Transcript Slides - Agenda INFN

Giorgia Favia
University supervisor: Luigi Palumbo
CERN supervisor: Michele Morvillo
① The LIU project, the PS RF 10 MHz system and motivations of the studies
② Improvements of the PS 10 MHz fast feedback
③ Ferrite magnetic properties studies and evaluation
④ Beam-cavity interaction studies and development of an impedance model
⑤ Validation of the upgrade and evaluation of the PS 10 MHz cavities impedance
The High-Luminosity LHC (HL-LHC) project aims to crank up the performance of the LHC in order to increase the potential
for discoveries after 2025. The objective is to increase luminosity by a factor of 10 beyond the LHC’s design value:
Beam intensity
Beam brightness
High intensity and high brightness beams can be achieved with
an upgrade of the LHC injectors → LHC Injectors Upgrade (LIU)
LIU-PS: increase the beam intensity from 1.7∙1011 ppb to
2.6∙1011 ppb within emittances of 1.9 μm at ejection from PS
The Proton Synchrotron (PS) determines the longitudinal structure of the LHC proton beam longitudinal structure (bunch spacing) as
the result of a sophisticated series of Radio Frequency (RF) gymnastics.
The 10 MHz RF system, which is responsible for beam acceleration, is seen as the most
probable impedance source and cause of beam losses.
10 MHz RF system:
11 cavities driven by amplifiers based on electron tubes (power up to 60 kW),
for radiation hardness and power dissipation constraints.
Ferrite loaded cavity
Gap voltage 0.5 - 10kVp
RF amplifier adopting:
Frequency range 2.8 – 10 MHz
3xYL1056 and 1xRS1084 tubes
Tuning via bias current that saturates
Housedthe
in the
cavity base
ferrite
BuiltFerrite
in 1975,
upgraded
in 1988
loss
resistance:
22kΩ/gap @3MHz
10kΩ/gap @10MHz
Beam loading represents the effect induced by the beam passage in an RF cavity.
The cavity-amplifier system is optimized for providing the maximum acceleration with the minimum RF input power.
𝜷
RF in a cavity
A short bunch arrives
and generates a wakefield
The field in the cavity has a phase and
an amplitude distortion
CURE: FAST FEEDBACK
When a bunch leaves the cavity, a beam induced voltage remains
VB = Zcav ∙ Ib
𝑍𝑐𝑎𝑣
𝑍
=
1 + 𝐴𝛽
 An upgrade of the fast feedback of the 10 MHz cavities is required to reduce their impedance
 Since the impedance is source of longitudinal instabilities, a development of the PS 10 MHz
cavity impedance model is necessary to plan possible cures.
UPGRADE OF THE 10 MHz CAVITIES
FAST FEEDBACK
• Three main amplification stages
• Grid circuit:
 50:200 load transformer
 variable inductance tuning the driver at the
same frequency of the cavity
 Inverter providing 180 phase shift local
feedback
• Local feedback
• Compensating network
• Anode feedback for cavity impedance reduction
Limits:
• RF input: up to 150 W available with consequent increase of
harmonics
• Large phase shift introduced by the first stage
• Lossy grid transformer
• Maximum input voltage allowed on the grid of the first stage
The aim of the upgrade is to maximize the loop gain of the feedback amplifier, still
providing a stable operation:
STANDARD CONFIGURATION:
Loop gain=26 dB → Impedance reduction ~20




Change of tubes working point;
New compensating network;
New grid circuit;
Additional improvements.
 Change of tubes working point;
VGRID
3A
1.5 A
VANODE
15 kV
6 dB
additional gain
 Change of tubes working point;
 New compensating network;
Phase shift 3-10MHz
30°→ 5°
 Change of tubes working point;
 New compensating network;
 New final grid circuit;
3 separated devices
• tuneable resonator coil;
• 50:200Ω load transformer;
• inverter.




NEW
3 MHz
10 MHz
DRIVER
LOAD
196 Ω
120Ω→177 Ω
Low Q
Unwanted resonance cancelled
Bias current reduced
Higher driver load




Change of tubes working point;
New compensating network;
New final grid circuit;
Additional improvements.
New FPGA based circuit for the bias
control of the new grid resonator.
 Direct conversion of the
cavity RF drive frequency
into bias current
New RF input driver



