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Simulation of the ELSA photo-injector with the TBCI-SF code
Nuclear Instruments and Methods in Physics Research A304 (1991) 333-335 North-Holland
333
Simulation of the ELSA photo-injector with the TBCI-SF code J.P. De Brion, S. Joly and F . Schumann
Centre d'Etudes de Bruyères-le- Châtel, Service de Physique et Techniques Nucléaires, BP No. 12, 91680 Bruyères-le- Châtel, France
A photo-injector has been designed to yield high-brightness electron bunches for the ELSA free electron laser. The electron beam is produced by a laser-driven photocathode, rapidly accelerated to relativistic energies in a single 144 MHz rf cavity and focused by a magnetic lens for bunch characteristics measurements . The TBCI-SF code is used to simulate the beam dynamics in the photo-injector and the associated diagnostic line . Given the initial laser parameters (pulse duration, radial and longitudinal distributions, illuminated area) and the rf fields, the longitudinal and transverse phase-space distributions are computed for the experimental setup. Results are presented and compared with recently obtained experimental data.
1. Introduction
z 00E- 007 ,
The high-brightness photo-injector developed for the IR-FEL experiment ELSA [1,2] consists of a laser-driven photocathode placed inside a 144 MHz rf cavity . A pulsed Nd : YAG laser illuminates a CsKZSb photocathode and generates electron bunches by photo-emission. Following the cavity, solenoid lenses focus the electron beam onto diagnostic devices where the beam characteristics are measured . A schematic diagram of the photo-injector is shown in fig. 1 . Up to 20 nC bunches have been produced and accelerated to a kinetic energy of about 1 MeV [3]. Electron beam dynamics in the photo-injector has been simulated by means of the TBCI-SF code and the results compared to experimental data .
Z (rn) Fig. 2. Distribution of the accelerating electric field along the axis of the 144 MHz rf gun cavity. 2. Beam dynamics simulation
spectrometer;
Distance z Icm) Fig. 1 Schematic diagram of the ELSA photo-injector .
The beam dynamics simulation is made using the TBCI-SF code [4,5] developed at DESY to study the dynamics of intense electron beams in the low-energy regime . The code is a 2 1' -dimensional, fully relativistic particle-in-cell code, solving Maxwell's equations (with the actual cavity geometry) and the Lorentz force equation self-consistently. Space-charge and wake-field effects are also taken into account. Electromagnetic fields in the rf gun cavity are precalculated using the URMEL code [6]; the amplitude and phase of the generated electric and magnetic fields can then be adjusted separately as variable parameters. The 144 MHz cavity had been designed to produce the highest possible accelerating field on the cathode,
0168-9002/91/$03 .50 © 1991 - Elsevier Science Publishers B.V . (North-Holland)
V. ACCELERATOR TECHNOLOGY
33 4
J. P. De Brion et al. / Simulation of the ELSA photo-injector 012
Urn "
~ . . .....
"'
O . (7
.
. . ... . . .... . . . .. '4
n
O S(i
,
. *r Fi~ (1JC
r;NO
Fig. 4. Evolution of the bunch dimensions as a function of distance from the photocathode . Fig. 3. Distribution of the magnetic field component duced by the focusing lens.
