6 x lop3 is the outstanding feature of this type of cathode. At MAMI we used InosGaosP cathodes in the investigation of the reaction 3fie(e’,e’n) at 8.55 MeV [37]. In this experiment the source had to deliver 50 uA beam to the accelerator for more than 600 h. Today GaAsoqsPo.as is the standard cathode type installed in the MAMI source of polarized electrons. The samples are grown by MOCVD techniques in the group of B. Yavich at the Ioffe Institute in St. Petersburg [38]. The installation procedure is similar to that described above for installation of InGaP cathodes. Chemical cleaning is skipped, however, because the crystals are delivered with a protecting arsenic layer at the surface. The layer is evaporated in vacuum in the first heat-cleaning process in the preparation chamber.
K. Aulenhatheu
Wavelength __
750 700
rf nl. I Nucl. Instr. and Meth. in Phjzs. Rex A 391 (1997)498-506
[nm]
650
600
550
NJ
-1
70
-2
T60 h c 50 .o z 40 .N j-j30
-3 c -4g s -5
a" 20 -6
503
Fig. 7. Ten different GaAsos5Poos-samples have been tested up to now. All show maximum spin polarization values in the range 70% and 80% at wavelengths around 830 nm. Values ofquantum efficiency between 0.2% and 0.5% at 830 nm are observed typically. The figure of merit = 2.3 x 10-j is below that of InGaP because of the lower quantum efficiency but this may be compensated partly at least by increased light power. GaAso9sPooS cathodes have been successfully applied in a measurement of polarization transfer in the quasielastic reaction D(c.e’ii) at 855 MeV. The MAMI source delivered 15-20 pA current spin-polarized to a degree of 75% in more than 1000h beamtime.
1.6 1.7 1.8 1.9 Photon
2.0 2.1 2.2 Energy [eV]
5. Operational experience Fig. 6. Photoelectron emission from strained InosGac,5P. Dependence of spin polarization P of emitted electrons and quantum efficiency QE on photon energy of light irradiating the cathode.
Wavelength
850
800
[nm]
750
700
-1 -2 -3 5 -4 g a0 -5
10
-6
I
0’ 1.4
-7 1.5 Photon
1.6 Energy
1.7 [eV]
1.8
Fig. 7. Photoelectron emission from strained G~As~~~P~~~~, Dependence of spin polarization P of emitted electrons and quantum efficiency QE on photon energy of light irradiating the cathode.
Nevertheless, we stick to the closed cycle even in use of samples with arsenic cap because the procedure also prevents contact of cathode holders with laboratory air. Fresh holders are heat-cleaned in the preparation chamber without a cathode attached to it. From this time on (hopefully) they never see dirty air again by virtue of the closed cycle handling. Fig. 7 shows the spin polarization of emitted electrons and the quantum efficiency as a function of wavelength of irradiated circularly polarized light measured in Ref. [25]. Polarization data of two different samples are shown. Maximum degrees of polarization of 7 1% and 78%, respectively, are observed at wavelengths around 830 nm. The quantum efficiency at 830 nm amounts to 2 x 10-j in the example of
The MAMI source of polarized electrons was successfully applied in several electron scattering experiments over more than 1600 h beamtime in recent months. Currents delivered to a target at the accelerator exit are restricted to moderate values below say 7.5 PA. There were two reasons for this limitation: one was the beam-induced cathode degradation discussed below which sets an upper limit of 50 PA approximately to the emission current, the other was the low standard capture efficiency of 15-20% at injection of the beam into the accelerator (see however Section 6). Apart from this limitation to moderate currents the source ran reliably. The main progress in operational reliability is due to the attachment of the load lock system presented above which allows to side-step more or less the nasty cathode degradation. Below a few remarks are made about experiences gained in running the source. Polarizatiurz. With the presently installed strained layer GaAsP cathodes electron beams spin-polarized to a degree of 75% are delivered regularly. The degree of polarization is measured from time to time at source energy of 100 keV by a Mott analyser installed in the spin rotator of the beam line [33]. It does not vary by more than 1% during a typical IO0h run if the quantum efficiency does not vary by more than a factor of 2. In one case. the quantum efficiency was allowed to drop to values around IO-’ which resulted in an increase of the spin polarization of emitted electrons to 84%. Similar increase of polarization with decrease of quantum efficiency is also reported by other experimenters
[161.
Quarztumcfficicwq and I(fe time. In Fig. 8 the time evolution of quantum efficiency QE of a strained layer GaAsP cathode is sketched. The raw data of a nm are plotted in which a constant current of I5 pA was continuously emitted. Variation of QE was compensated by readjustment of the light power shining on the cathode. QE is given in practical units of pA electron current per mW light power. lpA/mW corresponds to a QE of 0.15% at 830nm. One observes an overall decrease of QE with an l/e-time of 7 days. This is only achieved by occasionally evaporating
b)
rspot
q (-I crystal diameter
I
1
2
3
4
time/d
Fig. 8. Quantum efficiency of a strained layer GaAsP cathode as a function of time during a 5 day run. Emission current is held at 15 piA by adjusting the light power shining on the cathode. Labels
1Omm
Fig. 9. (a) Quantum efficiency in a 2 x 2 mm2 area of cathode surface around the 0.25 mm diameter emission spot after operation with an emission current of more than 50 nA for several hours. The efficiency QE is normalized to QEo at the start of the investigation. (b) The emission spot is chosen 3mm off cathode center. The location of the 2 x 2 mm” area at the cathode is indicated.
