Reviewer # 1:
Comment: Identical paper already issued by different author and review completed.
Reply: I am not sure how to reply on this, to the best of my knowledge, there is a paper on
damping of the crab cavity which involves detuning in this XB10. However, it is not identical,
though there are several papers on NLC_DDS by RMJ, et. al. This is subject to discussion
within us.
Reviewer # 2:
The article describes the design and fabrication of an alternative X band structure for the
compact linear collider project CLIC at CERN. Where the base line structure employs heavy
damping to control the transverse higher order modes, the approach here is to maximize the
detuning of the dipole bands combined with a small damping to avoid recoherence effects in
the
wake
field.
The publication describes the state of the project in a very thorough and comprehensible
manner. References are complete. The reviewer would only present the following minor
remarks:
* Section 2: The authors discuss the interleaving of eight different families of structures in
order to obtain the required wakefield suppression. This only works if the advancement of the
beta function over the set of eight structures is sufficiently small. I would recommend, that
this is mentioned in the text and the value of the phase advance from the current beam optics
setting
of
CLIC
is
given.
Reply: The interleaving of 8 structures (each with N=24 accelerating cells) will suppress the
wakefield recoherence within the acceptable limits. This will work when the betatron
wavelength is larger than the total proposed interleaved structure length. This is due to the
fact that the betatron wavelength will see only one single structure and not 8 different
structures. I have mentioned an approximate value of the betatron wavelength in my thesis
(For example, in CLIC main linacs, the betatron wavelength towards the beginning of the
lattice is 3.5 m and towards the end is 16.6 m). However, these numbers seem smaller than
expected. Alessandro and I will talk to Andrea Latina regarding the updated values.
* Section 4: The authors argue, that geometry for DDS_E has been chosen, because due to
its high aperture and high input power, it has the biggest probability of break downs. This
argument is a little bit confusing, because a high aperture lowers break down rates, where as
high power increases it. Given, that there are numerical criteria as P/C or the modified
Poynting vector, can the auhors explain the choice in terms of these quantities?
Reply: The optimised design is termed as DDS_E which incorporates the elliptical cavity
shape to satisfy the rf breakdown criteria. On the other hand, the beam dynamics criteria is
satisfied by interleaving 8 structures each with 24 accelerating cells. The average aperture
from structure number 1 to 8 gradually reduces. On line # 216 it is mentioned that the first
structure out of 8 in the DDS_E has been chosen, as it requires more power compared to the
remaining structures of DDS_E. As the aperture increases, the power requirement of the cell
or structure also increases, hence the ratio P/C does not change much, and this in general
leads to excessive power flow on the cavity walls, which potentially increases the breakdown
events. If the design of the cavity is such that increasing the aperture does not require more
input power (i.e. shunt impedance does not change) then P/C ratio will improve.
*
Reference 11, typo: replace 'new Journal of Physics' by 'New Journal of Physics'
Reply: Done
Reviewer # 3:
The paper has some very interesting results and should eventually be published. However the
paper is rather disordered and poorly written. This paper needs to be clearer before
publication.
Comments
on
the
Page 1 - CLIC frequency 11.9942 GHz not 12 GHz
manuscript
are:
Reply:Done
Page 2 - need some discussion of cell geometry and dimensions
The reviewer has not mentioned where he/she would like to see some discussion on the cell
parameters and geometry. I would describe it in a couple of lines in the penultimate section
on page 2 i.e. after line # 58 as follows:
Reply: The accelerating cells that are designed for the baseline structure are disc loaded and
are equipped with waveguides to damp the HOMs. The aperture radius ranges from 3.15 mm
to 2.35 mm with a linear taper and the iris thickness ranges from 1.6 mm to 1.0 mm [4-5]. In
designing CLIC_DDS, we attempt to closely match the geometry dimensions so as to satisfy
the design constraints, which are met by the CLIC_G baseline design. However, the disc
diameter in the baseline design is larger at least by a factor of 3, due to long waveguides.
Page 2- line 74 structure no structure
Reply:Done
Page 3 - line 86 - why do you need to alter the iris thickness to maintain a low group velocity,
I assume you are you changing some other parameter at the same time (such as iris radii) and
this
should
be
stated
in
this
sentence.
Reply: The group velocity is a function of iris parameters namely, radius and thickness.
Changing the iris radius is mainly governed by the requirement of the lowest dipole
bandwidth. For example, to achieve a lowest dipole bandwidth of ~3.3 GHz, the end cell iris
radii are 4.95 mm and 2.15 mm. For an iris radius of 4.95 mm, if the iris thickness is kept
similar to the baseline design (1.6 mm), then the group velocity will be as high as 6 to 7 %
(an approximate number). In order to reduce the group velocity (for a given iris) the iris
thickness is varied at the cost of shunt impedance.
