Conceptual Design of a Table-top Terahertz Free-electron Laser

Transcription

1 Journal of the Korean Physical Society, Vol. 59, No. 5, November 2011, pp Conceptual Design of a Table-top Terahertz Free-electron Laser Y. U. Jeong, S. H. Park, K. Lee, J. Mun, K. H. Jang, J. Y. Lee, J. Sunwoo, K. N. Kim, Y.-H. Cha and B. H. Cha Korea Atomic Energy Research Institute, Daejeon , Korea D. H. Kim Hanbat National University, Daejeon , Korea G. M. Kazakevich Budker Institute of Nuclear Physics, SB RAS, , Lavrentiev av. 11, Novosibirsk, Russia (Received 3 February 2011, in final form 30 August 2011) We have designed a table-top terahertz (THz) free electron laser (FEL). The main issue of the FEL design is to decrease radiation losses at the FEL resonator, except the outcoupling ratio. Also, reducing the number of undulator periods and the total undulator length is important to increase the FEL conversion efficiency and to reduce its size. The FEL consists of a magnetron-based microtron having an energy of 5 MeV, a strong electromagnetic helical undulator having a period of 25 mm, and a cylindrical waveguide-mode optical resonator. The total diameter of the microtron is approximately 60 cm, and the macropulse current is more than 50 ma. The condition for a low-loss high-gain oscillator of the table-top FEL has been studied by using a 2-D FEL code. The injection scheme of the electron beam to the undulator was optimized by calculating beam trajectories with a 3-D particle-in-cell (PIC) code. The average THz power was calculated to be 1 W with a tunable wavelength range from 200 µm to 500 µm. The size of the system is expected to be 1 2 m 2. The FEL is expected to be used for real-time imaging for security inspection. PACS numbers: Cr, q Keywords: Table-top free-electron laser, Terahertz radiation, Security inspection, Free-electron laser DOI: /jkps I. INTRODUCTION Recently, there has been rapid developments in science and technology in the terahertz (THz) range [1 5]. However, realization of powerful and compact sources is one of the bottlenecks and an important task in THz science and technology. If we consider the serious THz absorption by the atmosphere or materials and the limited sensitivity of THz sensors, the required power for real-time THz imaging is still far from the power levels of presently-available small-scale THz sources. We estimated the required power of a THz source for realtime imaging for security inspection by considering the minimum THz pulse energy for currently-available detector cells ( J), the number of detector pixels ( 10 4 ), absorption and losses of the THz radiation by media ( 10 9 ), and a signal-to-noise level of 100, and it was at the 1-W level, as shown in Fig. 1. We have developed a compact THz free-electron laser (FEL) with a conventional microtron accelerator driver [6,7]. The FEL has been used for basic research on THz yujung@kaeri.re.kr Fig. 1. (Color online) Estimate of THz power required for real-time imaging for security inspection by considering the minimum THz pulse energy for the currently available detector cell ( J),the number of detector pixels ( 10 4 ), absorption and losses of the THz radiation by media ( 10 9 ), and signal-to-noise level of 100. applications [8 10]. Based on that experience, we are developing a table-top THz FEL that can be used for