Cs indicate IOmin caesiation periods. caesium from a &-dispenser installed at a port of the gun chamber. Recaesiation times are indicated by Cs in Fig. 8. The amount of caesium evaporated at times Cs is only a few percent of the amount needed in a fresh NEA-preparation. The l/e-time of the QE-decay between two caesiations is 50 h approximately. The process of NEA-photocathode deterioration is not well understood up to now. We believe there are two main reasons for degradation of quantum efficiency. One is bombardment by positive ions that are produced by the electron beam in the residual gas of the gun chamber. Fig. 9 shows the quantum efficiency in a 2 x 2 mm2 area of cathode surface around the 0.25 mm diameter emission spot after operating the gun with an emission current of more than 50 uA for several hours. The efficiency QE is normalized to QEo at the start of the investigation. The emission spot is located 3 mm off cathode center. One sees clearly a deep groove beginning at the emission spot. In all cases investigated the groove extends to the center of the cathode. EGUN-simulations show that electrons starting off-axis follow a curved trajectory approaching the axis of the gun. Positive ions produced along the electron path impinge at the cathode on a line that is just the projection of the electron trajectory. We believe the ions dig the groove seen in Fig. 9 by bombardment. The grooves may be cured by heat-cleaning the crystal followed by another NEA-preparation. The second mechanism that in our mind reduces the quantum efficiency of NEA-photo-cathodes is the adsorption of chemically active molecules, in particular oxygen. They may originate from the residual gas but may also be desorbed from the vacuum chamber walls by electron stimulation [39,40]. Indeed the transmission of the electron beam from cathode to a Faraday cup 2m downstream of the beam line is better than 99%. Nevertheless, there may be some tiny scraping of beam halo that is produced, for
example. at the cathode by stray light outside the main spot, This may release enough oxygen to account for a slow over-oxidation of the cathode NEA surface. The time dependence of quantum efficiency between two caesiations observed in Fig. 8 is in agreement with this hypothesis: At times Cs caesium is applied to the cathode producing a recovery of QE. Recaesiation is stopped when the yield reaches its start value after having gone through a maximum. In the period following Cs the oxygen created by the beam enhances the quantum efficiency again up to another maximum but finally produces degradation. A similar time behavior of QE is seen in Refs. [39,40]. Rodway also proposes that growth of oxygen on a NEA cathode might be a mechanism for cathode degradation [4l]. Some caesium in the residual gas left from caesiation processes may counteract the detrimental influence of oxygen to some extent but cannot avoid it in the end. Once by accident the gun chamber got far too much caesium from a dispenser connected to a defective power supply. In the period following the accident cathode lifetimes increased by more than a factor of 3 compared to lifetimes in Fig. 8. We believe the prolongation was due to an increase in caesium partial pressure. Some experimenters propose to preserve the quantum yield of a NEA-cathode by a low-level continuous caesiation [28], which is a straightforward measure in case of low voltage sources, but may not be advisable in our 100 keV gun because of the danger of insulation breakdown. Ion sputtering as well as electron-stimulated molecule desorption are driven by the beam itself and increase with increasing current, It presently limits the emission of the MAMI gun to values below 50 uA approximately. Nevertheless extended beamtimes are possible by virtue of quick cathode exchange. E.g. in case of the investigation of the reaction 3fie(e. >n) mentioned above. Ina 5Ga0 XP-cathodes were run with emission currents between 50 and 60 uA. Cathodes were replaced all 2 1h. Replacement together with readjustment ofthe beamline took 3 h more in worst cases so
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Insfr. and Meth. in Phys. Rex A 391 (1997) 498-506
that a duty factor of 85% in beam availability was attained. At lower currents even better duty factors are possible. In the example of Fig. 8 a current of 15 uA was emitted continuously and the availability was better than 95%. Cathodes retracted back to the preparation chamber may be heatcleaned and prepared to NEA again so that even very long runs may be served by only two cathode samples.
505
Acknowledgements The work is supported by the Deutsche meinschaft, SFB 201. Projekt B2.
Forschungsge-
References [I] 11. Herminghaus, A. Feder, K.-H. Kaiser, W. Manz,
6. Conclusion and further developments Table 1 summarizes the present status of the MAMI source of polarized electrons. The source has proved to be adequate for running low to moderate current experiments at MAMI. The source beam current is not high enough to drive the Mainz race track microtron to its full capability of 100 PA; however, the bottle neck being the injection line of the accelerator, where the 100 keV d.c. beam produced at the source enters the RF-chopper-prebuncher system of the injector linac. The capture efficiency of this system. designed for a standard thermionic gun with ample current [42] is 1.5-2O%, so most of the polarized beam is lost there. In the above-mentioned investigation of the reaction )He(e,Gn) only 7.5 uA reached the target at 855 MeV of 50 uA emitted at the source. We follow two lines that will enhance the capture efficiency, namely (a) introduction of a more refined bunching system and (b) injection of an already micropulsed beam with a repetition rate equal to the 2.45 GHz MAMI-RF. The improvement of the bunching process is done by adding a harmonic prebunching cavity working at 4.9 GHz and by introducing a greater distance between the whole prebuncher and the graded-/i capture section of the injector linac [43]. This system was built in the end of last year already and first tests showed that capture efficiencies up to 50% are reachable now with injection of a dc. beam. Ps electron bunches may be emitted from a strained layer NEA GaAsP photo cathode. P. Hartmann of our group produced bunches as short as 8 ps using 100 fs light pulses from a mode-locked Ti : Sapphire laser [44]. No degradation of spin polarization was seen. In a joint experiment with Avramopoulos and Ciarocca from the university of Athens we injected a micropulsed electron beam with a repetition rate equal to the 2.45 GHz MAMI-RF into the accelerator using a mode-locked semiconductor laser developed at Athens [45]. In a first try capture efficiencies of 50% were achieved. The pulsed laser diode system published by Poelker [46] is another light source that should be capable of producing a 2.45 GHz pulsed light beam with photon energies in the near infrared. A mode-locked 2.45 GHzTi : Sapphire laser with ample power is being developed in our group [47], work is in progress to lock the laser to the RF of the accelerator. A capture efficiency near 100% seems to be in reach by combining both developments discussed above.
