DESIGN OF A 7-CELLS, HOM DAMPED,
SUPERCONDUCTING CAVITY FOR
THE STRONG RF FOCUSING EXPERIMENT AT DANE
David Alesini, Caterina Biscari, Roberto Boni, Alessandro Gallo, Fabio Marcellini,
Mikhail Zobov (INFN/LNF, Frascati (Roma));
Carlo Pagani (INFN/LASA, Segrate (MI); DESY, Hamburg)
CAVITY DESIGN CRITERIA
DAMPING OF HIGHER ORDER MODES
CONTROL OF THE TM010-LIKE MODE FREQUENCIES
COUPLED BUNCH LONGITUDINAL INSTABILITY
CHOICE OF THE EXTERNAL Q
INPUT COUPLING DESIGN
THE STRONG RF FOCUSING EXPERIMENT AT DANE HAS
BEEN PROPOSED TO CREATE AND OBSERVE A BUNCH LENGTH
MODULATION ALONG THE RING.
THE STRONG FOCUSING REGIME REQUIRES A VERY LARGE RF
GRADIENT THAT CAN BE ONLY REACHED BY A MULTI-CELL
SUPERCONDUCTING CAVITY.
TO DEMONSTRATE THE FEASIBILITY OF A HIGH LUMINOSITY
COLLIDER
BASED
ON
THIS
PRINCIPLE,
A
TOTAL
MULTIBUNCH CURRENT OF THE ORDER OF 1A HAS TO BE
STORED UNDER STABLE CONDITIONS IN THIS REGIME.
HIGH CURRENTS AND MULTIBUNCH STABILITY OPERATION
INVOLVE PROBLEMS, SUCH AS THE CURE OF HOMS, THAT
HAVE TO BE KEPT INTO ACCOUNT.
TESLA CAVITY
CAVITY PARAMETERS
RF frequency [MHz]
1288.9
Max RF voltage [MV]
8
R/Q geometric factor [W]
Quality factor (@ 1.8 K)
Cavity wall power [W @ 1.8 K]
Loaded quality factor
Cavity detuning for Beam
Loading (@ 8MV, Ib=1A) [kHz]
RF generator power [kW]
Cavity length [m]
390
1 · 1010
8
(14) ·107
-60
1
0.8
CAVITY PROFILE AND DIMENSIONS
Enlarged
beam pipe
For the experiment has been
designed a 7-cells cavity based
on the design of the TESLA
cavity. Main modifications are:
• basic cell dimensions has
been lightly changed to comply
with the DANE revolution
frequency.
• number of cells has been
changed from 9 to 7 to reduce
the number of the cavity HOMs
• beam tubes have been
enlarged to let most of the
HOMs propagate and be
damped by room-temperature
ferrite rings.
DAMPING OF HIGH ORDER MODES
transmission coefficient between the two enlarged beam tubes for the 2nd
and 3rd monopole bands. The highest quality factors are of the order of 103.
CONTROL OF THE TM010-LIKE MODE FREQUENCIES
The modes of the first longitudinal band, which
include the accelerating one, do not propagate in
the beam tubes and remain undamped.
Their resonant frequencies should not overlap
the bunch revolution harmonics to avoid
anomalous power exchange between the beam and
the cavity.
nominal resonant
frequencies and R/Q of
the non propagating
modes of the 1st
monopole band
Mode
f [GHz]
R/Q[W]
p/7
1266.73
1.10-2
2p /7
1269.90
1.4.10-5
3p /7
1274.53
1.10-1
4p /7
1279.72
4.10-4
5p /7
1284.45
1.4
6p /7
1288.95
1.6.10-3
modes of the 1st monopole band
CONTROL OF THE TM010-LIKE MODE FREQUENCIES
The real frequency values of these modes will be displaced by few hundreds kHz respect to the
nominal values, because of cell fabrication tolerances.
A cavity tuning, by controlled plastic deformation of the cells, is then necessary to precisely
position the frequency of the accelerating mode (p-mode) and restore a good field flatness.
