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Improvement of electron beam properties by reducing back-bombardment effects in a thermionic RF gun
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 507 (2003) 340–344
Improvement of electron beam properties by reducing back-bombardment effects in a thermionic RF gun Toshiteru Kii*, Koshiro Yamane, Isao Tometaka, Kai Masuda, Hideaki Ohgaki, Kiyoshi Yoshikawa, Tetsuo Yamazaki Institute of Advanced Energy, Kyoto University, Gokasyo, Uji, Kyoto 611-0011, Japan
Abstract In the Free Electron Laser (FEL) experiment, where a long beam macro-pulse is required, energy shift caused by an increase of current density at the cathode surface due to heating by back-streaming electrons is quite serious. It was numerically found that the low-energy component of the back-streaming electrons causes a serious effect. It was also found that the effect can be decreased by applying a transverse magnetic field by calculating time evolution of the cathode surface temperature with a one-dimensional thermal conduction model. r 2003 Elsevier Science B.V. All rights reserved. PACS: 41.60.Cr Keywords: Free-electron laser; RF gun; Electron injector
1. Introduction A thermionic RF gun is an economical approach to produce a high-brightness electron beam compared to a photocathode RF gun. However, some electrons which are not at the best accelerating phase are re-accelerated and return to the cathode, and heat up the cathode surface during the beam macro-pulse. This causes the current density at the cathode surface to increase and the extracted beam energy to decrease. This backbombardment problem is quite serious for FEL experiments, where long and stable electron beams are required. Owing to the problem, the beam *Corresponding author. Tel: +81-774-38-3422; fax: +81774-38-3426. E-mail address: [email protected] (T. Kii).
macro-pulse is usually limited up to several ms, and the repetition rate is also restricted, because additional power is given to the cathode surface from back-streaming electrons. In the case of highduty-operation, heating power from the backstreaming electrons becomes comparable to the heater power to the cathode, and thus stabilization by controlling the heater power becomes difficult. A photo-cathode RF gun is an almost perfect solution for the back-bombardment problem. However, it is not an economical and user-friendly solution for some industrial or commercial applications of FELs. This is because they require highly stable phase-controlled laser systems and expensive high-power lasers or high-quantum efficiency photocathodes with long lifetimes— currently not available. Also frequent cathode maintenance is required.
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)00942-2
ARTICLE IN PRESS T. Kii et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 340–344
On the other hand, a group at Stanford University [1] proposed to apply a transverse magnetic field to avoid this problem. In the case of our 4.5-cavity S-band thermionic RF gun [2–5], it was found that by using a two-dimensional (2D) simulation code KUBLAI [2,6,7] developed by our group, the high-energy back-streaming electrons even from around the exit of the RF gun are also important. Furthermore, by using a 1D heat conduction model, it was found that the lowenergy back-streaming electrons, which tend to be distributed widely on the whole cathode, have a more serious influence on the beam quality compared to the high-energy component [5]. In this study, we calculated electron trajectories in the RF gun, and the time evolution of the
341
cathode surface temperature during the macropulse, during application of dipole and quadrupole magnetic fields applied to the RF gun and used to decrease the back-streaming electrons.
2. Configuration of electromagnet Fig. 1 shows the cross-section of our 4.5 cell Sband RF gun and pole pieces of the electromagnets. A dipole or quadrupole electromagnet is installed just behind the RF gun. The calculated field distributions are shown in Figs. 2a and b. The dipole magnet generates a transverse magnetic field to sweep out the low-energy back-streaming electrons to the outside of the cathode. On the other hand, a quadrupole magnet defocuses the low-energy component in one direction with only small distortion to the outgoing electrons.
3. Evaluation
Fig. 1. Cross-section of our RF gun and magnets.
To evaluate the effects of the magnetic field, we used the PARMELA code [8] and the 1D heat conduction model [5]. PARMELA was used to calculate the trajectories and the kinetic energy of the back-streaming electrons. The 1D heat conduction code was used to estimate the time evolution of the beam properties due to the pulsed heat input by the back-streaming electrons.