H.v.d. Schmitt, Nucl. Instr. and Meth. 138 (1976) I. [2] M. Meyerhoff, D. Eyl, A. Frey, H.G. Andresen, J. Annand, K. Aulenbacher, J. Becker, J. Blume-Werry. T. Dombo. P. Drescher, J.E. Ducret, H. Fischer, P. Grabmayr, S. Hall, P. Hartmann, T. Hehl, W. Heil, J. Hoflmann, J.D. Kellie, F. Klein, M. Leduc. H. Moller, C. Nachtigall, M. Ostrick, E.W. Otten, R.O. Owens, S. Pliitzer, E. Reichert, D. Rohe, M. Schafer, L. D. Schearer, H. Schmieden, K. -H. Steffens. R. Surkau, T. Walcher, Phys. Lett. B 327 (1994) 201. [3] D. Eyl, A. Frey, II. Andresen. J. Annand. K. Aulenbacher, J. Becker, J. Blume-Weny, T. Dombo, P. Drescher. H. Fischer. P. Grabmayr, S. Hall, P. Hartmann. T. Hehl, W. Heil, J. Holfmann. J.D. Kellie, F. Klein, M. Meyerhoff, C. Nachtigall. M. Ostrick, E.W. Otten, R.O. Owens, S. Pliitzer, E. Reichert, R. Rieger, H. Schmieden, R. Sprengard, K.-H. Steffens. T. Walcher. Z. Phys. A 352
(1995) 211. [4] H.G. Andresen, J.R.M. Annand, K. Aulenbacher, J. Becker, J. Blume-Weny. T. Dombo. P. Drescher. J.E. Ducret. D. Eyl. H. Fischer. A. Frey, P. Grabmayr, S. Hall, P. Hartmann, T. Hehl. W. Heil, J. Hoffmann, J.D. Kellie, F. Klein, M. Leduc, M. Meyerhoff, H. Miiller, C. Nachtigall, M. Ostrick, E.W. Otten, R.O. Owens, S. Pliitzer, E. Reichert, D. Rohe, M. Schafer. L.D. Schearer, H. Schmieden, K.-H. Stelfens, R. Surkau, T. Walcher, Proceedings of SPIN94. Bloomington, September 94, Invited talk presented by E. Reichert. 1994. [5] S.D. Drell, A.C. Heam, Phys. Rev. Lett. 16 ( 1966) 908. [6] Jahresbericht 199441995, lnstitut fur Kemphysik der Johannes Gutenberg Universitit in Mainz. 1995. [7] E. Gatwin. D.T. Pierce, H.C. Siegmann. Helv. Phys. Acta 47 (1974) 393. [8] D.T. Pierce. F. Meier. P. Ziircher, Appl. Phys. Lett. 26 (I 975) 670. [9] D.T. Pierce, F. Meier, Phys. Rev. B 13 ! 1976) 5484. [lo] G. Lampel, C. Weisbuch, Solid State Commun. 16 (1975) 877. [I I] C.K. Sinclair. AlP Conf. Proc. 35, 1976. [I21 D.T. Pierce. R.J. Celotta, G.-C. Wang, W.N. Unertl, A. Galejs, C.E. Kuyatt, S.R. Mielczarek. Rev. Sci. Instr. 51 (1980) 478. [ 131 U. Kolac, M. Donath, K. Ertl, H. Liebl. V. Dose, Rev. Sci. lnstr. 59 (1988) 1933. [14] G.D. Cates. V.W. Hughes, R. Michaels. H.R. Schgfer, T.J. Gay, MS. Lubell, R. Wilson, G.W. Dodson. K.A. Dow, S.B. Kowalski, K. Isakovich, K.S. Kumar, M.E. Schulze, P.A. Souder. D.H. Kim, Nucl. lnstr. and Meth. A 278 ( 1989) 293. [15] W. Hartmann, D. Conrath. W. Gasteyer, H.-J. Gessinger, W. Heil, H. Kessler, L. Koch. E. Reichert. H.G. Andresen. T. Kettner, B. Wagner. J. Ahrens, J. Jethwa, F.P. Schafer, Nucl. Instr. and Meth. A 286 (1990) I, [16] R. Alley, H. Aoyagi. J. Clendenin, J. Frisch, C. Garden, E. Hoyt. R. Kirby, L. Klaisner, A. Kulikov, R. Miller.
G. Mulhollan, C. Prescott, P. Saez. D. Schultz. H. Tang, J. Turner, K. Witte. M. Woods, A.D. Yeremian, M. Zolotorev. Nucl. Instr. and Meth. A 365 (1995) I. [ 171 D.T. Pierce, in: R. Celotta. T. Lucatorta (Eds.). Experimental Methods m the Physical Sciences, vol. 29 A. Academic Press, New York, 1995. p. 1. [I81 M. Chatwell, J. Clendenin, T. Maruyama. D. Schultz (Eds.). SLAC pub-432: Proc. Workshop on Photocathodes for Polarized Electron Sources for Accelerators. Stanford, CA, 1993, Stanford Linear Accelerator Center, 1994. [I91 T. Maruyama, E.L. Garwin, R. Prepost. G.H. Zapalac. J.S. Smith, J.D. Walker, Phys. Rev. Lett. 66 (1991) 2376. [20] T. Nakanishi. H. Aoyagi. H. Horinaka, Y. Kamiya, T. Kato. S. Nakamura, T. Saka. M. Tsubata, Phys. Lett. A I58 (1991 ) 345. [21] T. Maruyama. E.L. Garwin. R. Prepost. G.H. Zapalac. Phys. Rev. B 46 (1992) 4261. [22] F. Meier, J.C. Griibli, D. Guartsco. A. Vaterlaus, Y. Yashin. Y. Mamaev, B. Yavich, I. Kochnev, Physica Scripta T 49 (1993) 574. [23] V. Alperovich. Y. Bolkhovityanov. A. Paulish, A. Terekhov, Nucl. Instr. and Meth. A 340 (1994) 429. [24] Y.A. Mamaev, J.C. Grobli, B.S. Yavich, Y.P. Yashin. F. Meier. A.V. Subashiev. N.N. Faleev, MS. Galaktianov, D. Guarisco. I.V. Kochnev, S.A. Starovoitov. A. Vaterlaus, E. Reichert, S. Pliitzer, in: M. Chatwell. J. Clendenin. T. Maruyama, D. Schultz (Eds.). Proc. Workshop on Photocathodes for Polarized Electron Sources fat Accelerators, Stanford, CA. 8-10 September, 1993; SLACReport 432 ( Stanford, USA ), pp. 157-l 73. SLAC. 1994. [25] P. Drescher, H.G. Andresen, K. Aulenbacher. J. Bermuth. T. Dombo, H. Euteneuer, N.N. Faleev. H. Fischer. M.S. Galaktionov. D.v. Harrach, P. Hartmann, J. Hoffmann. P. Jennewein. K.-H. Kaiser. S. Kobis, O.V. Kovalenkov, H.J. Kreidel. J. Langbein, Y.A. Mamaev. C. Nachtrgall. M. Petri, S. Plfitzer, E. Reichert. M. Schemies. K.-H. Steffens, M. Steigerwald. A.V. Subashiev, H. Trautner, D.A. Vinokurov. Y.P. Yashin, B.S. Yavich. Appl. Phys. A 63 (1996) 203. [26] P. Drescher. S. Pliitzer, E. Reichcrt. M. Schemies. V.L. Alperovich. Y.B. Bolkhovityanov. A.S. Jaroahevich. A.G. Paulish. H.E. Scheibler, AS. Terekhov, Nucl. Instr. and Meth. A 381 (1996) 169. [27] T. Omori, Y. Kurihari. T. Nakanishi, H. Aoyagi, T. Baba, T. Furuya. K. Itoga, M. Mizuta. S. Nakamura. Y. Takeuchi. M. Tsubata, M. Yoshioka, Phys. Rev. Lett. 67 (1991 ) 3294. [28] B. Letter. C.G. Baum. L. Grau, R. Niemeyer, W. Raith, M. Tondera. Z. Phys. D 33 (1995) 39. [29] W.B. Herrmannsfeldt. Electron trajectory program, SLAC Report 166, 1979. [30] K. Aulenbacher. Eine Quelle longitudinalpolarisierter Elektronen fur das MAMA-Beschleunigersystem, Ph.D. Thesis. [nstrtut fir Physik der Joh. Gutenberg Universitit. Mainz. 1994. [3 I] H. Aufbaus. Automatisierte Strahldiagnose und Phasenraumanpassung fur das Mainzer Mikrotron (MAMI), Ph.D. Thesis. lnstitut tir Kernphysik der Joh, Gutenberg Universitiit. Mamz, 198 I.