In addition this procedure can be used for tuning the resonant frequency of the other 6 modes
safely away from the DAFNE bunch revolution harmonics.
FREQUENCY SENSITIVITY OF EACH TM010 MODE AND FIELD FLATNESS OF THE p-MODE
AS A FUNCTION OF THE LONGITUDINAL DEFORMATION OF EACH CELL.
Frequency sensitivity
Electric field sensitivity
From previous results it is possible to calculate the cell deformations to implement in the final
tuning procedure to shift the resonant frequency of one of the trapped modes by a given quantity.
Example: assuming that a +200 kHz shift
of a selected mode in the band is needed.
Applying each of these sets of cell
longitudinal deformations:
• frequency of the relative mode is shifted
by +200kHz;
• frequency of pi-mode is unperturbed;
• field flatness degradation is minimized;
• frequency shifts of all the other modes
of the band is minimized.
independently of the selected mode, the
required cell deformations are
 0.3 mm, while the frequency shift of
the other modes is  200 kHz. The
accelerating mode R/Q reduction, due to
the loss of field flatness, is only few
percent.
COUPLED BUNCH LONGITUDINAL INSTABILITY
It could rise from the coupling with the synchrotron sidebands.
To reduce beam coupling impedance of 1st band monopoles the cavity has to be
powered through an efficient direct RF loop.
The use of a wideband, low group delay solid state amplifier is particularly suitable for
this task.
A comb filter tuned on the bunch revolution harmonics is necessary to avoid interaction
with powerful spectral lines that may saturate the loop.
The cavity can be
driven by
combining 4,
250W each, solid
state amplifiers.
Three of them are
already available
at LNF, as spare
for the DAFNE
longitudinal
feedback system.
OPTIMUM QEXT AND
MICROPHONIC NOISE
The input coupling factor of the SC
cavity is defined essentially by the
amount of microphonic noise that shakes
the SC cavity and continuosly displaces
its resonant frequency.
The optimum QEXT value depends on the
maximum tolerable frequency deviation
of the accelerating mode. If this figure
exceeds the 15 Hz value, the 1 kW RF
power available from our solid state
amplifier is no more sufficient to
guarantee 8 MV of accelerating voltage.
Depending on the amount of the
microphonic noise the optimal Qext of the
input coupler ranges from 2107 to 4107.
INPUT COUPLING DESIGN
Also the input coupler is of the same type of the TESLA one.
Like in the TESLA cavity, coupling coefficient variations will be possible by regulations,
through a bellows system, of the inner conductor penetration into the beam tube.
Q LOADED SENSITIVITY TO THE COUPLER INSERTION DEPTH
TARGET VALUE: QL = 2E+7
d [mm]
100
b [mm]
57
p [mm]
Q
45
54
55
58
61
1.225E6
9.439E6
7.428E6
1.298E7
1.922E7
HFSS INPUT
GEOMETRY
TRANSMISSION FREQUENCY RESPONSE OF
THE CAVITY. QEXT HAS BEEN CALCULATED
FROM 3dB BANDWIDTH.
FIELD FLATNESS FOR A GEOMETRY
WITH THE COUPLER INCLUDED.
CONCLUSIONS
A 7-cells SC cavity based on the TESLA design has been
proposed for the bunch length modulation experiment to be
made at DAFNE.
The basic cell has been slightly modified to resonate on the
420th harmonics of the bunch revolution frequency.
The high order modes, with the exception of modes of the 1st
monopolar (TM010) band, propagate in enlarged beam tubes
and are damped in room-temperature ferrite ring loads.
The control strategy of the TM010 band, which includes the
accelerating one, relies on detuning the modes from the bunch
revolution harmonics and impedance reduction through the
implementation of a direct RF feedback loop.
The TESLA cavity input coupler allows adjusting the coupling
coefficient, whose optimal value is defined by the amount of
microphonic noise affecting the cavity accelerating mode.