Dewa, Hideki

Ego, Hiroyasu

Inoue, Makoto

Iwashita, Yoshihisa

Noda, Akira

Okamoto, Hiromi

Shirai, Toshiyuki

Takekoshi, Hidekuni

Kyoto University Research Information Repository

Title High Power Dummy-Load for RF System in the LinacAuthor(s)Shirai, Toshiyuki; Dewa, Hideki; Ego, Hiroyasu; Inoue,Makoto; Iwashita, Yoshihisa; Noda, Akira; Okamoto, Hiromi;Takekoshi, HidekuniCitationBulletin of the Institute for Chemical Research, KyotoUniversity (1992), 70(1): 45-54Issue Date1992-03-30URL http://hdl.handle.net/2433/77433RightType Departmental Bulletin PaperTextversionpublisherKyoto UniversityBull. Inst.  Chem. Res., Kyoto Univ., Vol. 70, No. 1, 1992      High Power Dummy-Load for RF System in the Linac          Toshiyuki SHIRAI*, Hideki DEWA*, Hiroyasu EGO**, Makoto INOUE*,              Yoshihisa IWASHITA*, Akira NODA*, Hiromi OKAMOTO*and Hidekuni TAKEKOSHI***                            ReceivedFebruary10,1992    We have developed a high power dummy—load for the isolator of the RFQ linac. The body of the dummy—load is a modified waveguide and tapers down in order to absorb the RF power effectively. The cemented carbon is used as the RF absorber. The design is based on the transmission line theory. The return loss of the dummy—load is —33.1 dB on a low power measurement and —30.1 dB on a high power test at the RF frequency of 433.3 MHz. It is found that the dummy—load has enough performance for the isolator system.    KEY WORDS Dummy—load/RF absorber/Circulator/linac                           1. INTRODUCTION    In the accelerators, an RF electric field is often used to accelerate charged particles. In our laboratory, the peak RF power of 600 kW is fed to an RFQ linac and an Alvarez linac. The block diagram of the RF system in the linac is shown in Fig. 1 0. A klystron is chosen        IOscillator 433.3MHa 1W Amp.RFQ Cavity                                                     Circulator               VectorODirectional upler ®Coupler                  cocotroller 1111,•CoCoupler                  300W KlystronDummy-load                       Amp. (L-5773)      q Vector • Directional controller1111011'- Coupler                                                            DTL Cavity                      Buncher System               Figure 1. Schematic block diagram of the RF system of the linac. * ~# , i 77 z> #~ TA, ~ 1., WEB-, 1/1* .,E      Nuclear Science Research Facility, Institute for Chemical Research, Kyoto University   ** Zatcz : Present address : RIKEN, Wako-shi, Saitama 351-01, Japan.   *** frEAA3 : Present address : Hiroshima Denki Institute of Technology, Aki—ku, Hiroshima 731-02,       Japan. ( 45 )                     T.  SHIRAI, H. DEWA, M. INOUE, Y. IWASHITA, . NODA,                          H. OKAMOTO, H. EGO and H. TAKEKOSHI                  Table 1. Operating parameters of the RF power source     KlystronL5773 (Litton)     Frequency433.3 MHz      Maximum RF power1.2 MW         Required RF power for linac cavity 600 kWRF pulse width60 Iisec         Maximum pulse repetition rate 180Hzas a main RF amplifier, because it has high gain and a total RF system can be simple and reliable. The operating parameters of the RF power source are shown in Table 1.    When the power is reflected from the accelerating cavity, the operation of the klystron becomes unstable. So the high power RF isolator has been inserted between the klystron and the RFQ cavity in order to stabilize the operation of the klystron. It also protects the klystron from the reflected power. The isolator is composed of a high power circulator 2) and a dummy -load. The reflected power from the cavity is circulated into the dummy-load which absorbs the power. We have developed the dummy-load with the high power handling capability for the isolator. It can be also used in the case of the performance test of the high power RF devices.    We chose a waveguide as a body of the dummy-load and we adopted a forced air cooling. An RF absorber is glued on the inner surface of the dummy-load body. This type dummy-load has an advantage to a coaxial line one by the connectivity because the waveguide (WR 2100) is used as an RF power feeder in our linac system. The water-cooling is more efficient but the forced air cooling is simpler. The present dummy-load was designed so that the forced air cooling could remove the generated heat within the tolerable temperature rise.    In this paper, the design procedure and the performance of the dummy-load is described.                      2. DESIGN AND CONSTRUCTION 2-1 RF absorber    The RF absorber should have suitable electric resistivity (-10 Qcm) and high heat-resistance. The two materials were tested in this experiment. They are cemented carbon powder and SiC ceramic. The photograph of both the absorbers is shown in Photo 1. The size of the cemented carbon block is 17cm X 8cm X 1.5cm. The cemented carbon has an advantage to have controllable electric resistivity by changing the mixing ratio of the carbon and cement. This advantage is useful to optimize the dummy-load design. It, however, has poor thermal conductivity and the thickness of the RF absorber is limited because of the temperature gradient in it. High heat-resistant cement is used in this absorber.    