Aleksandrov, A

Arapov, L N

Crawford, C

Kuznetsov, G L

Logatchev, P V

Niell, F

Saewert, G W

Santucci, J K

Seryi, Andrei

Sharapa, A N

Shemyakin, A

Shiltsev, V D

Skarbo, B A

Sukhina, B

Wildman, D

CERN Document Server

Prototype “Electron Lens” Set-up for the Tevatron Beam-Beam CompensationC.Crawford, F.Niell, G.Saewert, J.Santucci, A.Sery, A.Shemyakin, V.Shiltsev, D.Wildman, FNALA.Aleksandrov, L.Arapov, G.Kuznetsov, P.Logachov, A.Sharapa , B.Skarbo, B.Sukhina, Budker INPAbstractA prototype “electron lens” for the Tevatron beam-beamcompensation project is commissioned at Fermilab. We de-scribe the set-up, report results of the first tests of the elec-tron beam, and discuss future plans.1 SCOPECompensation of beam-beam effects in the Tevatron withelectron beams is a promising technique to improve the col-lider performance [1]. The method implies that an antipro-ton beam propagates througha countertraveling low-energyhigh-current electron beam (“electron lens”). An impact ofthe negative electron space charge can reduce betatron tunespread within antiproton bunch and a bunch-to-bunch tunespread – the effects due to collision with intensive protonbeam which limit beam lifetime and luminosity.An experimental R&D program on beam-beam compen-sation was started at FNAL Beam Division early in 1998.The “electron lens” prototype has been designed, fabri-cated, assembled in the Linac Lab and commissioned inDecember 1998. The goal of the set-up is to study feasi-bility and properties of the electron beam required for thebeam-beam compensation. Currently, these studies are un-der way.Table 1 shows the Tevatron “electron lens” (TEL)design parameters and parameters of the prototype set-upoperation to date.Parameter units TEL PrototypeEffective length, m 2.0 1.96Electron current,max, A 2.2 3.05Electron energy, max kV 5-10 6.4CW modulation time, s 0.4(0.13) 0.5Solenoid field, kG 50 4Beam deviousness,rms mm 0.1 0.07Configuration 2 bends straightBeam shape control yes yes2 ”ELECTRON LENS” PROTOTYPEMajor components and systems of the “electron lens” proto-type are: electron gun, electron collector, modulator, powersupplies, magnetic system, vacuum system, control sys-tem. The set-up (in present configuration) is shown in Fig.1.Electrons are thermally emitted from a cathode of the gunand extracted toward the positive potential of an anode Ua.Then they propagate through some 2 m long beam pipewhich is under potential Up (usually, Up  Ua). Finally,electron beam is absorbed in a high efficiency collector ata smaller potential Uc  Ua. Some of the electrons can be on leave from BINP, Novosibirsk, Russialost and absorbed on the vacuum pipe walls or at other aper-ture limits, i.e., not in the designated place of the collector.Strong longitudinal magnetic fields (of the order of few kGall along the set-up) helps to keep these losses low.Figure 1: Electrical scheme of the “electron lens” proto-type.The high perveance electron gun is made in accordancewith a novel approach proposed in [2] based on use of aconvex cathode. The gun is immersed in 0.7-2 kG longi-tudinal magnetic field. It has a uniform current density pro-file, low transverse beam temperature and high perveance(these conditions can not be met in a standard Pierce geom-etry with a planar cathode). Fig.2 shows the gun construc-tion.Figure 2: Electron gun of the “electron lens”A 10 mm diameter 45o convex cathode is made of tung-sten impregnated with emitting oxides (made by HeatWaveCo., CA; see details in [3]). Fig.3 shows the maximum elec-tron current Je vs voltage between the anode and the cath-ode of the gun Ua. The maximum current is limited bynegative space charge of the electron cloud near the cath-ode and follows Child’s law Je = PU3=2a . Numericalsimulations of the gun yielded the perveance P = 4:9 10−6 A=V 3=2, while a fit of the measured current at theFig.3 gives somewhat larger perveance of P = 5:85 10−6 A=V 3=2.0-7803-5573-3/99/$10.00@1999 IEEE. 237Proceedings of the 1999 Particle Accelerator Conference, New York, 19990 1000 2000 3000 4000 5000 6000U_anode (V)050010001500200025003000I_collector (mA)Figure 3: Current in the collector vs anode voltage. Smoothline represents a fit accordingly to Child’s law with microp-erveance of P = 5:85.A possible explanation of excessive perveance (and cur-rent) can be imperfect alignment of the cathode and a con-trol electrode. The latter is a specially shaped ring (sim-ulated by computer) placed around the cathode which canvary the electron beam profile depending on its potentialrelative to the cathode. If the control electrode is groundedas well as the cathode Uce = 0, then the electron beam cur-rent profile has a smoothed rectangular shape. If Uce is neg-ative and more than 3.5 times the