A high repetition rate beam profile monitor is needed to measure the momentum distribution of each bunch in a 200 bunch train at the Brookhaven Accelerator Test Facility. We have designed a monitor using secondary emission from a microstrip delay line. The delay line converts position information into discrete time delays synchronous with the sampling rate of a fast transient digitizer. We present the results of initial electrical testing here

Gray, A. (State Univ. of New York, Stony Brook, NY (United States). Dept. of Physics)

Rogers, J.T.

Warren, J.B. (Brookhaven National Lab., Upton, NY (United States))

UNT Digital Library

Workshop on Adva_.cedBeam Instrumentation, 4/22-24/91,Tsukuba, Japan, Vol. i, pp. 141-144.BNL--46695. DE92 002068A HIGH REPETITION RATE BEAM PEOFILE MONITORJ.T.ROGERS', A. GRAY b,and J.B.W_ c _' _ 0 ;__i?a NSLS, B_.oo_%a_z_NationalL_, Upton,NY 11973 USAs Dept. 4 PhpM_.s, State Uni_ev_t9 of Nem Yod¢, 5fony Bvoot¢, NY 11794 USA¢ Inat.smentation Dir., Brookhaven N_tional Zabo.r_, lYp_on, NY 11973 USAAbstract: A high repetition rate beam profile monitor is needed to measure the momentumdistribution of each bunch in a 200 bunch train at the Brookhaven Accelerator Test Facility.We have designed a monitor using secondary emission from a microstrip delay Line. The delayline converts position information into discrete time delays synchronous with the samplingrate of a fast transient digitizer. We present the results of initial electrical testing here.1. IntroductionThe Accelerator Test Facility at Brookhaven National Laboratory is a genera[ purposefacility for accelerator and free electron laser research. It provides a 50 MeV electron beamsynchronized with N&YAG and CO= laser pulses, with the quadrupled Nd:YAG radiationbeing used to generate the electron beam from a photocathode. In its free electron lasermode, the facility produces a train of 200 bunches separated by 12.25 ns. Each bunch mayhave a slightly d_eren_ momentum distribution due to variations in the laser pulse durationor intensity, beam loading, or wake field effects.We wish to measure the momentum distribution of each bunch through its trausverseprofile in a dispersive section of the beamline. Rather than use a "harp" monitor withparallel transient recorders sensing the voltage on each wire, we use secondary emission froma meandering conductor separated by dielectric from a ground plane. This conductor formsa microstrip delay 1Lue. The d;rw-qions of the line are chosen so that the time delay betweenadjacent strips m_tches the sampling rate of a single transient digitizer. The use of a singledigitizer keeps the cost of the monitor from being prohibitively high.2. ConstructionThe conductor pattern of the prototype microstrip we have constructed is shown in Fig.1. The Lines represent gold conductor on a sin#e-crystal sapphire (Al=Oa) substrate. Thegroundplane underlying the pattern is also Au. Sapphire was chosen as the dielectric becauseof its high dielecl:ric constant (e/e0 = 9.3 to 11.5 depending on orientation) which allowsshorter line lengths for a given delay, low electrical loss tangent (3.0 - 8.6x10 -s at 10 GHz),and radiation hardness. The sapphire is in the form of a 500 pm thick wafer of the typeused in the semiconductor industry. Gold was chosen as the conductor bemuse of its highsecondary emission coefficient, good electrical conductivity, and oxidation resistance. Theconductor was applied in three steps..4, thin Cr layer was vapor deposited onto the sapphireto provide a base with good adhesion, followed by a vapor deposited Au layer, followed by a30 pm electroplaced Au layer. The microstrip was then e_ched using a phocoresist process.The length of a half-period of the line was chosen to match its delay to the 742 ps samplingdigitizer _. , , ,,- • _.... L_.intervalofa commercialtransient The ilnecomprises i4 suchaau per_ou_, _-o,Mr,CTE14 position %hannels" are ava_ble. The 12.25 ns interval between bunches is approximately16 samples, so that the monitor out'put ft,ore each bunch is temporally separste. The linewidth was chosen to make its impedance 50 _, matching the transien_ digitizer impedance.The unused end of the line is termin_ed in a matched load to prevent si@Lal re.flections.Fig. 1. Microstrip conductor pa_tern. The length oi"the vertical strips is 79 mm.3. Radiation effectsSecondary emission has long been used as a means of monitoring