1321 C. Nachtigall, Entwicklung eines hochpolarisierten Elektronenstrahles fir MAMI unter Ausnutzung der Photoelektr-onenemission von uniaxral deformicrten Ill-V-Halbleitem. Ph.D. Thesis. Institut fur Physik der Joh. Gutenberg Universitat, Mainz. 1996. [33] K.H. Steffens, H.G. Andrcsen. J. Blume-Werry, F. Klein. K. Aulenbacher. E. Reichert, Nucl. Instr. and Meth. A 325 (1993) 378. [34] K-H. Steffens, Konzeption und Optimierung eines 100keV Injektionssystems zur Erzeugung eincs longrtudinal polarisietten Elektronenstrahles an MAMI, Ph.D. Thesis, Institut fur Kernphysik der Joh, Gutenberg Universitat. Mainz. 1993. [35] H. Trautner. Konzeption und Aufbau eines kompakten c.w.TI : Saphir-Lasers fur die Quelle polarisiertcr Elektronen an MAMI. Diplomarbeit, lnstitut fur Physik der Joh. Gutenberg Universitat, Mainz. 1996. [36] C. Zimmennann. V. Vuletic, A. Hemmerich. L. Ricci. T.W. HLnsch. Opt. Lett. 20 (1995) 297. [37] F. Klein, Prog. Part. Nucl. Phys. 36 (1996) 53. [38] Y.A. Mamaev, Y.P. Yashin. A.V. Subashiev. M.S. Galaktionov, B.S. Yavich, O.V. Kovalenkov, D.A. Vinokurov, N.N. Faleev, Phys. Low-Dim. Struct. 7 (1994) 27. [39] H. Fischer, Untersuchungen zur Korrosion der (Cs.0 )Aktivierungsschicht van III-V-Halbleitcr-Photokathoden mn Hilfe der Riintgen-Photoelektronen-Spektroskopie. Ph.D. Thesis, lnstitut fur Physik der Joh, Gutenberg Universitat, Mainz, 1995. [40] H.J. &hope. XPS-Untersuchungen der Oxidation von GaAsP-Kathoden, Diplomarbeit, lnstitut mr Physik der Joh. Gutenberg Universitat. Mainz. 1996. (411 D. Rodway. M. Allenson, J. Phys. D: Appl. Phys. I9 (IS%) 1353. [42] H. Braun. H. Herminghaus, A. Streun. in: S. Tazzari Singapore, 1988. (Ed.), EPAC88, 2. World Scientific, pp. 997-999. [43] V.T. Shvedunov. M.O. Ihm. H. Euteneuer, K-H. Kaiser. T. Weis. in: S. Myers, A. Pacheco, R. Pascual, C. PetitJean-Genaz. J. Poole (Eds. ). EPAC96. 2, Institute of Physics Publishing, Bristol. 1996. pp. 155661558. [44] P. Hartmann. J. Bermuth, J. Hoffman”. S. Kobrs. E. Reichert, H.G. Andresen, K. Aulenbacher, P. Drescher. H. Euteneuer, H. Fischer. P. Jennewein. K.H. Kaiser, H.J. Kreidel. C. Nachtigall, S. Pliitzer. M. Schemies, K.-H. Steffens. M. Steigerwald. H. Trautner. D.v. Harrach, I. Altarev. R. Geiges. K. Grimm, T. Hammel, E. HeinenKonschak, H. Hofmann. E.-M. Kabul, A. Lopes-Ginja. F. Maas, E. Schilling, Nucl Instr. and Meth. A 379 (1996) 15. H. Avramopoulos. C.N. Papanicolas. [45] M. Ciarocca. A modelocked semiconductor laser for a polarized electron source, Nucl. Instr. and Meth., to be published. [46] M. Poelker. Appl. Phys. Lett. 67 (1995) 2762. [47] J. Hoffmann, P. Hartmann, C. Zimmermann. D.v. Harrach, E. Reichert, K. Aulenbacher. H. Euteneuer. K.-H. Haiser. S. Kiibis. M. Schemies. M. Steigenvald. H. Trautner, K. Grimm, T. Hammel. H. Hofmann, E.-M. Kabul& A. LopesGmja, F.E. Maas, P. Piatosa. E. Schilling. Nucl. lnstr. and Meth. A 383, (1996) 624.