B_ pro-
and then maximum beam energy, for a given maximum surface electron field which could be limited by breakdowns in the anode-cathode gap. Fig. 2 shows the electric field variation along the propagation z-axis ; a maximum electric field of 15 MV/m on the photocathode corresponds then to an accelerating voltage of 1 MV for the injector cavity . The static magnetic field generated by the actual magnetic lens has been precalculated using the PROFI code [7] ; for these simulations, we considered the lens placed in the anode nose of the cavity, and centered at 12 .5 cm from the cathode, only. The distribution of the magnetic field along the z-axis is shown in fig. 3; the field intensity can also be adjusted as a parameter. The lens is used to focus the electron bunch emerging from the cathode and to control the emittance growth throughout the injector [8]; it has to be noticed that the magnetic field on the cathode is almost zero as required for minimum emittance growth . The meshes used to calculate these dynamic and static external fields should be identical to the mesh used to calculate the current density and to advance the fields in time. Simulating the dynamics of short (- 50 ps) electron bunches in a 144 MHz cavity (radius _ 60 cm) implies a very large number of mesh points, the maximum of which is actually limited by the computer memory, for the version of the code we use now. For the normalized transverse emittance of the beam, we took the following definition : E = n
2 [ (r 2 )(r r2 ) _
the rf fields [9], which is a small effect here given the bunch length compared to the 144 MHz wavelength . The radial emittance has been calculated at the pepperpot position (see fig. 1), located at 78 .6 cm from the photocathode . 3. Results Simulation runs were carried out for the following parameters of the laser: pulse length : 100 ps, illuminated area : 1 cm2; pulse shape rectangular in both r- and z-directions . For the rf field, the maximum electric field on the cathode was set at 15 MV/m corresponding to an average kinetic energy of 1 MeV at the photo-injector exit . The bunch characteristics (energy, energy spread, radius, length and radial emittance) have been deternuned as a function of the charge contained in the bunch and of the magnetic field generated by the focusing lens ; these quantities are calculated at different locations along the propagation axis . We present here the results corresponding to q = 10 nC and a maximum magnetic field of 100 G. Fig. 4 shows the r vs z dimensions of the bunch at different locations; the focusing effect is observed around z = 30 cm but the radius is still increasing .
(rr')2)t/2
where r denotes the radial position of the particle and r' = ßyßr//3~ with 8r and 8, representing its radial and axial velocities . In an rf gun, the beam emittance growth is mainly due to space-charge effects, which are dominant while the beam is nonrelativistic, and to the time variation of
Fig. 5 Normalized radial emittance of a bunch passing through a detector located at z.
J. P. De Brion et al. / Simulation of the ELSA photo-injector The behaviour of the normalized emittance as
a
function of distance is shown in fig. 5; the emittance
reaches a maximum of 100m mmmrad just after the
lens and then decreases slowly to get a flat minimum of 757r mmmrad at about 54 cm . These results are in agreement with
other simulation codes [10]
estimates of
as far as the maximum
radius of the bunch is concerned. However, the situation is not that clear for the normalized emittance.
Preliminary experiments [2] with about the same parameters gave 90m mmmrad for the emittance as measured with the pepper-pot technique. More experimental data are necessary for a better comparison with the TBCI-SF predictions.
Acknowledgements T. Weiland (TH Darmstadt) and W.R . Novender
(PROFI Engineering) are greatly acknowledged for providing the TBCI-SF and PROFI codes.
33 5
References S. Joly et al ., A high-brightness photo-injector for a freeelectron laser proposal, EPAC, Rome (1988) p. 257. R. De-Cas et al ., Photo-emission studies at Bruyères -leChâtel for FEL applications, Bendor Workshop on Short Pulse High Current Cathodes, Bendor, France (1990) . S. Joly et al ., Progress report on the BRC photo-injector, EPAC, Nice (1990) . [4] F. Ebeling, P. Schütt and T. Weiland, Selfconsistent simulation of high power tubes with TBCI-SF, EPAC, Rome (1988) p. 678. P. Schütt, Zür Dynamik emes Elektronen-Hohlstrahls, Thesis, Institut filr Expenmentalphysik der Umversit5t Hamburg, DESY M-88-03 (1988) .
[6] U . Laustroer et al ., URMEL and URMEL-T User Guide, DESY M-87-03 (1987) . [7] W.R . Novender, The 2D/3D Static Nonlinear Field Program PROFI at DESY, DESY M-88-02 (1988). [8] B.E. Carlsten, Photoelectric Injector Design Code, Proc 1989 IEEE Particle Accelerator Conf ., Chicago (1989) p. 313. [9] K.J . Km, Nucl . Instr. and Meth . A275 (1989) 201 . [10] J. Fr6haut et al ., Beam dynamics studies in a low-frequency high-peak power laser-driven RF gun, EPAC, Nice (1990) .
