- Email: [email protected]
Improvement of beam macropulse properties using slim thermionic cathode in IAE RF gun
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 528 (2004) 408–411
Improvement of beam macropulse properties using slim thermionic cathode in IAE RF gun Toshiteru Kii*, Atsushi Miyasako, Syusuke Hayashi, Kai Masuda, Hideaki Ohgaki, Kiyoshi Yoshikawa, Tetsuo Yamazaki Institute of Advanced Energy, Kyoto University, Gokasyo, Uji, Kyoto 611-0011, Japan
Abstract A long beam macropulse is strongly required for free-electron lasers. RF guns can potentially produce high brightness electron beam using a simple and compact system. However, due to a back-bombardment, a cathode surface is overheated. Thus, it is difficult to maintain a constant beam current and beam energy during a macropulse. The use of a photo cathode with a short-pulsed laser is one of the solutions, but it affects the simplicity and the compactness of the RF guns. We studied a mechanism of back-bombardment and experimentally and numerically found that a low energy component of the back-streaming electrons plays an important role in cathode surface heating. r 2004 Elsevier B.V. All rights reserved. PACS: 41.60.Cr Keywords: Free-electron laser; RF gun; Electron injector
1. Introduction RF guns are widely used as electron injectors of accelerators. However, electrons that drop from the accelerating phase are decelerated and hit the cathode. Consequently, the cathode surface is overheated and the surface temperature increases. Then, the extraction of constant electron beam from the RF gun becomes difficult. To avoid the back-bombardment problem, a group at Stanford University proposed the appli*Corresponding author. Tel.: +81-774-38-3422; fax: +81774-38-3426. E-mail address: [email protected] (T. Kii).
cation of a transverse magnetic field and success in the production of a longer macropulse [1]. A group at the Science University of Tokyo was also successful in producing a long macropulse by using the slim LaB6 cathode [2]. We developed a two-dimensional (2-D) particle simulation code KUBLAI [3] and a 1-dimensional (1-D) heat conduction model [4] to study the backbombardment problem in our 4.5 cell thermionic RF gun. We found that the low energy backstreaming electrons, which distributed widely on the cathode, had a serious influence on the beam quality [5]. In this study, an effect of the magnetic field on back-streaming electrons has been analysed by measuring macropulse duration of
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.04.121
ARTICLE IN PRESS T. Kii et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 408–411
409
extracted electron beam and beam current. We also studied an effect of the cathode diameter by a particle simulation and time evolutions of the cathode surface temperature.
2. Experiment
Fig. 2. Arrangement of the dipole magnet.
1.0 transverse magnetic field
Field strength (a.u.)
0.8 0.6 0.4 0.2 0.0 0
5
10 15 Position Z (cm)
20
25
Fig. 3. Field distribution along a beam axis.
1.0
4.00
0.8
3.00
0.6
2.00
0.4
1.00
0.2
0.00
0.0 0
2
4
6
8
Beam current (A)
incident power reflection power beam current
5.00
Pin, Pref (MW)
Extracted beam current and effective beam macropulse duration were measured. Fig. 1 shows an experimental setup. A dipole magnet is located just behind a cathode to reduce back-streaming electrons. A disk-shaped dispenser cathode with a 6 mm diameter was used. The surface temperature of the cathode was kept at about 1000 C. Fig. 2 shows the arrangement of the dipole magnet attached to the RF gun. Field distribution along z-axis is shown in Fig. 3. In this paper, field strength is defined as the maximum field strength along the z-axis (z ¼ 0). A typical signal from a current transformer (CT) and an input and a reflected RF power are shown in Fig. 4. Due to the back-bombardment effect, beam loading increases during macropulse. Thus, the resonant frequency changes and reflected RF increases and extracted electron beam drops. Here, effective macropulse duration is defined as the time from the leading edge of the macropulse to the time when signals from the CT start swinging. Fig. 5 shows measured pulse duration with and without the magnetic field. As shown in Fig. 5, the pulse duration is improved with an increase in the magnetic field. On the other hand, by applying the magnetic field, extracted beam current decreases because the beam halo is stopped by the aperture of the RF gun (Fig. 6). In our case, about 20 G of a magnetic
10
Time (µs) Fig. 1. Experimental setup.
Fig. 4. Input and reflected RF power and extracted beam current when no magnetic field is applied.
ARTICLE IN PRESS T. Kii et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 408–411 Table 1 Input parameters for calculations
3.4
Frequency (MHz) Input RF power (MW) Current density on the cathode surface (A/cm2) Cavity voltage (1st half cell) (MV/m) Cavity voltage (2nd–5th cell) (MV/m)
3.2 3.0
0
10 20 30 40 Magnetic field (gauss)
50
Fig. 5. Measured effective beam macropulse duration as a function of the field strength of the magnetic field.
