Disturbance Observer Application for the Compensation of the Phase Drift of the LANSCE DTL LINAC Solid State Power Amplifier Sungil Kwon M. S. Barrueta L. Castellano J. M. Lyles M. Prokop P . Van Rooy P. Torrez

2025-04-26 0 0 543.13KB 4 页 10玖币

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Disturbance Observer Application for the Compensation of the Phase Drift of

the LANSCE DTL LINAC Solid State Power Amplifier*

Sungil Kwon†, M. S. Barrueta, L. Castellano, J. M. Lyles, M. Prokop, P. Van Rooy, P. Torrez

Los Alamos National Laboratory, Los Alamos, NM, USA

Abstract

The front end of Los Alamos Neutron Science Center

(LANSCE) linear accelerator uses four 201.25-MHz

Drift-Tube Linacs (DTLs) to accelerate the H+ and H-

beams to 100 MeV. Three of the 201.25-MHz DTLs are

tetrode. A 20-kW solid-state power amplifier (SSPA) is

used to provide ~15 kW drive power to the tetrode. The

SSPA is water-cooled and consists of 24 push-pull

LDMOS transistors operating at 45% of their power

saturation capability, providing ample power headroom

and excellent linearity. However, the phase of the SSPA

is perturbed at +/-20 degrees over a few ten minutes

partially caused by the temperature dependent phase

variation of the air-cooled SSPA driver circulator. This

phase variation consumes most of the phase control

margin of the cavity field feedback controller. In order to

mitigate the effect of the SSPA’s phase variation on the

cavity field, a disturbance observer controller (DOBC)

has been designed and implemented on the cavity field

control FPGA, which functions in parallel with the cavity

field feedback controller. In this paper, the DOBC design

and its function as well as its short- and long-term

performance are addressed.

1. INTRODUCTION

The capabilities of the Los Alamos Neutron Science

Center (LANSCE) experimental facilities include: 1) the

Lujan Center, which requires short, high-intensity proton

bunches in order to create short bursts of moderated neu-

trons with energies in the meV to keV range; 2) the Pro-

ton Radiography (pRad) Facility, which provides time-

lapse images of dynamic phenomena in bulk material (for

example, shock wave propagation) via 50-nsec-wide pro-

ton bursts, repeated at time intervals as short as 358 nsec

with programmable burst repetition rates; 3) the Weapons

Neutron Research (WNR) Facility, which provides un-

moderated neutrons with energies in the keV to MeV

range; 4) the Isotope Production Facility (IPF), which

uses the 100-MeV H+ beam to make medical radioiso-

topes; and 5) the Ultra Cold Neutron (UCN) Facility,

which creates neutrons with energies below the 𝜇𝑒𝑉 ener-

gy range for basic physics research [1]. For all beam spe-

cies, the beam pulse length is required to be adjusted.

The default beam pulse length is 625 usec and it can be

increased to 785 usec.

The ability of the digital low-level RF (DLLRF) con-

trol system to accommodate various beam loading condi-

tions is crucial for successful LANSCE operations in

which a wide variety of beam types of various levels of

beam loading are present in the accelerator’s RF cavities.

To provide the stable cavity field before the beam loading

and to minimize the perturbation of the cavity field

caused by beam loading, the LANSCE DLLRF control

system implements both a proportional-integral (PI) feed-

back controller (FBC) and feedforward controller capabil-

ities.

For a small peak current beam loading, the PI FBC is

sufficient to compensate for the beam loading in the cavi-

ty field. However, for high peak current beam loading, the

simple PI FBC is not sufficient and a feedforward control-

ler is crucial to the beam loading compensation capability.

Furthermore, in order to keep the stability of the closed

loop system against the external disturbance inputs, sys-

tem uncertainties, the PI FBC should provide the gain and

phase margin sufficiently.

At the LANSCE DTL tank1, a SSPA provides 15 kW

RF power to the final stage high amplifier Thales TH781

tetrode. While the SSPA guarantees ample power margin

and excellent linearity, it was observed that at the RF

turn-on transient of a few ten minutes, its phase is per-

turbed at 40 degrees. This transient phase perturbation

consumes most of the phase control margin of the PI FBC

and increases the possibility of the RF trip. In this paper,

a method to mitigate the effect of the phase variation on

the cavity field stability is addressed. For this purpose, the

phase variation of the SSPA is treated as input disturb-

ance and a DOBC is designed, implemented on the

DLLRF system to provide enough phase margin to the

PI FBC.

Figure 1. High-level functional diagram of the LANSCE digital

low-level RF control system.

________________________________________

* Work supported by U.S. Dept. of Energy

† email address: skwon@lanl.gov.

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DisturbanceObserverApplicationfortheCompensationofthePhaseDriftoftheLANSCEDTLLINACSolidStatePowerAmplifier*SungilKwon†,M.S.Barrueta,L.Castellano,J.M.Lyles,M.Prokop,P.VanRooy,P.TorrezLosAlamosNationalLaboratory,LosAlamos,NM,USAAbstractThefrontendofLosAlamosNeutronScienceCenter(LANSCE)linearaccelerato...

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Disturbance Observer Application for the Compensation of the Phase Drift of the LANSCE DTL LINAC Solid State Power Amplifier Sungil Kwon M. S. Barrueta L. Castellano J. M. Lyles M. Prokop P . Van Rooy P. Torrez.pdf

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Disturbance Observer Application for the Compensation of the Phase Drift of the LANSCE DTL LINAC Solid State Power Amplifier Sungil Kwon M. S. Barrueta L. Castellano J. M. Lyles M. Prokop P . Van Rooy P. Torrez

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