and Methods in Physics Research A 391
( 1997) 498-506
-_ ll!B ELSJZVIER
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SecllonA
The MAMI source of polarized electrons K. Aulenbacher a,*, Ch. Nachtigallb, H.G. Andresen”, J. Bermutha, Th. Dombo”, P. Drescherb, H. Euteneuer”, H. F&herb, D.v. Harrach”, P. Hartmann”, J. Hoffmanna, P. Jenneweina, K.H. Kaiser a, S. Kiibis”, H.J. Kreidel”, J. Langbeinb, M. Petri”, S. Pliitzerb, E. Reichertb, M. Schemiesb, H.-J. Sch6peb, K.-H. Steffensa, M. Steigerwaldb, H. Trautnerb, Th. Weis”
Abstract The present work describes the source of polarized electrons that is run at the 855 MeV race track microtron MAMI at the Johannes Gutenberg Universitiit in Mainz. The source is based on photoelectron emission from (III-V)-semiconductors. Presently strained layer InGaP- or GaAsP-cathodes are used, which are processed to negative electron affinity by coverage of the surface with a submonolayer of caesium and oxygen, Electron beams spin-polarized up to a degree of P=55% at a quantum efficiency of QE = 2% or P = 75% at QE = 0.4%, respectively, are obtained. The well-known but hitherto unsolved problem of limited cathode lifetime has been sidestepped by the attachment of an UHV load lock system to the source electron gun. It allows quick replacement of cathodes without breaking the gun vacuum. Availabilities in excess of 85% are obtained regularly in beamtimes longer than 100 h. The source was mainly applied in measurements of nucleon form factors via the reactions ‘H(e’, e’a), ‘D(Z,e’p’), ‘D(Z, e’ii), and 3Ge( Z, e’n). More than 1600 h beamtime have been accomplished in physics experiments with polarized electron beams at MAMI up to now. PACS:
79.60
1. Introduction The Mainz race track microtron cascade MAMI delivers a C.W. electron beam of energies up to 855MeV [l]. The physics program at MAMI proposes a series of experiments with polarized electron beams. Examples are measurement of electromagnetic nuclear formfactors [24], test of the Gerasimov-Drell-Hearn sum rule in photon proton scattering [5], and determination of s-quark contributions to the Pauli form factors of the nucleon in a parity violation experiment [6]. The present paper describes the source of polarized electrons applied in these investigations. The MAMI source of polarized electrons is based on photoelectron emission of (III-V)-semiconductor crystals. This process was discovered in 1970s by Pierce and collaborators [7-91 and Lampel and Weisbuch [iO] and is today the standard way to produce spin-polarized electron beams. * Corresponding author. Tel.: f49 6131 398504, fax: f49 6131 392964: e-mail: aulenbac@,kph.uni-mainz.de. 0168-9002/97/$17.00 PII SO 168-9002(
Examples of sources of polarized electrons applying photoelectron emission from GaAs or one of its relatives are discussed, e.g. in Refs. [ 1 l-1 61 A review of the work done before 1993 has been given by Pierce [ 171. A collection of papers describing recent developments may be found in Ref. [18] The degree of polarization available in emission from bulk GaAs is limited to values below 50% [9]. Reduction of the degeneracy between heavy hole and light hole bands at the valence band edge of (III-V)-semiconductor cathodes may allow higher polarization values. Application of strain [19-261 to the emissive cathode layer or growth of a super-lattice structure consisting of two materials with different gap energies [27] proved to be the most effective method in lifting the heavy hole-light hole degeneracy. In praxi strained layers of GaAs are the only new generation cathodes installed in sources of polarized electrons applied in real physics experiments [ 16,281 up to now. Degrees of spin polarization in the P = 80% range are achieved at quantum efficiencies around QE = 0.2% [ 161. The present work successfully applied strained layer InGaP cathodes emitting
Copyright @ 1997 Elsevier Science B.V. All rights reserved 97)00528-7
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electron beams spin-polarized up to a degree of P = 55% at a quantum efficiency of QE = 2% 1261 and strained layer GaAsP cathodes with P = 75% at QE = 0.4% [25]. The figures of merit P’. QE of 6 x lop3 and 2.3 x 10P3, respectively, are at least as high as that of the best strained layer (III-V)-cathodes described in literature. The transverse emittance of the beam at injection must stay below 1~ mm mrad at 100 keV in order to reach the high beam quality of MAMI in its standard operation with a thermionic gun. The gun in the present work is designed with help of the well-known EGUN-code of Herrmannsfeldt [29]. Emittance of the beam may be conveniently restricted by choosing a small diameter of the light spot at the cathode. The lifetime of negative electron affinity (NEA) cathodes installed in sources of polarized electrons is limited. The submonolayer of caesium and oxygen or fluorine that is necessary for attaining NEA may be disturbed by ions produced by the beam and poisoned by molecules present in the residual gas even if UHV in the low IO-” mbar range is maintained. This is particularly true in cases where the gun is run with high beam currents. The rate of ion production in the residual gas increases with current as well as the rate of molecules produced via electron-stimulated desorption from chamber walls by beam scraping. In the MAMI source described below the problem of limited cathode lifetime has been sidestepped by the attachment of an UHV load lock system to the gun chamber. It allows quick replacement of cathodes without breaking the gun vacuum. A load lock system is successfully used already at the SLAC source of polarized electrons [ 161.
2. Electron gun and its accessories The electron gun of the MAMI source of polarized electrons has a triode configuration. It is designed with help of the EGUN-code of Herrmannsfeldt [29]. Fig. l(a) sketches the layout of the gun electrodes [30]. Beam emittance was determined by a scanning wire method described in Ref. 1311. It was adjusted by decreasing the size of the laser spot. The gun is operated with a spot diameter of 0.25 mm. In this case, 90% of the beam current was found in an emittance volume of 0.5xmmmrad at 100 keV. Voltage breakdown may be a problem because of caesium introduced into the gun chamber by the so-called recaesiation procedure discussed below. Therefore, some care has been taken to get electrode shapes that preserve low electric field strengths at cathode and intermediate electrode surfaces. At the edge of the cathode electrode the maximum E-field is 2.95 MV/m, at the cathode itself 0.89 MV/m. and at the intermediate electrode 3.3 MV/m in the design shown in Fig. l(a). It has been pointed out by Alley and co-workers [16] that currents due to field emission should be kept below say 100 nA. This criterion is easily met by the gun of the
499
Fig. 1. (a) Section through the electrode configuration of the 100 keV electron gun: (b) elevator in down position.
present source. Field emission current of electrons from the cathode or the intermediate electrode are smaller than 10 n4 at 100 kV gun voltage. High voltage processing is possible while there is no crystal in the gun, but this has in fact not been necessary so far. The innermost cylindrical part, labeled elevator in Fig. 1, contains the conical photo cathode holder fitted in a conical seat [32]. The elevator may be rotated and moved down via a vacuum feedthrough not shown (BR2 in Fig, 2). Rotation is used to change the emission spot. During first use of the gun it turned out that in particular the central part of the photo cathode is damaged by ion bombardment. Therefore, the gun is operated now with the emission spot off-axis. This in turn allows to change the emission site on the cathode by just rotating the elevator. Beam steering is not affected by cathode rotation. Down movement of the elevator is needed in case of cathode exchange. Fig. l(b) sketches the elevator in downposition. The cathode holder is brought to the elevator and removed from it respectively by a fork fastened to a magnetic linear movement feedthrough, that serves for transport of cathodes between the cathode mount and the preparation chamber seen in Fig. 2. Fig. 2 presents a very schematic drawing of the load lock, preparation, and gun chamber assembly 1321. A new crystal is brought to the load lock chamber in a transport vessel filled with dry nitrogen. The vessel is bolted to a port of the load lock and not opened before pumpdown of the chamber. In this way, a cathode may be moved around after chemical cleaning without contact to laboratory air. Such a precaution was introduced by Terekhov and his co-workers [23]. A sample is moved from the load lock to the preparation chamber by a magnetic linear movement feedthrough MM 1 as soon as the residual pressure in the load lock chamber is in the 1O-’ mbar range. Up to six cathodes plus holders may be placed on a storage wheel in the preparation chamber. A cathode to be installed in the gun is heat-cleaned from the rear by radiation emitted from a hot tungsten filament. The sample is held at 580°C for half an hour.
