High Power Amplifiers

R.A. Yogi ESS RF Group Unit Leader for Spoke Power and RF Distribution

FREIA Group Unit Leader

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ESS: Superconducting 5 MW Linac

Frequency = 352.21 MHz Number of spoke resonators = 28 Maximum power to beam = 240 kW Maximum generator power = 300 kW

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Power Coupler

New design being developed at IPN Orsay

High Power Test Stand at Uppsala

High power testing needed !

Two high power RF chains are needed to test two spoke cavities at Uppsala test stand !

Baseline Design for ESS RF system

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Generation and distribution of the RF power from a single source to a single accelerating cavity

The capital and running cost of an accelerator is strongly affected by the RF power amplifiers: Capital cost: cost of the amplifiers gain of the power amplifier (decides number of stages) and hence size and weight of the amplifiers: (decides gallery requirements) Running cost: efficiency determines the electric power required and also amount of cooling needed Life time: replacement, maintenance schedule

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Importance of selection of RF source

RF Source Requirements

• Maximum RF power for a spoke resonator = 240 kW

• Considering LLRF overhead = 15% (Simulink model shows 12.5% power overhead)

• RF loss in distribution system = 5%, Power of RF source = 288 kW. Aim for 300 kW (4.0% extra, Why ? Will be explained in next slides)

• Beam pulse width = 2.86 ms, repetition rate = 14 Hz, fill time of the cavity:

Natural fill time = tf = = 135 µs, (QL = 1.5 x 106 ) RF pulse width = 3.1 ms Duty factor of the amplifier ≈ 4.28 %

• Spoke cavity band-width = 2.34 kHz

system band-width ≈ 100 times larger than spoke resonator band-width for tuning and regulation delay. 3 dB bandwidth ≥ 250 kHz.

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High Power RF Amplifier

Specifications of RF Amplifier: Frequency = 352.21 MHz Power = 300 kW 3dB band-width ≥ 250 kW Pulse width = 3.1 ms Pulse repetition rate = 14 Hz

No RF source exists at ESS specifications ! Hence development and prototyping is important !

Compared all the possible RF sources like Tetrode, Klystron, IOT, Solid state amplifier and selected Tetrode for the first high power RF chain. Simultaneously high power RF amplifier using solid state technology under development for the second chain.

Supported by AIR and TB at ESS

Comparison of Tetrodes

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TH781, TH391 and TH595 tetrodes can be used at ESS specifications. Typ. Performance

TH781 TH391 TH595

Maximum Frequency

250 MHz 450 MHz 450 MHz

Frequency (as per Thales catalogue)

200 MHz 352 MHZ 380 MHz 380 MHz

Power 200 kW 350 kW 200 kW 200 kW Pulse width cw 3.5 ms 100 µs 100 µs gain 12.2 dB 13 dB 15 dB 15 dB Efficiency 68.9% 55 %

(predicted) 67 % 67 %

Cavity (as per Thales catalogue)

TH18230B (200 kW, 380 MHz)

TH18528 (100 kW peak, 85-115 MHz)

Type of anode cooling & anode dissipation

Water, 250 kW Air, 12 kW Water, 40 kW

TH595 is selected for first high power RF chain. Output power of two tetrodes needs to be combined.

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Foot area:2.5m x 2.5m

Foot area:0.6m x 0.6m

Schematic for first RF chain

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SG: Signal generator, Φ: Phase shifter, A: Attenuator, A1 & A2: Preamplifiers, PA1 & PA2: High power amplifiers. Transmitted power and RF distribution systems are specified on top of schematic.

Loss in the connections are not considered due to the 4% margin on the generator power.

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Amplifier cavity TH 18595A

Tetrode TH595

Tetrode TH595 and cavity Th18595 A at Thales

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Schematic of RF station used at Thales

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Parameters ESS specifications

Test results at Thales

Frequency 352.21 MHz 352.21 MHz Power > 320 kW

(> 300 kW) > 400 kW

3 dB Band-width > 250 kHz 7 MHz Efficiency > 50 % 67 % Gain > 10 dB 15 dB

Conclusion for High power RF Amplifier

Comparison of target ESS specifications and achieved performance of TH595 and cavity

Thus TH595 shall be used for first high power amplifier chain.

Pre-amplifier

As gain of tetrode decreases due to aging, conservative gain of 13 dB is considered while calculating specifications of solid state amplifier. Thus output power of pre-driver = 8.7 kW. Aim for 10 kW.

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Specifications of Predriver: • Frequency = 352.21 MHz • Output power = 10 kW • Gain = 70 dB • Total efficiency = 55 % • Class of operation: AB

Selected solid state amplifier after comparing the available technologies. • Gain 70 dB • Efficiency > 50 - 55% (class AB) • Good reliability: Built in redundancy • Modular system, easy replacement possible • Off-shelf available system

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RF unit of 700W power RF 5 kW power module

Conclusion TH595 with amplifier cavity TH18595A can be used as high power amplifier Solid state amplifier can be used as pre-driver.

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Thank you ! Comments and suggestions are welcome

10 kW 352MHz Pre-amplifier

• Solid state amplifier: Gain 73 dB, so only one stage Good Reliability: Built in redundancy Efficiency > 50 - 55% (class AB) Modular system, so easy replacement possible • Triode: Low Gain 10 dB, so consists of three amplifier stages. It

requires pre-predriver triode (1 kW), and solid state low power amplifier (10 W) Complex system. 3 amplifier stages, their power supplies,

protections, hence low reliability. Efficiency: 70% 19

Features RF Distribution

No pressurization

Use of Ferrite loads

Distribution at 3 levels • Half height WR2300: 350kW • 6-1/8 inch, 50 Ω coax: 175 kW • 7/8 inch, 50 Ω coax: 10 kW

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Typical power sources used at 352 MHz are tetrodes, klystrons, IOTs and solid state amplifiers

Criteria of comparison: Power distribution scheme Lifetime Efficiency Gain Availability Costs

Too big for 350 kW power Size: 1 m x 1 m, weight: few tons Efficiency: 60 – 65% Modulator needed for power-supply,100 kV Gain: 37 dB Predriver not needed Life time: 40-50 khours Circulator needed for handling reflection Base line of one RF amplifier per cavity

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Klystron

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Solid state amplifier

70 modules of 5 kW each that are combined High reliability due to use of circulators and hybrid couplers. Size: very big 10 m2 Efficiency: 65% Few distributed power supply 50 V, 200 A (low voltage, high current) Gain: 37 dB Predriver not needed Life time: 50 khours

RF unit of 700W power

RF 5kW power module