Production of high bunch charge beams for the ElectronIon Collider (EIC) is a challenging task. High bunch charge (a few nC) electron beam studies at Jefferson Lab using an inverted insulator DC high voltage photo-gun showed evidence of space charge limitations starting at 0.3 nC, limiting the maximum delivered bunch charge to 0.7 nC for beam at -225 kV, 75 ps (FWHM) pulse width, and 1.64 mm (rms) laser spot size. The low extracted charge is due to the modest longitudinal electric field (Ez) at the photocathode leading to beam loss at the anode and downstream beam pipe. To reach the few nC high bunch charge goal, and to correct the beam deflection exerted by the non-symmetric nature of the inverted insulator photo-gun the existing photo-gun was modified. This contribution discusses the electrostatic design of the modified photo-gun obtained using CST Studio Suite’s electromagnetic field solver. Beam dynamics simulations performed using General Particle Tracer (GPT) with the resulting electrostatic field map obtained from the modified electrodes confirmed the validity of the new design

Benesch, J.

Delayen, Jean R.

Hernandez-Garcia, C.

Krafft, Geoffrey A.

Mamun, M.A.

Palacios-Serrano, Gabriel

Poelker, M.

Suleiman, R.

Wijethunga, S.A.K.

English

Old Dominion University

Old Dominion University ODU Digital Commons Physics Faculty Publications Physics 2021 Redesign of the Jefferson Lab -300 kV DC Photo-Gun for High Bunch Charge Operations S.A.K. Wijethunga Old Dominion University J. Benesch Jean R. Delayen Old Dominion University, jdelayen@odu.edu C. Hernandez-Garcia Geoffrey A. Krafft Old Dominion University, gkrafft@odu.edu See next page for additional authors Follow this and additional works at: https://digitalcommons.odu.edu/physics_fac_pubs  Part of the Engineering Physics Commons Original Publication Citation Wijethunga, S. A. K., Benesch, J., Delayen, J. R., Hernandez-Garcia, C., Krafft, G. A., Palacios-Serrano, G., Mamun, M. A., Poelker, M., & Suleiman, R. (2021) Redesign of the Jefferson Lab -300 kV DC photo-gun for high bunch charge operations. In L. Liu, J. Byrd, R. Neuenschwander, V.R.W. Schaa (Eds.), Proceedings of the 12th International Particle Accelerator Conference (pp. 2802-2805). JACoW. https://doi.org/10.18429/JACoW-IPAC2021-WEPAB092 This Conference Paper is brought to you for free and open access by the Physics at ODU Digital Commons. It has been accepted for inclusion in Physics Faculty Publications by an authorized administrator of ODU Digital Commons. For more information, please contact digitalcommons@odu.edu. Authors S.A.K. Wijethunga, J. Benesch, Jean R. Delayen, C. Hernandez-Garcia, Geoffrey A. Krafft, Gabriel Palacios-Serrano, M.A. Mamun, M. Poelker, and R. Suleiman This conference paper is available at ODU Digital Commons: https://digitalcommons.odu.edu/physics_fac_pubs/725 REDESIGN OF THE JEFFERSON LAB -300 kV DC PHOTO-GUN FOR HIGH BUNCH CHARGE OPERATIONS* S. A. K. Wijethunga†, J. R. Delayen, G. A. Krafft1, G. Palacios-Serrano Old Dominion University, Norfolk, Virginia, 23529, USA  M. A. Mamun, C. Hernandez-Garcia, R. Suleiman, M. Poelker, J. Benesch Thomas Jefferson National Accelerator Facility, Newport News, Virginia, 23606, USA 1also at Thomas Jefferson National Accelerator Facility, Newport News, Virginia, 23606, USAAbstract Production of high bunch charge beams for the Electron-Ion Collider (EIC) is a challenging task. High bunch charge (a few nC) electron beam  studies at Jefferson Lab using an inverted insulator DC high voltage photo-gun showed evi-dence of space charge limitations starting at 0.3 nC, limit-ing the maximum delivered bunch charge to 0.7 nC for beam at -225 kV, 75 ps (FWHM) pulse width, and 1.64 mm (rms) laser spot size. The low extracted charge is due to the modest longitudinal electric field (Ez) at the photocathode leading to beam loss at the anode and downstream beam pipe. To reach the few nC high bunch charge goal, and to correct the beam deflection exerted by the non-symmetric nature of the inverted insulator photo-gun the existing photo-gun was modified. This contribution discusses the electrostatic