500
up/down movement and rotation of storage wheel
chamber with storage-wheel Transport-vessel 5
BRl
I I/ I
G-gun
0ae
load-lock-c
ectron-beam
load-lock-sys
te
NFG-pump
Fig. 2. Assembly of electron gun, preparation chamber. and load lock chamber of the MAMI source of polarized electrons.
NEA-preparation is done at room temperature by evaporation of caesium to the cathode surface. Caesium source is a Cs-dispenser from S.A.E.S. GETTERS, Milano. During the process the cathode is irradiated by He-Ne-laser light and the photoelectron current emitted is registered. As soon as the emission current reaches a first maximum, oxygen is introduced into the preparation chamber via a leak valve. Oxygen flow is adjusted so that the rate at which the current increases is optimized. The procedure is stopped when the current goes through its maximum. Cathode exchange starts with transport of the old cathode from the gun (see Fig. l(b)) to a spare position in the storage wheel of the preparation chamber indicated in Fig. 2 and ends with placing the fresh cathode in the gun elevator. Movement between the two chambers is accomplished by a fork attached to the rod of a magnetic linear motion feedthrough MM2 sketched in Figs. l(b) and Fig. 2. Movement of the gun elevator is done with manipulator BR2. The whole process of cathode exchange takes 10 mins approximately. The vacuum system consisting of gun chamber. preparation chamber, and load lock is built in all-metal
UHV technique. Gun chamber and preparation chamber are evacuated by combinations of 110 I/s-iongetter + 500 l/s NEG pumps and 60 l/s iongetter + 300 l/s NEG pumps, respectively. Base pressures are 1 x lo-” and 3 x lo-” mbar, respectively, with main contribution coming from hydrogen. Both chambers are not opened to air again unless a vacuum break is forced by a component failure. Fig. 3 presents an overall view of the source together with the first part of the beamline between source and the injection point of the race track microtron. The preparation chamber and load lock are not drawn in this picture. The beam is guided by magnetic quadrupoles, a-magnets. solenoid lenses, and steering coils along the 25 m beamline [33,34]. A spin rotator incorporated in the line is used to adjust the orientation of the polarization vector with respect to the vector of momentum [33,34]. The transmission of the beam from the gun to a Faraday cup 2 m downstream is better than 99%. A high value of transmission not only saves current but is particularly important for avoiding electronstimulated desorption of molecules by beam scraping at the
K. Aukwbncher
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501
V-electrode
to load-lock
n-chamber umatic
valve
u
Not bakeable lm
I
Fig. 3. Overall view of the MAMI source of polarized electrons.
vacuum chamber walls. The transmission between gun and the distant injection point is better than 96%. The gun vacuum is isolated from the line vacuum by two-stage differential pumping indicated in Fig. 3. The circularly polarized light beam is introduced into the vacuum system via a view port at the first 90”-bend of the beamline. Spin polarization of the beam produced may be analysed by a Mott detector integrated in the spin rotator [33,34].
3. Laser sources and light beam optics Up to now two different laser light sources have been used. With InGaP-cathodes having an optimum wavelength at 670nm a commercially available dye laser pumped by an Argon ion laser was employed. ’ In case of strained GaAsP installed in the gun the dye laser was substituted by a Ti : Sapphire laser [35], that was built following a design of Zimmermann and colleagues [36]. The optimum wavelength for emission of polarized electrons from GaAsP is around 830 nm. Fig. 4 sketches the Ti: Sapphire laser used. It is tunable in the wavelength range 780-860 nm by adjusting the indicated Lyot-filter. Power up to 1.2 W is emitted at
’ Spectra Physics 2030cavity dumper.
15 Ar+-laser
+ 375B dye-laser
+ 3448
830nm with 5 W argon-ion pumplight. The linearly polarized laser light is transported via a 20 m mono-mode fiber to the source. Fig. 5 shows the elements arranged in an optical bench that guides the light to the cathode surface. The light emitted by the fiber exit is formed in a parallel beam by a microscope lens. A half-wave plate is used to adjust the orientation of the plane of linear polarization. A small fraction of the beam is reflected to a photodiode, whose output gives a measure of light power. The Glan prism produces a clean linear polarization of the light beam while the Pockels cell acts as a quarter wave plate and transforms the linear polarization into circular polarization. The degree of circular polarization achieved is better than 99.5%. The helicity of the light may be quickly switched from positive to negative sign by switching the polarity of the pockels-cell voltage. The switching is done at a frequency of 1 Hz in most experiments, the pockels-cell driver may be triggered externally at choice of the experiment to be served. Helicity switching is essential for elimination of instrumental asymmetries in an experiment. The beam expander widens the beam and focusses it on the cathode surface to a spot size of 0.25 mm. With the help of two totally reflecting prisms the spot position on the cathode may be adjusted. The two reflection planes are at right angles to each other so that the circular polarization is conserved.
502
LyotFilter
Fig. 4. C.w.-Ti : Sapphire-laser, dl = 35 mm, dl = 55 mm. Tuning range is 780-860 is emitted at 830 nm with 5 W argon-ion pump light.
fiber
,
nm by adjustment
of a Lyot-filter.