0.5 Beam current (A)
2856 3.0 10 15 B28
2.8
0.4
Nmber of particles (a.u.)
Effective pulse width (µs)
410
250 without mangnetic field 26 Gauss
200 150 100 50 0 0.0
0.3 0.2
0.5
1.0 1.5 Energy (MeV)
2.0
Fig. 7. Energy spectrum of back-streaming electrons.
0.1 0.0 0
10 20 30 40 Magnetic field (gauss)
50
Fig. 6. Extracted beam current as a function of applied field strength.
field is acceptable because the main component of extracted beam tends to be obstructed by the aperture. In this case, the improvement in macropulse duration was about 10%.
3. Evaluation To evaluate the effect of the dipole magnetic field to the beam properties, we used a particle simulation code PARMELA (version 3.30) [6] and the 1-D thermal conduction model [4]. The input parameters are shown in Table 1. Energy spectrum of the back-streaming electrons is shown in Fig. 7. A main component of back-streaming electrons lower than 250 keV is reduced by the dipole magnetic field. However, most of the low energy component remains when magnetic field is applied,
because very low energy component, which is very sensitive to the surface temperature, tends to change its direction close to the cathode, and returns to the almost same position on the cathode. Time evolution of the cathode surface temperature is also estimated by using the 1-D heat conduction model. Fig. 8 shows time evolutions when the magnetic field is applied and magnetic field of 20 G goes on and off. As shown in Fig. 8, when the magnetic field is applied, the gradient is 10% smaller than that without the magnetic field. This is consistent with the experimental results. To reduce low energy back-streaming electrons, using a thermionic cathode with smaller diameter appears to be effective, because the ratio of the electrons wiped out to those outside the cathode will increase. Thus, we also estimated an effect of a slim cathode with a 2 mm diameter by using the PARMELA and the 1-D heat conduction model. In our design, injection energy to an accelerator tube was designed to be 7 MeV [7], and input parameters were chosen as shown in Table 2.
ARTICLE IN PRESS T. Kii et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 408–411
75
1290 without magnetic field 26 gauss
1285
Beam current (a.u.)
Temperature (K)
411
1280 1275 1270
6mm 2mm
50
25
0 0.0
0.5
1.0 1.5 2.0 Time (µsec)
2.5
3.0
0
Fig. 8. Time evolution of the cathode surface temperature.
10 20 30 40 50 Magnetic field (gauss)
60
Fig. 9. Extracted beam current as a function of applied field strength.
Table 2 Input parameters for calculations 2856 7.4 10 22 B40
The estimated extracted beam current and time evolutions of cathode surface temperature are shown in Figs. 9 and 10. In case of the cathode with a 2 mm diameter, the reduction ratio when magnetic field is applied is larger compared to when the cathode with a 6 mm diameter is used. Thus, the combination of a slim cathode and dipole magnetic field will improve the macropulse duration of electron beam.
6mm 0 Gauss 6mm 32 Gauss 2mm 0 Gauss 2mm 32 Gauss
1285 Temperature (K)
Frequency (MHz) Input RF power (MW) Current density on the cathode surface (A/cm2) Cavity voltage (1st half cell) (MV/m) Cavity voltage (2nd–5th cell) (MV/m)
1280
1275
1270 0.0
0.5
1.0 1.5 2.0 Time (µsec)
2.5
3.0
Fig. 10. Time evolution of the cathode surface temperature.
slim cathode with a 2 mm diameter is used. It was found that the temperature growth during macropulse was effectively suppressed by the magnetic field.
4. Summary and conclusion References To extract a long pulse from the IAE 4.5 cell thermionic RF gun with thermionic cathode with a 6 mm diameter, we tested the effect of a magnetic field applied close to the cathode surface. It was experimentally found that the magnetic field can improve pulse duration, which is shown by the PARMELA and 1-D thermal conduction model. However, the improvement was not sufficient for FEL experiments. We estimated the time evolution of the cathode surface temperature during macropulse when a
[1] C.B. McKee, J.M.J. Madey, Nucl. Instr. and Meth. A 296 (1990) 716. [2] F. Oda, et al., Nucl. Instr. and Meth. A 483 (2002) 45. [3] K. Masuda, Development of Numerical Simulation Codes and Application to Klystron Efficiency Enhancement, Ph.D.Thesis, Kyoto University, 1997. [4] T. Kii, et al., Nucl. Instr. and Meth. A 483 (2002) 310. [5] T. Kii, et al., Nucl. Instr. and Meth. A 507 (2003) 340. [6] L.M. Young, J.H. Billen, PARMELA, LA-UR-96-1835, 2001. [7] H. Ohgaki, et al., Nucl. Instr. and Meth. A 507 (2003) 150.
