Results from the Impedance Reduction in the CERN SPS
E. Shaposhnikova, T. Bohl and T. Linnecar
CERN, Geneva, Switzerland
Abstract. The future use of the SPS as injector for the LHC requires both a high intensity and a high quality
beam. For many years the longitudinal single bunch instability was one of the most serious limitations to obtaining
the nominal beam parameters. The main sources of impedance causing this instability were found a few years
ago from measurements of the spectrum of unstable single bunches. Recently the major part of a programme to
reduce this impedance was completed. Reference beam measurements with RF on and RF off have allowed the
changes in bunch stability as well as in the coupling impedance to be seen.
IMPEDANCE IDENTIFICATION
The impedance reduction programme started in the SPS a
few years ago to prepare the old machine for its new role
as injector of LHC. Earlier estimations, based on measurements of microwave instability threshold, suggested
[1] that the LHC beam with nominal bunch parameters
(intensity N = 1.1 x 1011 and emittance £ = 0.35 eVs)
will be unstable already on the 26 GeV injection plateau.
This forecast was confirmed in 1999, when LHC beam
was injected for the first time into the SPS. Prior to this
[2], measurements with single long bunches and RF off
had allowed the dominant resonant impedances with high
R/Q to be seen as peaks in unstable beam spectra, Fig. 1.
Most of the impedance sources were identified very
quickly. They were
RF cavities
f
type
MHz
100
200
200
352
800
SW
SW
TW
SC
TW
R/Q
Q
kOhm
1.4
3.87
25
0.93
6.5
150
80
130
500
150
*i
w
< <;
I
f
Wl
Sj
200 400 600 800 1000 1200 1400 1600 1800 2000
Frequency [MHz]
FIGURE 1. Averaged (over 15 acquisitions) projection of
bunch spectra measured in 1999. Bunch parameters: N = 6.0 x
1010, £ = 0.22 eVs, bunch length T = 25 ns.
operation
mode
dipole magnets in the ring. This work was completed
during the 2000/2001 shutdown [3].
A series of beam measurements done before and after
the impedance reduction allowed both the improvement
in bunch stability and changes at particular frequencies
to be seen. The results for bunch stability are presented
in [4] and summarised at the end of this paper. Below
we will present results devoted mainly to the search and
elimination of the 400 MHz impedance source.
leptons
leptons
protons, ions
leptons
protons
Pumping ports 0 -1000) with total R/Q = (25
45) kOhm and Q^ '50 at 1.5,1.9,2.4 GHz etc.
However the source of impedance at 400 MHz which
can be seen as a dominant peak in Fig. 1 was not obvious. The high signal amplitude suggested some significant impedance, comparable to the impedance of the
200 MHz RF system, also seen well in this spectrum.
With the end of LEP running, all lepton equipment was
removed from the SPS ring including the 3 RF systems.
The two RF systems with the highest R/Q remain in use.
The pumping port impedance was proved, both by
measurements and by simulations, to be the main source
of microwave instability. The installation of complex
shields with sliding contacts involved displacing 400
INSTABILITY AT 400 MHZ
The instability at 400 MHz was first observed in 1996 in
the measurements with long single bunches injected into
the SPS with RF off at 26 GeV. It was seen on a spectrum
analyzer connected to the wideband pick-up as a strong
signal growing at frequencies around 400 MHz. The
instability was studied both below and above transition.
A signal at 400 MHz will always be seen in such
measurements as a second harmonic generated by the
nonlinearity of the instability developing due to the large
CP642, High Intensity and High Brightness Hadron Beams: 20th ICFA Advanced Beam Dynamics Workshop on
High Intensity and High Brightness Hadron Beams, edited by W. Chou, Y. Mori, D. Neuffer, and J.-F. Ostiguy
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62
MD
0.7
8000
.
6000
9
4000
0.6
200
400
600
800
1000
0.4
FIGURE 3. Real part of MKE impedance measured in the
laboratory with wire method. Solid and dotted lines: standard
and improved log formula. Data taken from [6].
0
0
i
0
8
MST septa did not give significant impedance, however bench measurements showed a clear peak at the
frequencies of interest. These cavity-like objects were
shielded during 1998-1999, but the instability was still
there. Measurements of A200 (reference) and A400 following these changes indicated a 50% increase in intensity
at which A400 ~ A20o in 2000, compared to similar measurements done in 1999 although with longer bunches
(26 ns). Later measurements showed a strong dependence of A400/A200 on bunch length which could also
have contributed to this result [5].
Another possible candidate for the 400 MHz
impedance was the electrostatic septum ZS. There
are 10 of them in the ring and these devices are very
difficult to shield. Fortunately bench measurements
showed small impedance at 400 MHz.
Bench measurement done on the MKE and MKP kickers suggested large impedance around 400 MHz. Results
of measurements for the real part of the MKE kicker
impedance are presented in Fig. 3. During the 2000/2001
shutdown some kickers were removed from the ring and
screens were installed in others. Measurements done in
2001 showed no instability. They are presented below.
