european gyrotron consortium (egyc)

KIT – University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz Association

KARLSRUHE INSTITUTE OF TECHNOLOGY, Institute for Pulsed Power and Microwave Technology (IHM)

www.kit.edu

Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER

European GYrotron Consortium (EGYC)

S. Kern1, J.-P. Hogge2, S. Alberti2, K. Avramides3, G. Gantenbein1, S. Illy1, J. Jelonnek1, J. Jin1, F. Li2, I. Gr. Pagonakis1, B. Piosczyk1, T. Rzesnicki1, M. K. Thumm1, I. Tigelis4, M. Q. Tran2 and the whole EU home team at EGYC

1 Karlsruhe Institute of Technology (KIT), Institute for Pulsed Power and Microwave Technology (IHM), Association EURATOM-KIT, D-76131 Karlsruhe, Germany2Centre de Recherche en Physique des Plasmas (CRPP), Association Euratom-Confédération Suisse,EPFL, CH-1015 Lausanne, Switzerland3National Technical University of Athens (part of HELLAS), School of Electrical and Computer Engineering, 9 Iroon Polytechniou st., GR15773 Athens, Greece4National and Kapodistrian University of Athens (part of HELLAS), Faculty of Physics, 157 84 Athens, Greece.

Institute for Pulsed Power and Microwave Technology

2 08.05.2012 S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands

Overview

Introduction

Summary and consequences of former experiments

Design modifications to the industrial CW prototype

Tests of the refurbished CW prototype

SAT and RF test

Post-test evaluations

Experiments with the short pulse pre-prototype

Future plans



Introduction

The EGYC consortium develops under F4E contract 170 GHz

gyrotrons in support of EU’s contribution to ITER’s ECRH system.

- EGYC is currently CRPP, KIT, HELLAS, IFP-CNR.

The industrial partner is Thales Electron Devices (TED).

Until now, the goal was a 2 MW coaxial cavity gyrotron.

Three CW prototypes at increasing pulse length goals (1s/60s/3600s)

foreseen, only one build so far.

A 1 MW conventional tube development as fallback was started 2008.

Due to essential delays, switch to the fallback solution is probable.

This presentation reports on latest results and future plans

for the coaxial cavity project



Summary of former experiments

In 2008 first experiments with CW prototype at CRPP:Serious problems with the electron gun:

low frequency oscillations (~100 MHz region)

low voltage standoff with magnetic field applied

-> Analysis shows the existence of potential traps in the gun

RF output beam pattern insufficient (77% Gaussian content)

-> Application of new launcher design methods

RF power limited to 1.4 MW in short pulse due to mentioned problems

Power capability of the collector successfully tested (2.2 MW/10s)

Pre-prototype short pulse tests at KIT until end 2008:Power limitation by limited magnetic field (6.7 T instead of 6.87 T)

-> Application of additional normal conducting (NC) coil

Indications of beam-tunnel oscillations (159 GHz)

-> Application of corrugated beam tunnel

Same RF beam as in prototype



Resulting modifications to the CW prototype

The CW prototype was refurbished with design improvements:

New gun design following improved design rules

New launcher design

Corrugated beam tunnel

Additional modifications in mechanical construction: Ion getter pumps moved to lower magnetic field

Large ceramic isolator mounting revised for lower force during bakeout

Validation tests with the short pulse pre-prototype at KIT in 2009:

Corrugated beam tunnel applied -> No parasitic oscillations

New launcher design as above -> 96 % Gaussian content

Achieving 6.87 T with NC coil -> operation at nominal parameters

-> nominal operation: 2.2 MW @ 30 % efficiency (w/o depr. collector) !



Mechanical issues with refurbished CW prototype

Refurbishment and modification started in 2009

Manufacturing problems caused massive delays - some clearly attributed to the attempt to refurbish a large device.

The refurbishment of the mirror box was considered critical by the manufacturer, after brazings became untight at bakeout. In particular, one RF absorber had to be removed.

Delivery date moved from summer 2010 to finally end September 2011.

The tube was untight on delivery.

It could be sealed and still showed good vacuum properties after pumping.

Achievable pulse length was unclear then.

After four days of successful RF operation, another RF absorber broke and flooded the tube with water, terminating any further experiment.

Reasons still have to be investigated in detail, ultimate cause is accidental operation in wrong mode rotation.

After experiment, additional problems of alignment were found.



SAT test of the prototype gyrotron

Site acceptance test (SAT) was successful:

Excellent HV stand-off without and with magnetic field

Cooling tests OK

Beam extraction tests OK (58 kV / 75 A after 2 days of conditioning)

Coaxial insert .vs. electron beam alignment OK

Body current higher than expected, but acceptable

Body .vs. electron beam alignment ~OK

(coarse measurement, made in x direction only)

-> The tube was formally accepted. Green light to proceed with RF tests

was given by TED on December 2nd 2011.



