- Email: [email protected]
RF Photoelectric injectors using needle cathodes
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 507 (2003) 323–326
RF Photoelectric injectors using needle cathodes J.W. Lewellena,*,1, C.A. Braub a
Advanced Photon Source, Argonne National Laboratory, 401/B2207, 9700 S. Cass Avenue, Argonne, IL 60439, USA b Department of Physics, Vanderbilt University, Box 1807, Station B, Nashville, TN 37235, USA
Abstract Photocathode RF guns, in various configurations, are the injectors of choice for both current and future applications requiring high-brightness electron beams. Many of these applications, such as single-pass free-electron lasers, require beams with high brilliance but not necessarily high charge per bunch. Field-enhanced photoelectric emission has demonstrated electron-beam current density as high as 1010 A/m2, with a quantum efficiency in the UV that approaches 10% at fields on the order of 1010 V/m. Thus, the use of even a blunt needle holds promise for increasing cathode quantum efficiency without sacrificing robustness. We present an initial study on the use of needle cathodes in photoinjectors to enhance beam brightness while reducing beam charge. Benefits include lower drive-laser power requirements, easier multibunch operation, lower emittance, and lower beam degradation due to charge-dependent effects in the postinjector accelerator. These benefits result from a combination of a smaller cathode emission area, greatly enhanced RF field strength at the cathode, and the charge scaling of detrimental postinjector linac effects, e.g., transverse wakefields and CSR. r 2003 Elsevier Science B.V. All rights reserved. PACS: 41.75.Fr; 41.75.Ht Keywords: RF photoinjector; Electron; Source
1. Introduction The electron beams generated by most electron sources can be characterized in a number of different fashions. These include total charge, bunch length or duration, energy spread, spot size and beam divergence. Transverse and longitudinal properties are often grouped and cross-correlated to give derived properties such as transverse and *Corresponding author. Tel.: +1-630-252-5252; fax: +1630-252-4732. E-mail address: [email protected] (J.W. Lewellen). 1 Supported by the US Department of Energy, Office of Basic Energy Sciences, under Contract No. W-31-109-ENG-38.
longitudinal emittances, and peak currents. More generally, the beam properties are often grouped and correlated according to the requirements for a particular application, yielding a single figure-ofmerit for the electron beam, either at the source or at the point of use. One common figure-of-merit is the normalized beam brightness, which for a Gaussian beam can be expressed as [1]: I Bn ¼ ð1Þ 2 2 2 4p b g ex ey where I is the peak beam current, ex;y are the (unnormalized) transverse rms emittances, b is the normalized velocity, and g is the Lorentz factor [2].
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)00938-0
ARTICLE IN PRESS 324
J.W. Lewellen, C.A. Brau / Nuclear Instruments and Methods in Physics Research A 507 (2003) 323–326
For a single-pass free-electron laser, a common figure-of-merit is the Pierce parameter r; given by 1=3 1=3 I I r¼ a 2 p ð2Þ sx en where sx is the transverse spot size, en is the average normalized transverse beam emittance, and a is a constant [3]. For both of these figures-ofmerit, larger is better. Various beam production and manipulation techniques can be evaluated rapidly by using such definitions. In general, the beam brightness can be improved by increasing the bunch charge, decreasing the bunch length, or reducing the transverse emittance. In some cases, reducing the bunch length can increase the beam emittance (e.g., CSRinduced emittance growth in a bunch compression chicane [4]). In many beam sources, increasing the charge can only be done at the expense of increasing the emittance also. Thus, the various tradeoffs must be carefully evaluated. The beam source of choice for many current and future applications of high-brightness beams is the RF photoinjector electron gun, or RF gun. These devices use strong electric field gradients (typically 50–150 MV/m) to rapidly accelerate short slugs of charge (typically 1–10 of the relevant RF phase) in order to preserve the transverse beam properties. Typical charges per bunch range from about 0.1 to 5 nC, depending on the RF frequency, gradient, cathode material, and application. While RF guns are starting to achieve the very good brightness numbers predicted by simulation and theory, there are many charge-dependent effects in the postinjector portion of the accelerator that can act to reduce the beam brightness, or to increase the difficulty of preserving the beam brightness through further acceleration. These include transverse wakefields, longitudinal wakefields, coherent synchrotron radiation in bunch compressors, etc. Secondary considerations, not directly related to the beam brightness but of definite concern for machine design, include the average beam power, average beam halo power, the physical aperture vs. beam size ratios, and the ability of diagnostics to detect and characterize the beam. These properties can have significant impacts upon the performance
and safety of an operational machine, and must be taken into account when new machines are designed. For many applications, such as single-pass freeelectron lasers, the beam brightness at the point of use is critical but the bunch charge is not. For such applications, it is reasonable to consider variations on the traditional RF gun design that are designed to optimize this specific tradeoff.
