|
|
|
Question #1:
The current layout of the megawatt proton beam line around the HE-beam splitter
(EHT4) with SHC4x and the magnetic septum ABS is shown in Fig. 1 (38 kB). The duration of the above described
procedure is estimated to last not much less than 60 s. It is also assumed that
(because of beam loading effects in the ring cavities and delays caused by proton beam
centering processes) the proton beam intensity cannot be raised faster than with 200 µA/s
so that the 6 mAs will be reached only after 8 s by just reaching 1.5 mA peak current.
This procedure will therefore reduce the availability of the proton beam on targets M, E and SINQ
by about 10 % and keep the mean time between failures (MTBF) at the low value of 10 min,
whereas already today, often during night shifts and over weekends it is found to be at 2
hours or even above.
Because no continuous proton beam of high intensity (megawatt) onto the target of UCN can be made
(thermal and shielding constraints), it is not possible to study the behavior
of the steering effects during the cranking-up procedure of the proton beam intensity
from injector 2. Because of all these reasons method A seems not to be very
attractive but cheap and possibly dangerous (has been discarded by now).
Method B: A fast kicker magnet (rise time < 1 ms is producing short (t = 5 ms ÷ 8 s) macro pulses of full proton beam intensity onto the target of the UCN source. Because of proton loss considerations the only position for a kicker is just in front of EHT4. The modified layout of the elements between AHC and EHT4 is drawn in Fig. 2 (11 kB). Space for the magnetic kicker is made by shifting QHA4 together with MHP5/6 some 40 cm closer towards QHA3. To gain this space the vacuum pump (PH11) and the vacuum valve (VHD1) have to be replaced by new models which require less space. The vacuum chamber at the location of the kicker has to consist of some sort of insulator like ceramics in order to avoid eddy currents which could slow down the kicking process. The steering effects of the splitter steering magnet (SHC4x) and the proposed kicker magnet in front of EHT4 (both 6 mrad) are compared in Fig. 3 (16 kB). Both produce a deviation of 40 mm at the location of the magnetic septum (ABS) which is required to get around its entrance collimator of 20 mm width. (Eventually the 2 present SHC4x steering magnet halves could be replaced by 2 new kicker magnets with ferrite yokes. But the stainless steel vacuum chamber of EHT4 will probably prevent fast rise times because of eddy currents.) The big advantages of the fast kicker method are, that it does not interfere with the accelerators and hardly diminishes the proton beam availability figure. Megawatt Proton Beam spill considerations (with method B) : During the transition of the megawatt proton beam between the targets and UCN half of the megawatt proton beam will be lost at the entrance collimator of the magnetic septum (ABS). The rest will be lost along both proton beam lines. But because of the fast transition time no interlock will be triggered during this time (< 1 ms). Unfortunately the kicked high intensity proton beam at the exit of EHT4 will be 7 mm further to the right than the proton beam deflected with SHC4x. Thus, for about 3 s every 10 minutes the kicker will produce some losses by protons hitting the EHT foils sitting in park position. ( With a megawatt proton beam size of 4 sigma = 20 mm all protons to the right of 3.4 sigma [340 ppm] will hit the splitter foil array and therefore be lost. See Fig.3. ) These losses could be reduced significantly by shifting the park position of EHT4 some 5 to 10 mm to the right and/or the proton beam center to the left. On the other hand, because of the low duty factor of only about 0.5 % these losses do not mean a big threat for the irradiated components, because they are comparable with the losses of the DC-splitting. Method B seems to be quite attractive but not cheap. Megawatt Proton Beam optics and centering: A possible proton beam envelope of the 1.5 mA proton beam intensity between the high intensity ring cyclotron extraction and the planned UCN target is shown in Fig. 4 (31 kB). The megawatt proton beam at the target (lead) of the UCN source is assumed to be about circular with a 4-sigma diameter of 11 cm. The correctness of the computed proton beam envelope (extrapolated from data gained with the megawatt proton beam on target M) should be checked with megawatt proton beam pulses of short duration (5 ms) and for that reason 2 pairs of harp monitors (present profile monitors will not work) are placed after ABK2. Additionally, the present electronics for the BPMs (beam position monitors) has to be replaced through a faster one (1 KHz) in order to be capable to detect the proton beam centers during some short (5 ms) high intensity test megawatt proton beam pulses [required for centering the proton beam see Fig. 5 (10 kB)]. A sketch of the layout of the proton beam line between ABK2 and the UCN-target is shown in Fig. 6 (61 kB).
Question #2: Table 1: Basic parameters for the 2 target thicknesses.
Table 2: Bend and Quad settings for the 2 target thicknesses.
|
Go to question #1: UCN with magnetic kicker.
proton beam envelopes for the 2
target E thicknesses (42 and 60 mm). The shown envelopes between Target E and
Collimator 3 (KHE3) are backward-extrapolations and correspond to the 2
transmitted proton beams (transmission = 57.5 or 69.3 %). In reality the proton beams in
this section are wider. Table 2 shows the bending magnet and quadrupole
settings for the 2 different lengths of target E. Of course, the 42 mm settings
have to be verified with some tuning and may finally change slightly. Also
the settings of the 5 steering magnets and the 2 pairs of halo-scraper slits
(not included in table 2) have to be tuned.