Line 88 need a figure showing geometry to accompany table.
Reply: Will be inserted.
Line 88 what phase advance is this structure? Why was this phase advance chosen.
Reply: Phase advance for CLIC is mentioned on line # 14. This phase advance was chosen
because it has maximum shunt impedance [R. Miller, LINAC’86].
Line 95 - This sentence doesn't make sense "are calculated using computational tool
(uncoupled mode),"
Reply: The synchronous frequencies and kick factors are calculated for single periodic
regular cells using computational tools such as HFSS [www.ansoft.com].
Line 107 What is a non-smooth kick factor?
Reply: In Fig. 8 the circuit model kicks observe some kincks in the frequency range of 15.5
to 16.5 GHz.
Page 111 what is the beam dynamics requirements for damping? How is this requirement
obtained?
Reply: The beam dynamics requirement is to suppress the wakefield below 6.6 V/pC/mm/m
for a beam charge of 3.72x109 particles per bunch [4-5]. This is referred from [4-5] and CLIC
beam dynamics group at CERN.
Line 112 - In what way are the electrical requirements not met? How are these requirements
calculated?
Reply: The electrical breakdown requirement (Esur < 260 MV/m) is referred from [4-5] and
are numerically calculated based on various experimental results of NLC and CTF2(3). In
this structure the surface electric field was excessively large (Esur/Eacc= 5).
Line 114 What is the CLIC-G structure, please reference this.
Reply:
Reference
Line 121 Needs a reference for zero crossing
[4-5].
Reply:
Reference
Line 153 and 162 Equation format has been corrupted in conversion to pdf
[7,18-19]
Reply: Will be done.
Line 157 These are the first three dipole passband NOT modes. Modes are short for the
eigenmodes of the structure and there are far more than three.
Reply: Will be done.
Line 157 Is the first three passband sufficient? What about higher passbands? The 5th dipole
passband can often have high shunt impedances in some structures?
Reply: For travelling wave accelerating structures operating at 2π/3 phase advance per cell, it
is observed [SLAC-PUB-9467] that the lowest dipole band has the largest kick factor, the
other, not so large however, significant kicks belong to the third and the sixth dipole band.
Hence we consider detuning and damping of the lowest dipole mode.
Line 171 - How is this requirement calculated?
Reply: Explained on line # 111 of page 3.
Line 212 This section requires an extensive discussion on the Q factors achievable in this
structure. The manifold optimisation is related to the surface fields in the cavity and the HOM
Q's so they cannot be decoupled.
The dipole mode Qs obtained in CLIC_DDS study are of the order of ~1000. The manifold
tip cuts into the cavity to sample the dipole field, the dipole energy travels through the
manifolds before being damped in the remotely located loads (SiC). The coupling of the
dipole modes to the manifold can in principle be increased to further reduce the Q. This can
be achieved by increasing the width of the manifold coupling slot or by extending the
penetration of the manifold tip further down the cavity. Both the approaches will exacerbate
the surface fields. Hence, lowering the dipole Q is constrained by the limit on the acceptable
surface fields [4-5].
Line 215 What do you mean by HOM couplers? The manifold is a HOM coupler, do you
omit that? I am a bit lost about what you are and are not simulating. The discussion of the Q
factors of the dipole modes should be in the section on structure optimisation and NOT in the
high
power
testing
chapter.
Reply: The manifold couples out the field from the cavity. The HOM coupler will couple out
the field from manifold to the damping loads. There is only one section (i.e section 4 starting
from line 213, page 6) describing both, the structure optimisation and high power testing,
because it is optimisation of a structure for high power testing.
Line 229 How is the spectral functioned calculated? How are the modes damped at the end of
the manifolds (absorbing boundary, HOM loads)?
Reply: The calculation and theory of Spectral function is referred from [6,9-11]. The purpose
of CLIC_DDS_A is to study the high power performance of the structure. In this structure no
attempt has been made to damp the HOM in the manifolds. However, CLIC_DDS_B will
serve this purpose and is being studied. RMJ: Could you help in answering this question?
Line 230 You only have the Q factors for the first dipole passband? What about the other
passbands? Does this structure meet the Q requirements for all dipole modes in all dipole
passbands?
Reply: Please refer to line 157.
Line 263 The conclusion/ final remarks is far too short. A single sentence is far from
adequate.
Reply: RMJ: Could you help in answering this question?