2. Needle cathodes For this study, we consider the use of a blunt needle cathode in a SLAC/BNL/UCLA-style 1.6cell p-mode S-band photoinjector. 2.1. Field enhancement The enhancement of the electric field at the tip of spheroid, embedded in an otherwise uniform electric field, is given by EEE0
2a b2 ; R¼ R lnð4a=RÞ a
ð3Þ
where E0 is the unperturbed field strength, a is semimajor radius of the needle, and b is the semiminor radius of the needle, the semimajor axis is taken to be parallel with the unperturbed field [5]. For our initial studies we used a needle design with a flat-top tip, with a radius of 200 mm, and chamfered out at a 45 angle to a needle stalk radius of 300 mm. This provides a field enhancement of about a factor of three above the bulk cavity gradient in the absence of the needle. A 50mm emission radius is assumed; this keeps the edge of the electron beam sufficiently far from the needle edge so as to avoid the strongest radial variations in the field. 2.2. Quantum efficiency Field-enhanced photoelectric emission has demonstrated electron-beam current density as high as 1010 A/m2, with a quantum efficiency (QE) in the UV that approaches 10% at fields on the order of 1010 V/m. The use of needle cathodes may be
ARTICLE IN PRESS J.W. Lewellen, C.A. Brau / Nuclear Instruments and Methods in Physics Research A 507 (2003) 323–326
325
seen as a path towards robust cathodes with high QE figures. 2.3. Thermal emittance reduction
2.4. Current densities The maximum current density from a cathode is limited by the space-charge force at the cathode surface; when the magnitude of the field from the beam equals the magnitude of the RF field at the cathode surface, emission stops. In a conventional photoinjector running at 120 MV/m (on-crest) gradient, the expected limit is roughly 70 kA/cm2. For the type of needle described above, the corresponding limit is 200 kA/cm2.
3. Gun design and simulation The gun design used for these simulations is based on the SLAC/BNL/UCLA 1.6-cell p-mode design [7]. The only major alteration to the gun is the addition of a needle cup-and-holder to the back plane of the gun. This geometry, shown in Fig. 1, serves several purposes. First, it places the base of the needle in a low-field region, which makes engineering design considerably simpler. Second, it provides a rounded surface to mate with the back wall of the cavity, easing arcing concerns.
Fig. 1. Cathode plate geometry for the needle-cathode gun.
4
Ez [normalized]
Recent reports from conventional photoinjector experiments list total beam emittances approaching the simulation values; while this represents long-awaited developments in the field, it also means that thermal emittance now represents much of the total emittance in high-brightness beams from conventional photoinjectors. Unless a means of reducing the thermal emittance is found, the normalized emittance limit for a conventional photoinjector with Cu or Mg cathode appears to be approximately 0.5 mm for a 1 mm radius laser spot on the cathode. The beam thermal emittance scales as the radius of the laser spot on the cathode [6]. With an emission spot radius of 50 mm, the thermal 1 emittance should be approximately 20 of that of a photoinjector with a 1-mm cathode spot radius.
2
0 1 .10
3
0.01
0.1 z [cm]
1
10
Standard Cathode Needle Cathode
Fig. 2. On-axis field profiles for standard vs. needle-cathode pmode photoinjector cavities. The horizontal axis is plotted on a log scale to facilitate showing the fields in the cathode region.