V. ACCELERATOR TECHNOLOGY
333
Simulation of the ELSA photo-injector with the TBCI-SF code J.P. De Brion, S. Joly and F . Schumann
Centre d'Etudes de Bruyères-le- Châtel, Service de Physique et Techniques Nucléaires, BP No. 12, 91680 Bruyères-le- Châtel, France
A photo-injector has been designed to yield high-brightness electron bunches for the ELSA free electron laser. The electron beam is produced by a laser-driven photocathode, rapidly accelerated to relativistic energies in a single 144 MHz rf cavity and focused by a magnetic lens for bunch characteristics measurements . The TBCI-SF code is used to simulate the beam dynamics in the photo-injector and the associated diagnostic line . Given the initial laser parameters (pulse duration, radial and longitudinal distributions, illuminated area) and the rf fields, the longitudinal and transverse phase-space distributions are computed for the experimental setup. Results are presented and compared with recently obtained experimental data.
1. Introduction
z 00E- 007 ,
The high-brightness photo-injector developed for the IR-FEL experiment ELSA [1,2] consists of a laser-driven photocathode placed inside a 144 MHz rf cavity . A pulsed Nd : YAG laser illuminates a CsKZSb photocathode and generates electron bunches by photo-emission. Following the cavity, solenoid lenses focus the electron beam onto diagnostic devices where the beam characteristics are measured . A schematic diagram of the photo-injector is shown in fig. 1 . Up to 20 nC bunches have been produced and accelerated to a kinetic energy of about 1 MeV [3]. Electron beam dynamics in the photo-injector has been simulated by means of the TBCI-SF code and the results compared to experimental data .
Z (rn) Fig. 2. Distribution of the accelerating electric field along the axis of the 144 MHz rf gun cavity. 2. Beam dynamics simulation
spectrometer;
Distance z Icm) Fig. 1 Schematic diagram of the ELSA photo-injector .
The beam dynamics simulation is made using the TBCI-SF code [4,5] developed at DESY to study the dynamics of intense electron beams in the low-energy regime . The code is a 2 1' -dimensional, fully relativistic particle-in-cell code, solving Maxwell's equations (with the actual cavity geometry) and the Lorentz force equation self-consistently. Space-charge and wake-field effects are also taken into account. Electromagnetic fields in the rf gun cavity are precalculated using the URMEL code [6]; the amplitude and phase of the generated electric and magnetic fields can then be adjusted separately as variable parameters. The 144 MHz cavity had been designed to produce the highest possible accelerating field on the cathode,
0168-9002/91/$03 .50 © 1991 - Elsevier Science Publishers B.V . (North-Holland)
V. ACCELERATOR TECHNOLOGY
33 4
J. P. De Brion et al. / Simulation of the ELSA photo-injector 012
Urn "
~ . . .....
"'
O . (7
.
. . ... . . .... . . . .. '4
n
O S(i
,
. *r Fi~ (1JC
r;NO
Fig. 4. Evolution of the bunch dimensions as a function of distance from the photocathode . Fig. 3. Distribution of the magnetic field component duced by the focusing lens.