The SiC ceramic of a honeycomb structure was tested. The weight is lighter and the relative dielectric constant is lower than those of the cemented carbon. The size is 6cm X 4cm X 4cm. It has good thermal conductivity but less controllability of the electric resistivity in a laboratory.    The RF absorber is glued on the inner surface of the dummy-load body by the silicone rubber glue which is heat resistant up to 250 °C. (46)                High Power Dummy-Load for RF System in The Linac  //,      Photo. 1. The photograph of the RF absorbers. The left is the cemented carbon              block and the right is the SiC ceramic. 2-2 Design procedure    A schematic view of the dummy-load is shown in Fig. 2. The body of the dummy-load is made of a modified waveguide for an easy connection to the existing waveguide system. The unique feature of the structure is that the height of the body tapers down. For this geometry, the efficiency of the RF absorption is improved than that of straight waveguide body. The body of the dummy-load is divided into four sections, that is, a matching section, a taper section, a flat section and an end section. Each section has a different role described in the following sub sections.    According to the transmission line theory, the input impedance of the dummy-load should be equal to the characteristic impedance of the waveguide for the reflectionless connection.       AlieW --f- d     ShortingAir/ To                 Termination Waveguide h PAW/ Agir53.3  cm      End L3 LiL2            Section            FlatTaper Matchin9 Sectio • Section Section                    Figure 2. Schematic view of the high power dummy-load. (47)                     T. SHIRAI, H. DEWA, M. INOUE, Y. IWASHITA, A. NODA,                           H. OKAMOTO, H. EGO and H. TAKEKOSHIThe dummy-load is designed so that this condition is satisfied. The input impedance of the dummy-load is derived from the characteristic impedance of all sections. 2-2-1 End section    The thick end section is desirable in order to absorb the RF power effectively. The SiC ceramic is used in this section because of its good thermal conductivity.    The characteristic impedance  Zo of the waveguide is 3)        Wttoz —z~2 Zo=iS—WEo,~o—(a)(l) There co is an angular frequency of the RF, E o is a dielectric constant of free space and ,u o is an magnetic permeability of free space. The quantity a is the width of the waveguide. In the end section, the dielectric constant of free space E o is substituted by a complex dielectric constant a' of the absorber. The characteristic impedance Z, in this section is given by z       Z,=---- , az=W z ego(7r),£' =E-1---°(2)  za where a is a dielectric constant and a is an electric conductivity of the absorber. 2-2-2 Flat section    In the flat section, an energy density of the RF is high because its height is low compared to the normal waveguide. Therefore, the efficiency of the RF absorption is high although the RF absorber glued on the wall is thin. The cemented carbon blocks are used as the RF absorber in this section.    From the Maxwell equation in the waveguide, the following equation is obtained,      0Hr+{co2£,u—(a)2—az}Hz=O(3) where H, is the z-component of magnetic field and /3 is the z-component of the propagation constant. Supposed that the solution of this equation is Hz1=Aexp (j73 iy)+Bexp (—jS y) {0y<(b—d)}, H,2=Cexp (,IQyzy)+Dexp (—IQyzy) {y?(b—d)},(4) where 13 Yl and 3 yz are the y-components of the propagation constants and the quantity b is the height of the waveguide. From eqs. (3) and (4), the dispersion relation is z        WZIua) —/32z=QYi {0__y<(b—d)},       w2E21I—( a)Z-Nz=NY2{y>(b—d)},(5) (48)               High Power Dummy-Load for RF System in The Linac where  E 1 and s 2 are complex dielectric constants. The boundary condition of eqs. (3) is         BHZ,    ay(y=b)= 0,          HA( y==Hj2(y=b—d),        8HZ'(y= b—d) =8H~(y= b— d), H y=O) =O.(6) From eqs. (4) and (6), the following relation is obtained jtan ($1(I)+~2j tan {i3y2(b—d)}=O.(7) WC1We From eqs. (5) and (7), the propagation constant /3 z is obtained. The characteristic impedance Z, is             cog   Z,=a•(8) 2-2-3 Taper section    The height of the flat section is much lower than that of the normal waveguide. This tapered section is installed because a sudden change of the waveguide geometry causes the reflection of the RF power. The characteristic impedance in this section is calculated numerically by the same method as in the flat section with variable quantity b. The cemented carbon blocks are used as the RF absorber in this section. 2-2-4 Matching section    The RF would be reflected at the boundary between the taper section with RF absorber and the normal waveguide by the abrupt impedance change. In this section, the absorber which is the cemented carbon blocks, covers the half of the inner surface area and decrease the total reflection at the boundary (see Fig. 2). 