Technical Design Report Super Fragment Separator

DRAFT
Fast-extracted beams on a graphite wheel target
In view of the complexity of the operation of a liquid-metal target, it is desirable to use the rotating-wheel
target also for fast-extracted beams, as long as the critical parameters of graphite
(temperature, pressure) are not exceeded. This will be the case for low-Z projectile particles or low
beam intensities. Since the beam intensity at the FAIR facility will increase gradually in different
steps, one could use this option during the early phases. In case of the highest beam intensities one
can achieve lower-specific-energy conditions by extending the beam-spot size. In this context it is
important to remember that a wider beam spot in the dispersive (x-) direction will deteriorate the
ion-optical resolution, see Figure 2.4.13. An extended beam spot in y-direction will not affect the
resolution but reduce the transmission. Going to σy = 12 mm instead of 2 mm will reduce the
transmission into the CR without changes in the optics by a factor of three. In order to operate a
graphite wheel in the fast-extraction mode, there are several critical questions to be answered:
• What are the maxima (positive and negative) of the pressure waves that traverse a graphite
target after the impact of the short ion pulse?
• What are the technical limits of these pressure peaks that allow safe operation? Since the
technically acceptable operating temperature is about 1800 °C [65], one would need to
know the limiting pressure values at this temperature.
First calculations were performed with the 2-dimensional code BIG-2 [72] for three different
benchmark beams: argon, xenon and uranium. For each of these beams, different cases were
studied using different beam intensities – 10 10 , 10 11 , 10 12 ions/spill, and different sizes of the beam
spot with σx = 1mm and σy = 2, 6, 12 mm assuming two-dimensional Gaussian shapes. In each case,
the single spill was assumed to have a length of 50 ns while the beam energy was taken to be 1 A
GeV. For the uranium beam a target thickness of 3 g/cm 2 was assumed, while for the two other
beams it was 8 g/cm 2 . Calculations were performed for both, single-shot and multiple-shot cases.
Details of the calculations are given in Ref. [72]; here only a summary of results is presented.
The limiting values of temperature and pressure defining the regime where the target would survive
a single shot are taken as T = 2000 K and P = 150 MPa 2 , respectively. For 10 10 ions/spill, the
target would survive the irradiation with all three beams having nominal sizes of σx = 1 mm, σy = 2
mm. In the case of 10 11 ions/spill, the target would survive an argon beam with σy = 2 mm and a
xenon beam with σy = 6 mm, while none of the beam-spot sizes considered is acceptable for uranium.
For the highest intensity of 10 12 ions/spill and nominal beam spot sizes of σx =1 mm, σy = 2
mm, only in the case of argon the graphite target would survive a single shot. For the two other
beams the pressure and/or the temperature are above the critical limits for all three beam-spot sizes
considered. In order to use a solid graphite target with the highest intensity, calculations have
shown that one would need σx = 2.5 mm, σy = 12 mm for xenon and σx = 4 mm, σy = 12 mm for
uranium [72]. This would, on the other hand, decrease the resolving power of the Super-FRS. For
these cases, one should consider to use the liquid-metal jet target.
2 For the preliminary calculations presented in this report, a critical pressure value of 150 MPa is assumed. Since a
detailed study on values of critical parameters is under way, some conclusions on the operating regime might change.
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