Power up to
I .ZW
beam expander lens
/’
i., /
glan
2~52 Al-l\
6 15)
I
Inr
,* *’
;/’
glassplate :
I
-
1 I
+-
pockels-cell
shutter
prism 2 (deflects the beam out of the plane of drawing)
-+I--
by moving the prisms the light spot at the cathode is adjusted
photo diode (monitors light power) Fig. 5. Optical elements for adjusting
beam spot at cathode and attaining
4. Strained layer cathodes Two different types of strained layer semiconductor photocathodes have been employed so far. One is InGaP the other is GaAsP. InGaP is produced by liquid-phase epitaxy at the Institute of Semiconductor Physics in Novosibirsk by the group of Bolkhovityanov and Terekhov [23]. In the present work, samples with composition Ino&ao.sP and layer thickness of 0.6 urn are used [26]. The installation procedure follows the closed cycle recipe proposed by Terekhov [23]. The main idea of the cycle is to avoid contact of cleaned sample surfaces with possibly poisonous laboratory atmosphere: in a first step the samples are introduced into a glove bag filled with dry nitrogen. The transport vessel mentioned above is also attached to the bag in this step. InGaP cathodes are chemically cleaned in the bag in a 1: 5 mixture of HCl and 2-propanol. After cleaning the cathodes are mounted in the transport vessel. The vessel is tightly closed, is detached from the bag, and bolted to a port of the load lock seen in Fig. 2. The load lock is pumped down then overnight by a turbo pump. The transport vessel is opened again when a residual pressure around 1O-* mbar is reached. The samples are transported to the preparation chamber and prepared to NEA as described above.
circular light polarization
In Fig. 6 spin polarization of emitted electrons and the quantum efficiency is shown as a function of photon energy of irradiating light. The data have been measured in a separate investigation [26]. The highest degree of spin polarization is observed at a wavelength of 670 nm. In the example of Fig. 7 the maximum value is P = 52%. We tested several samples, the mean of all maximum polarization values is 55%. The qlmnhlm efficiency is high compared to other strained cathodes described in literature In all cases tested we got quantum efficiencies in excess of 1% at 670 nm, the mean value being 2%. The figure ofmerit P’. QE = 6 x lop3 is the outstanding feature of this type of cathode. At MAMI we used InosGaosP cathodes in the investigation of the reaction 3fie(e’,e’n) at 8.55 MeV [37]. In this experiment the source had to deliver 50 uA beam to the accelerator for more than 600 h. Today GaAsoqsPo.as is the standard cathode type installed in the MAMI source of polarized electrons. The samples are grown by MOCVD techniques in the group of B. Yavich at the Ioffe Institute in St. Petersburg [38]. The installation procedure is similar to that described above for installation of InGaP cathodes. Chemical cleaning is skipped, however, because the crystals are delivered with a protecting arsenic layer at the surface. The layer is evaporated in vacuum in the first heat-cleaning process in the preparation chamber.
K. Aulenhatheu
Wavelength __
750 700
rf nl. I Nucl. Instr. and Meth. in Phjzs. Rex A 391 (1997)498-506
[nm]
650
600
550
NJ
-1
70
-2
T60 h c 50 .o z 40 .N j-j30
-3 c -4g s -5
a" 20 -6
503
Fig. 7. Ten different GaAsos5Poos-samples have been tested up to now. All show maximum spin polarization values in the range 70% and 80% at wavelengths around 830 nm. Values ofquantum efficiency between 0.2% and 0.5% at 830 nm are observed typically. The figure of merit = 2.3 x 10-j is below that of InGaP because of the lower quantum efficiency but this may be compensated partly at least by increased light power. GaAso9sPooS cathodes have been successfully applied in a measurement of polarization transfer in the quasielastic reaction D(c.e’ii) at 855 MeV. The MAMI source delivered 15-20 pA current spin-polarized to a degree of 75% in more than 1000h beamtime.
1.6 1.7 1.8 1.9 Photon
2.0 2.1 2.2 Energy [eV]
5. Operational experience Fig. 6. Photoelectron emission from strained InosGac,5P. Dependence of spin polarization P of emitted electrons and quantum efficiency QE on photon energy of light irradiating the cathode.
Wavelength
850
800
[nm]
750
700
-1 -2 -3 5 -4 g a0 -5
10
-6
I
0’ 1.4
-7 1.5 Photon
1.6 Energy
1.7 [eV]
1.8
Fig. 7. Photoelectron emission from strained G~As~~~P~~~~, Dependence of spin polarization P of emitted electrons and quantum efficiency QE on photon energy of light irradiating the cathode.
Nevertheless, we stick to the closed cycle even in use of samples with arsenic cap because the procedure also prevents contact of cathode holders with laboratory air. Fresh holders are heat-cleaned in the preparation chamber without a cathode attached to it. From this time on (hopefully) they never see dirty air again by virtue of the closed cycle handling. Fig. 7 shows the spin polarization of emitted electrons and the quantum efficiency as a function of wavelength of irradiated circularly polarized light measured in Ref. [25]. Polarization data of two different samples are shown. Maximum degrees of polarization of 7 1% and 78%, respectively, are observed at wavelengths around 830 nm. The quantum efficiency at 830 nm amounts to 2 x 10-j in the example of
The MAMI source of polarized electrons was successfully applied in several electron scattering experiments over more than 1600 h beamtime in recent months. Currents delivered to a target at the accelerator exit are restricted to moderate values below say 7.5 PA. There were two reasons for this limitation: one was the beam-induced cathode degradation discussed below which sets an upper limit of 50 PA approximately to the emission current, the other was the low standard capture efficiency of 15-20% at injection of the beam into the accelerator (see however Section 6). Apart from this limitation to moderate currents the source ran reliably. The main progress in operational reliability is due to the attachment of the load lock system presented above which allows to side-step more or less the nasty cathode degradation. Below a few remarks are made about experiences gained in running the source. Polarizatiurz. With the presently installed strained layer GaAsP cathodes electron beams spin-polarized to a degree of 75% are delivered regularly. The degree of polarization is measured from time to time at source energy of 100 keV by a Mott analyser installed in the spin rotator of the beam line [33]. It does not vary by more than 1% during a typical IO0h run if the quantum efficiency does not vary by more than a factor of 2. In one case. the quantum efficiency was allowed to drop to values around IO-’ which resulted in an increase of the spin polarization of emitted electrons to 84%. Similar increase of polarization with decrease of quantum efficiency is also reported by other experimenters
[161.
Quarztumcfficicwq and I(fe time. In Fig. 8 the time evolution of quantum efficiency QE of a strained layer GaAsP cathode is sketched. The raw data of a nm are plotted in which a constant current of I5 pA was continuously emitted. Variation of QE was compensated by readjustment of the light power shining on the cathode. QE is given in practical units of pA electron current per mW light power. lpA/mW corresponds to a QE of 0.15% at 830nm. One observes an overall decrease of QE with an l/e-time of 7 days. This is only achieved by occasionally evaporating
b)
rspot
q (-I crystal diameter
I
1
2
3
4
time/d
Fig. 8. Quantum efficiency of a strained layer GaAsP cathode as a function of time during a 5 day run. Emission current is held at 15 piA by adjusting the light power shining on the cathode. Labels
1Omm
Fig. 9. (a) Quantum efficiency in a 2 x 2 mm2 area of cathode surface around the 0.25 mm diameter emission spot after operation with an emission current of more than 50 nA for several hours. The efficiency QE is normalized to QEo at the start of the investigation. (b) The emission spot is chosen 3mm off cathode center. The location of the 2 x 2 mm” area at the cathode is indicated.
