Arguments to choose the Frequency for a new 180 MeV Linac and the associated Front-End Test Stand at RAL
In case the ion source fails to deliver the required currents and duty cycles a beam funnel could help to achieve the necessary beam intensity. Up to now there have been two experiments for a beam funnel after an RFQ. One of them is well documented and was constructed as a one-leg funnel at 5 MeV with a 425 MHz strongly collimated (40 %) input beam [4], [5]. It was found that 25-40mA beams can pass the funnel structure with 100 % transmission without a major transverse or longitudinal emittance increase (within measurement precision). However, no reference was made to the used duty factor (which leads to assume that the duty factor was very low) or if the longitudinal beam properties satisfied the conditions for injection into a higher frequency RF system. Other critical points are the cooling of the funnelling device and the beam dynamics after the funnel. Any development of beam halo can only be measured after several focusing periods and was not assessed in the experiment. Since the beam will not see a constant field along the length of a bunch, different portions of the bunch will experience different deflection forces, which will yield transverse oscillations after the funnel device. More recent efforts towards funnelling which will be detailed in the following fall in two categories: a) funnelling at an energy of around 20-30 MeV (so far theoretical work), b) low energy RFQ funnels (theoretical and experimental work) For recent high-power proton linac studies (e.g. ESS [1] [2], CONCERT [3]) a merging of the beams at an energy of 20-30MeV is suggested to avoid beam blow-up through space charge. Furthermore, the space charge forces in the two chopper lines would be relaxed putting less constraints on the beam dynamics design in this area. The required frequency doubling demands that the subsequent accelerating structure is reasonably efficient at the higher frequency. For high-intensity linacs this limits the frequency after the funnel to ≈ 400 MHz if one wants to avoid a high number of single RF sources (see section 4). Neither CCDTL, SDTL nor traditional DTL structures are practical at higher frequencies for this energy. Higher energies for beam funnelling are excluded because of the required voltages for the funnelling device. The 2nd scheme is studied by IAP suggesting beam funnelling within a funnel-RFQ [6]. With this device every eventual beam loss will take place in the RFQ, at low energy. Up to now these funnels still suffer from emittance growth (≈ 60 %) but given enough time for R&D they represent an interesting option for the 352/704MHz and 400/800MHz option: in case of too low source currents one could replace the source and RFQ part with a half-frequency front-end instead of building two complete legs including source, RFQ, chopper line, and first DTL tank. Since the RFQ basically defines the longitudinal phase space, the risk of additional phase and energy jitter is much lower than for scenarios with two frequency jumps at later stages in the linac.
Beam-dynamics design and performance of the RF deflector in the Los Alamos single-beam funnel experiment
A single-beam funnel experiment exploring the beam dynamics and physics issues of a discrete-element funnel has been completed at Los Alamos National Laboratory (LANL). The only known beam dynamics issue of concern in a two-beam funnel not addressed was the beam-beam interaction, which is negligible. The RF deflector, in which beam merging occurs, is a key component of a discrete-element funnel. The beam dynamics design procedure and experimental results for the RF deflector are reported.<<ETX>>
Experimental energy barriers to anions transporting through nanofiltration membranes.
Environmentally relevant contaminants fluoride, chloride, nitrate, and nitrite face Arrhenius energy barriers during transport through nanofiltration (NF) membranes. The energy barriers were quantified using crossflow filtration experiments and were in the range of 7-17 kcal·mol(-1), according to ion type and membrane type (Filmtec NF90 and NF270). Fluoride faced a comparatively high energy barrier for both membranes. This can be explained by the strong hydration energy of fluoride rather than other ion properties such as bare ion radius, fully hydrated radius, Stokes radius, diffusion coefficient, or ion charge. The energy barrier for fluoride decreased with pressure, indicating an impact of directional force on energy barriers. The influence of temperature-induced pore radius variability and viscosity on energy barriers was considered. The novel link between energy barriers and ion properties emphasizes the importance of ion hydration and/or partial dehydration mechanisms in determining transport in NF.