Bossart, Rudolf

Juillard, M

Lalot, M

Lussignol, Y

Magne, C

Mosnier, A

Phung, B

CERN Document Server

HIGH RESOLUTION BPM FOR FUTURE COLLIDERSC. Magne, M. Juillard, M. Lalot, A. Mosnier, B. Phung, DAPNIA/SEA, CEA-Saclay, FranceY. Lussignol, DAPNIA/SIG, CEA-Saclay, FranceR. Bossart, CERN, Geneva, SwitzerlandAbstractA beam position monitor using a coaxial reentrant RFcavity has been designed and tested. Main advantages arethe small size, the high precision, excellent linearity, andthe broadband operation. The measured resolution is 10microns on the low current beam of the TTF linac. Theexpected resolution is 700 nm for the high current beamof TTF.1   INTRODUCTIONHigh resolution beam position monitors (BPM) will beneeded for the future linear colliders. Large band-widthswill also be important to operate fast feedback systems.We have designed and tested on TTF (Tesla Test Facility)a broadband BPM which can also be operated at liquidhelium temperature.The TTF superconducting linac at DESY is now inoperation with a low charge injector delivering bunches at216 MHz. In the next step an RF gun will provide highcharge (8 nC) bunches at the frequency of 1 MHz. TheBPM is suited to both operating modes. In the first casethe tests have shown a resolution of 8 m m. In the secondcase a resolution of 0.7 m m is expected.2   PRINCIPLE OF THE BROADBANDBPM WITH A REENTRANT CAVITYA reentrant cavity (fig.1) has a small size and the broadband is well adapted to the measurement of high chargeshort-bunched beams, such as they will be in futurecolliders [1]. The beam coupling impedance isadvantageously weak. The measurement frequency ischosen well below the monopole (TEM) and dipole(TE011) modes of the cavity, and below the cut offfrequency fc of the dipole TE11 mode of the cavityconsidered as a coaxial transmission line (table 1). Thebeam excites an "evanescent" TE011 mode [2], inproportion of its displacement off axis, damped towardsthe bottom of the reentrant cavity. The energy isconcentrated in the gap where the antennae  are located.The higher order modes are damped much more stronglyso their contribution is negligible and the linearity of themeasurement is ensured.If U1, U2, U3, and U4 are the voltages induced on eachantenna, Um the voltage induced by the monopolar modeon each antenna, I the beam current, x and y the beamdisplacements from the axis, then the transfer impedanceis defined by:Z UItm=and the dipolar constant L  is defined by:x = LU1 - U3U1 + U3y = LU2 - U4U2 + U4.Zt and L are calculated analytically in [2] and the BPMsensitivity isS = 2ZtL.3   FABRICATION OF THE CAVITYThe BPM precision at room and cryogenic temperatures(cooling without deformation) relies on the axialsymmetry obtained by machining the cavity with a lathe.The beam pipe radius is 78 mm and the gap of thereentrant cavity is 8 mm. A demountable cavity wasfabricated (figure 1) and severe tolerances were applied onthe radii and the perpendicularity of the assembling surface(2/100). The concentricity of the cavity walls actuallymeasured on the cavity is better than 20 m m.Figure 1: BPM cavityThe feedthrough have a standing wave ratio below 1.02at 650 MHz. They ensure vacuum tightness between thelinac 10-11 vacuum and the 2 K helium bath. To achievebroadband operation the cavity is strongly overcoupled byfixing the antenna tips to the inner diameter of the cavity.The external Q factor is about 4.4   RF MEASUREMENTS OF THECAVITY ALONEThe transmissions between two opposite antennae havebeen measured with a network analyser for two positionsof the antennae: pulled out or fully pushed. The first323open-2000-13723/08/1998position (resonant mode) is used to spot the preciseresonance frequencies and the second one (broadband mode)to visualize the bandwidth really used in the operationmode (figure 2 and table 1).Fig. 2: Cavity transmission, broadband(1) and resonant(2)5   SIGNAL PROCESSINGA 180°  hybrid junction is connected to each pair ofopposite antennae, yielding directly the sum anddifference RF voltages proportional to the beam currentand position. These RF signals are then filtered,amplified, and demodulated with a superheterodynereception. To achieve a good balance between the 4 portsthe hybrid coupler is placed in a box as close as possibleto the cavity. And a "perpendicular balance" is done: Areference signal is input through the horizontal antennaewhile attenuators and phase shifters are adjusted to cancelthe vertical power output.figure 3: BPM electronicsThe acquisition board yields analog output for noisemeasurements and digitized 14 bit output for the controlsystem. The software used is EPICS.6   CALIBRATION ON A TEST BENCHA test bench has been assembled to calibrate the BPMwithout beam, at room temperature. The beam issimulated by a moveable rod. The cw RF powerequivalent to the beam current is injected throughtransition cones as shown on figure 11. The cones have areflection coefficient of 0.25.figure 4: Test bench for the BPM cavity and electronicsThe characteristics measured with a network analyserare compared with the theoretical values in table 1. Thelinearity is shown in fig. 5. The noise measured is20 mV for a 4 V/mm signal, so the resolution is 5 m m.