THE GENERATION OF 400 MW RF PULSES AT X-BAND USING RESONANT DELAY LINES *

Transcription

1 SLAC PUB 874 3/1999 THE GENERATION OF 4 MW RF PULSES AT X-BAND USING RESONANT DELAY LINES * Sam G. Tantaw, Arnold E. Vleks, and Rod J. Loewen Stanford Lnear Accelerator Center, Stanford Unversty P.O. Box 4349, MS 33, Stanford CA9439 Abstract We present theory and expermental data for a resonant-delay-lne pulse-compresson system. The system s fed by two hgh power klystrons at X-band. The output power s four tmes bgger than the nput power. The system produces flat-top output pulses. The system uses evacuated room-temperature copper delay lnes as a means of storng energy. These lnes acheved a qualty factor greater than 4.3x1 5, wth total losses due to external components measured at 4%. We compare theory wth expermental results. Contrbuted to 1999 IEEE MTT-S Internatonal Mcrowave Symposum Anahem, Calforna, June 13-19, 1999 * Work supported by Department of Energy contract DE AC3 76SF515.

2 THE GENERATION OF 4 MW RF PULSES AT X-BAND USING RESONANT DELAY LINES Sam G. Tantaw, Arnold E. Vleks, and Rod J. Loewen Stanford Lnear Accelerator Center, Stanford Unversty P.O. Box 4349, MS 33, Stanford CA9439 Abstract-We present theory and expermental data for a resonant-delay-lne pulse-compresson system. The system s fed by two hgh power klystrons at X-band. The output power s four tmes bgger than the nput power. The system produces flat-top output pulses. The system uses evacuated room-temperature copper delay lnes as a means of storng energy. These lnes acheved a qualty factor greater than 4.3x1 5, wth total losses due to external components measured at 4%. We compare theory wth expermental results. I. INTRODUCTION Rado frequency pulse compresson usng resonant Delay Lnes s proposed as a method for achevng the hgh-power flat rf pulse requred to drve the Next Lnear Collder (NLC [1-]. In order to acheve pulse compresson, energy from an ncomng rf pulse s stored n hgh Q resonant delay lnes. Whle chargng, energy that leaks out of the delay lnes s, to great extent, canceled by the reflected ncdent rf. To dscharge the lnes, the phase of the ncomng pulse s reversed so that the reflected sgnal from the nputs to the lnes adds constructvely wth the emtted feld from the stored energy n the lnes for the duraton of one round trp tme of rf n the lne. The system suffers from two types of losses that reduce ts effcency: ntrnsc losses and fnte conductvty losses. By desgn, some of the nput energy s mmedately reflected at the delay lne entrance durng the chargng phase. Addtonally, after the phase reversal, the energy nsde the lnes s not dscharged completely at the desred compressed pulse tme perod. Unfortunately, the couplng coeffcent to the lne that maxmzes the energy storage makes the energy dscharge from the lne far from optmum. The system s desgned for an optmum couplng coeffcents at a specfc compresson rato to maxmze the output power. Durng the perod of tme the rf energy spends nsde the storage lne part of t s lost smply due to the fnte qualty factor of the lnes. Smlar losses occur from the fnte conductvty of the components used to manpulate the nput and output sgnals. We descrbe the expermental procedures and the measurements performed on the hgh-power X-band prototype bult at the Stanford Lnear Accelerator Center (SLAC. Also wth the Communcatons and Electroncs Department, Caro Unversty, Gza, Egypt

3 3 II. THEORY Consder a wavegude delay lne termnated by a short crcut and coupled at ts nput wth a couplng rs. The lossless scatterng matrx representng the rs s untary. At a certan reference plane the matrx takes the followng form: 1/ R j(1 R S = 1/. (1 j(1 R R In wrtng Eq. (1 we assumed a symmetrcal structure for the rs two port network. The forward and reflected felds around the rs are related as follows: 1/ V = R V j( 1 R V, ( 1 1 = 1/ j( 1 R RV V. (3 Wth the excepton of some phase change, the ncomng sgnal V at tme nstant t s the same as the outgong sgnal V at tme nstant t τ ; where τ s obvously the round trp delay through the lne;.e. jβl V ( t = V ( t τ e ; (4 where β s the wave propagaton constant wthn the delay lne, and l s the length of the lne. Substtutng from Eq. (4 nto Eq. (3 we get 1/ jβl V ( t = j(1 R ( t RV ( t τ e. (5 Durng the chargng phase we assume a constant nput,.e., V 1 ( t = Vn whch equals a constant value. We, also, assume that all the voltages are equal to zero at tme t <. Hence, substtutng the soluton of the dfference equaton (5 nto Eq. (4 leads us to wrte jβl 1 ( Re 1/ jβl V ( = j (1 R e Vn jβl. 1 Re (6 In Eq. (6 V ( means the ncomng wave n the tme nterval τ t < ( 1 τ and. Substtutng from Eq. (6 nto Eq. ( we get jβl 1 ( Re ( = Vn R (1 R e jβl 1 Re (7 If the delay lne has small losses ( β has a small magnary part, at resonance the term jβl jβl e = p. (8 where p s a postve real number close to 1. Eq. (7 becomes 1 ( R p ( = Vn R (1 R p. 1 R p (9 After the energy has been stored n the lne one may dump part of the energy n a tme nterval τ by flppng the phase of the ncomng sgnal just after a tme nterval ( n 1 τ,..e., Vn t < ( n 1 τ ( t = Vn ( n 1 τ t < nτ (1 otherwse. The output pulse level durng the tme nterval ( n 1 τ t < nτ can be calculated from Eq. ( wth the ad of Eq.(6. The result s n1 1 ( R p Vout = ( n 1 = Vn R (1 R p 1 R p (11 Indeed, ths s the essence of the pulse compresson system. The maxmum power gan of the system s lmted. Usng Eq. (11, the power gan as n s Vout p = R ( 1 R ;(1 Vn 1 R p n whch has a maxmum value of 17 1 (1 p MaxmumPowerGan = 8 p p (13 at 3