2-3 Numerical calculation    Dividing a transmission line into n sections, the input impedance Z; at the i-th section is given by following recurrence equation ;           Z,_I+jZ,;tan(RZ;L~(1<iGn),(9)          Z;=Z~;Z,,+jZi_1 tanC 8z,L) where Z; is the characteristic impedance, L; is the length and /3,, is the complex propagation (49)                     T. SHIRAI, H. DEWA, M. INOUE, Y. IWASHITA, . NODA,                         H. OKAMOTO, H. EGO and H. TAKEKOSHIconstant in the i—th section. At the end,  Zo  =  0 because the dummy—load is terminated by the shorting plate. The return loss L (dB) is defined as    L=10 log(P'e),(10)                      Pin which gives the level of reflection, where Pin is the input power of the dummy—load and P,ef is the reflected power. The return loss of the dummy—load is calculated by the following rela-tion ;    L=201og \ Zn+Z P(11)                                    n where Z is a characteristic impedance of the waveguide.    Figure 3 shows the dependence of the calculated return loss on the electric resistivity of the cemented carbon block with various geometries. In this calculation, the length of the taper section L 2 is fixed at 90 cm which corresponds to the wavelength in the waveguide. The quantity L 3 is determined to be 4.0 cm and h is limited by the size of available ceramic. The relative dielectric constant of the ceramic is 2.8 and the electric resistivity is 910 52cm. The cemented carbon block should be thin to reduce the temperature difference inside. The length of the flat section L1 is determined so that the reflection would become minimum. As the —0-.. h=8 cm, d=1.0 cm —II— h=8 cm, d=1.5 cm —0— h=8 cm, d=1.0 cm—•• h=6 cm, d=1.5 cm                                                     Optimum length of   -Lr- h=4 cm, d=1.0 cm —,—h=4 cm,d=1.5 cmthe flat section (Ll)-.... Return loss 035-------------------0      /~~~~/~/—~~(cm)(dB)  .fi"25 MIEN _20AN.•20MIMI I.\a0  F.a0 ~~~,~15M •I'10 kW_1050  10MEV 5 III -60  5001111111.70    1010010002090 60 80 100               Resistivity (Clem)Length of the taper section L2 (cm) Figure 3. Dependence of the calculated re-Figure 4. Calculation results of the return          turn loss on the electric resistivityloss and the optimum length of L          of the cemented carbon block withas a function of the taper length          various geometries. The length ofL2.          the taper section and the end           section are fixed at 90 cm and 4.0            cm, respectively. ( 50 )               High Power Dummy-Load for RF System in The Linac results, the following parameters were determined as  d=1.5 cm, h= 6.0 cm and the electric resistivity of the cemented carbon block was 20 S2cm.    The cemented carbon blocks which had been made were found to have higher electric resistivity (80±20 Qcm) than we expected. The relative dielectric constant was about 8.0. Therefore the length of L, and L2 were reoptimized for the RF absorber. Figure 4 shows the return loss and the optimum length of L, as a function of the taper length L2.  The length L, and L2 were determined to be 30 cm and 89 cm, respectively. 2-4 Heat generation    The maximum heat generation is estimated to be about 1 W/cm2 when the input average power is 10 kW, which corresponds to the peak RF power of 1 MW and the duty factor of 1%.                   3. MEASUREMENT SETUP AND RESULTS 3-1 Low power measurement    The return loss is measured at the low RF power level by the standing wave method using the slotted line. The schematic block diagram of the measurement setup is shown in Fig. 5 (a). The standing wave in front of the dummy-load is as follows ; Am e1 +Arfe-i1'=A,,,e'kz(1+pej(e- 4),(12) where p is given by p=Aret(13)               Am • A;a and Aref are complex amplitudes of the input RF wave and reflected wave, respectively.    (a)1 W RFWaveguide             Amp.---------- (WR2100) ----------------                             Coagai -----------Waveguide Dummy —©wavegriuide transition ---------------Slotted line Load                                     LIRF power                                                                                   meter      (b)300 W RFWaveideDirectional Amp.21coupler -----------                                      (WR2100)p      RFKlystron ------------------ Dummy      Controller®(L5773 -----------------------------Load                                                        RF diode Digitizing                                                                    detector oscilloscope      Figure 5. Schematic block diagram of the low power (a) and the high power              measurement setup (b) of the return loss of the dummy—load.( 51 )                    T. SHIRAI, H.  DEWA, M. INOUE, Y. IWASHITA, A. NODA,                         H. OKAMOTO, H. EGO and H. TAKEKOSHI -F Before modification -it- After modification --------            WANI1 A -35,~— MEM'~.                                                