Fig. 2. (a) Field distribution of dipole magnet and (b) Field distribution of quadrupole magnet.
ARTICLE IN PRESS T. Kii et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 340–344
342
3.1. Magnetic field strength
Table 1 Input parameters for calculations
In our S-band RF gun, the energy of the main component of the back-streaming electrons is several hundred keV. The field strength of the magnetic field was chosen to be 7.6 G. The relationship between back-streaming beam energy and position shift on the cathode is shown in Fig. 3.
Frequency Input RF power Current density on the cathode surface Cavity voltage (first-half cell) Cavity voltage (second to fifth cell)
The schematic drawing of the model is shown in Fig. 4, and input parameters and applied fields are shown in Tables 1 and 2. Because the magnetic field cannot be added at the same z position as the cavities, the first-half cavity is cut into two pieces, and the magnetic field is applied so that the integrated field intensity is the same as the real field. The particle distribution of the back-streaming electrons on the cathode without any magnetic
∆X [mm]
8 6 4 2 0.2
0.4
0.6
0.8
1.0
E [MeV] Fig. 3. Back-streaming beam energy E vs. position shift DX when a transverse magnetic field of 7.6 G is applied.
1.312 [cm]
0.8 [cm]
cathode 0.6 [cm]
magneticfield 0.01 [cm] 5.248 [cm]
MHz MW A/cm2 MV/m MV/m
Table 2 Applied field to the first cavity
3.2. Effects of extra magnetic field
0 0.0
2856 7.4 10 22 B40
Field type Without magnetic field Dipole magnetic field Quadrupole magnetic field
Strength — 7.6 560
G G/m
field, with a dipole field, and with a quadrupole field are shown in Figs. 5a–c, respectively. It is found that the dipole magnetic field can remove the main component of the back-streaming electrons effectively. The low-energy component, less than 300 keV is not reduced under the model, because the low-energy electrons never reach to the region where the magnetic field is applied. Although the very low-energy component is not removed, total back-streaming beam power is reduced to 60%. In this condition, the beam emittance grows about 40%. On the other hand, a quadrupole magnetic field does not change the beam emittance, but no improvement is found at the field gradient of 560 G/m. To estimate the possibility of a reduction of the back-streaming electrons without emittance growth, we tried to increase the field gradient. It is found that at the field gradient of 56 kG/m, the back-streaming beam power would be reduced to 90% with only a small change of the emittance, but such highquadrupole fields are not realistic. 3.3. Beam quality in the macro-pulse
1.312 [cm]
Fig. 4. Schematic drawing of the RF gun for PARMELA.
To evaluate the beam quality in the macropulse, we improved the 1D thermal conduction code to treat heat density in the cathode along the beam axis. Without a magnetic field, a tempera-
ARTICLE IN PRESS T. Kii et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 340–344
No magnetic field
1310
0.4
Without magnetic field With dipole field
TC [°C]
Ee [MeV]
1300
0.3 0.2
343
1290 1280
0.1 1270 0.0
0.0 0 1 2 3 4 Distance from center of the cathode [mm]
(a)
With magnetic field (Dipole-type)
0.5
1.0
1.5 2.0 Time [µs]
2.5
3.0
Fig. 6. Time evolution of cathode surface temperature TC during macro-pulse.
Ee [MeV]
0.4
density on the cathode changes from 43% to 28%. In the real experiment, the low-energy component will be reduced more, and thus more improvement is expected.
0.3 0.2 0.1 0.0
(b)
0 1 2 3 4 Distance from center of the cathode [mm] With magnetic field (Quadrupole-type)
Ee [MeV]
0.4 0.3 0.2 0.1 0.0 (c)
0 1 2 3 4 Distance from center of the cathode [mm]
Fig. 5. Energy and radial distribution of the back-streaming electrons. (a) Without magnetic field, (b) with dipole magnetic field and (c) with quadrupole magnetic field.
