The Argonne Wakefield Accelerator has been successfully commissioned and used for conducting wakefield experiments in dielectric loaded structures and plasmas. Although the initial wakefield experiments were successful, higher drive beam quality would substantially improve the wakefield accelerating gradients. In this paper we present a new 1-1/2 cell L-band photocathode RF gun design. This gun will produce 10-100 nC beam with 2-5 ps rms pulse length and normalized emittance less than 100 mm mrad. The final gun design and numerical simulations of the beam dynamics are presented

Conde, M E

Gai, W

Konecny, R

Power, J G

Schoessow, P

CERN Document Server

DESIGN AND CONSTRUCTION OF A HIGH CHARGE AND HIGHCURRENT 1 – 1/2 CELL L-BAND RF PHOTOCATHODE GUN∗M.E. Conde+, W. Gai, R. Konecny, J.G. Power and P. SchoessowArgonne National Laboratory, Argonne, ILAbstractThe Argonne Wakefield Accelerator has been successfullycommissioned and used for conducting wakefieldexperiments in dielectric loaded structures and plasmas.Although the initial wakefield experiments weresuccessful, higher drive beam quality would substantiallyimprove the wakefield accelerating gradients. In this paperwe present a new 1-1/2 cell L-band photocathode RF gundesign. This gun will produce 10 - 100 nC beam with 2– 5ps rms pulse length and normalized emittance less than100 mm mrad. The final gun design and numericalsimulations of the beam dynamics are presented.1  INTRODUCTION High current short electron beams have been a subject ofintensive studies [1].  One of the particular uses for thistype of beam is in wakefield acceleration applications.High current (kA) short electron beam generation andacceleration did not materialize until the advent of RFphotoinjector technology[2]. Although most photocathodeRF gun development has been concentrated on highbrightness, low charge applications such as free electronlaser injectors, there have been several relatively highcharge rf photocathode based electron sources built andoperated[3,4,5].  In general, there are two approaches toattaining high peak current.  One approach is to generatean initially long electron bunch  with a linear head-tailenergy variation that is subsequently compressed usingmagnetic pulse compression.  The advantage of magneticcompression is that it is a well-known technology and canproduce sub-picosecond bunch lengths.  However, due tostrong longitudinal space charge effects, this technology islimited to relatively low charges (<10 nC). Another approach is to directly generate short intenseelectron bunches at the photocathode and then acceleratethem to relativistic energies rapidly using high axialelectric fields in the gun [3].  The advantage of thisapproach is that it can deliver very high charges, forexample, 100 nC if one uses an L-band gun.  This wouldsatisfy the requirements of most electron driven wakefieldexperiments for both plasma and dielectric structures, ifthe pulse length is short enough (< 10 ps FWHM).  So far, ∗ This work  is supported by the Department of Energy, High EnergyPhysics Division, Advanced Technology Branch under the ContractNo. W-31-109-ENG-38.+ Email: conde@hep.anl.gov the Argonne Wakefield Accelerator (AWA) hasdemonstrated the capability of producing 100 nC, 25 35 ps (FWHM) electron beams at 14 MeV.  Thisunprecedented performance was obtained using a half cellphotocathode gun cavity and two standing wave iris-loaded linac sections [6].  The AWA machine has reachedits design goal and has been used for dielectric wakefield[7] and plasma [8] experiments. The initial results areencouraging [9]. Achieving higher gradients in wakefieldexperiments would require the drive electron pulse to beeven shorter and have a lower emittance. In this paper, wediscuss the design of a new RF photocathode gun with thecapability of producing 10 - 100 nC with 2 - 5 ps (rms)pulse lengths.2  DESIGN CONSIDERATIONSIn order to generate high charge and short bunchlengths from a photocathode RF gun, the electric field onthe cathode surface has to be very intense. In this way theelectrons leaving the cathode surface are quicklyaccelerated to