Operating frequency                              360 400 490. 480 520                                                   Frequency (MHz)      Figure 6. Frequency dependence of the measured return loss at the low power mea-              surement set up. The quantity 0 is a phase difference between the A in and A,ef. The quantity /3 is a propagation constant along the waveguide. The picked up power in the slotted line is proportional to the square of the real part of the expression (12) ; A; 2{1±p2+2pcos (0-2az)},(14) The measured data are fitted by the expression (14) and the return loss are determined from the quantity p.    The squares in Fig. 6 show the frequency dependence of the return loss (before modification). Then, we replaced the ceramic blocks in the end section with the cemented carbon blocks in order to improve the return loss. The return loss is measured at the various thickness L 3 of the end section. After all, the thickness of the end section was changed to be 3.0 cm and the resurn loss in this condition is shown by solid circles in Fig. 6 (after modification). The return loss became —33.1 dB at the RF frequency of 433.3 MHz after this modification. The measured data is different from the calculation results. It is considered to be caused by the nonuniformity of the cemented carbon material and the approximation used in the characteristic impedance calculation at the taper section. 3-2 High power test    At the high power operation, it is anticipated that the return loss changes from the results of low power measurement because of the temperature rise. The schematic block diagram of the high power test setup is shown in Fig. 5 (b). The input and reflected power of the dummy-load are picked up by the directional coupler which has — 60 dB coupling and the signals are monitored by the digitizing oscilloscope (HP 54503A) through the RF diode detectors.    The RF pulse shapes of the input and output power of the dummy-load are shown in Fig. 7 when the input RF power is 900 kW. The dependence of the return loss on the peak RF power level is shown in Fig. 8. The return loss at the peak RF power of 920 kW is — 30.1 dB. The power dependence of the return loss is considered to be caused by the harmonics in the ( 52 )               High Power Dummy—Load for RF System in The Linac  Rp susil1n0 1r100er          -20 -----------------------------------------------------------    (a) .«.JiI I ...I. .-h       ---.e=—~°I -----'I-25             -,_~-1.-1-1- . ,.:.1~,..~..w1.~~.-.1i..O-30 570.0-6 us 0.000 us770.000 us          10.0.0 vs/Elv -35  Figure 7. The RF pulse shapes of the input and output power of the dummy—40 --------------------------------         load. The RF frequency is 433.3200 400 600 800 1000         MHz. Time base is lqusec/div.Input RF Power (kW)          (a) : The input power from the kly-              stron to the dummy—load, 600 Figure 8. The dependence of the return loss          kW/div.on the peak RF power level. The          (b) : The reflected power from theRF frequency is 433.3 MHz and             dummy—load, 1kW/div.the duty factor is 1.0%.          (c) : The input power to the kly-              stron, 50W/div. output RF power of the klystron.                             4. CONCLUSION    We have developed the high power dummy—load based on the transmission line theory. Owing to the tapered waveguide body, the total length is short and the connection becomes easy. The measured return loss of the dummy—load was — 30.1 dB at the RF peak power of 920 kW and the duty factor of 1%. At present, the dummy—load is attached to the circulator and is used for the linac operation.    In addition, the performance of the klystron and the circulator was measured with this dummy—load. We confirm that the available output peak power of the klystron is more than 1 MW, which well covers the required power of the accelerating cavity. We also confirm that the isolation of the circulator is more than - 25 dB under any condition 2). This value is high enough for the isolator system.                         ACKNOWLEDGEMENT    The authors would like to thank TOKAI KONETSU KOGYO CO., LTD. who supplied the SiC ceramic. We would like to thank the stuff of Toray Industries, Inc. for their advice about the RF absorber.                            ( 53 )                  T.  SHIRAI, H. DEWA, M. INOUE, Y. IWASHITA, A. NODA,                        H. OKAMOTO, H. EGO and H. TAKEKOSHI                          REFERENCES 1) Y Iwashita, et al. ; "System of 7 MeV—Proton Linac", Bull. Inst. Chem. Res. Kyoto Univ., 68, No.    2 (1990). 2) T. Shirai, et al. ; "High power circulator for the 433.3 MHz proton linac", Nucl. Instr. and Meth.    Submitted. 3) H. A. ATWATER, "Introduction to Microwave Theory", McGraw Hill (1962). ( 54 )

High Power Dummy-Load for RF System in the Linac

http://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/77433/1/chd070_1_045.pdf

High Power Dummy-Load for RF System in the Linac

Abstract

Similar works

Full text

Available Versions

Kyoto University Research Information Repository