ture increase of 33 C in 3 ms is estimated. By applying a transverse dipole magnetic field, 40% of beam power is reduced, but the surface temperature is reduced by only 24%. This is due to the very low-energy electrons, which are not removed under the simple model, depositing their energy very close to the cathode surface and making a larger contribution to the cathode temperature than the high-energy electrons. Time evolution of the cathode surface with and without dipole magnetic field calculated using the 1D code are shown in Fig. 6. According to the technical data of the cathode material [9], the current
4. Summary and conclusion Effects of several external magnetic fields on the performance of a RF gun were calculated using PARMELA and a 1D thermal conduction model. In the case of our 4.5 cell RF gun, it is found that the dipole magnetic field will effectively reduce the low-energy component of the back-streaming electrons. A quadrupole magnetic field will be good for the beam emittance, but the reduction effect of the back-streaming effect is small. By using a 1D thermal conduction model, it is found that at least 24% of the back-streaming beam power will be reduced by the dipole magnet, and the increase in current density during the beam macro-pulse of 3 ms will be reduced more than 30%.
References [1] C.B. McKee, John M.J. Maday, Nucl. Instr. and Meth. A 296 (1990) 716. [2] Y. Yamamoto, et al., Nucl. Instr. and Meth. A 393 (1997) 443. [3] K. Masuda, et al., Proceedings of the 12th Symposium on Accelerator and Technology, Waco, Japan, 1999. [4] R. Ikeda, Experiments and analysis on characteristics of 4.5-cavity S-band thermionic RF gun, Master Thesis, Kyoto University, 2000 (in Japanese). [5] T. Kii, et al., Nucl. Instr. and Meth. A 483 (2002) 310.
ARTICLE IN PRESS 344
T. Kii et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 340–344
[6] K. Masuda, Development of numerical simulation codes and application to klystron efficiency enhancement, Ph.D. Thesis, Kyoto University, 1997. [7] T. Inamasu, Numerical analysis of electron beam in an RF gun, Bachelor Thesis, Kyoto University, 1996.
[8] L.M. Young, J.H. Billen, PARMELA, LA-UR-96-1835, 2001. [9] Technical data book of the cathode material, Heat Wave corp.
Nuclear Instruments and Methods in Physics Research A 507 (2003) 340–344
Improvement of electron beam properties by reducing back-bombardment effects in a thermionic RF gun Toshiteru Kii*, Koshiro Yamane, Isao Tometaka, Kai Masuda, Hideaki Ohgaki, Kiyoshi Yoshikawa, Tetsuo Yamazaki Institute of Advanced Energy, Kyoto University, Gokasyo, Uji, Kyoto 611-0011, Japan
Abstract In the Free Electron Laser (FEL) experiment, where a long beam macro-pulse is required, energy shift caused by an increase of current density at the cathode surface due to heating by back-streaming electrons is quite serious. It was numerically found that the low-energy component of the back-streaming electrons causes a serious effect. It was also found that the effect can be decreased by applying a transverse magnetic field by calculating time evolution of the cathode surface temperature with a one-dimensional thermal conduction model. r 2003 Elsevier Science B.V. All rights reserved. PACS: 41.60.Cr Keywords: Free-electron laser; RF gun; Electron injector
1. Introduction A thermionic RF gun is an economical approach to produce a high-brightness electron beam compared to a photocathode RF gun. However, some electrons which are not at the best accelerating phase are re-accelerated and return to the cathode, and heat up the cathode surface during the beam macro-pulse. This causes the current density at the cathode surface to increase and the extracted beam energy to decrease. This backbombardment problem is quite serious for FEL experiments, where long and stable electron beams are required. Owing to the problem, the beam *Corresponding author. Tel: +81-774-38-3422; fax: +81774-38-3426. E-mail address: [email protected] (T. Kii).
macro-pulse is usually limited up to several ms, and the repetition rate is also restricted, because additional power is given to the cathode surface from back-streaming electrons. In the case of highduty-operation, heating power from the backstreaming electrons becomes comparable to the heater power to the cathode, and thus stabilization by controlling the heater power becomes difficult. A photo-cathode RF gun is an almost perfect solution for the back-bombardment problem. However, it is not an economical and user-friendly solution for some industrial or commercial applications of FELs. This is because they require highly stable phase-controlled laser systems and expensive high-power lasers or high-quantum efficiency photocathodes with long lifetimes— currently not available. Also frequent cathode maintenance is required.
