126 research outputs found
Etude thermique de la plaque de couplage a 6m du RFQ d'IPHI
Une etude thermique de la plaque de couplage et de la barre de stabilisation dipolaire associee (Figure 1) a ete menee. Le but est de prevoir sur ces pieces les temperatures atteintes, les containtes, et les deformations attendues lors du fonctionnement nominal du RFQ d'IPHI
Explication de l'écart entre les réglages déduits d'une mesure par perturbation du champ électrique et du champ magnétique
Transverse emittance dilution due to coupler kicks in linear accelerators
One of the main concerns in the design of low emittance linear accelerators
(linacs) is the preservation of beam emittance. Here we discuss one possible
source of emittance dilution, the coupler kick, due to transverse
electromagnetic fields in the accelerating cavities of the linac caused by the
power coupler geometry. In addition to emittance growth, the coupler kick also
produces orbit distortions. It is common wisdom that emittance growth from
coupler kicks can be strongly reduced by using two couplers per cavity mounted
opposite each other or by having the couplers of successive cavities
alternation from above to below the beam pipe so as to cancel each individual
kick. We therefore analyze consequences of alternate coupler placements.
We show here that for sufficiently large Q values, alternating the coupler
location from before to after the cavity leads to a cancellation of the orbit
distortion but not of the emittance growth, whereas alternating the coupler
location from before and above to behind and below the cavity cancels the
emittance growth but not the orbit distortion. These compensations hold even
when each cavity is individually detuned, e.g. by microphonics. Another
effective method for reducing coupler kicks that is studied is the optimization
of the phase of the coupler kick. This technique is independent of the coupler
geometry but relies on operating on crest. A final technique studied is
symmetrization of the cavity geometry in the coupler region with the addition
of a stub opposite the coupler, which reduces the amplitude of the off axis
fields and is thus effective for off crest acceleration as well.
We show applications of these techniques to the energy recovery linac (ERL)
planned at Cornell University
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External Q studies for APT superconducting cavity couplers
Coupling coefficients for the APT superconducting cavity couplers have been predicted using an improvement of the method previously developed for the French Trispal project. The authors here present the method and a proof of the formula used to compute the external Q. Measurements on a single-cell copper cold model exhibited a very good agreement against simulation. Then, they established that the original coupler design lead to an insufficient coupling in {beta} = 0.64 cavities. Different solutions were proposed to fix this problem, like combining impedance discontinuities in the line and an off-centered disc end tip. Finally, it was decided to increase the beam tube diameter though it has some influence on the cavity end-cell performance
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Sparking rates measured on the CRITS RFQ
During the test of the LEDA injector on the CRITS RFQ, an automatic data acquisition system has been implemented. The purpose was to measure the sparking rate of this CW RFQ. The RF level has some influences on vacuum, but there is no evidence of any reciprocal effect. The raw sparking rate is very difficult to interpret, since burst of sparks bias the statistics. A more convenient and useful interpretation is the number of sparking seconds. At the nominal field level (1.75 Kilp), the sparking-second rate is 0.5 per minute without beam. It strongly depends on the field, with a logarithmic law: 4.5 decade/Kilp. With beam, the sparking rate jumps to 3.0 per minute. As far as tested, it depends neither on the beam current (20 to 80 mA) nor on the field (1.5 to 1.7 Kilp tested). With sparking rates as measured here, one could not hope to build an RFQ that would be free of sparks over a several months continuous operation. Such a requirement, based on an extrapolation of the curves presented here, would lead to a maximal electric field much lower than the Kilpatrick value, an unreasonable requirement for a functional RFQ. A conclusion is that a sparkless RFQ is hopeless, even with a very carefully conditioned cavity. It will probably be necessary to deal with a few sparks per day, and the linac must be able to restart automatically after a short beam interruption
Experimental determination of impedance and delay time of the 100 meander transmission line for the SPIRAL2 Single Bunch Selector
International audienceThe Spiral2 driver requires a Single Bunch Selector to reduce the bunch repetition rate at the experimental targets. A 100 Ω meander line is used in the beta 0.04 medium energy line of the Spiral2 driver. The non standard characteristic impedance figure helps to reduce the pulsed power but introduces the problem of calibrated measurements. The paper describes the results of the different methods used to measure the impedance and the delay of the electrodes
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Suppression of the 1 MHz beam current modulation in the LEDA/CRITS proton source
Earlier operation of a microwave proton source exhibited an approximate 1-MHz modulation in the beam current. This oscillation could cause instabilities at higher energies in the linac, as the low-level RF control for linac operation rolls off at 200 kHz. Tests on a dummy load show the modulation is created by the magnetron itself: at a typical power level required for the source operation (680W), the 1-MHz sideband level was as high as {minus}4 dB from carrier. Since the magnetron exhibited better behavior at higher levels, a RF power attenuator is inserted to force the magnetron to run at a 50% higher power level for the same final power in the load. This attenuator is made of two antennas plunged in the waveguide and connected to dummy loads by a coaxial line. As the antenna are separated by a quarter of the guided wavelength, mismatching effects approximately cancel each other. The antenna length is experimentally adjusted to obtain the {minus}1.8 dB attenuation required. Magnetron operation at the higher power level gives a beam current spectrum free of the 1-MHz modulation, showing the coherent beam noise is not generated by plasma chamber phenomena
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