Nuclear Instruments and Methods in Physics Research A 528 (2004) 408–411
Improvement of beam macropulse properties using slim thermionic cathode in IAE RF gun Toshiteru Kii*, Atsushi Miyasako, Syusuke Hayashi, Kai Masuda, Hideaki Ohgaki, Kiyoshi Yoshikawa, Tetsuo Yamazaki Institute of Advanced Energy, Kyoto University, Gokasyo, Uji, Kyoto 611-0011, Japan
Abstract A long beam macropulse is strongly required for free-electron lasers. RF guns can potentially produce high brightness electron beam using a simple and compact system. However, due to a back-bombardment, a cathode surface is overheated. Thus, it is difficult to maintain a constant beam current and beam energy during a macropulse. The use of a photo cathode with a short-pulsed laser is one of the solutions, but it affects the simplicity and the compactness of the RF guns. We studied a mechanism of back-bombardment and experimentally and numerically found that a low energy component of the back-streaming electrons plays an important role in cathode surface heating. r 2004 Elsevier B.V. All rights reserved. PACS: 41.60.Cr Keywords: Free-electron laser; RF gun; Electron injector
1. Introduction RF guns are widely used as electron injectors of accelerators. However, electrons that drop from the accelerating phase are decelerated and hit the cathode. Consequently, the cathode surface is overheated and the surface temperature increases. Then, the extraction of constant electron beam from the RF gun becomes difficult. To avoid the back-bombardment problem, a group at Stanford University proposed the appli*Corresponding author. Tel.: +81-774-38-3422; fax: +81774-38-3426. E-mail address: [email protected] (T. Kii).
cation of a transverse magnetic field and success in the production of a longer macropulse [1]. A group at the Science University of Tokyo was also successful in producing a long macropulse by using the slim LaB6 cathode [2]. We developed a two-dimensional (2-D) particle simulation code KUBLAI [3] and a 1-dimensional (1-D) heat conduction model [4] to study the backbombardment problem in our 4.5 cell thermionic RF gun. We found that the low energy backstreaming electrons, which distributed widely on the cathode, had a serious influence on the beam quality [5]. In this study, an effect of the magnetic field on back-streaming electrons has been analysed by measuring macropulse duration of
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.04.121
ARTICLE IN PRESS T. Kii et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 408–411
409
extracted electron beam and beam current. We also studied an effect of the cathode diameter by a particle simulation and time evolutions of the cathode surface temperature.
2. Experiment
Fig. 2. Arrangement of the dipole magnet.
1.0 transverse magnetic field
Field strength (a.u.)
0.8 0.6 0.4 0.2 0.0 0
5
10 15 Position Z (cm)
20
25
Fig. 3. Field distribution along a beam axis.
1.0
4.00
0.8
3.00
0.6
2.00
0.4
1.00
0.2
0.00
0.0 0
2
4
6
8
Beam current (A)
incident power reflection power beam current
5.00
Pin, Pref (MW)
Extracted beam current and effective beam macropulse duration were measured. Fig. 1 shows an experimental setup. A dipole magnet is located just behind a cathode to reduce back-streaming electrons. A disk-shaped dispenser cathode with a 6 mm diameter was used. The surface temperature of the cathode was kept at about 1000 C. Fig. 2 shows the arrangement of the dipole magnet attached to the RF gun. Field distribution along z-axis is shown in Fig. 3. In this paper, field strength is defined as the maximum field strength along the z-axis (z ¼ 0). A typical signal from a current transformer (CT) and an input and a reflected RF power are shown in Fig. 4. Due to the back-bombardment effect, beam loading increases during macropulse. Thus, the resonant frequency changes and reflected RF increases and extracted electron beam drops. Here, effective macropulse duration is defined as the time from the leading edge of the macropulse to the time when signals from the CT start swinging. Fig. 5 shows measured pulse duration with and without the magnetic field. As shown in Fig. 5, the pulse duration is improved with an increase in the magnetic field. On the other hand, by applying the magnetic field, extracted beam current decreases because the beam halo is stopped by the aperture of the RF gun (Fig. 6). In our case, about 20 G of a magnetic
10
Time (µs) Fig. 1. Experimental setup.
Fig. 4. Input and reflected RF power and extracted beam current when no magnetic field is applied.
ARTICLE IN PRESS T. Kii et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 408–411 Table 1 Input parameters for calculations
3.4
Frequency (MHz) Input RF power (MW) Current density on the cathode surface (A/cm2) Cavity voltage (1st half cell) (MV/m) Cavity voltage (2nd–5th cell) (MV/m)
3.2 3.0
0
10 20 30 40 Magnetic field (gauss)
50
Fig. 5. Measured effective beam macropulse duration as a function of the field strength of the magnetic field.
