Search Results (6 total)

The Rapid Cycling Synchrotron (RCS) of the J-PARC complex in Tokai, Japan, is designed to accelerate a high intensity proton beam from 181 MeV, and later 400 MeV to 3 GeV in 20 ms within the 40 ms machine cycle. The beam power up to 1 MW demands a stable beam control to avoid excessive losses and activation of the accelerator chain. The fully digital control system is based on quadrature modulation and demodulation. In the amplitude control loops standard FIR filters separate the harmonics (h=2) and (h=4) after down conversion. For the phase loops at (h=2) and (h=4), intended to damp synchrotron oscillations, the delay in a FIR filter would limit the loop stability. Cascaded integrator comb filters, also called CIC filters, provide a shorter delay because they filter the longitudinal beam signal only where it is necessary. The notches are located at multiples of the revolution frequency of the proton beam. For fixed frequency accelerator applications, digital comb filters with fixed clock frequency are widely used to improve loop stability. For variable frequency accelerator applications, as in a proton synchrotron, where the frequency swing is larger than the notch width, usually the clock frequency of the comb filter is variable and chosen to be an integer multiple of the particle revolution frequency. At J-PARC RCS, the clock frequency has to be fixed. Tracking the frequency would require a variable noninteger number of filter taps. Here we present a filter, based on the weighted output of 2 CIC filters with variable length, and one tap difference. The filter function looks like a CIC with smoothly varying coefficients, where the notches follow the revolution frequency of the proton beam. The delay of this filter is approximately half of the corresponding FIR filter, so that the phase loops have a higher stability margin.

High resolution ac calorimetric measurements have been carried out near the smectic-A–smectic-C phase transition in a racemic mixture of 4-(1-methylheptyloxycarbonyl)phenyl 4^|IH-octyloxybiphenyl-4-carboxylate. The heat capacity data show a distinct pretransitional excess above the transition temperature as well as below it. The data have been analyzed in detail with the renormalization-group expression with correction-to-scaling terms. It was found that the data show Gaussian tricritical behavior. It was also found that the present data support the first-order correction exponent Δ₁ to have a value 0.5. The value of nonuniversal amplitude ratio A^-/A⁺, and the possibility of the existence of the logarithmic corrections have also been discussed.

High resolution ac calorimetric measurements have been carried out for two liquid-crystal systems: 4-(1-methylheptyloxycarbonyl) phenyl 4′-octyloxybiphenyl-4-carboxylate (MHPOBC), and 2-fluoro-4-{[(1-trifluoromethyl) undecyloxy] carbonyl} phenyl 4′-(dodecyloxy) biphenyl-4-carboxylate (12BIMF10). The heat-capacity anomaly around the smectic-A to the chiral-smectic-C transition has been analyzed in detail. It is revealed that the heat anomaly for both systems shows a crossover from three-dimensional XY critical behavior to tricritical behavior. All the data are described well with a crossover function which has been obtained from a modification of the original Rudnick-Nelson-type expression.

High resolution ac calorimetric measurements have been carried out near the smectic-A—smectic-C phase transition in a racemic mixture of 4-(1-methylheptyloxycarbonyl)phenyl 4′-octyloxybiphenyl-4-carboxylate (MHPOBC). The heat capacity data show a distinct pretransitional excess above the transition temperature as well as below it. Analysis of the data reveals Gaussian tricritical behavior of this transition. It is also found that the anomaly is described well with the renormalization-group expression with corrections-to-scaling terms.

High resolution ac calorimetric measurements have been carried out near the smectic-C_α^*–smectic-A phase transition in an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl)phenyl4’-octyloxybiphenyl-4-carboxylate. A clear deviation from the extended mean-field Landau behavior was seen. The data have been analyzed using a renormalization-group expression, including the correction-to-scaling terms. It was found that the heat capacity anomaly is described well with the three-dimensional XY model, in agreement with the theoretical prediction. It was also found that the second-order correction terms play quite an important role.

The momentum dependence of the analyzing power A_y in proton-proton elastic scattering has been measured in small steps using an internal target during polarized beam acceleration from 1 to 3 GeV/c. The momentum bin size ranges from 5 to 18 MeV/c. The relative uncertainty of A_y is typically less than 0.01 for each momentum bin. Narrow structures have been found in the two-proton invariant mass distribution of A_y.