positive anode potential,−Uce  3:5  Ua, then no current comes from the cathode(full emission suppression). At the intermediate cases, thenegative control electrode potential 0 < −Uce < 3:5  Uasuppresses the emission from the areas of the cathode whichare close to the electrode, the total current is reduced andthe profile becomes ”bell-shape-like”. Fig.4 demonstratesan example of such a shape when the total current is reducedon about 30% from 0.22 A (at Uce = 0) to 0.16 A (otherparameters: Bgun = 2 kG, Bcoll = 0:5 kG, main solenoidfield 2 kG, Ua = 1:9 kV, Ucoll=2.5 kV, Upipe=2.5 kV).The electron beam current profile measurements are pos-sible using a profile analyzer installed in the collector. Thecollector is able to absorb about 5-10 kW of electron beampower on its water cooled walls. There are no additionalelectrodes in the collector for repelling secondary electrons(low energy electrons born after the incoming electrons hitthe collector walls) and reflected part of the primary beam.To lower the outcoming stream of electrons, the collectoris designed to work in a magnetic field quickly decreasingfrom about 2 kG at the collector entrance to almost zeroat its backplate. Good collector efficiency allowed us toachieve very small relative losses of electrons - typicallyof the order of 5  10−4 (minimum 10−4). The losses wereof extreme concern at the stage of commissioning becausea) they caused intensive outgassing from the vacuum pipewall, b) there was a fear of excessive heat release at the an-ode (if the electron current goes onto it), c) beam pipe po-tential was kept by a high voltage power supply with max-imum current of 2.5 mA and that sets a limit on maximumallowable losses. Routinely, positive potential at the collec-tor is 1.5-2 times less than the anode potential and that isenough to keep the losses low.-0.20 -0.10 0.00 0.10 0.20Vertical displacement in main solenoid (cm)05101520253035Current in profile analyzer, uAFigure 4: Vertical beam scan: analyzer current vs beam de-flection in main solenoid.The backplate of the collector has a tiny hole 0.2 mm indiameter and an additional Faraday cup behind it to measurethe current which goes through the hole. This current is ameasure of the electron beam current density. Using dipolecorrectors we are able to move the electron beam across thehole and measure the current profile like one presented inFig.4. Usually a magnetic field Bcoll of a few hundredsGauss helps to increase the current density (as the adiabaticinvariant in the magnetic field is Ba2 = const, a is electronbeam size) and, thus, the current coming through the hole,to easily detectable level of dozens of A.The magnetic system of the ”electron lens” prototypeconsists of three solenoid magnets, 17 dipole correctors andindependent power supplies. Two solenoid magnets (50 cmlong and 28 cm inner diameter) produce longitudinal mag-netic fields in the gun and collector (4 kG maximum with2.7 kA of the coil current, typical values are 1 kG in the gunand 0 in the collector). The main solenoid (1.96 m long, 20cm inner diameter) provides 4 kG with 10 kA of the coil cur-rent. One of the goals of the prototype is to get a straightelectron beam in the main solenoid with a deviation lessthan 0.1 mm rms. Because low temperature electrons in astrong magnetic field just follow the magnetic field lines,we paid special attention to the magnetic field quality. Theways to keep the field distortions low are a) precise coilfabrication and winding, b) special measures to distributeevenly the return current in 8 rods of the main solenoid, c)dipole corrector coils to compensate the field imperfectionson axis of the main solenoid, d) a magnetic shield over themagnets. With these steps we have achieved the field errorsB?=Bk of about 10−4 in the main solenoid, about 10−3in the central region of the gun and collector solenoids, andabout 0.01 in the gap between the main and gun solenoidmagnets. Most of imperfections take place either near theends of the magnets, or in the middle of the main solenoid,where two of its sections are connected together (see details238Proceedings of the 1999 Particle Accelerator Conference, New York, 1999in [4]). The resultingdeviousness of the magnetic field lines(and, therefore, the electron beam) is about 0.05 mm rms ineach transverse plane, or about 0.07 mm total. The vacuumpipe, gun and collector were installed into the solenoidsonly after the measurements and correction of the magneticfield were completed.Figure 5: Current modulation in the “electron lens” proto-type with frequency 1 MHz. Upper line - current in the col-lector 0.2 A/div (0.43 A peak-to-peak), lower line - anodevoltage 1 kV/div (1.5 kV peak-to-peak).A linear ”electron lens” will be used to compensatebunch-to-bunch