B_ pro-
and then maximum beam energy, for a given maximum surface electron field which could be limited by breakdowns in the anode-cathode gap. Fig. 2 shows the electric field variation along the propagation z-axis ; a maximum electric field of 15 MV/m on the photocathode corresponds then to an accelerating voltage of 1 MV for the injector cavity . The static magnetic field generated by the actual magnetic lens has been precalculated using the PROFI code [7] ; for these simulations, we considered the lens placed in the anode nose of the cavity, and centered at 12 .5 cm from the cathode, only. The distribution of the magnetic field along the z-axis is shown in fig. 3; the field intensity can also be adjusted as a parameter. The lens is used to focus the electron bunch emerging from the cathode and to control the emittance growth throughout the injector [8]; it has to be noticed that the magnetic field on the cathode is almost zero as required for minimum emittance growth . The meshes used to calculate these dynamic and static external fields should be identical to the mesh used to calculate the current density and to advance the fields in time. Simulating the dynamics of short (- 50 ps) electron bunches in a 144 MHz cavity (radius _ 60 cm) implies a very large number of mesh points, the maximum of which is actually limited by the computer memory, for the version of the code we use now. For the normalized transverse emittance of the beam, we took the following definition : E = n
2 [ (r 2 )(r r2 ) _
the rf fields [9], which is a small effect here given the bunch length compared to the 144 MHz wavelength . The radial emittance has been calculated at the pepperpot position (see fig. 1), located at 78 .6 cm from the photocathode . 3. Results Simulation runs were carried out for the following parameters of the laser: pulse length : 100 ps, illuminated area : 1 cm2; pulse shape rectangular in both r- and z-directions . For the rf field, the maximum electric field on the cathode was set at 15 MV/m corresponding to an average kinetic energy of 1 MeV at the photo-injector exit . The bunch characteristics (energy, energy spread, radius, length and radial emittance) have been deternuned as a function of the charge contained in the bunch and of the magnetic field generated by the focusing lens ; these quantities are calculated at different locations along the propagation axis . We present here the results corresponding to q = 10 nC and a maximum magnetic field of 100 G. Fig. 4 shows the r vs z dimensions of the bunch at different locations; the focusing effect is observed around z = 30 cm but the radius is still increasing .
(rr')2)t/2
where r denotes the radial position of the particle and r' = ßyßr//3~ with 8r and 8, representing its radial and axial velocities . In an rf gun, the beam emittance growth is mainly due to space-charge effects, which are dominant while the beam is nonrelativistic, and to the time variation of
Fig. 5 Normalized radial emittance of a bunch passing through a detector located at z.
J. P. De Brion et al. / Simulation of the ELSA photo-injector The behaviour of the normalized emittance as
a
function of distance is shown in fig. 5; the emittance
reaches a maximum of 100m mmmrad just after the
lens and then decreases slowly to get a flat minimum of 757r mmmrad at about 54 cm . These results are in agreement with
other simulation codes [10]
estimates of
as far as the maximum
radius of the bunch is concerned. However, the situation is not that clear for the normalized emittance.
Preliminary experiments [2] with about the same parameters gave 90m mmmrad for the emittance as measured with the pepper-pot technique. More experimental data are necessary for a better comparison with the TBCI-SF predictions.
Acknowledgements T. Weiland (TH Darmstadt) and W.R . Novender
(PROFI Engineering) are greatly acknowledged for providing the TBCI-SF and PROFI codes.
33 5
References S. Joly et al ., A high-brightness photo-injector for a freeelectron laser proposal, EPAC, Rome (1988) p. 257. R. De-Cas et al ., Photo-emission studies at Bruyères -leChâtel for FEL applications, Bendor Workshop on Short Pulse High Current Cathodes, Bendor, France (1990) . S. Joly et al ., Progress report on the BRC photo-injector, EPAC, Nice (1990) . [4] F. Ebeling, P. Schütt and T. Weiland, Selfconsistent simulation of high power tubes with TBCI-SF, EPAC, Rome (1988) p. 678. P. Schütt, Zür Dynamik emes Elektronen-Hohlstrahls, Thesis, Institut filr Expenmentalphysik der Umversit5t Hamburg, DESY M-88-03 (1988) .
[6] U . Laustroer et al ., URMEL and URMEL-T User Guide, DESY M-87-03 (1987) . [7] W.R . Novender, The 2D/3D Static Nonlinear Field Program PROFI at DESY, DESY M-88-02 (1988). [8] B.E. Carlsten, Photoelectric Injector Design Code, Proc 1989 IEEE Particle Accelerator Conf ., Chicago (1989) p. 313. [9] K.J . Km, Nucl . Instr. and Meth . A275 (1989) 201 . [10] J. Fr6haut et al ., Beam dynamics studies in a low-frequency high-peak power laser-driven RF gun, EPAC, Nice (1990) .
V. ACCELERATOR TECHNOLOGY