12
10
FIGURE 2. Mode amplitude at 200 MHz and 400 MHz as
a function of bunch intensity measured at 26 GeV in 2000
(T = 21 ns, e = 0.24 eVs).
impedance of the 200 MHz TW RF system. However
particle simulations show [5] that the ratio of maximum
amplitudes of the 400 and 200 MHz signals generated
by the TW cavity impedance, A400/A200 — ^ an^ me
results of the measurements shown in Figs. 1,2 can be
explained only by the presence of significant impedance
at 400 MHz. This impedance could also be a cause of the
single bunch instability observed at 26 GeV with RF on.
Estimations show that for a cavity-like object to have
its lowest resonant frequency, TM010, at 400 MHz, a
radius around 30 cm is required. The list of such objects
found in 1996 and studied later is presented in Table 1.
TABLE 1. Possible 400 MHz band sources in
the SPS in 1996.
n I Element
No.
1
400 MHz, LHC prototype cavity
MSL, lepton injection
MKLE electron extraction
MKLP positron extraction
MKA antiproton injection
1
2
1
1
2
2
MSE, extraction septum
MST, extraction septum, thin
ZS electrostatic septum
10
6
10
3
MKE extraction
MKP proton injection
3
3
RESULTS
Measurements of mode amplitude at 200 MHz and
400 MHz as a function of intensity are presented in
Fig. 4. Compared to Fig. 2, the amplitude of the 400 MHz
signal stays well below the 200 MHz signal.
Measurements of growth rates of the 400 MHz instability do not need any calibration, but they are difficult
due to the impure exponential character of signal growth.
Growth rates of the signal at 200 MHz and 400 MHz,
measured in 2000 and 2001 as a function of intensity assuming exponential growth are shown in Fig. 6.
The spectrum measured in 2001 with bunch parameters similar to those in Fig. 1 is presented in Fig. 7. The
400 MHz signal is now significantly lower and appears
as the second harmonic of the signal at 200 MHz. Peaks
at 1.5 and 1.9 GHz have disappeared as well due to the
pumping ports shielding.
The first group in this Table contains elements which
were removed from the ring in 1998 (400 MHz cavity)
and during 2000/2001 shutdown (lepton equipment). Unlike the second group in Table 1, they were never seriously suspected of being a source of single bunch instability due to their small R/Q. The second group of elements was carefully studied due to their large quantity.
Calculations of a simplified model of the MSE and
63
* 200 MHz
MD 20.08.01
35
A 400 MHz
f= 200 MHz
f= 400 MHz
^,30
d
V25
(D 15
2
3
Np [10*10]
4
FIGURE 6. 200 MHz and 400 MHz mode growth rate as a
function of bunch intensity in 2001 (T = 22 ns, £ = 0.2 eVs).
Bunch intensity/10
FIGURE 4. Mode amplitude at 200 MHz and 400 MHz as
a function of bunch intensity measured at 26 GeV in 2001
(1 = 21 ns, £ = 0.24 eVs).
MD 25.07.00
jj
a ma a
2.5
a
^0.04
f= 400 MHz
5
£=0.24 eVs
| 0.03
It
f i
E
03
D n HE
n
200 400 600 800 1000 1200 1400 1600 1800 2000
Frequency [MHz]
n
n n
0.5
f= 200 MHz
10
FIGURE 7. Bunch spectra in 2001. Bunch parameters: N =
6.0 x 1010, £ = 0.22 eVs, T = 25 ns.
12
N/101'
REFERENCES
FIGURE 5. 200 MHz and 400 MHz mode growth rate as a
function of bunch intensity in 2000 (T = 21 ns, £ = 0.24 eVs).
1. T. Linnecar, E. Shaposhnikova, Requirements for beam
parameters in the SPS when used as LHC injector, CERN
SL Note/94-87 (RF), 1994.
2. T. Bohl, T. Linnecar, E. Shaposhnikova, Measuring the
resonance structure of accelerator impedance with single
bunches, Phys. Rev. Lett., 3109, 1997.
3. P. Collier et al, Reducing the SPS impedance, Proc. EPAC
2002.
4. T. Bohl, T. Linnecar, E. Shaposhnikova, Impedance
reduction in the CERN SPS as seen from longitudinal beam
measurements, Proc. EPAC 2002.
5. E. Shaposhnikova, 400 MHz impedance - where are we?
Proc. LHC Workshop 2001, Chamonix, CERN-SL-2001003 DI.
6. F. Caspers et al, Impedance measurement of the SPS MKE
kicker by means of the coaxial wire method, PS/RF/Note
2000-004, 2000.
7. H. Burkhardt, G. Rumolo, F. Zimmermann, Coherent beam
oscillations and transverse impedance in the SPS, Proc.
EPAC 2002.
Comparison of the measurements of quadrupole frequency shift from 1999 and 2001 gives a factor 2.5 reduction in effective inductive impedance ImZ/n. Measurements of bunch length as a function of intensity demonstrate a factor 7 decrease in the slope of the bunch lengthening curve and a more than 10 times increase in microwave instability threshold for the range of bunch parameters used in the experiment.
In 2002 an LHC beam with bunch intensity up to
1.4 x 1011 was stable at 26 GeV.
Results for transverse impedance reduction are presented in [7].
We are grateful to PS and SL operation groups for
providing us with various beams during our studies.
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