Energy regulation of excimer laser high-voltage power supplies in laser-induced fluorescence and micro-welding
An excimer laser supply charges a pulse forming network capacitor bank to 20 kV, at a repetition rate of 100 pulses per second. The laser tube is a krypton fluoride type, 30 mm bore, 1 m long, filled with a gas mixture of krypton, fluorine, and neon at 3 bar. The pulse energy is regulated at 500 mJ per pulse, measured by a pyroelectric detector at the laser output. The supply uses a resonant charging topology, with the capacitor bank charging from 0 to 20 kV in 8 ms, then discharging through the laser tube in 20 ns.
The pulse forming network consists of 20 capacitors, each 0.1 μF at 30 kV, connected in parallel. The total capacitance is 2 μF, and the stored energy at 20 kV is 400 J. The discharge current peaks at 20 kA, and the current risetime is 5 ns. The discharge electrodes inside the laser tube are copper, 50 mm wide, with a 10 mm gap. The electrode surface erodes slowly over 10 million pulses, and the erosion rate is monitored by measuring the pulse energy drift.
The resonant charging inverter runs at 20 kHz, with a 540 V DC bus. The inverter switches IGBT modules, and the resonant inductor is 2 mH. The charging current is limited to 10 A, and the capacitor bank voltage reaches 20 kV in 8 ms. The charging voltage is measured by a high-voltage divider, and the feedback loop adjusts the inverter duty cycle to hold the target voltage. The loop bandwidth is 1 kHz, fast enough to compensate for the pulse-to-pulse energy variation.
The pulse energy regulation loop uses the pyroelectric detector signal as feedback. The detector outputs a 1 V signal for 500 mJ, and the signal feeds the control board ADC. The control board adjusts the charging voltage setpoint based on the measured pulse energy. If the pulse energy drifts 5 percent below the target, the charging voltage is raised by 1 percent, and the energy returns to the target within 10 pulses. The loop once overshot when the laser tube gas pressure drifted, and the pulse energy surged to 600 mJ. The overshoot was corrected by adding a rate limiter to the charging voltage setpoint.
The laser tube gas mixture is refilled every 10 million pulses, and the gas purity is checked by a gas chromatograph. The fluorine concentration is critical, and a 5 percent drop in fluorine reduces the pulse energy by 10 percent. The gas refill takes 30 minutes, during which the supply is offline. The gas cylinders are stored in a ventilated cabinet, and the cabinet exhaust routes to a scrubber.
The high-voltage cable connects the supply to the laser tube, through a 3 m shielded cable. The cable is a 30 kV silicone type, with the outer jacket grounded at both ends. The cable once developed a hairline crack in the jacket, and the leakage current rose from 2 μA to 20 μA. The leakage did not trip the 100 μA overcurrent threshold, but the extra 18 μA added noise to the charging voltage measurement. The cable was replaced at the next scheduled maintenance, and the leakage returned to 2 μA.
The supply cabinet is 800 mm wide, 1000 mm deep, 1800 mm tall, with IP54 protection. The cabinet contains the inverter, resonant inductor, capacitor bank, thyratron switch, and control board. The cabinet cooling fans run on a thermal switch, starting at 35 degrees and stopping at 25 degrees. The fans once failed in summer, and the cabinet temperature reached 55 degrees. The electrolytic capacitors on the DC bus reached 65 degrees, above the 60 degree rating. The fan fault alarm now routes to the central monitoring room, and the fan is replaced within 10 minutes of alarm.
The control board uses a 16-bit ADC, sampling the charging voltage at 1 kHz. The ADC reference is a buried-zener type, with a drift of 2 ppm per degree Celsius. The cabinet temperature is held at 25 plus or minus 2 degrees by the fan and heater, and the reference drift over a 24 hour operation stays below 10 ppm, equivalent to 0.02 V at 20 kV. The drift is within the 0.1 percent acceptance limit, and the reference is recalibrated annually against a 0.01 percent standard divider.
The safety interlock on the laser tube enclosure was tested during commissioning. A technician opened the enclosure while the laser was running, and the supply stopped charging within 20 ms. The residual charge on the capacitor bank bled through a 10 MΩ resistor within 100 ms. The interlock response time was logged at 18 ms, within the 25 ms design limit. The enclosure carries a laser radiation warning label, and entry requires safety glasses. The non-authorized personnel are not permitted inside the laser bay.
The maintenance log lists routine tasks. Each shift, the operator records the charging voltage, pulse energy, and cabinet temperature. Weekly, the technician inspects the cable jacket and the thyratron grid current. Monthly, the insulation resistance is measured with a 2500 V megohmmeter, and the reading should exceed 1000 MΩ. Quarterly, the divider is calibrated against a 0.1 percent standard. Annually, the ADC reference is recalibrated. The log is signed at each step, and the record is filed in the plant archive.
The input side of the supply draws 400 V three phase at 20 A. A 80 kVA isolation transformer sits between the plant bus and the supply, to isolate the laser tube ground from the plant ground. The isolation transformer has a 2500 V insulation rating between primary and secondary. The input cable is copper, 16 mm², rated for 50 A, routed through a cable tray. The input breaker is a 63 A molded case type, with electronic trip set to 1.05 times rated current long delay. The input power factor is 0.96, measured with a power analyzer during commissioning.
The high-voltage transformer inside the supply cabinet is oil insulated, with a 20 L oil volume. The oil temperature reaches 48 degrees after 8 hours of continuous operation at 20 kV and 100 pulses per second, against an ambient of 25 degrees. The silica gel breather on the transformer tank changes color after 12 months, and the gel is replaced before the color reaches half saturation. The oil breakdown voltage is tested annually, and the last measurement showed 40 kV per 2.5 mm gap, well above the 25 kV replacement threshold. The transformer sound level is 52 dB at 1 m, measured with a sound level meter during commissioning.
