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TIARA-REP-WP7-2014-005
Test Infrastructure and Accelerator Research Area
Status Report
Design report of a 3 MW power amplifier
Montesinos, E. (CERN) et al
10 February 2014
The research leading to these results has received funding from the European Commissionunder the FP7-INFRASTRUCTURES-2010-1/INFRA-2010-2.2.11 project TIARA (CNI-PP).
Grant agreement no 261905.
This work is part of TIARA Work Package 7: ICTF R&D Infrastructure.
The electronic version of this TIARA Publication is available via the TIARA web site athttp://www.eu-tiara.eu/database or on the CERN Document Server at thefollowing URL: http://cds.cern.ch/search?p=TIARA-REP-WP7-2014-005
TIARA WP7 D7.4
2014-02-10
Eric.Montesinos@cern.ch
Novel pulsed Diacrode® RF power amplifier tests at LANL
Author(s):
Eric Montesinos, CERN
John Lyles, Los Alamos Neutron Science Center (LANL)
Alan Grant, STFC Daresbury Laboratory
Kevin Ronald, SUPA and the Department of Physics, University of Strathclyde
Keywords: TIARA ICTF Diacrode RF power amplifier test
Summary
The Ionisation Cooling Test Facility at the Rutherford Appleton Laboratory is the only facility
in the world capable of providing the necessary infrastructure to develop the technologies
required for muon cooling for the Neutrino Factory and Muon Collider communities. MICE is
expected to be completed by 2019-2020 after which the six-dimensional (6D) ionization
cooling R&D programme will be initiated. It is crucial that the ICTF infrastructure be
sustainable until at least 2028. It is therefore essential that preparations be made to replace the
triode-based amplifiers in a future upgrade. For this reason, it is proposed to carry out a design
study of a novel tetrode-based high-power amplifier using a Diacrode®*
that will form the
basis of a future upgrade to the ICTF RF power system. Recent work on a similar amplifier
system at Los Alamos National Laboratory provides insight to a proven design that is
currently commercially available. This proposal will summarize that design as it is becoming
a new standard for 200 MHz RF power systems to replace older triode-based amplifiers that
use legacy or obsolete vacuum electron devices. The project also serves to lay the groundwork
for future full scale deployment of 200MHz amplifiers for the cooling channels for a Muon
Collider or Neutrino factory.
1. Introduction
A review of the state of development of tetrode, and more specifically, Diacrode®
-based
amplifier systems, has been carried out in the international community. One goal of the
review was to identify recommendations of “best practice” that can be incorporated into the
TIARA ICTF design. Contact has been made with Los Alamos National Laboratory (LANL)
where tests of a Diacrode®-based high power amplifier system are undertaken exploiting an
installation of commercially manufactured units. It is important to consider the results of the
tests performed and the LANL amplifier and system design, during the development of the
amplifier for future use in the ICTF. Extensive details of the LANL amplifier and the system
* Diacrode
® is a unique modification of a tetrode vacuum electron device, having a double-
ended RF geometry for higher power capability at 200 MHz. It is produced by Thales
Electron Devices in Thonon les Bains, France [2].
- 2 -
design are included in the previous report [1] (http://cds.cern.ch/record/1510945). This current
paper reports on additional tests performed with the LANL amplifier configured to match the
ICTF pulse and duty requirements and summarizes the industrialisation of LANL Diacrode®-
based RF amplifier.
During 2013, the LANL amplifier has been operated in the mode required by the TIARA
ICTF project: a 1 ms pulse at a repetition rate of 1 Hz. CERN provided a spare Diacrode®
tube to LANL for collaboration.
2. The LANL Test Facility
A high power test facility was constructed simultaneously with the development of the new
power amplifier at LANL. A block diagram of major components of the RF test facility is
shown in figure 1. Details about the amplifier cascade consisting of solid-state preamplifier
and Intermediate Power Amplifier have been published [3].