Developed in house
Up to 400W available;
Reduced harmonic
content.
Improvement of the internal connections
 damping of unwanted resonances
NEW CONFIGURATION:
Loop gain @10 MHz=30 dB
 A new prototype amplifier has been built and
installed in the PS ring during the winter technical
stop in 2015
 Higher loop gain is required at 10 MHz, which is the
accelerating frequency of the LHC-type beams.
 It has been tested when operated with beam
 Higher loop gain
 Stable operation in the full operating range
FERRITE STUDIES
Ferrite is used as core of the 10 MHz cavity and of the final grid resonator of the amplifier. Measurements of ferrite material
properties have been performed for the characterization of the existing elements and the development of new devices
𝑍 = 𝑗𝜔𝐿0 𝜇 ′ − 𝑗𝜇′′ = 𝑗𝜔𝐿𝑒𝑞 = 𝑗𝑋 + 𝑟
𝑍𝑓 = 50
inductance
1 + 𝑆11
1 − 𝑆11
losses
 Shorted coaxial line approximation
 Rectangular cross section approximation
𝑍 −𝑙 = 𝑗𝑍0 𝑡𝑎𝑛𝛽𝑙
𝑍𝑓 = 𝑗𝜔𝐿𝑒𝑞 = 𝑗𝑍0
2𝜋 𝜇
𝑍0 𝜇𝑙
𝜇
𝑙=𝑗
𝑐𝑜
𝑐
Z𝑓 = 𝑗𝜔𝐿𝑒𝑞 = 𝜔𝜇0 𝜇
𝐴𝑒
𝑙𝑚
When exposed to a magnetic orthogonal or parallel bias field, a reduction of the ferrite incremental permeability can
be observed, as well as a reduction of the losses
BIAS FIELD
𝜇𝑟 =
1 ∆𝐵
𝜇0 ∆𝐻
COMBINED BIAS TECHNIQUE
RF FIELD
RF FIELD
BIAS FIELD
𝜇𝑟 =
1 𝐵
𝜇0 𝐻
An optimum combination of the two
bias fields reduces the losses by a factor
of two for the ferrites under test.
BEAM-CAVITY INTERACTION ANALYSIS
A complete and accurate impedance model of the 10 MHz cavities is essential to understand the nature of the
beam instabilities and to plan possible cures.
Beam crossing the
gaps in series
 Impedance measurements;
 Pspice simulations;
 CST simulations.
A dedicated measurement campaign has been carried out. The voltage induced by a single bunch beam at
different harmonics has been evaluated.
The impedance is computed from the beam induced voltage:
In order to benchmark the measurements numerical simulations have been carried out.
The beam is simulated by means of two current sources delayed in time of 4 ns (time of flight between gaps)
CST offers the possibility to evaluate the beam coupling impedance. Geometry and materials properties of the cavity
are included into simulations results
Ferrite curves fitted by CST:
Since the total cavity impedance depends on the final tube resistance and is reduced by the fast feedback action, the
possibility to combine Pspice and CST has been explored
Good matching between
measurements and simulations
Different cases have been studied and several CST solvers have been used to compute the coupling impedance, validate
the results and understand the beam-cavity interaction
Case 1)
2) AC
wakefields
co-simulations
simulations
 The beam, being longitudinally longer than the time of
flight between the two gaps, excites both gaps almost
simultaneously.
 The gaps are coupled and, as a consequence, the voltage
re-distributes between the two gaps.
 The two cavity halves are equivalent, thus the same
voltage is induced on both gaps, making the role of the
coupling bar geometry irrelevant.
Each PS 10 MHz accelerating cavity consists of two ferrite-loaded lambda/4 resonators:
• both resonators oscillate in phase, as their gaps are electrically connected by short bars.
• they are in addition magnetically coupled via the bias loop used for cavity tuning.
Gap relays short-circuit the gaps when the cavity are not in use
Measurements of beam induced voltage in the two half-cavities indicate that the coupling as seen by the beam is not as tight
as expected.
 The bars connecting the two gaps do not provide a perfect coupling, the voltage does not re-distribute between the two gaps.
 The magnetic coupling seems to do not influence the distribution of the induced voltage between the gaps.
VALIDATION OF THE
UPGRADED AMPLIFIER
Measurements have been performed on a PS
cavity, when driven by a standard amplifier and
by the upgraded one
A reduction of the impedance by
a factor two has been achieved
at 10 MHz
Measurements performed
varying the input power driving
the amplifier demonstrate that a
stable operation can be provided
Measurements of the impedance of all PS cavities provide an important input for the present campaign
which aims at an impedance reduction and provide cures to instabilities, such as coupled-bunch
instabilities (see L. Ventura presentation)
In the framework of the LIU upgrade important results have been achieved:
 A new amplifier prototype has been developed, installed in the PS and tested with beam
→ Impedance reduction up to a factor of two, compared with standard amplifier performance;
→ A stable operation is provided in the operating frequency range and at the operational level;
→ A measurements technique of ferrite properties has been developed, and an innovative
method for biasing the ferrites has been tested.
 A proper evaluation of the 10 MHz cavity impedance has been provided and demonstrated
→ Pspice simulations have proved to be a powerful means for the synthesis of a complex system,
as well as for the characterization of single circuit elements and of their contribution to the
whole system behaviour;
→ An innovative Pspice-CST model has been developed, which has been fundamental in the
beam-cavity interaction studies.

TO DO NEXT:

Explore the possibility to integrate a solid-state amplifier in the chain by numerical
simulations and experiments;
Upgrade of all PS amplifiers to translate the impedance reduction into an improved beam
performance.