The AGS Booster is a rapid cycling proton and heavy ion synchrotron. Anomolous, intensity dependent losses in Au(32+) beams have been observed in the AGS Booster. No collective signal is expected, or observed, but increasing the number of injected ions decreases the beam lifetime. The loss rates for Au(32+) are compared with those for Au(15+)

Ahrens, L.

Blaskiewicz, M.

Calvani, H.

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1997 Particle Accelerator Conf., Vancouver, B.C., Canada, BNL-638 56 May 12-16, 1997. ANOMOLOUS, INTENSITY DEPENDENT LOSSES IN Au(32+) BEAMS 3 '  2.5 2 '  1.5 1 '  0.5 RECEIVED M. BIaskiewicz*, L. Ahrens, H.Calvani, AGS Dept. BNL,, Upton NY 11973-5000 USA . * Abstract homolous, intensity dependent losses in Au(32+) beams have been observed in the AGS Booster. No collective sig- nal is expected, a r  observed, but increasing the number of injected ions decreases the beam lifetime. The loss rates for Au(32+) are compared with those for Au( 15+) . 1 INTRODUCTION The AGS Booster is a rapid cycling proton and heavy ion synchrotron. The beam pipe has an average radius of 7 cm and the Booster's circumference is C = 202 m with be- tatron tunes w 4.8. When accelerating gold the pressure of the backround gas is P w 2 x Torr. The in- jection momentum of the ions is 9 GeV/c and the ex- traction momentum of 70 GeV/c was reached in 0.55 s for Au(15+) and 0.10 s for Au(32+) . A strongly inten- sity dependent loss in the Au( 15+) beam has been reported previously[l]. In this note we report a qualitatively similar behavior for the Au(32+) . Machine studies with Au(15+) were conducted in September 1994 and with Au(32+) in January 1997. In both cases the Booster magnet cycle was modified to in- clude porches of constant field, so that data could be col- lected at a fixed beam energy. The data consisted of digi- ti& current transformer traces taken under various condi- tions. During the magnetic porch the number of ions in the ring as a function of time N ( t )  was fitted by an exponen- tial N ( t )  M NO exp(-at) where a is the inverse life time or loss rate. For some of the data there was clear evidence that the losses were not a simple exponential, but without a theory we had no reasonable parameterization. 2 REVIEW OF Au(15+) DATA AND Au(32+) DATA Figure 1 shows the equilibrium loss rate as a function of the number of injected Au(15+) ions. The low intensity loss rates M O.ls-' are roughly consistent with scattering off residual gas at a pressure of 2 x 10-l' Torr. The lack of a coherent signal, and the insensitivity of the loss rate to the presence of rf suggests that the intensity dependent loss rates are due to the beam creating targets with which it sub- sequently scatters. This hypotheses is also consistent with the observation that the instantaneous loss rate depends on the intensity earlier in the cycle and on previous cycles. The dependence of the loss rate on machine history was observed in two ways. In equilibrium, where the same number of Au( 15+) ions were injected every cycle, the av- erage loss rates are as shown in Figure 1. What is not Work supported by United States Department of Energy JUN 2 5 4 1  1 1 I I I I 3.5 1 B + B + X X Q  X Q t 0 8  Q X $S ++ 0' I I I I I I 0 5 10 15 20 25 30 injected iondl 0% Figure 1: Au( 15+) loss rate versus number of injected ions for momenta of: 9 GeVk (diamonds), I 5  Gevk (crosses), 30GeVk (squares), and 60 GeVk (Xs) obvious from the figure is that the number of ions at the end of the Booster cycle was not monotonic with the num- ber of injected ions. For example, with a momentum of p = 30 GeV/c with 2 x lo9 ions injected there were 1 x lo7 ions remaining after 1.5s, while for 1 x lo9 ions injected there were 2 x 10' ions remaining after 1.5s. The history dependence of the loss rates was observed when the beam was turned off for several cycles and the machine was allowed to "cooI". The loss rate for the first cycle after cooling was M 50% smaller than the equilibrium value, and loss rates on subsequent cycles monotonically approached the equilibrium value. Taken together these observations provide compelling evidence that the Au( 15+) beam influences the background gas. We also note that the background gas is affected before a significant fraction of the beam is lost, since enhanced loss rates are apparent very early in the cycle. Figure 2 shows the equilibrium loss rate as a function of the number of injected Au(32+) ions. A clear trend in the Au(32+) loss rate with intensity is apparent. The Au(32+) data were not as good as the Au( 15+) data because of intensity limitations and generally smaller loss rates. At- tempts to measure memory affects and variations in the loss rate during a given Booster cycle were inconclusive. On the other hand, we see no reason to believe a qualitatively ~ T R I B U T f O N  OF THE DOCUMENT IS UNUMrrOiD yA ~ ferent loss mechanism for the two cases. 3 DISCUSSION Straight lines were fit to the loss rates in Figures 1 and 2, yielding the change in loss rate with the number of injected ions as a function of momentum. These derivatives are shown in Figure 3. For scattering off residual gas the loss rate is given by where v is the beam velocity and nj and oj ( v )  are the den- sity and total charge changing cross section for scatterers of the jth type. Only nj would vary with beam intensity. As- sume that any scatterers are uniformly distributed within the vacuum chamber and that only one species is respon- sible for the change in loss rate with intensity. Then the change in the number of scatterers N ,  with the number of injected ions Nb is given by where V, = 3. lm3 is the total vacuum volume in the ring. Set u = a1610-16cm2, p = pgGeV/c, and da/dNb = ~ ' l o - ~ s - ~ .  