Cs indicate IOmin caesiation periods. caesium from a &-dispenser installed at a port of the gun chamber. Recaesiation times are indicated by Cs in Fig. 8. The amount of caesium evaporated at times Cs is only a few percent of the amount needed in a fresh NEA-preparation. The l/e-time of the QE-decay between two caesiations is 50 h approximately. The process of NEA-photocathode deterioration is not well understood up to now. We believe there are two main reasons for degradation of quantum efficiency. One is bombardment by positive ions that are produced by the electron beam in the residual gas of the gun chamber. Fig. 9 shows the quantum efficiency in a 2 x 2 mm2 area of cathode surface around the 0.25 mm diameter emission spot after operating the gun with an emission current of more than 50 uA for several hours. The efficiency QE is normalized to QEo at the start of the investigation. The emission spot is located 3 mm off cathode center. One sees clearly a deep groove beginning at the emission spot. In all cases investigated the groove extends to the center of the cathode. EGUN-simulations show that electrons starting off-axis follow a curved trajectory approaching the axis of the gun. Positive ions produced along the electron path impinge at the cathode on a line that is just the projection of the electron trajectory. We believe the ions dig the groove seen in Fig. 9 by bombardment. The grooves may be cured by heat-cleaning the crystal followed by another NEA-preparation. The second mechanism that in our mind reduces the quantum efficiency of NEA-photo-cathodes is the adsorption of chemically active molecules, in particular oxygen. They may originate from the residual gas but may also be desorbed from the vacuum chamber walls by electron stimulation [39,40]. Indeed the transmission of the electron beam from cathode to a Faraday cup 2m downstream of the beam line is better than 99%. Nevertheless, there may be some tiny scraping of beam halo that is produced, for
example. at the cathode by stray light outside the main spot, This may release enough oxygen to account for a slow over-oxidation of the cathode NEA surface. The time dependence of quantum efficiency between two caesiations observed in Fig. 8 is in agreement with this hypothesis: At times Cs caesium is applied to the cathode producing a recovery of QE. Recaesiation is stopped when the yield reaches its start value after having gone through a maximum. In the period following Cs the oxygen created by the beam enhances the quantum efficiency again up to another maximum but finally produces degradation. A similar time behavior of QE is seen in Refs. [39,40]. Rodway also proposes that growth of oxygen on a NEA cathode might be a mechanism for cathode degradation [4l]. Some caesium in the residual gas left from caesiation processes may counteract the detrimental influence of oxygen to some extent but cannot avoid it in the end. Once by accident the gun chamber got far too much caesium from a dispenser connected to a defective power supply. In the period following the accident cathode lifetimes increased by more than a factor of 3 compared to lifetimes in Fig. 8. We believe the prolongation was due to an increase in caesium partial pressure. Some experimenters propose to preserve the quantum yield of a NEA-cathode by a low-level continuous caesiation [28], which is a straightforward measure in case of low voltage sources, but may not be advisable in our 100 keV gun because of the danger of insulation breakdown. Ion sputtering as well as electron-stimulated molecule desorption are driven by the beam itself and increase with increasing current, It presently limits the emission of the MAMI gun to values below 50 uA approximately. Nevertheless extended beamtimes are possible by virtue of quick cathode exchange. E.g. in case of the investigation of the reaction 3fie(e. >n) mentioned above. Ina 5Ga0 XP-cathodes were run with emission currents between 50 and 60 uA. Cathodes were replaced all 2 1h. Replacement together with readjustment ofthe beamline took 3 h more in worst cases so
K. Adenbaclwr
et al. INud.
Insfr. and Meth. in Phys. Rex A 391 (1997) 498-506
that a duty factor of 85% in beam availability was attained. At lower currents even better duty factors are possible. In the example of Fig. 8 a current of 15 uA was emitted continuously and the availability was better than 95%. Cathodes retracted back to the preparation chamber may be heatcleaned and prepared to NEA again so that even very long runs may be served by only two cathode samples.
505
Acknowledgements The work is supported by the Deutsche meinschaft, SFB 201. Projekt B2.
Forschungsge-
References [I] 11. Herminghaus, A. Feder, K.-H. Kaiser, W. Manz,
6. Conclusion and further developments Table 1 summarizes the present status of the MAMI source of polarized electrons. The source has proved to be adequate for running low to moderate current experiments at MAMI. The source beam current is not high enough to drive the Mainz race track microtron to its full capability of 100 PA; however, the bottle neck being the injection line of the accelerator, where the 100 keV d.c. beam produced at the source enters the RF-chopper-prebuncher system of the injector linac. The capture efficiency of this system. designed for a standard thermionic gun with ample current [42] is 1.5-2O%, so most of the polarized beam is lost there. In the above-mentioned investigation of the reaction )He(e,Gn) only 7.5 uA reached the target at 855 MeV of 50 uA emitted at the source. We follow two lines that will enhance the capture efficiency, namely (a) introduction of a more refined bunching system and (b) injection of an already micropulsed beam with a repetition rate equal to the 2.45 GHz MAMI-RF. The improvement of the bunching process is done by adding a harmonic prebunching cavity working at 4.9 GHz and by introducing a greater distance between the whole prebuncher and the graded-/i capture section of the injector linac [43]. This system was built in the end of last year already and first tests showed that capture efficiencies up to 50% are reachable now with injection of a dc. beam. Ps electron bunches may be emitted from a strained layer NEA GaAsP photo cathode. P. Hartmann of our group produced bunches as short as 8 ps using 100 fs light pulses from a mode-locked Ti : Sapphire laser [44]. No degradation of spin polarization was seen. In a joint experiment with Avramopoulos and Ciarocca from the university of Athens we injected a micropulsed electron beam with a repetition rate equal to the 2.45 GHz MAMI-RF into the accelerator using a mode-locked semiconductor laser developed at Athens [45]. In a first try capture efficiencies of 50% were achieved. The pulsed laser diode system published by Poelker [46] is another light source that should be capable of producing a 2.45 GHz pulsed light beam with photon energies in the near infrared. A mode-locked 2.45 GHzTi : Sapphire laser with ample power is being developed in our group [47], work is in progress to lock the laser to the RF of the accelerator. A capture efficiency near 100% seems to be in reach by combining both developments discussed above.
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