Figure 1. Block Diagram of Test System at LANL
3. Testing of Diacrode® RF power amplifier at LANL
In March 2012, tests were performed at LANL. The goal was to prove the peak power
capability of the Diacrode®. A peak power of 3 MW was obtained with the first Thales
Diacrode®, serial number A04, property of LANL. Both grid power supplies were near their
peak ratings, so this was the peak power limit of the valve installed in the circuit at the LANL
test facility.
- 3 -
Ea Ia Eg2 Eg1 Pin Pout Η Pg
kV A kV -V kW MW % dB
29.4 153 1.6 430 96 3.0 65.3 14.9
Table 1 : Test results with 300 microseconds pulse width, 30 pulses per second repetition rate
Figure 2: 3 MW peak power achieved with the first LANL Diacrode®
Extensive testing of the prototype power amplifier at Los Alamos, at higher duty factors of up
to 12% with 120 pulses per second, proved that the amplifier design was stable and reliable.
In March of 2013, the TH628 Diacrode® that had been on loan from CERN (serial number
0102) was installed in the prototype amplifier and the pulse parameters were set to closely
mimic the MICE requirements, operating at a peak power of 2 MW. The other parameters
were as per MICE requirement; 201.25 MHz, 1 millisecond pulse length, and one pulse per
second repetition rate. RF power envelopes were captured using Boonton model 4542 peak
power meters. The results are shown in figures 3 and 4, with two horizontal time scales.
Figure 3:Forward (blue) and Reflected (red) Power
- 4 -
Figure 4: Same signals with expanded time axis
There were no problems reaching these parameters except for the higher than normal reflected
power of 21 kW peak. This was due to the low temperature of the ionic solution used in the
water filled RF load. At higher duty factor this value is less than 1 kW. The amplifier was able
to provide 2 MW as expected.
Figure 5: Tube Anode (blue), G1 (magenta) and G2 (cyan) currents
Figures 5 shows the corresponding DC currents in the Diacrode®, while operating at 2 MW
peak output power. With anode voltage of 23 kV DC and 143 Amperes peak current, the
corresponding anode efficiency was 60.8%. The power gain of the stage was 13.5 dB.
4. Subsequent Testing of Commercial Amplifiers
The prototype power amplifier was operated with the original 1998 LANL tube, the CERN
tube, and two new production tubes. It accumulated 5173 hours of high power testing, before
it was removed from service to install the first commercial amplifier.
A tender for manufacturing the LANL-designed PA was issued in 2012 and the work was
awarded to Continental Electronics Corporation of Dallas, Texas. This company had built the
original triode amplifiers in service at the LANSCE DTL since 1968. Their capabilities in
high power amplifier construction are significant, having made all of the 200 MHz DTL RF
amplifiers used at LANL, BNL and Fermilab. After a ramp up period for scope of work
definition and planning in collaboration with LANL RF and mechanical engineers, the
company delivered the first amplifier in October of 2013. The new amplifier was built to
print, using the LANL design and drawings. The only testing done at the manufacturer was
mechanical measurements and capacitor HV tests, along with operating the tuners. This
reduced time and expense as LANL already had a fully functional test facility.
- 5 -
One Power Amplifier was delivered for testing in October and two more were delivered in
December. Three have been fully tested so far, with 1278 hours of accumulated operating
time. Four more power amplifiers are planned for purchase in the next two years. Four new
TH628 Diacrodes® have been tested and accepted and 1 more is in manufacturing. Two
intermediate power amplifiers have been procured and delivered from Betatron Electronics,
with 2 more expected in the coming years. The lab and the fabricator jointly designed these,
based on the LANL prototype. This amplifier uses a Thales TH781 tetrode, and matching
TH18781 cavity amplifier. One Intermediate Power Amplifier will drive two combined
TH628s at each RF station, with a maximum capability of 200 kW peak power.
Installation of the first two amplifiers begins in February of 2014, after the removal of one
DTL RF station of the original triode design. The same work will replace one more RF station
in each of 2015 and 2016, until all of the LANSCE high power 201 MHz stations are
converted to the Diacrode® - based RF power system.