Finally, it places the tip of the needle level with the original back wall of the cavity, which helps maintain the whole-beam longitudinal dynamics of this gun design. These changes lower the shunt impedance of the gun by about 30%, and require some alteration to the outer radii of the cathode and full cells to maintain both the resonant frequency at 2.856 GHz and to maintain what would be a 1:1 field balance between the cells, were the needle not present. Fig. 2 shows the on-axis fields. While it is possible that additional modifications to the back plane of the cathode cell would allow a needle cathode to be used with an otherwise ‘‘stock’’ p-mode gun, we have not yet explored such an option.
ARTICLE IN PRESS 326
J.W. Lewellen, C.A. Brau / Nuclear Instruments and Methods in Physics Research A 507 (2003) 323–326
The RF cavity fields were generated using the SUPERFISH group of codes; PARMELA v3.11 was used for the electron beam dynamics calculations. Due to the rapidly changing electric field in the vicinity of the needle tip, the gun was broken into three logical ‘‘cavities.’’ The first cavity models the region around the cathode tip proper, the second cavity extends to the entrance of the full cell, and the third cavity models the remainder of the gun. The gun is followed by an emittance compensation solenoid, an approximately 1-mlong drift, and a single SLAC-style 3-m TWCG linac section. Beam properties are calculated two meters after the end of the capture linac section. The charge per bunch was kept fixed at 20 pC. The beam distribution was taken to be uniform in both transverse and longitudinal dimensions. The bunch length, launch phase, solenoid current, and linac phase were varied in order to optimize the electron beam brightness following the capture linac section.
Table 1 Beam parameters for needle-cathode vs. conventional cathode gun Parameter
Conventional gun
Needlecathode gun
Charge Bunch duration Cathode radius Cathode gradient Norm. emittance
1 nC 10 ps 1 mm 120 MV/m 1 mm
20 pC 4 ps 50 mm 350 MV/m 0.11 mm
Est. thermal emittance Cathode current density Normalized RMS brightness Relative pierce parameter
0.6 mm 3.2 kA/cm2 20 A/mm2
0.03 mm 200 kA/cm2 83 A/mm2
1
0.77
cathode tips for greater field enhancement; and extending the optimization loop to include cavity geometry. Experimentally, we hope to be able to test a variety of needle cathodes within 1–2 years.
4. Results and discussion References The optimized beam parameters are shown in Table 1, along with nominal beam parameters from a standard p-mode gun for comparison, and some other parameters of interest. The results to date are very encouraging and compare quite favorably to a conventional gun, especially when it is noted that the needle-cathode gun operates quite well at the expected spacecharge limited current density of 200 kA/cm2. The results are achieved at relatively low absolute charge levels; combined with the smaller spot size, this should greatly ease the difficulties of transporting high-brightness beams through a postinjector accelerator. Theoretical and simulation plans for the future include trying alternate needle and cavity geometries, finer needles in particular; using radiused
[1] C.A. Brau, ‘‘What brightness means’’, The physics and applications of high-brightness beams, Sardinia, Italy, 2002, Proceedings in press. [2] Martin Reiser, Theory and Design of Charged Particle Beams, Wiley, New York, 1994. [3] M. Xie, Design optimization for an X-ray free electron laser driven by SLAC Linac, Proceedings of the 1995 Particle Accelerator Conference, Vol. 180, 1996, Dallas, TX. [4] B.E. Carlsten, T.O. Raubenheimer, Phys. Rev. E 51 (1995) 1453. [5] C.A. Brau, Modern Problems in Classical Electrodynamics, Oxford, England: Oxford University Press, 2003. [6] W.S. Graves, et al., Measurement of thermal emittance for a copper photocathode, Proceedings of the 2001 Particle Accelerator Conference, Vol. 2227, 2001, Chicago, IL. [7] D.T. Palmer, et al., Microwave measurements of the BNL/ SLAC/UCLA 1.6 cell photocathode RF gun, Proceedings of the 1995 Particle Accelerator Conference, Vol. 979, 1996, Dallas, TX.