design of the modified photo-gun obtained us-ing CST Studio Suite’s electromagnetic field solver. Beam dynamics simulations performed using General Particle Tracer (GPT) with the resulting electrostatic field map ob-tained from the modified electrodes confirmed the validity of the new design. INTRODUCTION Delivering high bunch charge beams for the EIC cooler to meet the ion beam high luminosity specification has been a challenging task. At Jefferson Lab, we studied space charge limitations for generating electron beams using a DC high voltage photo-gun with inverted insulator geom-etry, biased at -225 kV, with a bialkali-antimonide photo-cathode and a commercial laser [1]. At -225 kV, the ex-tracted charge at the dump was only ~ 0.3 nC. Increasing the laser power yielded ~ 0.7 nC but with significant vac-uum activity in the beamline stemming from beam loss [1].  DC high voltage photo-guns have been employed in a variety of accelerator facilities to produce both polarized and non-polarized beam such as the Continuous Electron Beam Accelerator Facility (CEBAF), Free Electron Lasers (FELs), Energy Recovery Linacs (ERLs), etc. The majority of these photo-guns have a Pierce geometry at the cathode front to focus the beam [2-5]. In addition, inverted insula-tor geometry photo-guns like the JLab design serve to con-nect the high voltage cable to the cathode electrode, and have a shield to minimize the electric field at the insulator-metal-vacuum interface known as the triple point junc-tion [3]. However, the Pierce geometry reduces the Ez at the cathode, thus increasing space charge effects and re-ducing bunch charge extraction. Additionally, the inverted insulator and triple point junction shield combine to intro-duce asymmetric electric fields in the anode-cathode gap which then result in deflecting the beam vertically at the exit of the anode, causing difficulty in beam steering, and ultimately beam losses [2-5]. This paper presents the elec-trostatic design of the modified photo-gun resolving the above design issues using CST Studio Suite’s electromag-netic field solver to obtain a higher Ez at the cathode while keeping the beam on-axis in comparison with the original photo-gun [6]. Further, beam simulations conducted using GPT software [7], implementing the electrostatic field map obtained from the modified electrodes will also be pre-sented. ORIGINAL PHOTO-GUN DESIGN Figure 1 shows the 3D CST model of the original photo-gun design which mainly includes a 15 cm diameter spher-ical cathode electrode with a 25⁰ Pierce focusing geometry. The angle terminates on a 1.2 cm diameter aperture expos-ing the flat photocathode. The electrically isolated anode with slightly curved front face to match to the Pierce ge-ometry is located 9 cm downstream of the cathode front. The inverted ceramic insulator, triple point junction shield, and array of eight non-evaporable getter (NEG) pump modules complete the photo-gun assembly [2].   Figure 1: Left: CST model of the original gun. Right: Pho-tograph of the cathode electrode assembly mounted to the inverted insulator. The spherical photocathode is 15 cm in diameter.  ___________________________________________  * Authored by Jefferson Science Associates, LLC under U.S. DOE Con-tract No. DE-AC05-06OR23177. Additional support comes from JSA in-itiatives fund program and Laboratory Directed Research and Develop-ment program. † wwije001@odu.edu 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-WEPAB092WEPAB092ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI2802MC2: Photon Sources and Electron AcceleratorsT02 Electron SourcesHV cable HV cable rubba Ceramic insulator l""ltIIlr 'r--ill-- J.:~;0~=:c1d "9'illl~ "-- -,rlff-- Triple poUltjunction Anode NEG modules This gun operated reliably at -300 kV high voltage and 10−12 Torr vacuum without field emission or high voltage breakdown.  