current in electronlinaa _,4. Previous investigators have found secondary emission coefficients of 3.5% 2 and5.2% ' from double-sided A1 foils. Extrapo_ to a single Au _e, we expect a secondaryemission coe_cient of 2 to 3%. SiSal sn-en_hs, within the bandwidth of the transientdigitizer, vary with the incident profile width, but will be of the order of 10 to 100 mV pernC bunch charge.B.ad_tion induced conductivity in sapphire has been measured by several authors s's'T.ExtrapoLating from the results of van Lint ct e_.,s who find a conductivity of 10-s fl -L con-lfor irradiation at 3. "_]s/s with a 30 MeV electron beam, we find that the shunt resistanceoi"a strip to ground will be approxim_ely 104 _1 for 200 bunches of InC each. This shuntresistance should not measurably a/t"ect the performance of the monitor. It does, however,suggest another mode of operation in which the line is biased relative to the monitor, so thatthe radiation induced current through the dielectric becomes the si_al.The monitor will be locally heated by the beam. The instantaneous temperature risefrom 200 bunches will be highest in the Au layers (210°C), and the average temperature in&4. Electrical measurementsPrelhninary electrical tests have been made of the prototype device. The propagationvelocity in the line is accurately determined by measurement of the resonant frequenciesin the _ resonator, shown at the right side of Fig. 1. The velocity is 1.15 x 10s m/s.Measurement of the capacitance of the disk on the right side of Fig. 1 gives the dielectricconstant _/_ = 11.2. The capacitance per unit length of the line was measured to be1.98 × 10 -l° F/ro. Combining these values we find the line impedance to be 44 _1. Theattenuation in the Line has been measured using the ring resonator to be 0.07 dB/cm.The impulse response of each line segment (strip) was measured by capacitively couplinga swept signal generator to a single line and observing the microstrip out'put with a networkanalyzer. This analyzer is equipped with software which calculates impulse response througrhFourier analysis. The results are shown in Fig. 2 for strips 1, 5, 9, and 13. When the signalmust propagate through greater line lengths, it becomes progressively smaller due to the lineattenuation. Dispersion in the line is not apparent. Coupling between adjacent periods ofthe line is a problem and results in the dip following the signal.The e_'ect of attenuation and interperiod coupling will be somewhat improved in thenext prototype monitor by adjustments of the line width, len_h, and spacing. R,m_h,_ngsmall e_ects _ be removed in numerical post-processh_.V II V tVSTART Q. • •ST_P 12. • n=Fig. 2. Impulse response of the monitor to a capacitively coupled signal applied to strips(from bottom) 1,5,9, and 13. The vertical scale is arbitrary.S. ConclusionsElectrical measurements of a high repetition rate profile monitor using a meanderingmicrosu'ip show the principle to be useful, and also point out problems in interline coupLingwhich we plan to solve in a second prototype. ILadiation induced e/_ects in such a device arebelieved to be understood, and tests in an electron beam are scheduled for the immediate'..,.fixture.AcknowledgementsWe would like to thank the members of the ATF coll_bora_on and NSLS sta/_', especiallyL Ben-Zvi and M_ Wood/e, for their contr/but/_Is and suggestions. This work was supportedby the U.S. Department of EnerKy.Eefeeene._t Model 6880B DiKitizer , LeCroy Corporation, Chestnut Ridge, N'Y, USA.2 CJ. Karzn_k, Eev. Sd. Instr. Ss, IS46 (1964).z S.A.. Blankenbur_, J.K. Cobb, and J.J. Muray, Nucl. Instr. Me_h. 39, 303 (1966).' LA. Prudnikov, G.L. SaiBaganskii, E.B. Buhanov, and B.V. Zabrodin, Pribory i T_l_h,_-,,_¢sp_Ca 4,, 33 (1977).s VJL.J. van Lint, 3.W. Harrity, and TJ_. Flanaipm, IEEE Trans. Nucl Sci. NS-15, 194(19es).e LC. Hushes, Phys. Eev. B 19, 5318 (1979).¢ E.W. _, BJ_. Eme, A.N. Goland, and GJ. Dienes, Phys. Eev. B 21, 3610 (1980).DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-once herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not ncccs,_arily state or reflect those of theU,ited States Government or any agency thereof.

A high repetition rate beam profile monitor

https://digital.library.unt.edu/ark:/67531/metadc1052582/m2/1/high_res_d/5046321.pdf

A high repetition rate beam profile monitor

Abstract

Similar works

Full text

Available Versions

UNT Digital Library