0.5 Beam current (A)
2856 3.0 10 15 B28
2.8
0.4
Nmber of particles (a.u.)
Effective pulse width (µs)
410
250 without mangnetic field 26 Gauss
200 150 100 50 0 0.0
0.3 0.2
0.5
1.0 1.5 Energy (MeV)
2.0
Fig. 7. Energy spectrum of back-streaming electrons.
0.1 0.0 0
10 20 30 40 Magnetic field (gauss)
50
Fig. 6. Extracted beam current as a function of applied field strength.
field is acceptable because the main component of extracted beam tends to be obstructed by the aperture. In this case, the improvement in macropulse duration was about 10%.
3. Evaluation To evaluate the effect of the dipole magnetic field to the beam properties, we used a particle simulation code PARMELA (version 3.30) [6] and the 1-D thermal conduction model [4]. The input parameters are shown in Table 1. Energy spectrum of the back-streaming electrons is shown in Fig. 7. A main component of back-streaming electrons lower than 250 keV is reduced by the dipole magnetic field. However, most of the low energy component remains when magnetic field is applied,
because very low energy component, which is very sensitive to the surface temperature, tends to change its direction close to the cathode, and returns to the almost same position on the cathode. Time evolution of the cathode surface temperature is also estimated by using the 1-D heat conduction model. Fig. 8 shows time evolutions when the magnetic field is applied and magnetic field of 20 G goes on and off. As shown in Fig. 8, when the magnetic field is applied, the gradient is 10% smaller than that without the magnetic field. This is consistent with the experimental results. To reduce low energy back-streaming electrons, using a thermionic cathode with smaller diameter appears to be effective, because the ratio of the electrons wiped out to those outside the cathode will increase. Thus, we also estimated an effect of a slim cathode with a 2 mm diameter by using the PARMELA and the 1-D heat conduction model. In our design, injection energy to an accelerator tube was designed to be 7 MeV [7], and input parameters were chosen as shown in Table 2.
ARTICLE IN PRESS T. Kii et al. / Nuclear Instruments and Methods in Physics Research A 528 (2004) 408–411
75
1290 without magnetic field 26 gauss
1285
Beam current (a.u.)
Temperature (K)
411
1280 1275 1270
6mm 2mm
50
25
0 0.0
0.5
1.0 1.5 2.0 Time (µsec)
2.5
3.0
0
Fig. 8. Time evolution of the cathode surface temperature.
10 20 30 40 50 Magnetic field (gauss)
60
Fig. 9. Extracted beam current as a function of applied field strength.
Table 2 Input parameters for calculations 2856 7.4 10 22 B40
The estimated extracted beam current and time evolutions of cathode surface temperature are shown in Figs. 9 and 10. In case of the cathode with a 2 mm diameter, the reduction ratio when magnetic field is applied is larger compared to when the cathode with a 6 mm diameter is used. Thus, the combination of a slim cathode and dipole magnetic field will improve the macropulse duration of electron beam.
6mm 0 Gauss 6mm 32 Gauss 2mm 0 Gauss 2mm 32 Gauss
1285 Temperature (K)
Frequency (MHz) Input RF power (MW) Current density on the cathode surface (A/cm2) Cavity voltage (1st half cell) (MV/m) Cavity voltage (2nd–5th cell) (MV/m)
1280
1275
1270 0.0
0.5
1.0 1.5 2.0 Time (µsec)
2.5
3.0
Fig. 10. Time evolution of the cathode surface temperature.
slim cathode with a 2 mm diameter is used. It was found that the temperature growth during macropulse was effectively suppressed by the magnetic field.
4. Summary and conclusion References To extract a long pulse from the IAE 4.5 cell thermionic RF gun with thermionic cathode with a 6 mm diameter, we tested the effect of a magnetic field applied close to the cathode surface. It was experimentally found that the magnetic field can improve pulse duration, which is shown by the PARMELA and 1-D thermal conduction model. However, the improvement was not sufficient for FEL experiments. We estimated the time evolution of the cathode surface temperature during macropulse when a
[1] C.B. McKee, J.M.J. Madey, Nucl. Instr. and Meth. A 296 (1990) 716. [2] F. Oda, et al., Nucl. Instr. and Meth. A 483 (2002) 45. [3] K. Masuda, Development of Numerical Simulation Codes and Application to Klystron Efficiency Enhancement, Ph.D.Thesis, Kyoto University, 1997. [4] T. Kii, et al., Nucl. Instr. and Meth. A 483 (2002) 310. [5] T. Kii, et al., Nucl. Instr. and Meth. A 507 (2003) 340. [6] L.M. Young, J.H. Billen, PARMELA, LA-UR-96-1835, 2001. [7] H. Ohgaki, et al., Nucl. Instr. and Meth. A 507 (2003) 150.