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.

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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