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)00942-2
ARTICLE IN PRESS T. Kii et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 340–344
On the other hand, a group at Stanford University [1] proposed to apply a transverse magnetic field to avoid this problem. In the case of our 4.5-cavity S-band thermionic RF gun [2–5], it was found that by using a two-dimensional (2D) simulation code KUBLAI [2,6,7] developed by our group, the high-energy back-streaming electrons even from around the exit of the RF gun are also important. Furthermore, by using a 1D heat conduction model, it was found that the lowenergy back-streaming electrons, which tend to be distributed widely on the whole cathode, have a more serious influence on the beam quality compared to the high-energy component [5]. In this study, we calculated electron trajectories in the RF gun, and the time evolution of the
341
cathode surface temperature during the macropulse, during application of dipole and quadrupole magnetic fields applied to the RF gun and used to decrease the back-streaming electrons.
2. Configuration of electromagnet Fig. 1 shows the cross-section of our 4.5 cell Sband RF gun and pole pieces of the electromagnets. A dipole or quadrupole electromagnet is installed just behind the RF gun. The calculated field distributions are shown in Figs. 2a and b. The dipole magnet generates a transverse magnetic field to sweep out the low-energy back-streaming electrons to the outside of the cathode. On the other hand, a quadrupole magnet defocuses the low-energy component in one direction with only small distortion to the outgoing electrons.
3. Evaluation
Fig. 1. Cross-section of our RF gun and magnets.
To evaluate the effects of the magnetic field, we used the PARMELA code [8] and the 1D heat conduction model [5]. PARMELA was used to calculate the trajectories and the kinetic energy of the back-streaming electrons. The 1D heat conduction code was used to estimate the time evolution of the beam properties due to the pulsed heat input by the back-streaming electrons.
Fig. 2. (a) Field distribution of dipole magnet and (b) Field distribution of quadrupole magnet.
ARTICLE IN PRESS T. Kii et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 340–344
342
3.1. Magnetic field strength
Table 1 Input parameters for calculations
In our S-band RF gun, the energy of the main component of the back-streaming electrons is several hundred keV. The field strength of the magnetic field was chosen to be 7.6 G. The relationship between back-streaming beam energy and position shift on the cathode is shown in Fig. 3.
Frequency Input RF power Current density on the cathode surface Cavity voltage (first-half cell) Cavity voltage (second to fifth cell)
The schematic drawing of the model is shown in Fig. 4, and input parameters and applied fields are shown in Tables 1 and 2. Because the magnetic field cannot be added at the same z position as the cavities, the first-half cavity is cut into two pieces, and the magnetic field is applied so that the integrated field intensity is the same as the real field. The particle distribution of the back-streaming electrons on the cathode without any magnetic
∆X [mm]
8 6 4 2 0.2
0.4
0.6
0.8
1.0
E [MeV] Fig. 3. Back-streaming beam energy E vs. position shift DX when a transverse magnetic field of 7.6 G is applied.
1.312 [cm]
0.8 [cm]
cathode 0.6 [cm]
magneticfield 0.01 [cm] 5.248 [cm]
MHz MW A/cm2 MV/m MV/m
Table 2 Applied field to the first cavity
3.2. Effects of extra magnetic field
0 0.0
2856 7.4 10 22 B40
Field type Without magnetic field Dipole magnetic field Quadrupole magnetic field
Strength — 7.6 560
G G/m
field, with a dipole field, and with a quadrupole field are shown in Figs. 5a–c, respectively. It is found that the dipole magnetic field can remove the main component of the back-streaming electrons effectively. The low-energy component, less than 300 keV is not reduced under the model, because the low-energy electrons never reach to the region where the magnetic field is applied. Although the very low-energy component is not removed, total back-streaming beam power is reduced to 60%. In this condition, the beam emittance grows about 40%. On the other hand, a quadrupole magnetic field does not change the beam emittance, but no improvement is found at the field gradient of 560 G/m. To estimate the possibility of a reduction of the back-streaming electrons without emittance growth, we tried to increase the field gradient. It is found that at the field gradient of 56 kG/m, the back-streaming beam power would be reduced to 90% with only a small change of the emittance, but such highquadrupole fields are not realistic. 3.3. Beam quality in the macro-pulse
1.312 [cm]
Fig. 4. Schematic drawing of the RF gun for PARMELA.