2 Journal of the Korean Physical Society, Vol. 59, No. 5, November 2011 security inspections. The goal of our new project is to develop a table-top THz FEL with a 1-W average power. In this paper, we present the design status of the FEL including the compact microtron accelerator, the undulator, and the waveguide oscillator. II. DESIGN TARGET OF THE TABLE-TOP THZ FEL To realize a table-top THz FEL, we need a compact accelerator, a short beamline, and an undulator, all with good, compact self-shielding. The target wavelength of the FEL for security inspections is determined to be µm by considering the spectroscopic and the transmittance properties of materials. The target power of the THz radiation is 1 W. The system should be a tabletop system or have a standing-rack style to be used at airports, ports or gates of facilities. To reduce radiation hazard and shielding load, we will use a rather lowenergy electron beam of 5 MeV. To get the target wavelength range, we set the undulator period to be 25 mm, and the magnetic field strength should be 3 7 kg to generate the target THz wavelength range. We will develop a short-length undulator having a period number of less than 30. The main reason for the short undulator is to achieve a higher efficiency for conversion of the THz power from the electron beam to radiation, as well as compactness. Because the conversion efficiency from the electron beam to radiation is proportional to the linewidth of the FEL radiation, and the linewidth of the FEL radiation is inversely proportional to the period number of the undulator. The total length of the system might be approximately 2 m. Fig. 2. (Color online) Dependence of the measured beam current of the microtron macropulse on the electron energy. We have developed a conventional microtron accelerator that is driven by the radio-frequency (RF) source of a magnetron. The electron energy from the microtron ranges from 4.5 to 6.5 MeV depending on the turn number of electrons passing through the RF cavity and the magnetic strength of its main magnet. The macropulse duration of the electron beam is 5.5 µs, and the pulse current is up to 70 ma for a 4.5-MeV electron beam. The beam power is more than 10 W for a 10-Hz macropulse repetition rate, as shown in Fig. 2. In the case of the microtron, the electron beam power, which is electron energy times its current, decreases slightly with increasing electron energy due to energy and momentum filtering during the acceleration. Therefore, the electron current is decreased when the energy is increased, as shown in Fig. 2. If we can upgrade the repetition rate to 100 Hz, the beam power could be more than 100 W. III. COMPACT MICROTRON ACCELERATOR We have studied and compared compact accelerators as candidates for the table-top FEL. The candidates for the accelerators were microtron or RF linear accelerators with a thermionic gun or RF gun. If we consider the beam power, RF system, cost, shielding load, and size, a microtron driven by a magntron RF driver could be the best choice, except for the low beam power. The microtron can generate quite good parameters for the electron beam. The energy spread and the emittance could be less than 0.5% and 5 mm mrad in the energy range of 4 6 MeV, respectively. The good quality of the electron beam has an advantage of reducing the electron beam envelope in a strong undulator to increase the interaction with a strongly confined radiation mode in a small-gap waveguide [6,7]. The required beam power of the accelerator is approximately 100 W. Based on our experience, we will develop a high-repetition-rate microtron having a beam power of 100 W. IV. UNDULATOR AND WAVEGUIDE-MODE RESONATOR We have designed a hybrid electromagnetic (EM) helical undulator. The undulator has two superposed hybrid EM planar undulators developed for the Korea Atomic Energy Research Institute (KAERI) THz FEL [1,2], which are placed at 90 rotation and phase shift of 1/4 period relative to each other. Blocks of permanent magnets (PMs) are placed between adjacent iron poles to reduce the magnetic field saturation caused by coils. The dependence of the magnetic field strength on the current in the coils is shown in Fig. 3 and was calculated by using a 3-D simulation code. We can get a field strength of 3 7 kg on the axis of the undulator by changing the current from A. The value of the magnetic field meets the requirement for the undulator. We will use a cylindrical waveguide for the THz FEL oscillator to confine the THz mode. The diameter of the waveguide is just 4 mm, and we can significantly

3 Conceptual Design of a Table-top Terahertz Free-electron Laser Y. U. Jeong et al Fig. 3. Calculated magnetic field strength on the axis of the designed helical undulator. Fig. 5. (Color online) Dependence of the FEL operating wavelength on the undulator magnetic field strength for the conditions of a 5-MeV electron energy, 25-mm undulator period, and a 2-mm radius of the cylindrical waveguide. Fig. 4. (Color online) Electric and magnetic field distribution of the TE 11 mode in a cylindrical waveguide. increase the interaction between electrons and the THz oscillation mode. The ratio of the mode cross-section between free-space and waveguide mode is more than 10, which increases the small signal gain of the FEL with the same ratio. The dispersion relation of the cylindrical waveguide can be expressed as follows. ω 2 c 2 = k2 + x2 mn Rwg 2, (1) where R wg is the radius of the waveguide and x mn meets the condition of J n(x mn ) = 0 for the TE mode and J n (x mn ) = 0 for the TM mode. By combining this relation and the FEL dispersion relation, we can write the relation as follows. ω c β z(k + k u ). (2) We can get the mode-matching condition of the waveguide-mode FEL; k = ( γ 2 z 1 ) k u ± γ z (γ 2 z 1) k 2 z ( xmn R wg ) 2. (3) Fig. 6. (Color online) Calculated loss of the TE 11 mode of a cylindrical waveguide having an 1-m length and 4-mm diameter compared with that of the TE 21 mode for the THz spectral range from 100 to 600 µm. In the case of R wg = 4 mm, the fundamental TE 11 mode can be excited in the FEL. The field distribution of the TE 11 mode is shown in Fig. 4. The FEL wavelength for a waveguide of 2-mm radius could be calculated for the designed helical undulator having a period of 25 mm and a field strength of 3 8 kg, which is shown in Fig. 5. We found that the magnetic field strength of 4 7 kg met the requirement for a FEL wavelength range of µm. The calculated loss of the TE 11 mode for the wavelength range of µm was 16 11% for a 1-m-length and a 4-mm-diameter waveguide, as shown in Fig. 6. V. SIMULATION RESULTS We have calculated the small-signal gain of the waveguide-mode FEL. The length of the undulator should be determined by considering the gain and the