RF tests of the prototype gyrotron (Dec. 5.-9.)

After 4-5 days of RF conditioning, the tube delivered

2 MW / 170 GHz (short pulse ~1ms)

with an efficiency of 45 %

at 75 A (nominal value) and 90.5 kV (60 kV + 30.5 kV), (nominal: 90kV, 55kV + 35kV)

No particular optimization,no evident sign of saturation, not finally conditioned.


9 08.05.2012

Simulation pre-prototype 96 % GC refurbished prototype

Comparison

S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands

RF beam pattern of the prototype

Mode TE 34,19 Mode TE 35,19 ?

Logbook shot #10410, 2011-12-05


10 08.05.2012

Positive results

2 MW / 170GHz short pulse

efficiency of 45 % (using depressed collector), non-optimized, at nominal beam parameters 75 A, 90.5 kV,depression voltage 30.5 kV (nominal: 35 kV)

Very good RF beam pattern-> second validation of launcher redesign

No low frequency oscillations

Excellent voltage standoff-> validation of gun redesign / design principles

No evidence for parasitical RF (160GHz range)-> nth validation of corrugated beam tunnel

All design modifications were verified to a high degree !!


RF tests of the prototype gyrotron: Summary

Unclear observations

Unexpectedly high starting currents (~60 A !)

Tube prone to operation in wrong mode rotation-> results in high stray radiation-> ultimate cause for broken internal absorber !

Body current -> compare to pre-prototype tests

These effects could be related to misalignment !

Nevertheless, the risk of total loss of a tube due to a breaking absorber is inacceptable

-> redesign of internal absorber scheme

Negative results

Unexpected, but acceptable body current

Unclear alignment situation

Absorber broken, total loss of tube

No long pulses were possible !


11 08.05.2012S. Kern et al., Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER, EC17 Workshop, 07-11 May 2012, Deurne, The Netherlands

Post-test evaluations: Alignment

The magnet alignment (ASG magnet at CRPP) could only be checked after the experiment.

Results of subsequent check: Magnetic axis is shifted by 0.7 mm in y-direction (check with tube indicating good alignment done in x-direction only !).

Additional result: Due to some loose stabilisation rods, the tube can be easily bend by millimeters, resulting in basically undefined alignment !

-> The relative body-beam alignment along ycould have been anything between good and bad !

-> Investigations on the influence of tube alignment are needed for evaluation of the observations !


12 08.05.2012

Experiments with the KIT pre-prototype are currently running in short pulse in the following configuration:

Further improved launcher design: smoothed launcher surface

Electron gun refurbished by TED:

New emitter ring

New cathode and anode shape, identical to CW prototype-> this includes a small halo shield with ~2mm electron beam clearance

Ready for depressed collector operation

Gyrotron housing reworked to fit into 220 mm bore hole-> KIT OI magnet was equipped with cooled CW NC coil


Experimental setup of the pre-prototype


13 08.05.2012

Start / End of the probe

measurement

Alignment procedure and measurements

Emission uniformity test and alignment of coaxial insert

be

fore

th

e a

lign

me

nt

dR/Idipol=0.1mm/A Shift of the electron beam position using dipole coils

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-10

-5

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5

10

I Xd

ipo

l / A

IyIYdipol

/ A

Verification of the gyrotron position


Concentricity of the coaxial insert with respect to the electron beam: δR ~ 0.04mm

Concentricity of the electron beam with respect to cavity wall: δR ≤ 0.1 mm

-> excellent alignment conditions


14 08.05.2012

Comparison to CW prototype tests:

Low starting currents (10A)

No unusual danger of operating in wrong rotation

Good voltage standoff

But: strong LF oscillations during startup – due to gun rear part or small remaining potential traps?

Body current at design parameters -> can be avoided by parameter settings-> but calls for investigations: reasons are unclear, halo shield design ?

- electron beam radius appears 1.8 mm larger than expected !

Current results of the ongoing short-pulse tests are:

RF power:1.9 MW @ 28% efficiency (w/o depr. collector), not optimized

Reduced stray radiation: 4% instead of 7%

Good beam pattern

Next steps:

Further conditioning

Preparation for depressed collector test

Tests of the influence of misalignment


Latest experimental results of the pre-prototype

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New Launcher / q.o. system measurements

The smoothed launcher shows an equally good RF beam pattern as its predecessor.