To evaluate the beam quality in the macropulse, we improved the 1D thermal conduction code to treat heat density in the cathode along the beam axis. Without a magnetic field, a tempera-
ARTICLE IN PRESS T. Kii et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 340–344
No magnetic field
1310
0.4
Without magnetic field With dipole field
TC [°C]
Ee [MeV]
1300
0.3 0.2
343
1290 1280
0.1 1270 0.0
0.0 0 1 2 3 4 Distance from center of the cathode [mm]
(a)
With magnetic field (Dipole-type)
0.5
1.0
1.5 2.0 Time [µs]
2.5
3.0
Fig. 6. Time evolution of cathode surface temperature TC during macro-pulse.
Ee [MeV]
0.4
density on the cathode changes from 43% to 28%. In the real experiment, the low-energy component will be reduced more, and thus more improvement is expected.
0.3 0.2 0.1 0.0
(b)
0 1 2 3 4 Distance from center of the cathode [mm] With magnetic field (Quadrupole-type)
Ee [MeV]
0.4 0.3 0.2 0.1 0.0 (c)
0 1 2 3 4 Distance from center of the cathode [mm]
Fig. 5. Energy and radial distribution of the back-streaming electrons. (a) Without magnetic field, (b) with dipole magnetic field and (c) with quadrupole magnetic field.
ture increase of 33 C in 3 ms is estimated. By applying a transverse dipole magnetic field, 40% of beam power is reduced, but the surface temperature is reduced by only 24%. This is due to the very low-energy electrons, which are not removed under the simple model, depositing their energy very close to the cathode surface and making a larger contribution to the cathode temperature than the high-energy electrons. Time evolution of the cathode surface with and without dipole magnetic field calculated using the 1D code are shown in Fig. 6. According to the technical data of the cathode material [9], the current
4. Summary and conclusion Effects of several external magnetic fields on the performance of a RF gun were calculated using PARMELA and a 1D thermal conduction model. In the case of our 4.5 cell RF gun, it is found that the dipole magnetic field will effectively reduce the low-energy component of the back-streaming electrons. A quadrupole magnetic field will be good for the beam emittance, but the reduction effect of the back-streaming effect is small. By using a 1D thermal conduction model, it is found that at least 24% of the back-streaming beam power will be reduced by the dipole magnet, and the increase in current density during the beam macro-pulse of 3 ms will be reduced more than 30%.
References [1] C.B. McKee, John M.J. Maday, Nucl. Instr. and Meth. A 296 (1990) 716. [2] Y. Yamamoto, et al., Nucl. Instr. and Meth. A 393 (1997) 443. [3] K. Masuda, et al., Proceedings of the 12th Symposium on Accelerator and Technology, Waco, Japan, 1999. [4] R. Ikeda, Experiments and analysis on characteristics of 4.5-cavity S-band thermionic RF gun, Master Thesis, Kyoto University, 2000 (in Japanese). [5] T. Kii, et al., Nucl. Instr. and Meth. A 483 (2002) 310.
ARTICLE IN PRESS 344
T. Kii et al. / Nuclear Instruments and Methods in Physics Research A 507 (2003) 340–344
[6] K. Masuda, Development of numerical simulation codes and application to klystron efficiency enhancement, Ph.D. Thesis, Kyoto University, 1997. [7] T. Inamasu, Numerical analysis of electron beam in an RF gun, Bachelor Thesis, Kyoto University, 1996.
[8] L.M. Young, J.H. Billen, PARMELA, LA-UR-96-1835, 2001. [9] Technical data book of the cathode material, Heat Wave corp.