-3.0 -2.0 -1.0 0.0 1.0 2.0 3.0rod position (mm)-10.0-5.00.05.010.0detected voltage (V)linear regression coefficient = 0.99996Fig.5: BPM test on benchThe precision after calibration is given in table 2.Micrometric shifters should be used to evaluate therelative precision better.Table 1 (measurement frequency = 0.650 GHz)calculated measuredfc           (GHz) 1.0f  TEM   (GHz) 1.24 1.20f  TE011 (GHz) 1.7 1.658Zt          ( W ) 6.2 8L           (mm) 37.5S  (mV/mm/mA) 0.331 0.26Table 2absolute precision 0.2mmrelative precision <50 m m3247   MEASUREMENTS WITH THE TTFBEAMOne BPM has been tested inside the capture cavitycryostat at 2 K and the other at room temperature.   a)     Absolute precision     D  : The BPM error D bpm and thealignment error D align give:D  = D bpm+ D align = 0.2 mm + 0.1 mm = 0.3 mm.   b)     Beam     current      measurement:  The beam current ismeasured between 0.1 mA and 8 mA. By adding a highgain preamplifier the dark current emitted by the cavity (aslow as 100 nA) can also be measured.   c)     Resolution   : The measured noise is 40 mV when theRF amplifier is 54dB and the video amplifier has a gain of100. With the 5 V/mm BPM calibration for a beamcurrent of 8 mA this leads to a resolution of 8 m m.   e)     Linearity:  The x and y beam displacements weremeasured with an 8 mA, 35 m s, 10 Hz beam whilehorizontal and vertical steerers were varied. The steerers are1.513 m upstream. Their linearity is such thatd x/Isteerer = 2.188mm/A with a precision of.002 mm/A over the range -4V, +4V. (The DC powersupplies are stabilised to 10-3).-2.0 -1.0 0.0 1.0 2.0steering coil current (A)-5.00.05.0beam position  (mm)Figure 6: Measurement of the TTF beam positionThe linearity in y is observed with a regression coefficientof R = 0.9996 (fig. 6). In that example a fluctuation ofthe beam position is seen with a standard deviation of150 m m, about 5% of the beam size.8   EXPECTED RESOLUTION IN THECASE OF INJECTOR 2The short bunch signal is filtered at the output of thehybrid coupler. Bessel filters of order 7 have been chosenas they have the less distorted time-domain response andshortest rise time. The 4 MHz bandwidth Bessel filterentirely determines the time response of the electronicchain. The 10 ps bunch can be considered as a Diracimpulse and the input current is: Iin[A]=8x10-9xd (t).Calculating the output current from the transfer functionof the filter we find an envelope amplitude of 8x106xIinwith a time response width fitting inside the 1 m s bunchseparation. A comparison with the current of theinjector #1 shows that a factor 12 is gained on thesensitivity. As the noise level will be unchanged (sincethe same electronics are kept except for a modifiedacquisition card) the resolution will be: 8 m m / 12 =700 nm.9   CONCLUSION   a)     Results    of   the   tests   (low    bunch    charge)  The axial symmetry of the RF cavity allowed a highfabrication precision (2/100) and the absolute precision ofthe BPM was measured on a test bench (0.2 mm).On the Tesla Test Facility the BPM was tested at2 K and at 300 K; the beam position is measured in theranges 0 to 1 mm, and 1 mm to 10 mm; the beamcurrent and the dark current can be measured. The pass-band is 4 MHz due to the RF filter chosen. Theacquisition time is 1 m s. The resolution (at low bunchcharge) is 8 m m.   b)    Plans   for   injector    2   (high    bunch    charge:    8     nC)By modifying slightly the acquisition card  (changingthe sample-and-hold amplifier) the BPM is presently beingadapted to the high charge 1 MHz bunches of theinjector #2. The resolution will be 700 nm if nothingelse is changed (same RF amplifier, same RF filter).   c)     New   technical    developmentsChanging the RF amplifier to a low noise amplifierand broadening the pass-band of the RF filter from 4 MHzto 20 MHz will increase the resolution in the case of highbunch charge beams. A resolution of 100 nm should beobtained at 8 nC bunch charge.   d)     New    applicationsIt is possible to use the BPM without the presentelectronics (4 MHz band-width) and benefit from the realbandwidth of the BPM cavity and hybrid coupler (about200 MHz) if a degraded resolution (about 100 m m) isaccepted. This is presently tested to measure the HOMimpedances of the TTF 9-cell superconducting cavities.Resonances due to the HOM are excited by an offset of thebeam position and a modulation of the beam current atfrequencies from 5 MHz to 100 MHz. The growth of theresonances inside the 35 m s bunch train is observed withan oscilloscope and a spectrum analyser.An other application is to build a new acquisitionboard with flash components. In that case the BPM wouldhave a 25ns measurement time and could be inserted in afast  position feedback loop.10  REFERENCES[1] R. Bossart, “High precision beam position monitorusing a re-entrant coaxial cavity” Proceedings ofLINAC94, KeK, (1994).[2] R. Bossart, “Microwave beam position monitor usinga re-entrant coaxial cavity,” CERN report PS 91-59(1992).325

High Resolution BPM for Future Colliders

Abstract

Similar works

Full text

Available Versions

CERN Document Server