The output divider is a series chain of 100 MΩ high-voltage glass glaze resistors, totaling 1 GΩ. The divider tap at 1:1000 feeds the control board ADC. The divider is calibrated quarterly against a 0.1 percent standard divider. The calibration drift over 12 months was 0.3 percent, equivalent to 60 V at 20 kV. The drift is within the 1 percent acceptance limit, but the divider is scheduled for resistor replacement at the 24 month mark.
The thyratron switch is a hydrogen thyratron, rated for 30 kV and 30 kA peak current. The thyratron filament is heated to 1500 degrees, and the filament life is 2000 hours. The thyratron grid current is monitored, and a 20 percent increase in grid current signals end of life. The thyratron once failed mid-run, and the capacitor bank did not discharge, leaving the laser tube unenergized. The failure was caught by the grid current monitor, and the thyratron was replaced within 2 hours.
The laser tube cooling system is a closed loop with deionized water, conductivity below 1 μS per cm. The water flow is 10 L per minute, and the water temperature is held at 20 degrees by a chiller. The chiller once failed, and the water temperature rose to 30 degrees. The laser tube electrode temperature reached 60 degrees, and the pulse energy dropped 15 percent. The chiller was repaired within 4 hours, and the pulse energy returned to normal. The water conductivity is measured weekly, and the ion exchange resin is replaced when the conductivity exceeds 1 μS per cm.
The customer acceptance test was run over three days. Day one covered the pulse energy stability, running 1 million pulses and measuring the energy drift. Day two covered the pulse energy regulation, injecting a 5 percent gas pressure dip and measuring the compensation. Day three covered the safety interlocks, opening the enclosure and verifying the shutdown response. The test report was signed by the customer representative, and the supply entered the 12 month warranty period.
The plant environment around the supply cabinet is held at 23 plus or minus 3 degrees Celsius, with 45 to 65 percent relative humidity. Winter heating raises the room temperature to 26 degrees, and the cabinet fan runs continuously. Summer cooling failure once pushed the room temperature to 32 degrees, and the cabinet internal temperature reached 50 degrees. The ADC reference drifted 0.3% during that week, equivalent to a 60 V error at 20 kV. The drift was corrected after the cooling unit was repaired, and the supply was recalibrated. The plant floor is epoxy coated, cleaned daily with a vacuum cleaner, and the illumination is 300 lux with LED fixtures at 24 V.
The safety interlock on the supply cabinet door was tested during commissioning. A technician opened the door while the supply was running, and the output dropped to zero within 20 ms. The residual charge on the capacitor bank bled through a 10 MΩ resistor within 100 ms. The interlock response time was logged at 18 ms, within the 25 ms design limit. The cabinet door carries a high voltage warning label, and entry requires a sign-in log. The non-authorized personnel are not permitted inside the power supply bay.
The laser process data is recorded on the laser workstation, with the run number, the pulse energy, the repetition rate, and the run time. The data is stored for 5 years, and is available for customer audit. The quality department reviews the data weekly, and trends the pulse energy drift. The data system generates a weekly report, showing the process yield and the defect rate. The report is distributed to the plant manager and the customer.
The laser bay safety audit is conducted annually, by an internal safety team. The audit covers the laser radiation, the high-voltage system, and the gas supply. The audit report lists the findings, and the corrective actions are scheduled within 30 days. The last audit found two minor findings: a worn door seal, and a faded laser warning sign. Both findings were corrected within the scheduled time. The audit record is filed in the quality system.
The laser bay spare parts inventory includes: thyratrons, pulse capacitors, fuses, and pyroelectric detectors. The inventory is stocked at 20 percent of the annual consumption, and is reviewed monthly. The inventory is stored in a locked cabinet, and the key is held by the bay supervisor. The parts are logged out when used, and the log is reviewed by the quality department. The inventory was once depleted when a thyratron failed unexpectedly, and the stock was replenished within 2 days.
The laser bay operates two shifts per day, 6 days per week. The day shift runs the laser-induced fluorescence experiments, and the evening shift runs the micro-welding production runs. The product changeover happens between shifts, and the optics are swapped. The shift handover includes a status report, listing the completed runs, the pulse energy, and the tube gas pressure. The handover log is reviewed by the bay supervisor, who allocates resources for the next shift.
The laser process is qualified per ISO 11554, with the pulse energy and beam profile tested. The test coupons are prepared alongside the production runs, and the coupons are tested after each batch. The pulse energy is measured by a pyroelectric detector, and the beam profile is measured by a beam profiler. The test results are recorded in the quality system, and the batch is released only when all tests pass. The process has maintained the qualification for 4 years, with no major findings in the last 2 audits.
The laser tube gas supply system is maintained by the maintenance department, with weekly inspection and monthly preventive maintenance. The gas cylinders are stored in a ventilated cabinet, and the gas pressure is regulated at 0.1 MPa. The gas flow is controlled by a mass flow controller, calibrated quarterly. The gas supply system has a 99 percent uptime target, and the downtime is logged in the maintenance record. The system once had a gas leak, and the leak was fixed within 2 hours, with no production loss.
The supply has run for 16 months. The thyratron has been replaced once, at 8 million pulses. The capacitor bank shows a 5 percent capacitance loss, still within the 10 percent threshold. The unresolved issue is the pulse energy drift when the laser tube gas pressure changes. The drift is compensated by the regulation loop, but the compensation lags by 10 pulses. A faster detector is under evaluation, and the upgrade is scheduled for the next control board revision.