4 4 1 8(1 p R = (14 p 4 p Clearly the maxmum power gan s lmted to 9 as p 1 Furthermore, ths maxmum s greatly affected by the losses n the delay lne; for example, the gan s lmted to 7.46 f the lne has a 1% round trp power losses. III. SYSTEM IMPLEMENTATION Fgure 1 shows the pulse-compresson system. It uses two 41.6-meter long cylndrcal copper wavegudes as delay lnes, each 1.65 cm n dameter and operatng n the TE 1 mode. In theory, these over-moded delay lnes can form a storage cavty wth a qualty factor Q > 1x1 6. A shortng plate, whose axal poston s controllable to wthn ±4 µm by a stepper motor, couplng to the lnes do not excte hgher order modes provded that they are perfectly concentrc wth the wavegude axs. A compact low-loss mode converter exctes the TE 1 mode just before each rs. These mode transducers, known as wrap-around mode converters, were developed specfcally for ths applcaton. Both mode converters are connected to two arms of a hgh-power, overmoded, planer 3-dB hybrd. Ths hybrd s also desgned specally for ths applcaton so that t can handle the super hgh power produced by ths system. Therefore, the reflecton from the lnes exts through the thrd arm of ths hybrd whle the nput to the lnes s fed through the fourth arm. The dstance from the rses to the center of the hybrd has been adjusted to wthn ±13 µm to maxmze ths transmsson. The rs reflecton coeffcent s was H-Plane Over-moded Hybrd Input Irs Output Delay Lnes Wrap-around Mode Converter Fgure 1. Resonant Delay Lne Pulse compresson system termnates each of the delay lnes. The nput of the lne s tapered down to a cm dameter wavegude at whch the mode TE s cut-off; hence, the crcular rses whch determne the optmzed for a compresson rato of 8. 4

5 5 IV RESULTS The power gan was measured for a seres of compresson factors. A least-squares fttng of these measurements to theory s shown n Fgure 3. The round trp losses was found to be.45%, ndcatng an ntrnsc Q for the lnes of 4.3x1 5. The external losses are 4%, and the rs reflecton coeffcent s.74. The rs was desgned usng a mode matchng code to have a reflecton coeffcent of.73, the optmum value for a compresson rato of 8. Power (MW Input Power Output Power Tme ( M cro Seconds Fgure 4. Hgh-power pulse compressor output. system. The output of the system agrees well wth theory. The over-moded components and mode converters have proven to be capable of handlng ths level of rf power at X-band. ACKNOWLEDGMENT Fgure 3. The ponts are measured power gans. The curve represents the least-squares ft. Ths pulse compresson s fed by two hgh power klystrons operatng at GHz. The each klystron s capable of producng 5 MW of rf power. Hence, the maxmum nput power avalable for ths compressor s a 1 MW. Fgure 4. Shows the nput and output of the compressor. The output power level s about 3 MW. The maxmum power acheved by the system at the tme of wrtng ths paper. Gong up n power s foreseen as the components get processed, and the vacuum pressure nsde the system mproves wth tme. Work supported by Department of Energy contract DE-AC3-76SF515 REFERENCES [1] R. D. Ruth et al., "The Next Lnear Collder Test Accelerator," Proceedngs of the IEEE Partcle Accelerator Conference, Washngton DC, May 1993 [] P. B. Wlson, Z. D. Farkas, and R. D. Ruth, "SLED II: A New Method of RF Pulse Compresson," Lnear Accl. Conf., Albuquerque, NM, September 199; SLAC- PUB-533 V. CONCLUSION We presented an mplementaton of a super hgh power, resonant-delay-lne pulse- compresson 5