relativistic velocities, minimizing the bunchlengthening and the emittance growth that the spacecharge forces produce [10,11]. There is also bunchlengthening and transverse emittance growth at the exitiris of the gun cavity due to the defocusing forces of theRF fields. Thus, this effect also calls for high acceleratinggradient and high beam energy at the exit of the gun. It istherefore desirable to have a multicell gun with highaccelerating gradient. Practical considerations (mainly afinite amount of RF power) limit the design to 1 − 1/2cells. The choice for our new gun design is a Brookhaventype 1− 1/2 cell cavity [12]  scaled up to L band operation.This gun will be followed by one of the present linac tanksthat exist at the AWA facility.A detailed numerical study [13, 14] of this gun wasperformed with the computer codes SUPERFISH andPARMELA [15]. Table 1 summarises the parameters usedin the simulations. These extensive numerical simulationsshowed a strong dependence of bunch length andemittance with respect to the accelerating gradient in thegun cavity. Based on these studies, it was decided that anaccelerating gradient of 80 MV/m on the cathode surfacewas a good operating point. This requires 10 MW of RFpower to be coupled into the gun cavity, which still leavesenough power to run one of the linac tanks. Thisaccelerating gradient yields good values of emittance andbunch length, while still not high enough to make the RFconditioning of the gun a challenging task. (In fact, we0-7803-5573-3/99/$10.00@1999 IEEE. 2030Proceedings of the 1999 Particle Accelerator Conference, New York, 1999recently conditioned a duplicate of the present AWA gunup to a gradient of 125 MV/m [16].)TABLE 1. The gun design parameters as calculated usingSUPERFISH Inner Radius of the Cell, b (cm) 9.03 Radius of the iris, a (cm) 2.75 Width of the iris, d (cm) 1.5 Aperture of the exit (cm) 2.5 Operating frequency (GHz) 1.3 Initial beam radius (cm) 1 Quality factor, Q 26008 Shunt impedance (MΩ/m) 36.47 3  REALIZABLE GUN DESIGNIn order to reach a design that can actually be built, onehas to take into account all the other constraints besidesthe optimization of the electron beam parameters. Theremust be enough space around the gun to allow for awaveguide to couple RF power into the cavity. Space mustalso be available for tuning plungers, RF pickup probes,vacuum pumping and cooling channels. All these spacerequirements limit the size and constrain the location ofthe solenoids. Certainly one must also be careful not todrive the solenoid steel into saturation.Figure 1 shows a drawing of the complete gun andsolenoids design. Two solenoids are exactly next to eachother, with the photocathode plane as their plane ofsymmetry. This maximizes the space available for the RFpower coupler over the full cell of the gun. The tuningplunger in the full cell is located diametrically opposite tothe RF coupler, both being at the equator line of the fullcell. An RF pickup probe is located half way along thecircumference of the full cell between the RF coupler andthe tuning plunger. The half cell has both the tuningplunger and the RF pickup probe on the back plate of thecell. This breaks the symmetry of the half cell, but it isacceptable in our L-band size cavity. The perturbation ofthe field lines over the relatively small size cathode area isnegligible. The tuning plungers and RF pickup probes willallow us to verify and, if necessary, to adjust theparameters of the two cells, in order to achieve the rightresonance frequency for the pi mode and field balance inthe cavity. The cooling channels are drilled along thecylindrical wall of the gun, and also run over part of theback and front plates of the cavity.Numerical simulations of this final design yield valuesfor the emittance and bunch length that are slightly worsethan the ones obtained in reference [13, 14]. This resultsfrom the fact that the location and dimensions of thesolenoids are not dictated only by the optimization of thebeam parameters, but also by other physical constraints.Figure 1:  Photocathode RF gun2031Proceedings of the 1999 Particle Accelerator