Nuclear Instruments and Methods in Physics Research A 507 (2003) 323–326
RF Photoelectric injectors using needle cathodes J.W. Lewellena,*,1, C.A. Braub a
Advanced Photon Source, Argonne National Laboratory, 401/B2207, 9700 S. Cass Avenue, Argonne, IL 60439, USA b Department of Physics, Vanderbilt University, Box 1807, Station B, Nashville, TN 37235, USA
Abstract Photocathode RF guns, in various configurations, are the injectors of choice for both current and future applications requiring high-brightness electron beams. Many of these applications, such as single-pass free-electron lasers, require beams with high brilliance but not necessarily high charge per bunch. Field-enhanced photoelectric emission has demonstrated electron-beam current density as high as 1010 A/m2, with a quantum efficiency in the UV that approaches 10% at fields on the order of 1010 V/m. Thus, the use of even a blunt needle holds promise for increasing cathode quantum efficiency without sacrificing robustness. We present an initial study on the use of needle cathodes in photoinjectors to enhance beam brightness while reducing beam charge. Benefits include lower drive-laser power requirements, easier multibunch operation, lower emittance, and lower beam degradation due to charge-dependent effects in the postinjector accelerator. These benefits result from a combination of a smaller cathode emission area, greatly enhanced RF field strength at the cathode, and the charge scaling of detrimental postinjector linac effects, e.g., transverse wakefields and CSR. r 2003 Elsevier Science B.V. All rights reserved. PACS: 41.75.Fr; 41.75.Ht Keywords: RF photoinjector; Electron; Source
1. Introduction The electron beams generated by most electron sources can be characterized in a number of different fashions. These include total charge, bunch length or duration, energy spread, spot size and beam divergence. Transverse and longitudinal properties are often grouped and cross-correlated to give derived properties such as transverse and *Corresponding author. Tel.: +1-630-252-5252; fax: +1630-252-4732. E-mail address: [email protected] (J.W. Lewellen). 1 Supported by the US Department of Energy, Office of Basic Energy Sciences, under Contract No. W-31-109-ENG-38.
longitudinal emittances, and peak currents. More generally, the beam properties are often grouped and correlated according to the requirements for a particular application, yielding a single figure-ofmerit for the electron beam, either at the source or at the point of use. One common figure-of-merit is the normalized beam brightness, which for a Gaussian beam can be expressed as [1]: I Bn ¼ ð1Þ 2 2 2 4p b g ex ey where I is the peak beam current, ex;y are the (unnormalized) transverse rms emittances, b is the normalized velocity, and g is the Lorentz factor [2].
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)00938-0
ARTICLE IN PRESS 324
J.W. Lewellen, C.A. Brau / Nuclear Instruments and Methods in Physics Research A 507 (2003) 323–326
For a single-pass free-electron laser, a common figure-of-merit is the Pierce parameter r; given by 1=3 1=3 I I r¼ a 2 p ð2Þ sx en where sx is the transverse spot size, en is the average normalized transverse beam emittance, and a is a constant [3]. For both of these figures-ofmerit, larger is better. Various beam production and manipulation techniques can be evaluated rapidly by using such definitions. In general, the beam brightness can be improved by increasing the bunch charge, decreasing the bunch length, or reducing the transverse emittance. In some cases, reducing the bunch length can increase the beam emittance (e.g., CSRinduced emittance growth in a bunch compression chicane [4]). In many beam sources, increasing the charge can only be done at the expense of increasing the emittance also. Thus, the various tradeoffs must be carefully evaluated. The beam source of choice for many current and future applications of high-brightness beams is the RF photoinjector electron gun, or RF gun. These devices use strong electric field gradients (typically 50–150 MV/m) to rapidly accelerate short slugs of charge (typically 1–10 of the relevant RF phase) in order to preserve the transverse beam properties. Typical charges per bunch range from about 0.1 to 5 nC, depending on the RF frequency, gradient, cathode material, and application. While RF guns are starting to achieve the very good brightness numbers predicted by simulation and theory, there are many charge-dependent effects in the postinjector portion of the accelerator that can act to reduce the beam brightness, or to increase the difficulty of preserving the beam brightness through further acceleration. These include transverse wakefields, longitudinal wakefields, coherent synchrotron radiation in bunch compressors, etc. Secondary considerations, not directly related to the beam brightness but of definite concern for machine design, include the average beam power, average beam halo power, the physical aperture vs. beam size ratios, and the ability of diagnostics to detect and characterize the beam. These properties can have significant impacts upon the performance
and safety of an operational machine, and must be taken into account when new machines are designed. For many applications, such as single-pass freeelectron lasers, the beam brightness at the point of use is critical but the bunch charge is not. For such applications, it is reasonable to consider variations on the traditional RF gun design that are designed to optimize this specific tradeoff.