5. Design Plan for ICTF
Diacrode® amplifier 2D drawings procured by CERN have all been redesigned to 3D Catia
drawings. In addition to these CERN drawings, upon request, LANL can also provide support
for the TIARA ICTF/MICE integration of their amplifiers. LANL dimensional assembly
drawings, units in cm, are shown in figures 6 to 8. All these drawings are essential when the
implementation of the Diacrode® amplifier in the ICTF Hall will be considered.
These drawings have been incorporated with the overall ICTF 3D models used to plan the
space allocations and requirements for the equipment required for the MICE experiment by
the Daresbury drawing office. This enables us to understand and visualise how the Diacrode
based amplifier systems will be incorporated into the ICTF as part of a future upgrade.
Although slightly larger in footprint than the existing triode amplifiers, the diacrode systems
are in fact slightly shorter and comfortably fit under the mezzanine floor in place of the
triodes. This is illustrated in figure 9 (a) and (b) which shows the space behind the shield wall
(note the magnetic screening function of the shield wall will become redundant in a future
evolution of the MICE experiment, which will enable a rather simple and elegant routing of
the RF output lines to the cavities).
Figure 10 shows the top view of the mezzanine, showing the amplifiers sitting below the floor
(the current triode valve systems actually protrude through the apertures in the floor). It also
shows the layout of the power supplies for the first amplifier system. The pre-amplifiers need
be no larger than the present system (in fact the greater gain of the diacrode systems means
that the current preamplifiers are significantly larger than the minimum requirement). At the
same time the diacrodes can achieve the ICTF power requirements with higher efficiency and
significantly lower anode bias voltage, less than two thirds the voltage required by the triodes,
this means the power supplies (at least for the duty cycle pattern planned for the ICTF) need
be no larger than at present. Clearly the system will offer the potential, with suitable power
supplies and cooling infrastructure, for a substantial increase in duty cycle or peak power as
demonstrated at LANL.
- 6 -
Figure 6: Top view of the LANL Diacrode® power amplifier
- 7 -
Figure 7: Rear view of the LANL power amplifier
- 8 -
Figure 8: Side view of the LANL Diacrode® amplifier
Figure 9: 3D models showing the proposed installation of a diacrode based amplifier system in
the ICTF
- 9 -
Figure 10: Illustration of the upper mezzanine floor, notice the diacrode based amplifiers can be
accommodated completely under the floor.
6. Summary
The project has developed a 3MW capable amplifier system and demonstrated this performance
in an extensive sequence of experimental tests which have explored a range of peak power and
duty cycle ranges, spanning the 2MW, 1ms, 1Hz standard requirements of the ICTF to a peak
power performance of 3MW in 300microsecond pulses at 30Hz. These tests have also
demonstrated the reliability of the amplifier system. The amplifiers have developed in such a
way that they can be accommodated in the tight confines of the RF power station in the ICTF.
The engineering drawings produced by the amplifier design team have been mapped into the
ICTF by the hall design team to produce a realistic installation proposal. It is not anticipated
that the pre-amplifiers or power supplies need to increase in size unless the ICTF duty or peak
power requirements are uprated. Moreover the demonstration of the performance of these
amplifiers, exploiting modern valve technology, supported by the transfer of knowledge in the
design of these systems to industry, also proven by the tests of the first commercially produced
systems within this project, ensures the community can plan to exploit ionisation cooling in a
future neutrino factory or muon collider [4] with very high confidence in the availability of a
modern, reliable and sustainable RF drive system.
References
[1] E. Montesinos, J. Lyles, “Novel pulsed RF power amplifier design”, TIARA-REP-WP7-
2013-002, 2013.
[2] C. Robert, “Diacrode® TH628”, International Vacuum Electronics Conference, 2007, pp.
225-226
[3] J. Lyles, S. Archuletta, J. Davis, et al., “Progress on New High Power RF System for
LANSCE DTL”, PAC’07, Albuquerque, NM, June, 2007, pp. 2382-2384
[4] S. Geer, “Neutrino beams from muon storage rings: Characteristics and physics potential”,
Phys. Rev. D:, 1998, 57, pp6989-6997
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