Conference, New York, 1999The degradation is however very small, and the gun is stillexpected to generate very short bunches with lowemittance. Results of numerical simulations withPARMELA are shown in Fig. 2. These plots showemittance, bunch length, energy and radial coordinate  as afunction of the longitudinal coordinate along theaccelerator for a bunch charge of 40 nC. At the exit of thelinac the code predicts a normalized rms emittance of 66mm mrad and an rms bunch length of 3.7 ps. 4  DISCUSSION AND SUMMARYWe have reached a final design for the new AWAphotocathode RF gun. This gun will dramatically improvethe capabilities of our program to study wakefieldacceleration in dielectric loaded structures and plasmas.The electron beam produced by this gun is expected toexcite wakefields in plasmas with accelerating gradients inexcess of 1 GeV/m with a plasma density of ~1014 /cm3.In dielectric loaded structures, this beam will also makea significant improvement over present attainablegradients.  One can use this beam to directly demonstratecollinear wakefield acceleration gradients in excess of 50MV/m corresponding to 200 MW of RF power generatedin 30 GHz dielectric structures.It is worth pointing out that the present AWAphotocathode RF gun has achieved unprecedented valuesof charge per bunch, and has allowed us to advance theunderstanding of wakefield acceleration in plasmas and indielectric structures. However, the present gun wasdesigned when only a very limited amount of RF powerwas available for the experiment (2 MW). Thus, the beamparameters, namely, bunch length and emittance, sufferedserious limitations due to this relatively low level of RFpower. The newly designed gun will take advantage of thehigher level of RF power now available in the facility,yielding better beam parameters and, consequently, higheraccelerating gradients in the wakefield accelerationexperiments. 5  REFERENCES1. C. Travier, Proceedings of Advanced Acceleration ConceptsWorkshop, Edited by P. Schoessow, AIP Proceedings, No. 335,p.57, 1994.2. J. Fraser et al., IEEE Trans. Nucl. Sci., NS-32, p.1719 (1985).3. P. Schoessow et al., Proceedings of Particle AcceleratorConference, p.976, 1995.4. B. Carlsten, et al., Proceedings of Particle AcceleratorConference, p.985, 1995.5. E. Colby et al., Proceedings of Particle Accelerator Conference,p.967, 1995.6. M.E. Conde et al., Proceedings of Particle AcceleratorConference, p.1996, 1997.7. P. Schoessow et al., Proceedings of Particle AcceleratorConference, p.639, 1997.8. N. Barov, M.E. Conde, W. Gai, J. Rosenzweig, Phys. Rev. Lett.,Vol. 80, No. 1, p.81, 1998.9. P. Schoessow et al., Journal of Applied Physics, Vol. 84, No. 2,p.663, 1998.10. K.J. Kim, Nuclear Instrumentation and Methods, A275, p.201,1989.11. L.Serafini, J. Rosenzweig, Physical Review E, Vol. 55, No. 6,p.7565, 1997.12. K. Batchelor et al., Proceedings of European Particle AcceleratorConference,  p.541, 1990.13. W. Gai et al., Nucl. Instr. and Meth. A 410, p.431, 1998.14. W. Gai et al., Proceedings of Advanced Acceleration ConceptsWorkshop, Baltimore, 1998.15. SUPERFISH and PARMELA, Las Alamos National Lab. ReportLA-UR-96-1834, 1997 and LA-UR-96-1835, 1996.16. C.H. Ho et al., Proceedings of Linac98 Conference, Chicago,1998.z (cm)0 50 100 150 200r (mm)010203040energy (MeV)05101520z (cm)0 50 100 150 200rms bunch length (ps)01234rms normalized emittance (mm mrad)020406080100bunch lengthemittanceFigure 2: Numerical simulations of the beam along the gunand linac structures: (a) energy and trajectories in thetransverse plane; (b) bunch length and emittance.2032Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

Design and construction of a high charge and high current 1 - 1/2 cell L-band RF photocathode gun

Conde, M. E.

Gai, W.

Konecny, R.

Power, J. G.

Schoessow, P.

UNT Digital Library

Design and construction of a high charge and high current 1-1/2 cell L-Band RF photocathode gun.

http://digital.library.unt.edu/ark:/67531/metadc624483/

Design and construction of a high charge and high current 1 - 1/2 cell L-band RF photocathode gun

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