Measured stray radiation is essentially reduced to

4 % of the RF output power

Former results:7 % original KIT design5.5 % IAP design

window

thermal image 85 mm from window

thermal image 1000 mm from window

burned paper spot



Future plans

2nd Industrial CW prototype: formal decision pending, 1MW decision probable

Agreement on necessary modifications on scientific side achieved

In view of their future relevance, KIT will continue experiments with coaxial cavity gyrotrons – if necessary on stretched time scales.

The KIT short-pulse pre-prototype gyrotron will be sequentially extended for longer pulse lengths.

This is made possible through a modular approach which enables an easy exchange of components.

Next experimental steps with this “Modular Gyrotron Concept“:

(1) Test other components in short pulse: ‚different launchers, modified electron guns, different beam tunnels

(2) Add CW collector and CVD window -> ~100 ms pulse length

(3) Replace remaining short pulse components (cavity,q.o. system, beam tunnel, gyrotron housing) by cooled CW parts -> 10 s pulse length

In parallel: broad band operation tests (around 140 GHz /1.8MW already done)



Modular gyrotron setup steps

Current step:

First depressed collector tests

Launcher test

CW gun with prototype shape- cathode nose easily exchangeable

2nd: short pulse component tests

Beam tunnel, launcher, anode shape

3rd: CW collector and window

Increase of pulse length to ~ 100 ms

4th: fully CW compatible

CW cooling added, new RF absorber scheme

New redesigned electron gun

Increase of pulse length up to 10s (KIT power supply limit)



Plans for a hypothetical 2nd CW prototype

Component, topic, observation

Internal absorbers, mirror box

Launcher, Q.o. system

Halo shield

Collector Beam tunnel, parasitic modes

Tube alignment

Modes with wrong rotation

High starting currents

High body current

Better cooling of shaft

Any other tube compo-nent

Summary Changes

Remove and discuss replacement:Preferably two relief windows with or without stainless steel pipes

Replace the launcher by the new version, presently under test (lower stray radiation already proven)

Increase halo shield radius by 0.5-2mmTo be short term optimized

No change

No change

Revised by TED; possibility for controlled alignment tests added

No change

No change

No change

No change

No change

Next step activities

cooled stainless steel pipes coated/uncoated/inside carbon tubes- relief windows- mirror vessel partly or totally coated (CrO,

CR2O3) with

external cooling

Consider tube inspection



Summary

The latest test results with EU 2 MW 170 GHz coaxial cavity tubes were reported.

CW prototype results:

2 MW / 170 GHz / 45 % non-optimized in very short time

All design modifications validated

No long pulse validation of RF performance or cooling (except collector)

Tube damaged, internal RF absorber scheme needs redesign

Short pulse pre-prototype results:

1.9 MW 28 % (without depressed collector, not finally conditioned)

Smoothed launcher validated: stray radiation reduced to 4 %

Depressed collector operation under preparation

Future plans:

2 MW or 1 MW ITER development: F4E decision pending

Coaxial experiments towards long pulse continue at KIT



The authors acknowledge gratefully the continuing support of TED and F4E staff in these projects, in particular F. Albajar, F. Cismondi and T. Bonicelli at F4E as well

as R. Marchesin, C. Lievin, F. Legrande and P. Benin at TED.

This work was supported by Fusion for Energy under Grant F4E-2009-GRT-049 (PMS-H.CD)-01 and within the European Gyrotron Consortium (EGYC). The views

and opinions expressed herein do not necessarily reflect those of the European Commission. EGYC is a collaboration among CRPP, Switzerland; KIT, Germany;

HELLAS, Greece; IFP-CNR, Italy.

Thank you for your attention !

Acknowledgement


21 08.05.2012

clearance ~ 2mm

4. Body current

has been observed at the nominal magnetic field parameters increasing of the magnetic field compression (= reduction of the beam radius ~0.2 mm only)allows to reduce the body current to 0 the investigation of the several magnetic field profiles shows that the problem appearsin the region of anode, at the halo-shield (and not at the launcher cut) however, the calculated clearance between the halo-shield is ~2mm, - it indicates that some additional “electron flow” exists outside the regular beam flow / or the thickness of the beam is higher than expected

possible reasons:- field emission at the edges in the rear part of the gun

- additional electron emission from the regions around the emitter(i.e. due to bad thermal isolation)

to avoid the body current flow, a different magnetic field configuration has been proposed- it was necessary to energize of the bucking-coil separately from the main coil

Experimental results with the modular gyrotron



22 08.05.2012

4. Body current

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Body current vs. beam current- strong, linear dependence

- body current relatively high, about 3% of the current of the electron beam!

Body current vs. cathode voltage- nearly linear dependence, it means: no field-emission effect

Experimental results with the modular gyrotron


european gyrotron consortium (egyc)

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