4 Journal of the Korean Physical Society, Vol. 59, No. 5, November 2011 Table 1. Design parameters for the KAERI table-top THz FEL. Electron Beam Energy (peak current) 5 MeV (0.5 A) Emittance <5 mm mrad Energy Spread 0.4% Fig. 7. Dependence of the calculated small-signal gain of the waveguide-mode FEL on the undulator field for field strengths from 3 to 8 kg. Undulator (Helical Type Period (Number of Periods) 25 mm (28) Peak Magnetic Induction (K-value) 4 7 kg (1 1.8) Waveguide Mode & Radius TE11, 2 mm THz Beam Radiation Wavelength mm (frequency) ( THz) Average Power 1 W THz Micropulse - Pulse Duration ps - Power kw - Repetition Rate 2.8 GHz THz Macropulse - Pulse Duration 4 µs - Repetition Rate 100 Hz Fig. 9. (Color online) Dependence of the evolution of the calculated oscillation power of the waveguide FEL on the output coupling ratio. Fig. 8. (Color online) (a) Trajectories and (b) beam envelope of the electrons incident on the undulator, as calculated by using a 3-D PIC code. loss of the waveguide mode. The length of the undulator was determined to be 700 mm, and the length of the FEL oscillator could be less than 1 m. The calculated results for the dependence of the gain on the undulator field strength are shown in Fig. 7. The duration of the electron macropulse is 5 µs. The FEL build-up time is less than 1 µs, which means that the small-signal gain for the target wavelength range of µm is enough for FEL oscillation. The trajectories of the electrons incident on the undulator were calculated by using a 3-D particle-in-cell (PIC) code. We found that the trajectories could be optimized for our small-diameter waveguide-mode FEL. The calculated trajectories and the beam envelope are shown in Fig. 8. We can see that the electron beam from the microtron can be transported through the 4- mm-diameter waveguide in the helical undulator. The evolution of the oscillation power of the waveguide FEL were calculated by a 2-D FEL simulation code for different output coupling ratios, which is shown in Fig. 9. We find that the optimal output coupling ratio generates

5 Conceptual Design of a Table-top Terahertz Free-electron Laser Y. U. Jeong et al more than 30 kw of THz FEL pulse power, which results in an average output power of 1 W for a repetition rate of 100 Hz. In conclusion, we have summarized the design parameters of the table-top THz FEL in Table 1. It should be noted that the results can be upgraded by further researches to find better ideas for the FEL. ACKNOWLEDGMENTS This work was supported by the World Class Institute (WCI) Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology of Korea (MEST) (NRF Grant Number: WCI ). REFERENCES [1] R. Kohler, A. Tredicucci, F. Beltram, H. E. Beere, E. H. Linfield, A. G. Davies, D. A. Ritchie, R. C. Iotti and F. Rossi, Nature 417, 156 (2002). [2] Q. Wu and X-C. Zhang, Appl. Phys. Lett. 67, 3523 (1995). [3] K. Kawase, Y. Ogawa, Y. Watanabe and H. Inoue, Opt. Express 11, 2549 (2003). [4] M. Abo-Bakr, J. Feikes, K. Holldack and G. Wustefeld, Phys. Rev. Lett. 88, (2003). [5] M. A. Dem yanenko, D. G. Esaev, B. A. Knyazev, G. N. Kulipanov and N. A. Vinokurov, Appl. Phys. Lett. 92, (2008). [6] Y. U. Jeong, B. C. Lee, S. K. Kim, S. O. Cho, B. H. Cha, J. Lee, G. M. Kazakevitch, P. D. Vobly, N. G. Gavrilov, V. V. Kubarev and G. N. Kulipanov, Nucl. Instrum. Methods Phys. Res., Sect. A 475, 47 (2001). [7] Y. U. Jeong, G. M. Kazakevitch, B. C. Lee, S. K. Kim, S. O. Cho, N. G. Gavrilov and J. Lee, Nucl. Instrum. Methods Phys. Res., Sect. A 483, 195 (2002). [8] G. N. Zhizhin, A. K. Nikitin, G. D. Bogomolov, V. V. Zavialov, Y. U. Jeong, B. C. Lee, S. H. Park and H. J. Cha, Opt. Spectrosc. 100, 734 (2006). [9] H. J. Cha, Y. U. Jeong, S. H. Park, B. C. Lee and S-H. Park, J. Korean Phys. Soc. 49, 354 (2006). [10] Y. U. Jeong, G. M. Kazakevitch, H. J. Cha and S. H. Park, Nucl. Instrum. Methods Phys. Res., Sect. A 575, 58 (2007).