Due to the Pierce geometry the Ez at the photocathode was 2.5 MV/m at -350 kV, which is relatively small for extracting nC bunches. Additionally, the asymmetric na-ture of the inverted photo-gun leads to significant beam de-flection; Fig. 2 illustrates the beam trajectory obtained us-ing GPT simulations. The left figure shows a slight beam deflection (~3 mm at 1 m) in x due to the asymmetry in placing NEG pumps at the bottom of the vacuum chamber. The right figure shows a significantly larger beam deflec-tion (~3.3 cm at 1 m) due to the inverted insulator and triple point junction shield. Standard steering magnets placed just at the exit of the gun chamber can correct these deflec-tions and minimize the beam loss.  Figure 2: Beam trajectory (Left) x and (Right) y. MODIFIED PHOTO-GUN DESIGN The Ez at the photocathode depends on the anode-cath-ode gap and on the Pierce geometry. The anode-cathode gap is a trade-off between achieving sufficiently higher Ez for charge extraction but not too high to minimize the risk of field emission. Fig. 3 shows Ez variations with the an-ode-cathode gap from 4 cm to 9 cm without the Pierce ge-ometry (flat cathode front and flat anode front). The cath-ode is placed at 0 m with applied high voltage -350 kV. According to previous experience, the field emission can be negligible (after high voltage conditioning) when the gradient is maintained below ~10 MV/m [3]. Hence, though the 4 cm gap gives the highest Ez at the cathode, since it approaches -10 MV/m we settled on the 5 cm gap with no Pierce geometry. It provides -7.8 MV/m Ez at the cathode.  Figure 3: Ez vs z for different cathode-anode gaps. Next, we focus on the beam deflection. The asymmetry in the x direction can be minimized by replacing the large NEG modules with NEG sheets which also helps to reduce the electric field at the bottom of the cathode electrode. The significantly large beam deflection in y cannot be removed without risking field emission and high voltage break-down, e.g, if we decide to remove the triple point junction shield. Instead after a thorough examination of the design we found out that it can be cancelled by lowering the anode aperture by a few mm. Fig. 4 illustrates the beam deflection variation with anode aperture shift.    Figure 4: GPT simulations of beam deflection in y for dif-ferent anode aperture shifts. Beam deflection in y is very sensitive to the anode aper-ture shift. According to our simulation results the beam de-flection can be completely cancelled by shifting the anode aperture 1.6 mm in the negative y direction.  Figure 5 shows the electrostatic field distribution inside the modified photo-gun at -350 kV biased cathode. The new design maintains the -10 MV/m electric field limit everywhere inside the chamber.   Figure 5: CST simulation results showing the electrostatic field distribution of the modified gun biased at -350 kV. The horizontal (left) and vertical (right) electric field pro-files in the anode-cathode gap of the modified photo-gun design are illustrated in Fig. 6. The color map (a) shows the Ex distribution and the (b) graph shows the magnitude of Ex along each of the colored dotted lines shown on the field map. The (c) and (d) panels show the corresponding results for Ey. In both cases the vertical position of each dotted line is indicated with respect to the beam axis. According to (b) Ex is symmetric along the axis but (d) shows that Ey is dis-torted first due to the inverted insulator and triple point junction screening electrode and then due to the shifted an-ode aperture. 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-WEPAB092MC2: Photon Sources and Electron AcceleratorsT02 Electron SourcesWEPAB0922803ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.§.o.oo -O.oi~., - ,-.,- ,.-, - ,-.,- ,.-, - ,-,- ,.,- ,-.,- ,.,- ,-,- ,.,~ i[ml 0.0 -1.0 -2.0 -3.0 I -4.0 -5.0 '" -6.0 -7.0 -8.0 -9.0 -10.0 z [m] 3.0 e 2.0 ~ ,:: .9 u 1.0 e 'o >, ·= 0.0 ,:: .9 -~ - 1.0 0. E Jl -2 .0 -3.0 0.1 0. 2 - 4cm - S cm - 6 cm - 7 cm - 8cm - 9 cm - +2 mm - + I mm - 0mm - - Imm - - 1.6mm - -2 mm 0.0 0.2 0.4 0.6 0.8 1.0 z [m]  Figure 6: (a) Ex, (c) Ey variations along the dotted colored lines between the anode cathode gap when biased at -350 kV where each color line position is indicated with respect to the x axis (b) and y axis (d). Figure 7 illustrates the GPT simulations of the beam tra-jectory (left) in x and (right) in y. With the shifted anode aperture, the beam is centered in both directions.   