tune spread in the Tevatron antiprotonbeam. The minimum bunch spacing in the Tevatron, thatis 396 ns in the Run II and 132 ns in the TEV33 upgradeof the collider. A straightforward way to get needed CWmodulationof the current is to vary the anode voltage Ua(t).The anode is modulated using a grid driven, 25 kW tetrode(EIMAC 4CW25000B) in series with a 5 kOhm resistorconnected between the tetrode’s plate and the 10 kV, 1.5ADC plate power supply. The anode is attached directly tothe plate of the tetrode resulting in the 70 pF anode capaci-tance being added in parallel with the 25 pF tube output ca-pacitance. The tetrode is operated in a grounded cathodeconfiguration with the grid being driven by a 150 W solid-state amplifier. A 60 Hz de-rippling circuit is connected inseries with the DC grid bias to remove any unwanted 60Hz modulation due to the AC filament supply. Fig.5 shows500 ns modulation of the anode voltage (1.5 kV peak-to-peak, 3 kV maximum)- see lower line, and the total elec-tron current ( 0.43 A peak-to-peak, 0.7 A maximum) - seeupper line. Future upgrades to the modulator include lower-ing the plate series resistance and obtaining a higher currentDC plate supply. These two upgrades will allow a greaterdegree of anode modulation at higher frequencies. A newall solid-state modulator, utilizing MOSFET technology, isalso under development [5].The “electron lens” control system utilizes Internet RackMonitors (IRMs) for all of its settings, read backs, con-trol and status. The IRM is a general purpose intelligentdata acquisition system based on the VME chassis and theMotorola MVME-162 processor. It contains the processor,network connection and the analog and digital I/O in thesame chassis. The IRM’s capabilities are expandable viatwo VME slots. As its name suggests, this device is a nodeon the world-wide Internet. Experimenters can access theIRM via any terminal with permission and the right inter-facing software (LabVIEW, FNAL’s ”Local Station”, etc.)A local database within each IRM allows it to function as astand-alone control system for the equipment it controls.High vacuum during routine operation is provided by adiode ion pump with 50 l/s rate. A turbomolecular pumpwith liquid nitrogen trap is used for rough pumping and dur-ing baking. A T-crossing located in a gap between the mainsolenoid and the gun solenoid provides a connection of themain pipe with the vacuum pumps. The entire vacuum sys-tem was assembled and baked at 250-300o C over few days.The main concern during the system commissioning wasoutgassing of the internal surfaces of the collector and thevacuum pipe due to currents of primary and secondary elec-trons. The beam current was increased step by step, keep-ing the vacuum below 10−6 Torr (in order to to reduce thecathode emission capability) and the electron current to thepipe less than 2.5 mA. Continuous increase of the producedelectron charge in the system resulted in steady vacuum im-provement. Finally, we achieved the design goal of 2 A onDecember 23, 1998. A month later, after installation of anadditional collector power supply, we got more than 3 A ofelectron current. At the present time, the vacuum withoutthe electron beam is about 2  10−9 Torr, while 1 A currentleads to pressure of (3 − 5)  10−8 Torr. The set-up oper-ation has become easy and very reliable. It takes about tenminutes (mostly to heat the cathode) to get a high currentelectron beam.Our future plans at the “electron lens” prototype includestudies of the collector efficiency vs the set-up parame-ters, electron beam profile measurements with a thin wire inmain solenoid; and installationof diagnostic tools to controlthe amount of ions and secondary electrons in the system.A new (smaller size) electron gun and collector are underconsideration.Our sincere thanks to Prof.L.Tecchio (INFN, Padova,Italy) for lending two short solenoid magnets. We aregrateful to A.Makarov, S.Nagaitsev, A.Warner, D.Wolffand V.Yarba for valuable contribution to the set-up de-sign. We acknowledge technical assistance of L.Bartelson,A.Chupira, D.Douglas and B.Ogert.3 REFERENCES[1] V.Shiltsev, et.al, “Compensationof Beam-Beam Effects in theTevatron with Electron Beams”, FNAL-Pub-98/260 (1998);see also these Proceedings.[2] A.Sharapa, et.al, NIM A, 406 (1998), p.169.[3] J.L.Cronin, Microwave Journal, Sept. 1979, p. 57.[4] C.Crawford, et.al, these Proceedings.[5] F.Niell, ”Electron Gun Beam Extraction with MOSFETS ”,this Proceedings.239Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

Prototype 'electron lens' set-up for the Tevatron beam-beam compensation

Prototype 'electron lens' set-up for the Tevatron beam-beam compensation

Abstract

Similar works

Full text

Available Versions

CERN Document Server