The beams are non-relativistic so dN,/dNb = 1.9 x 106cr'/(pga16), and for Au(15+) at p = 30 GeV/c, dN,/dNb = 1 x 104/als. For Au(32+) a tp  = 20 GeV/c, dN,/dNb = 7 x 1o3/uI6. For 616 = 10 there are several hundred scatterers generated by each Au( 15+) ion. For a beam velocity v = 0.15~ a gas pressure of P = 2 x 10-l'Torr, and a low intensity loss rate of Q = 0.25s-1 the data predict a loss cross section of ut = 0.87A2. This is a rather large cross section and to model the intensity de- pendent iosses even larger cross sections appear necessary. To model the losses, assume that hydrogen (H2) is the main vacuum component at low beam intensity. After the beam is injected it interacts with the hydrogen via a total cross section a: which can result in charge exchange 07 or just momentum transfer. When the hydrogen molecule hits the wall, with an energy large compared to kT, it des- orbs M molecules of type A. Molecules of type A remain in the beam pipe for a time TA before sticking on the wall. Since no significant rise in pressure is noted, assume that the amount of hydrogen is essentially constant. Then, In equation (1) Nb is the number of gold ions in the beam, N A  is the number of molecules of type A, NH is the num- ber of molecules of H2 and a? and uf are the charge ex- change cross sections for Ha and molecules of type A, re- spectively. It is assumed that molecules of both types are distributed uniformly throughout the entire volume of the vacuum chamber V, . In equation (2) there are M molecules of type A liberated by each molecule that is scattered by the beam, and uf is the cross section for momentum trans- fer between the beam ions and molecules of type A. These equations were numerically integrated using a 4s repetition period with beam present for 2s each cycle, as was done during the Au(15+) study. The calculation was continued until a nearly periodic solution was obtained and average loss rates were calculated. If we assume that the molecule in question is CO and that the stripping cross sections vary as the square of the target atomic numbers[3] then rt = 50A2. Assume that two CO molecules are liberated per energetic molecular impact and that the CO lifetime is TA = 20 s. Additionally, assume that the total cross sections for momentum transfer are ten times the cross sections for charge transfer, u? = 10A2 and uf = 500A2. With these assumptions the modeled Au(15+) loss rates as a function of the injected number of ions are shown in Figure 4. The loss rate is monotonic in the cross sections and significantly smaller cross sections did not accurately model the Au( 15+) data The change in gas composition with beam intensity should be measurable using mass spectroscopy. Such a measurement will provide additional constraints on any model. Since the Au(32+) Coulomb field is significantly greater than the field of Au( 15+) we expect any change in backround gas composition to be present when accelerating significant amounts of Au(32+). Hence, this model can be tested without returning to Au( 15+) running. For the Au(32+) data significantly smaller values of a$ are indicated. On the other hand, if the far field Coulomb force is responsible for the momentum transfer cross sec- tion at, then at should be larger for Au(32+) than for Au(15+) . If this is the case the charge exchange cross sections for Au( 15+) are much greater than simple scaling laws would indicate. 4 ACKNOWLEDGEMENTS We thank H.C. Hseuh for providing valuable information with regard to vacuum conditions in the AGS Booster. 5 REFERENCES [l J M. Blaskiewicz,L.A. Ahrens, H.C. Hseuh, T. Roser, Y. Shoji, [2] H.C. Hseuh, BNL BoosterTech Note # 1 16, (1988). [3] J3D. Betz, Rev. Mod. Phys., V o l a ,  pg 465 (1972) K. &no PAC95. 0.7 1 I I 1 I 0.6 - I3 - 0 Q QQQ a 5 0.4 - 0 rn - 0 0 +st + 0 +f 0.5 - - Q Q  A Y - 3 ++ $9; * 0.3 - 8 - -a %  + 02 - t o m  - 0.1 - - 2.5 - 0 I I I I I 0 5 10 15 20 25 30 injected iondl 0% 2 -  Figure 2: Au(32+) loss rate versus number of injected ions for momenta of: 20 GeVk (diamonds), 27 Gevk (crosses), - 1.5 - F -40 Gev/c (squares) Q c. E cn r0 1 -  - -0.5 - 0.2 1 I I I 1 cn 32+ - c 0.18 - 0.16 - - c 0.14 - 1 s  ----. - - 0 -  4 0 - ,' '., ,* . 1 I I I I ,' '\ 0 5 10 15 20 25 30 injected iondl 0% 8. 0 Q - - - Figure 4 Modeled Au( 15+) loss rate s-l versus number of '\ '. , , '. injected ions. 8. y " .L g 0.12 - s 0.1 - 1 3 0.08 - 1 , a S I , , -. Q I rn f e 2 0.06 - rn - 0 0.04 - 0 -0.02 - .... ... ....... .... ......... ......................... ................................ l 'P I Ic -g) 0.02 - c L 3  A= 0 0 10 20 30 40 50 60 ion momentum (GeV/c) Figure 3: change in loss rate with the number of in- jected ions, dcr/dNb (~~10'~) versus ion momentum, p (GeV/c). The error bars are one standard deviation. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their emplcyees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or pmcess disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, rccom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Portions of this document may be illegible in electiorric image prodock are produced f m n  the best available original dOCtXXlmt 

Anomolous, intensity dependent losses in Au(32+) beams

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Anomolous, intensity dependent losses in Au(32+) beams

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