Figure 7: Beam trajectory (left) x and (right) y. This beam deflection can also be corrected by tilting the anode and it will be discussed in comparison with the an-ode aperture shift in a journal publication.  Figure 8 provides charge extraction simulation results using GPT with the original and modified photo-gun elec-tric field maps. Hence, the extracted charge at the cathode doubled with the increased Ez [1].  Figure 8: GPT simulations of the extracted charge at the cathode vs initial bunch charge from original and modified gun designs. (300 kV, 50 kHz, 75ps (FWHM), 1.64 mm (rms)). CONCLUSION Jefferson Lab’s -300 kV photo-gun was re-designed us-ing CST Studio Suite’s electromagnetic field solver to ob-tain a higher Ez at the photocathode for high bunch charge operations and to correct beam trajectory deflections inher-ent to the inverted insulator geometry design. The longitu-dinal electric field magnitude Ez was increased from  -2.5 MV/m to -7.8 MV/m by removing the Pierce geometry and decreasing the anode-cathode gap from 9 cm to 5 cm. The realization of the anode aperture shift effect on beam trajectory represents a significant advancement for photo-gun designs with inverted insulator geometry. A 1.6 mm vertical downward shift of the anode aperture is a simple design change to eliminate beam deflection. Our simula-tion results show that extracted bunch charge from the pho-tocathode should increase from 4.6 nC to 10.2 nC before accounting for beam loss. Modified parts have already been built, polished and are ready for assembly and com-missioning. ACKNOWLEDGEMENT This work is supported by the Department of Energy, under contract DE-AC05-06OR23177, JSA initiatives fund program, and the Laboratory Directed Research and Development program.  REFERENCES [1] S. A. K. Wijethunga et al., “Space charge effect in low energy magnetized electron beam”, in Proc. 12th Int. Particle Accel-erator Conf. (IPAC’21), Campinas, Brazil, May 2021, paper WEPAB093, this conference.  [2] P. Adderley et al., “Load-locked dc high voltage gas photo-gun with an inverted-geometry ceramic insulator”, Phy. Rev. ST Accel.Beams, vol. 13, p. 10101, 2010. doi:10.1103/PhysRevSTAB.13.010101 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-WEPAB092WEPAB092ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI2804MC2: Photon Sources and Electron AcceleratorsT02 Electron Sources12.0 u .s i10.o 0 .c <O ~ 8.0 = E 6.0 g ., e> "' 4.0 .c (.) 'O * 2.0 l!! )( w ur_o.3 -0.5 -0.7 • Original gun design .02 x = 2mm - x = I mm - x = 0mm - x = - 1 mm - x = -2mm • Modified gun design Bunch Charge [nCI z [m] V/m l e+7 9e+6 .... 7e+6 6e+6 Se+6 ie+6 3e+6 2e+6 le+6 EO.l > 0. 8 6-0.1 ui""_o .3 -0.5 -0.7 0.02 - y = 2mm - y = I mm - y = 0mm - y = -1 mm - = -2 mm 0. 8 z[m] [3] C. Hernandez-Garcia et al., “Compact 300 kV DC inverted insulator photo-gun with biased anode and alkali-antimonide photocathode”, Phy. Rev. ST Accel.Beams, vol. 22, p. 113401, 2019. doi:10.1103/PhysRevAccelBeams.22.113401 [4] G. Palacios-Serrano et al., “Electrostatic design and condi-tioning of a triple point junction shield for 200 kV DC high voltage photo-gun”, Review of scientific instruments, vol. 89, p. 104703, 2018. doi:10.1063/1.5048700 [5] N. Nishimori et al., “Experimental investigation of an opti-mum configuration for a high-voltage photoemission gun for operations at > 500 kV”, Phy. Rev. ST Accel.Beams, vol. 17, p. 053401, 2014. doi:10.1103/PhysRevSTAB.17.053401 [6] CST Studio Suite’s, http://www.cst.com  [7] General Particle Tracer, http://www.pulsar.nl/gpt   12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-WEPAB092MC2: Photon Sources and Electron AcceleratorsT02 Electron SourcesWEPAB0922805ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI-

Redesign of the Jefferson Lab -300 kV DC Photo-Gun for High Bunch Charge Operations

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Redesign of the Jefferson Lab -300 kV DC Photo-Gun for High Bunch Charge Operations

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