2. Needle cathodes For this study, we consider the use of a blunt needle cathode in a SLAC/BNL/UCLA-style 1.6cell p-mode S-band photoinjector. 2.1. Field enhancement The enhancement of the electric field at the tip of spheroid, embedded in an otherwise uniform electric field, is given by EEE0
2a b2 ; R¼ R lnð4a=RÞ a
ð3Þ
where E0 is the unperturbed field strength, a is semimajor radius of the needle, and b is the semiminor radius of the needle, the semimajor axis is taken to be parallel with the unperturbed field [5]. For our initial studies we used a needle design with a flat-top tip, with a radius of 200 mm, and chamfered out at a 45 angle to a needle stalk radius of 300 mm. This provides a field enhancement of about a factor of three above the bulk cavity gradient in the absence of the needle. A 50mm emission radius is assumed; this keeps the edge of the electron beam sufficiently far from the needle edge so as to avoid the strongest radial variations in the field. 2.2. Quantum efficiency Field-enhanced photoelectric emission has demonstrated electron-beam current density as high as 1010 A/m2, with a quantum efficiency (QE) in the UV that approaches 10% at fields on the order of 1010 V/m. The use of needle cathodes may be
ARTICLE IN PRESS J.W. Lewellen, C.A. Brau / Nuclear Instruments and Methods in Physics Research A 507 (2003) 323–326
325
seen as a path towards robust cathodes with high QE figures. 2.3. Thermal emittance reduction
2.4. Current densities The maximum current density from a cathode is limited by the space-charge force at the cathode surface; when the magnitude of the field from the beam equals the magnitude of the RF field at the cathode surface, emission stops. In a conventional photoinjector running at 120 MV/m (on-crest) gradient, the expected limit is roughly 70 kA/cm2. For the type of needle described above, the corresponding limit is 200 kA/cm2.
3. Gun design and simulation The gun design used for these simulations is based on the SLAC/BNL/UCLA 1.6-cell p-mode design [7]. The only major alteration to the gun is the addition of a needle cup-and-holder to the back plane of the gun. This geometry, shown in Fig. 1, serves several purposes. First, it places the base of the needle in a low-field region, which makes engineering design considerably simpler. Second, it provides a rounded surface to mate with the back wall of the cavity, easing arcing concerns.
Fig. 1. Cathode plate geometry for the needle-cathode gun.
4
Ez [normalized]
Recent reports from conventional photoinjector experiments list total beam emittances approaching the simulation values; while this represents long-awaited developments in the field, it also means that thermal emittance now represents much of the total emittance in high-brightness beams from conventional photoinjectors. Unless a means of reducing the thermal emittance is found, the normalized emittance limit for a conventional photoinjector with Cu or Mg cathode appears to be approximately 0.5 mm for a 1 mm radius laser spot on the cathode. The beam thermal emittance scales as the radius of the laser spot on the cathode [6]. With an emission spot radius of 50 mm, the thermal 1 emittance should be approximately 20 of that of a photoinjector with a 1-mm cathode spot radius.
2
0 1 .10
3
0.01
0.1 z [cm]
1
10
Standard Cathode Needle Cathode
Fig. 2. On-axis field profiles for standard vs. needle-cathode pmode photoinjector cavities. The horizontal axis is plotted on a log scale to facilitate showing the fields in the cathode region.
