NSLS-II INJECTION CONCEPT. Page: 3 of 5
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NSLS-II INJECTION CONCEPT
T. Shaftan#, I. Pinayev, J. Rose, X.J. Wang, J.B. Murphy, E. D. Johnson, J. Bengtsson, A.
Blednykh, S. Chouhan, S. L. Kramer, S. Krinsky, S. Pjerov, B. Podobedov, G. Rakowsky, T.
Tanabe, J.M. Wang, L.H. Yu, BNL, Upton, NY 11973, U.S.A.
Currently the facility upgrade project is in progress at
the NSLS (at Brookhaven National Laboratory). The goal
of the NSLS-II is a 3 GeV ultra-low-emittance storage
ring that will increase radiation brightness by three orders
of magnitude over that of the present NSLS X-ray ring.
The low emittance of the high brightness ring's lattice
results in a short lifetime, so that a top-off injection mode
becomes an operational necessity. Therefore, the NSLS-II
injection system must provide, and efficiently inject, an
electron beam at a high repetition rate.
In this paper, we present our concept of the NSLS-II
injection system and discuss the conditions for, and
constraints on, its design.
NSLS-II INJECTION REQUIREMENTS
A fuller description of the NSLS-II can be found in our
other publications [1,2,3]. The project features a third-
generation storage ring with optics based on 24 TBA
cells. Table 1 lists the NSLS-II parameters relevant to the
Table 1: Design parameters of NSLS-II
Energy 3 GeV
Operating current 500 mA
RF frequency 500 MHz
Number of bunches -700 out of -1,000
Charge per bunch 1.55 nC
Estimated lifetime 3 hours (-6 hours with
3rd harmonic cavity')
Length of straight section 7 m
In the following, we discuss two options for the full-
energy injector: a booster (repetition rate of a few Hz
depending on the choice of a ramping power supply), or a
linac (tens or hundreds of Hz).
The NSLS-II injection system must allow the storage
ring to be filled rapidly. For the duration of the initial fill,
we may write
Atfil b = - N , (1)
where Nb is the number of bunches in the ring, Iil/I is the
ratio of current in the injected bunch to the nominal
current in the storage ring bucket,f nj is the repetition rate,
and N. is the number of bunches in the macropulse (for
Using expression (1) and taking 3Hz and 60Hz for the
repetition rate in the single-bunch mode, we obtain,
respectively, 233 seconds (-4 min) and 11 seconds
correspondingly. We note that we assumed lossless
injection, and the ideal case "one injected bunch per
bucket". This might be difficult to achieve; consequently,
the initial fill can take longer, extending to tens of minutes
for a low- repetition-rate injection system.
Between fills, the stored beam current decays to 50%
of that at injection. These changes cause corresponding
variations in the heat load that entail thermal drifts in the
mechanical alignment of both the machine and the
This combination of the short lifetime with the high
average current of the NSLS-II justifies implementing the
top-off injection mode. This mode maintains the current
and, therefore, the heat load, within a fraction of a
percent, so eliminating the drifts and greatly stabilizing
operations. Furthermore, the average luminosity of a light
source approximately doubles by continually maintaining
a maximum current. The use of the top-off injection mode
is foreseen for many light sources, and already has been
implemented in some (APS , SLS , SPRING-8 ).
Table 2 shows the specifications for top-off injection
that satisfy users' requirements.
Table 2: Top-off injection parameters
Stability of average current -0.5%
Time between injections in top-off >1 min
Bunch-to-bunch variation of current <20%
The first requirement follows from constancy of the
heat load and already has been achieved in existing
facilities. The second requirement is defined by the
duration of user's experiments that are sensitive to the
injection transients. The last requirement is somewhat
arbitrary; however, experience with top-off injection at
SLS demonstrates substantial intensity-correlated orbit
oscillations for an uneven bunch pattern .
For the time interval between top-off cycles we get
AtTO = -r -ln 1-Mb2NnAtfm 1 2
where r is the lifetime, Alb/Ib is the bunch-to-bunch
variation of current in the pattern, and Atr is the time
interval for a single top-off cycle. To assess the stability
of the ring's average current we can write
AISRI SR AtTO Iv, (3)
Using (2), we obtain the duration of the top-off cycle,
At,, equal to 6 seconds for a repetition rate of 3Hz, and 1
minute for the interval between the top-off cycles (Table
2). We note that this value is unacceptably large,
signifying that 10% of the overall beam time is spent on
A system with a higher repetition rate of injection
eliminates this problem. For example, for the repetition
'Estimate is based on the measured data from the SLS .
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SHAFTAN, T.; PINAYEV, I.; ROSE, J.; WANG, X. J. & AL., ET. NSLS-II INJECTION CONCEPT., article, May 16, 2005; United States. (https://digital.library.unt.edu/ark:/67531/metadc1412587/m1/3/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.