Finally, it places the tip of the needle level with the original back wall of the cavity, which helps maintain the whole-beam longitudinal dynamics of this gun design. These changes lower the shunt impedance of the gun by about 30%, and require some alteration to the outer radii of the cathode and full cells to maintain both the resonant frequency at 2.856 GHz and to maintain what would be a 1:1 field balance between the cells, were the needle not present. Fig. 2 shows the on-axis fields. While it is possible that additional modifications to the back plane of the cathode cell would allow a needle cathode to be used with an otherwise ‘‘stock’’ p-mode gun, we have not yet explored such an option.
ARTICLE IN PRESS 326
J.W. Lewellen, C.A. Brau / Nuclear Instruments and Methods in Physics Research A 507 (2003) 323–326
The RF cavity fields were generated using the SUPERFISH group of codes; PARMELA v3.11 was used for the electron beam dynamics calculations. Due to the rapidly changing electric field in the vicinity of the needle tip, the gun was broken into three logical ‘‘cavities.’’ The first cavity models the region around the cathode tip proper, the second cavity extends to the entrance of the full cell, and the third cavity models the remainder of the gun. The gun is followed by an emittance compensation solenoid, an approximately 1-mlong drift, and a single SLAC-style 3-m TWCG linac section. Beam properties are calculated two meters after the end of the capture linac section. The charge per bunch was kept fixed at 20 pC. The beam distribution was taken to be uniform in both transverse and longitudinal dimensions. The bunch length, launch phase, solenoid current, and linac phase were varied in order to optimize the electron beam brightness following the capture linac section.
Table 1 Beam parameters for needle-cathode vs. conventional cathode gun Parameter
Conventional gun
Needlecathode gun
Charge Bunch duration Cathode radius Cathode gradient Norm. emittance
1 nC 10 ps 1 mm 120 MV/m 1 mm
20 pC 4 ps 50 mm 350 MV/m 0.11 mm
Est. thermal emittance Cathode current density Normalized RMS brightness Relative pierce parameter
0.6 mm 3.2 kA/cm2 20 A/mm2
0.03 mm 200 kA/cm2 83 A/mm2
1
0.77
cathode tips for greater field enhancement; and extending the optimization loop to include cavity geometry. Experimentally, we hope to be able to test a variety of needle cathodes within 1–2 years.
4. Results and discussion References The optimized beam parameters are shown in Table 1, along with nominal beam parameters from a standard p-mode gun for comparison, and some other parameters of interest. The results to date are very encouraging and compare quite favorably to a conventional gun, especially when it is noted that the needle-cathode gun operates quite well at the expected spacecharge limited current density of 200 kA/cm2. The results are achieved at relatively low absolute charge levels; combined with the smaller spot size, this should greatly ease the difficulties of transporting high-brightness beams through a postinjector accelerator. Theoretical and simulation plans for the future include trying alternate needle and cavity geometries, finer needles in particular; using radiused
[1] C.A. Brau, ‘‘What brightness means’’, The physics and applications of high-brightness beams, Sardinia, Italy, 2002, Proceedings in press. [2] Martin Reiser, Theory and Design of Charged Particle Beams, Wiley, New York, 1994. [3] M. Xie, Design optimization for an X-ray free electron laser driven by SLAC Linac, Proceedings of the 1995 Particle Accelerator Conference, Vol. 180, 1996, Dallas, TX. [4] B.E. Carlsten, T.O. Raubenheimer, Phys. Rev. E 51 (1995) 1453. [5] C.A. Brau, Modern Problems in Classical Electrodynamics, Oxford, England: Oxford University Press, 2003. [6] W.S. Graves, et al., Measurement of thermal emittance for a copper photocathode, Proceedings of the 2001 Particle Accelerator Conference, Vol. 2227, 2001, Chicago, IL. [7] D.T. Palmer, et al., Microwave measurements of the BNL/ SLAC/UCLA 1.6 cell photocathode RF gun, Proceedings of the 1995 Particle Accelerator Conference, Vol. 979, 1996, Dallas, TX.